Mexico City Basin Wind Circulation During the MCMA-2003 Field

Atmos. Chem. Phys., 5, 2267–2288, 2005 www.atmos-chem-phys.org/acp/5/2267/ Atmospheric SRef-ID: 1680-7324/acp/2005-5-2267 Chemistry European Geosciences Union and Physics

Mexico City basin wind circulation during the MCMA-2003 ﬁeld campaign

B. de Foy1, E. Caetano2, V. Magana˜ 2, A. Zitacuaro´ 2, B. Cardenas´ 3, A. Retama4, R. Ramos4, L. T. Molina1, and M. J. Molina1 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, USA 2Centro de Ciencias de la Atmosfera,´ Universidad Nacional Autonoma´ de Mexico,´ Mexico 3General Direction of the National Center for Environmental Research and Training (CENICA), National Institute of Ecology (INE), Mexico 4Secretar´ıa del Medio Ambiente, Gobierno del Distrito Federal, Mexico Received: 22 March 2005 – Published in Atmos. Chem. Phys. Discuss.: 2 May 2005 Revised: 28 July 2005 – Accepted: 11 August 2005 – Published: 26 August 2005

Abstract. MCMA-2003 was a major field campaign in- 1 Introduction vestigating the atmospheric chemistry of the Mexico City Metropolitan Area (MCMA) in April of 2003. This paper The Mexico City Metropolitan Area (MCMA) is situated in- ◦ describes the wind circulation patterns during the campaign side a basin at 2240 m altitude and 19 N latitude. The basin both within the Mexico City basin and on the regional scale. is surrounded by high mountains on three sides as shown in “Time roses” are introduced to concisely analyze the diurnal Fig. 1. Basin drainage flows in the MCMA are therefore wind patterns. Three episode types were identified that ex- modified by gap flows as well as plateau winds and valley plain the conditions encountered: “O3-South”, “Cold Surge” effects further afield. To the west and south there is an el- and “O3-North”. These can be diagnosed from a combina- evated rim of which the highest point is the Volcan Ajusco tion of synoptic and basin observations based on whether at 3930 m above sea level. To the east is a north/south bar- the day was predominantly cloudy, or whether the O3 peak rier consisting of three peaks, with the Popocatepetl´ reach- was in the north or south of the basin. O3-South days have ing an elevation of 5465 m. To the north, the basin extends weak synoptic forcing due to an anti-cyclone over the east- into the Mexican Plateau and the arid interior of the country, ern Pacific. Strong solar heating leads to northerly flows in with the Sierra de Guadalupe creating a small 800 m bar- the basin and an evening shift due to a gap flow from the rier above the basin floor. The regional topography is just south-east. Peak ozone concentrations are in the convergence as important as that of the basin. To the south, at low alti- zone in the south of the city. Cold Surge days are associated tude, are Cuernavaca and the valley of Cuautla. Immediately with “El Norte” events, with strong surface northerlies bring- to the north-east is the bottom of the Sierra Madre Oriental ing cold moist air and rain. Stable conditions lead to high parallel to coastal plains 50 km away and the coast of the concentrations of primary pollutants and peak ozone in the Gulf of Mexico 200 km away. To the north-west and west city center. O3-North days occur when the sub-tropical jet is is the Sierra Madre Occidental which forms a barrier to the closer to Mexico City. With strong westerlies aloft, the circu- Pacific, 300 km away. Molina and Molina (2002) describe lation pattern is the same as O3-South days except for a wind in greater detail the MCMA and provide the current state of shift in the mid-afternoon leading to ozone peaks in the north understanding of its air quality. of the city. This classification is proposed as a means of un- The Mexico City basin meteorology is usually classified derstanding pollutant transport in the Mexico City basin and into three seasons: the cold dry season from November to as a basis for future meteorological and chemical analysis. February, the warm dry season from March to April and the Furthermore, model evaluation and design of policy recom- rainy season from May to October. These are due to the two mendations will need to take into account the three episode basic patterns on the synoptic scale: dry, westerly flow from types. November to April with anticyclonic conditions, and moist easterly flows due to the weaker trade winds for the other half of the year. Correspondence to: B. de Foy ([email protected])

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N Mexican Plateau convected aloft into the drainage flows of the mountains Plateau Winds at night. Bossert (1997) analyzed three different episodes Sierra Madre Oriental Cerro Catedral which he called the synoptic, thermal and regional flows. In the first case, southerly flow aloft leads to strong cool Gulf Breeze Nevado de Toluca southerly flow in the basin and rapid pollutant washout to the north. In the second case, very weak synoptic flows lead to a situation dominated by thermal heating in the basin. Up- Pachuca slope flow leads to basin venting to the upper troposphere. Mexico City Lastly, north-westerly synoptic flow leads to a situation fa- Afternoon Jet (Chalco Passage) voring the arrival of cool moist air from the Gulf of Mexico Cuernavaca into the basin. This leads to pollutant accumulation in the south of the basin followed by mountain venting as for the second case. Fig. 1. Mexico City basin and surrounding areas. The mountains surrounding the basins can be clearly seen, along with the MexicanThe Plateau next international campaign was the IMADA-AVER to the north-west, theFig. Sierra Madre1. Mexico Oriental andCity the basin drop to theand Gulf surrounding of Mexico plain areas. to the north-east The mountains and the low-lying Cuernavaca and Cuautla valley to the south.surrounding The location the of the basins meteorological can be stations clearly at Cerro seen, Catedral along (CATE), with Nevado the de Mexi- Toluca (NEVA)Boundary and Pachuca Layer Experiment held in February and March (PACH) are indicated. Black arrows show 3 main wind features described in the literature: plateau winds, Gulf breezes and theof afternoon 1997 jet (Doran et al., 1998). Fast and Zhong (1998) into the basin. can Plateau to the north-west, the Sierra Madre Oriental and the drop to the Gulf of Mexico plain to the north-east and the low-lying analyzed the meteorological factors associated with inho- Cuernavaca and Cuautla valley to the south. The location of the me- mogeneous ozone concentrations using mesoscale models. teorological stations at Cerro Catedral (CATE), Nevado de Toluca Thermal flows were found similar to those described by (NEVA) and Pachuca (PACH) are indicated. Black arrows show 3 Bossert (1997), but with significant vertical wind direction main wind features described in the literature: plateau winds, Gulf shear leading to recirculation over the basin and possible re- breezes and the afternoon jet into the basin. entrapment of air pollutants in the northern part of the basin. Other days had similar thermal flows but with no vertical shear and weaker flows aloft, combined with increased at- Due to its complex physiography, the meteorology of the mospheric stability leading to pollutant accumulation in the MCMA cannot be simplified to one individual factor but de- south of the city. Residence times were analyzed with a La- pends rather on the interplay of the basin with the Mexican grangian model showing very little multi-day accumulation. plateau and the lower coastal areas. Jauregui (1988) ana- Whiteman et al. (2000) found very strong daytime heat- lyzed the surface wind and temperature measured in the city Atmos. Chem. Phys., 5, 1–40, 2005 www.atmos-chem-phys.org/acp/5/1/ing which was followed by rapid cooling in the late after- and their impact on SO2 levels. For the dry season when anti- noon. This could only be explained by horizontal advection cyclonic conditions and westerly flows aloft dominate, weak of cold air from the south. Furthermore, flows were described northerly flows were found during the morning, strengthen- that were not typical basin flows but rather were due to the ing in the afternoon with an increased westerly component plateau-basin interactions. The coastal flows described by and turning into downslope drainage flows at night. Of par- Bossert (1997) were found to be held back by the moun- ticular significance was the effect of the urban heat island tains and only led to an influx of cold air when the plateau- which may increase the drainage flows but reduces the up- basin thermal balance reverses later in the day. In addition, slope flow to the west during the day. The surface energy Whiteman et al. (2000) noted an absence of nocturnal inver- balance was measured by Oke et al. (1992) showing large sions due to urban effects. Doran and Zhong (2000) ana- ground heat fluxes capable of modifying the local circula- lyzed the strong southerly gap flow from the Cuautla valley tion. Jauregui and Luyando (1999) looked at the contribution through the Chalco passage. This correlates with both the of the attenuation due to aerosols to the radiation budget. In- north/south surface temperature gradient in the basin and a creased radiation attenuation correlated with reduced wind similar gradient aloft between the basin and Acapulco on the speeds and temperatures but increased relative humidity and Pacific coast to the south. Jazcilevich et al. (2003) further air pollution. analyzed these episodes using the Multiscale Climate Chem- The Mexico City Air Quality Research Initiative (MARI) istry Model (Grell et al., 2000). Transport of CO from the conducted a field campaign in February 1991 involving ver- south-west to the northern part of the city resulting from the tical profiling with both a LIDAR and an aircraft (Streit mountain-basin flow was indicative of direct, convective re- and Guzman, 1996). Nickerson et al. (1992) show some circulation of air. of the profiles obtained for particulate matter, SO2 and O3, Analysis of monitoring data from the Ambient Air Moni- focussing on a day with weak northerly winds leading to toring Network (Red Automatica´ de Monitoreo Atmosferico,´ high pollutant levels and one with stronger southerly winds RAMA) has provided valuable insights into the basin dy- leading to cleaner air. Williams et al. (1995) modeled the namics. Raga and Le Moyne (1996) analyzed basin-wide episodes to reproduce the light northeast winds with pol- monitoring data and identified a pattern of transport from the lution accumulation in the south-west of the basin. La- north-west to the south-east using nonlinear analysis. Raga grangian dispersion modeling found entrapment of particles et al. (1999) identified evidence of the upslope flow on the

Atmos. Chem. Phys., 5, 2267–2288, 2005 www.atmos-chem-phys.org/acp/5/2267/ B. de Foy et al.: Mexico City wind circulation 2269 southern edge of the basin by looking at measurements of primary and secondary pollutants. While the former are di- luted by up to 50% as the flow moves up the slope, the concentration of the latter increases. A lag time of 2 h was found for the transport from the basin floor to the edge of the basin. At night, increasing levels of O3 at basin sites is traced to downward transport due to the drainage flows. Prin- cipal component analysis of O3 distributions was carried out by Klaus et al. (2001). Four eigenvectors were found to account for 55% of the variance in O3 concentrations. These components correspond to the north/south transport of pollutants, the east/west slope flows, the center/periphery due to drainage flows and urban effects and a north-east/south-west component due to moisture and precipitation flows. The majority of pollution studies to date have concentrated on episodes with O3 peaks in the south of the city. This paper examines this episode type during the MCMA-2003 campaign, but also expands the analysis by adding two additional Fig. 2. Map of Mexico. Circles indicate radiosonde launch categories of importance to air quality studies. Section 10 sites: Mexico City (GSMN), Acapulco (ACAP), Veracruz (VER), will suggest a classification of the existing circulation mod- Monterrey (MTY), Guadalajara (GUAD) and Manzanillo (MANZ). els in terms of the episode types proposed here. Surface stations are indicated by diamonds: Chinatu (CHIN), Presa Allende (ALLE), Pachuca (PACH), Huejutla (HUEJ), R´ıo Tomatlan (TOMA), Angamacutiro (ANGA), Instituto Mexicano de Tecnolog´ıa del Agua (IMTA), Atlacomulco (ATLA), Nevado 2 Field campaign description de Toluca (NEVA), Puerto Angel (PUER), U. Tec. de Tec- machalco (UTEC), Izucar´ de Matamoros (IZUC), Huimilpan MCMA-2003 was designed to be towards the end of the dry (HUIM), Tampico (TAMP), Soto la Marina (SOTO), Matamoros season when solar radiation is intense but daily convective (MATA), Jalapa (JALA), Tuxpan (TUXP), Alvarado (ALVA), Cd. rainfall has not yet begun. In addition, it included school Aleman´ (CDAL), La Cangrejera (CANG), Zacatecas (ZACA) and vacations as well as Holy Week and Easter when traffic is Los Colomos (LOSC). Some stations are included for completeness greatly reduced in the whole city, providing a natural experi- but not labeled as they were not used. ment for the impact of emissions reductions. The four goals of the field campaign were (1) to develop improved volatile Section 4 will provide an overview of the field campaign organic compounds (VOC) and nitrogen oxides (NO ) emis- x meteorological conditions and wind patterns at CENICA and sions inventories; (2) to develop a better understanding of proposes a classification and circulation scheme for the Mex- formaldehyde sources and atmospheric chemistry; (3) to in- ico City basin. The surface wind patterns are analysed at vestigate fine particulate components and sources; and (4) to the regional scale (Sect. 6) and at the basin scale (Sect. 7) develop improved understanding of odd nitrogen (NO ) lev- y showing the differences between the proposed episodes. Sec- els and partitioning. In this context, the main meteorological tion 8 extends this analysis to the measurements available questions were (1) the nature of the mixed layer; (2) the im- aloft from radiosondes. The impact of the circulation pat- pact of carry-over from day-to-day; and (3) the evaluation of terns on the diurnal meteorological profiles at CENICA and mesoscale model performance. the pollution levels in the city will be described in Sect. 9. This paper describes the basin circulation patterns occur- After the discussion in Sect. 10, a catalogue of events has ring during the campaign and classifies the days of the cam- been included in Sect. 11 to highlight photochemical and me- paign into three types of episodes. This lays the foundation teorological events that are of particular interest. for further work to answer the meteorological questions of the field campaign. By classifying the days, it will be possible to look at the behavior of the mixed layer in relation to the 3 Measurement description basin circulation. Developing circulation patterns will guide the analysis of carry-over by identifying both the days where The MCMA-2003 field campaign was based at the National accumulation is expected to occur and those where the basin Center for Environmental Research and Training (Centro Na- is flushed clean. Finally, the paper lays out a framework cional de Investigacion´ y Capacitacion´ Ambiental, CENICA) for evaluating models in terms of the dominant flow features super-site working together with the Aerodyne mobile lab- encountered in the basin for multiple episodes grouped to- oratory (Kolb et al., 2004). The analysis is based on the gether. monitoring site at CENICA and on the monitoring networks www.atmos-chem-phys.org/acp/5/2267/ Atmos. Chem. Phys., 5, 2267–2288, 2005 B. de Foy et al.: Mexico City wind circulation 17

2270 B. de Foy et al.: Mexico City wind circulation

O3-South Cold Surge O3-North H L

B. de Foy et al.: Mexico City wind circulation 17 H

O3-South Cold Surge O3-North Fig. 4. GOES-12 satellite images for O3-South (14 April 11:45 CDT), Cold Surge (8 April 16:45 CDT) and O3-North (24 April 7:15 CDT). AreasH of high and low pressure are markedL by H and L. Arrows show significant wind directions: surface sea breezes for O3-South, low level jet for Cold Surge and westerlies aloft for O3-North. Images courtesy of GOES Biomass Burning Monitor- ing Team, UW-Madison, Cooperative Institute for Meteorological Fig. 3. Map of the MCMA showing the monitoring stations. The Satellite Studies (CIMSS). star is theB. de CENICA Foy et supersite. al.: Mexico Circles City arewind National circulation Meteorolog- 17 ical Service surface stations: SMN Headquarters (GSMN), Es- cuela Nacional de Ciencias Biologicas´ (ENCB), MexicoH Interna- tional Airport (AERO), Tezontle (TEZO), Pimentel (PIME) and Presa Mad´ın (MADI). Triangles and diamonds are RAMA stations by geographicalO3-S sector.outh Those referred to in textC are:old TlalnepantlaSurge O3-North (TLA), Azcapotzalco (AZC), Tacuba (TAC),Fig. Villa 4. de lasGOES-12 Flores satellite images for O3-South (14 April (VIF), Xalostoc (XAL), San Agustin (SAG),11:45 Chapingo CDT), (CHA), Cold Surge (8 April 16:45 CDT)O and3-So O3-Northuth (24 Cold Surge O3-North La Merced (MER), Cuajimalpa (CUA), PedregalApril (PED),7:15 CDT). Tlalpan Areas of high and low pressure are markedL by H H Day Afternoon Day (TPN), Tlahuac (TAH). Closed circles denoteand availability L. Arrows of meteo- show significant wind directions: surface sea breezes Night Day rological observations. Urban area of 1995 shownfor O3-South, in beige, shading low level jet for Cold Surge and westerlies aloft for corresponds to topography. O3-North. Images courtesy of GOES Biomass Burning Monitor- ing Team, UW-Madison, Cooperative Institute for Meteorological Satellite Studies (CIMSS). operated by the Mexican National Weather Service (Servi- cio Meteorologico´H Nacional, SMN) and by RAMA. Detailed measurements during the campaign available from LIDAR, Night SODAR, tethersonde, pilot balloon, radiometers and tempo- rary monitoring sites installed on the MCMA boundary will Night Fig. 4. GOES-12 satellite images for O3-South (14 AprilFig. 4. GOES-12 satellite images for O3-South (14 April 11:45 CDT), Cold Surge (8 April 16:45 CDT) and O3-North (24 be presented11:45 CDT), separately. Cold Surge (8 April 16:45 CDT) and O3-North (24 April 07:15 CDT).C Areasold Su ofrg highe E andven lowing pressure are marked by H Afternoon FigureApril2 shows 7:15 CDT). surface Areas and of upper-air high and low stations pressure usedO3- areSo inu markedt theh by H O3-North and L. Arrows show significant wind directions: surface sea breezes currentand study L. Arrows on the show national significant scale. wind The directions: automatic surface weatherDa seay breezesAfternoon Night Day for O3-South, low level jet for Cold Surge andD westerliesay aloft for stationsfor (EHCA) O3-South, are low operated level jet by for SMN Cold Surge(see http://smn.cna. and westerlies aloft for O3-North. Images courtesy of GOES Biomass Burning Monitor- O3-North. Images courtesy of GOES Biomass Burning Monitor- gob.mx/productos/emas/emas.html) and report 10-min data ing Team, UW-Madison,Fig. 5. Circulation Cooperative model Institute for the Mexico for Meteorological City basin for O3-South, ing Team, UW-Madison, Cooperative Institute for Meteorological in three hour blocks via satellite. The observations were Satellite StudiesCold (CIMSS). Surge and O3-North. Weak drainage flows on O3-South days averagedSatellite to one Studies hour (CIMSS). intervals for this analysis. The SMN give way to strong north-easterly winds. A weak southerly jet forms radiosondes are launched at 12:00 UTC outside of Mexico in the Chalco passage in the evening. Cold Surge days have predom- City. The Mexico City radiosondes are launched from the (GSMN), Escuelainantly northerly Nacional flow de Ciencias with a westerly Biologicas´ component (ENCB), at night turning to easterly flow from the Gulf in the afternoon. On O3-North days, the SMN headquarters (GSMN), WMO ID 76679 (also MEX) Mexico International Airport (AERO), Tezontle (TEZO), Pi- day-time flow is from the north-west and gives way to a strong after- which is on the western edge of the basin floor every 6 h at mentel (PIME) and Presa Mad´ın (MADI). RAMAN operatesight 00:00, 06:00, 12:00 and 18:00 UTC. noon jet that starts in the pass but expands over the entire southern Night 32 monitoringbasin sites rim. in the MCMA. Of these, 15 are equipped Figure 3 shows the observation network in the basin. with weather stations, 23 with CO monitors and 20 with O3 Cold Surge CENICA is in theO3-S south-eastouth of the city. There are six SMNEveninmonitors.g The dataO3- isNo archivedrth at one-minute time resolutionAfternoon Day Afternoon Day surface weather stations in the basin: SMN HeadquartersNight and averagedDay to one hour for reporting and analysis. The

Atmos. Chem. Phys., 5, 2267–2288, 2005Fig. 5. Circulation model for the Mexicowww.atmos-chem-phys.org/acp/5/1/ City basin www.atmos-chem-phys.org/acp/5/2267/ for O3-South, Atmos. Chem. Phys., 5, 1–40, 2005 Cold Surge and O3-North. Weak drainage flows on O3-South days give way to strong north-easterly winds. A weak southerly jet forms in the Chalco passage in the evening. Cold Surge days have predominantly northerly flow with a westerly component at night turning to easterly flow from the Gulf in the afternoon. On O3-North days, the day-time flow is from the north-west and givesNig wayht to a strong after- Night noon jet that starts in the pass but expands over the entire southern basin rim. Evening Afternoon

Fig. 5. Circulation model for the Mexicowww.atmos-chem-phys.org/acp/5/1/ City basin for O3-South, Atmos. Chem. Phys., 5, 1–40, 2005 Cold Surge and O3-North. Weak drainage flows on O3-South days give way to strong north-easterly winds. A weak southerly jet forms in the Chalco passage in the evening. Cold Surge days have predominantly northerly flow with a westerly component at night turning to easterly flow from the Gulf in the afternoon. On O3-North days, the day-time flow is from the north-west and gives way to a strong afternoon jet that starts in the pass but expands over the entire southern basin rim.

www.atmos-chem-phys.org/acp/5/1/ Atmos. Chem. Phys., 5, 1–40, 2005 B. de Foy et al.: Mexico City wind circulation 17

O3-South Cold Surge O3-North H L

Fig. 4. GOES-12 satellite images for O3-South (14 April 11:45 CDT), Cold Surge (8 April 16:45 CDT) and O3-North (24 April 7:15 CDT). Areas of high and low pressure are marked by H and L. Arrows show signiﬁcant wind directions: surface sea breezes for O3-South, low level jet for Cold Surge and westerlies aloft for O3-North. Images courtesy of GOES Biomass Burning Monitor- ing Team, UW-Madison, Cooperative Institute for Meteorological Satellite Studies (CIMSS).

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O3-South Cold Surge O3-North Day Afternoon Day Night Day

Night

Evening Afternoon

Fig. 5. Circulation model for the Mexico City basin for O3-South, Cold Surge and O3-North. Weak drainage flows on O3-South days give way toFig. strong 5. north-easterlyCirculation model winds. for A the weak Mexico southerly City jet basin forms for in O3-South, the Chalco passage in the evening. Cold Surge days have predominantly northerlyCold flow Surge with and a westerly O3-North. component Weak drainage at night flowsturning on to O3-South easterly flow days from the Gulf in the afternoon. On O3-North days, the day-time flow isgive from way the north-westto strong north-easterly and gives way winds. to a strong A weak afternoon southerly jet jet that forms starts in the pass but expands over the entire southern basin rim. in the Chalco passage in the evening. Cold Surge days have predominantly northerly flow with a westerly component at night turning to symboleasterly on the flow map from shows the Gulf the geographical in the afternoon. sectors On O3-North used by days,high the ozone levels in the north of the city, and will be referred RAMAday-time in classifying flow is from the stations. the north-west and gives way to a strong after-to as “O3-North”. Thenoon timezone jet that in starts the MCMAin the pass was but Central expands Standard over the entire Time southern (CST=UTC–6)basin rim. before 6 April and daylight saving time (CDT=UTC–5) thereafter. The field campaign policy spec- GOES-12 cloud imagery is shown in Fig. 4 for representa- ified the use of local time for data storage and analysis, a tive days of the 3 episode types (14, 8, and 24 April). Su- convention that will be followed here with times in CDT un- perimposed are arrows showing dominant transport direc- less marked otherwise. Solar noon during the campaign was tion of air masses important to the flow in the Mexico City withinwww.atmos-chem-phys.org/acp/5/1/ 5 min of 13:35. Atmos. Chem. Phys., 5, 1–40, 2005 basin. The proposed circulation patterns in the basin for each episode are shown in Fig. 5. These will be discussed in greater detail once the supporting data has been presented. 4 Synoptic episodes

April is a transition month between the hot dry season and Surface and 500 hPa weather maps are shown in Figs. 6 the wet season (Jauregui, 1988). There are therefore weather and 7 for representative days of the 3 episode types (14, events occurring from both kinds of seasons in brief succes- 8, and 24 April 13:00). O3-South days occur when a high sion. Typically, the mean April synoptic flow is dominated pressure system is located over the eastern Pacific leading to by a low level anticyclonic circulation over central south- subsidence and clear skies. Strong sea breezes develop un- ern Mexico and westerly winds aloft north of 20◦ N. This der these conditions and are clearly signaled by the surface leads to subsidence over the Mexico basin with weak sur- convection cloudiness on the Gulf coast as the warm moist face winds favorable to the development of thermally driven air from the Gulf rises overland. For the Cold Surge, a sur- circulations. This pattern was observed for two episodes dur- face high over Texas with convergence aloft leads to strong ing MCMA-2003: 14–17 April and 1–3 May. They will be northerly flow of cold polar air down the Gulf of Mexico. It referred to as “O3-South” as, under these conditions, ozone accumulates moisture and is partially held back by the Sierra levels are highest in the south of the city. An important phe- Madre Oriental leading to afternoon flows over the moun- nomenon leading to low temperatures and rains in Mexico is tains and strong flows through the isthmus of Tehuantepec. “el Norte” or “Cold Surge” (Schultz et al., 1998 and Mosino˜ Mild cloudiness is associated with a low pressure trough off Aleman´ and Garc´ıa, 1974). This occurred twice during the the western coast of Mexico containing warmer tropical air. campaign, on 8–10 and 20–22 April, and once before the be- For O3-North days, the situation is dominated by the west- ginning of the campaign on 29–31 March. The remaining erly winds that reach down to the latitude of the MCMA, days of the campaign had unperturbed westerlies with very with the subtropical jet clearly marked by the band of clouds weak anticyclonic conditions to the south and a strong sub- that form beneath. This situation corresponds to low pressure tropical jet just to the north of the MCMA. This results in systems over the US but very little activity to the south. www.atmos-chem-phys.org/acp/5/2267/ Atmos. Chem. Phys., 5, 2267–2288, 2005 6 B. de Foy et al.: Mexico City wind circulation

5 20 20 10 −24 101220 1010 −23 5560−215600 −18 5820 15 1012 1022 1014 1016 5580 5620 −15 1018 −20 −13 5640 −16 −13 1024 −22 −17 5740 5760 −14 −16 1020 5720 −14 1014 −19 5800 −15 5780 30 5840 30 25 5700 1016 5660 ° 25 ° −15 5820 30 N 20 30 N 5680 35 25 5820 5840 1018 25 −13 30 40 −13 −12 −14 20 5860 1016 1022 1018 5760 −13 5860 −14 1020 25 1024 5780 5800 −11 −12 1020 −11 5880 5840 1018 30 25 −12 −10 35 20 5840 25 30 30 5860−8 −9 5900 −10 25 20 5880 −10 −11 5880 35 1022 −7 −9 25 −8 −9 5900

° 30 ° 20 N 20 N 1020 5900 5920 −8 1020 35 −7 −7 1018 25 −7 −6 25 1018 30 30 35 5920 −6 30 5920 1018 −5 −6 5920 25 1018 30 −5 30

5920 −5 ° ° −5 10 N 1016 10 N 1016 −5 1016 5920 ° Sea−Level Pressure (hPa) ° Height (gpm) 120 W o ° 120 W o ° Isotherm ( C) ° ° 90 W Isotherm ( C) ° ° 90 W 110 W 100 W 110 W 100 W 18Wind (0.2−9.5 m/s) B. de Foy et al.: Mexico City wind circulation Wind (0.8−39.8 m/s) 62272 B. de Foy et al.: Mexico B. City de Foy wind circulation et al.: Mexico City wind circulation (a) 14 April (O3-South) (a) 14 April (O3-South) 6 B. de Foy et al.: Mexico City wind circulation

5 20 15 −14 10 5800 5660−26 20 1022 −16 5640 5580 −23 1024 1026 −24 −18 10 15 1034 5 10 1018 −23 −22−21 101220 1010 20 −23 −22 5600 5560−215600 −18 5820 15 1012 1016 1022 5620 1014 5580−20 −15 1020 −19 −20 1018 1024 −13 5760 −20 5680 25 5640 −16 −13 −21 56605680 1024 −22 −17 5740 155760 −14 −16 5620 1020 1030 −14 5800 −15 −25 5700−16 1020 −19 5720 −19 1014 5800 −15 5700 −24 5720 1030 5780 5 5780 30 15 1028 5840 −13 −17 −14 30 20 15 −17 57405720 −15 20 1018 1032 5700 25 20 1028 −18 5740 1016 10 −24 ° 25 ° 1022 ° 5660 ° 101220 1010 30 25 −23 −15 5820 30 −13 −11 30 N N 30 N 5680 5560−215600 −18 5820 N 20 15 1012 1016 1022 5620 1014 25 1022 5580−20 −15 35 25 1018 5820 5840 −13 −13 −14 20 25 −22 5640−17 −16 −14 −16 1018 1020 1024 −13 5740 5760 −16 5760 30 −14 −19 5720 1014 1026 −14 5800 −15 40 1016 1024 15 −131016−12 5780 5800 5780 20 30 5840 5800 30 30 35 5860 −12 1022 20 −15 1016 25 5700 −13 1018 30 20 5760 −13 5860 5820 1016 1026 −14 1020 25 30 5660 −14 30° 102425 1018 5780 580020 −11 30° −12 −15 5820 −9 N 35 1020 N 5680 5840 5820 1020 20 1014 15 1028 −11 5880 30 35 25 1024 5820 5840 5820 1018 25 −12 −12 −10 25 1018 1020 1018 −13 30 10 5840 1018 −10 −11 −14 5840 35 40 5840 −13 −12 −11 20 20 25 25 1022 −13 25 1016 2025 5860 −12 5860 25 1016 1022 30 30 1018 −8 −9 1014 5900 5760 −10 −13 5860 20 5860 −14 5860 25 −10 25 20 1020 −11 35 1024 5880 −10 5780 58005880 −11 −12 −11 −9 1022 1014 −7 −9 −9 5840 5840 5880 −6 1020 1016 −9 −11 5880 −7 25 1018 30 30 5900 −12 −8 25 ° −8 ° ° 30 ° −10 20 N 20 N 20 N 25 −10 20 N 35 30 5880 1014 5840 1020 20 1016 5920 −8 5860 −9 35 25 5900 −7 −7 1020 −10 5900 1018 25 30 −8 −9 −7 −8 30 25 5860 5900 25 20 35 30 1012 40 −6 5880 −10 −11 5880 −8 −7 −6 35 1012 −7 −9 25 1018 30 30 102225 35 35 25 35 30 1012 −9 5920 −8 5900 −75880 −5 ° 30 30 −6 ° −4 20 N 30 5920 20 N 1018 25 −5 −6 1020 1014 5920 5920 −8 −6 5900 35 30 5900 −7 −7 30 25 1018 1020 30 1018 25 −5−7 −4 1012 −5 −6 30 3025 1018 30 −5 5900 35 30 35 5920 −3 B. de Foy et al.: Mexico City wind circulation 19 −6 30 5920 201018 5920 B. de Foy et al.: Mexico City wind circulation −5 −5 −6 −4 1010 5920 ° 1012 −5 ° ° 1016 1012 25 ° −2 10 N 1018 10 N 30 10 N 10 N 1016 1012 −5 5900 −5 30 5920 −3 1016 Height (gpm) 5880 ° Sea−Level Pressure (hPa) ° Sea−Level Pressure (hPa) ° Height (gpm) ° 120 W o ° 120 W o 120 W o ° ° 5920 120 W o ° ° ° 90 W ° ° ° 90 W −5° 90 W ° ° 90 W Isotherm ( C) 110 W Isotherm ( C) 110 W Isotherm ( C) 110 W Isotherm ( C) 110 W ° 100 W 100 W ° 100 W −5 100 W Wind (0.2−9.5 m/s) 10 N Wind (0.1−14.0 m/s) 1016 Wind (0.8−39.8 m/s) 10 N Wind (1.0−38.7 m/s) 1016 −5 1016 5920 ° Sea−Level Pressure (hPa) ° Height (gpm) 120 W o ° 120 W o ° Isotherm ( C) ° ° 90 W Isotherm ( C) ° ° 90 W 110 W 100 W 110 W 100 W (a) 14 April (O3-South)Wind (0.2−9.5 m/s) (b) 8 April (Cold Surge) (a) 14 April (O3-South)Wind (0.8−39.8 m/s) (b) 8 April (Cold Surge)

(a) 14 April (O3-South) (a) 14 April (O3-South)

15 20 −14 1002 5680 1010 −14 10 15 1012 5800 15 5660−26 5700 5640 5520 998 1004 1022 −16 5640 5580 −23 5560−20 −16 1024 1026 1006 1008 −18 15 10 1018 1000 −23 5540−19 1034 5 1014 −22−21 20 1012 20 −22 5600 −17 Fig. 6. Surface weather maps for (a) 14 April (O3-South), (b) 8 −13 −15 5580−18 5600 56405660 1016 1008 −12 1020 −19 5680 −20 5660 1024 25 5760 −20−21 5660 5620 25 1014 5680 5700 −14 15 20 20 5620 5720 April (Cold Surge)1030 and (c) 24 April (O3-North) at 13:00 CDT. Thick 5800 −15 −25 5700−16 −17 1020 5700 −19 −16 1010 −24 5720 5740 1030 25 1018 20 5780 −14 5720 5740 1028 15 15 blue lines for sea level pressure contours, thin red lines for surface15 1004 −13 −17 −14 −13 −15 15 25 −17 57405720 −15 20 10 5800 −13 5700 5680 1018 5660−26 −14 1032 1022 −16 1028 1024 1026 −18 5740 5640 5580 −23 −18 5720 15 25 isotherms and wind vectors are shown. 1034 5 10 1018 −23 −22−21 ° 1022 20 ° 1012 ° 1006 −11 −22 ° 5600 5760 30 N 25 30 N 30 N −13 30 N 25 25 1020 −19 −20 1022 1024 5760 −20 5680 5760 5740 25 −14 −21 56605680 5760 20 25 30 25 5620 −12 1030 15 30 −25 −13 1020 30 −16 5800 5760 −15 −19 5700−16 1026 5700 −24 5780 5720 5780−12 1030 −11 1016 1024 15 1016 5800 5780 5780 −12 15 1028 −13 −14 5780 15 35 5800 −17 57405720 −17 −15 35 20 −12 30 20 1018 1032 30 −15 5800 1028 1006 5820 −13 −18 5740−11 30 20 1008 −10 1026 1010 −10 −11 30 ° 1022 40 ° −14 5800 5800 1018 30 2025 1012 30 −13 −10 −11 N 30 N −9 35 1020 25 1022 1008 5820 5820 1014 1028 1014 5820 15 1016 20 1024 −14 20 30 5820 −12 −10 −9 −9 1020 1018 −16 5820 5760 10 1026 5840 5840 −9 5840 1018 1016 1024 15 1016 1010 −11 5800 5840 25 −11 57805840 5800 −8 25 25 1022 35 25 −13 −12 −8 −8 25 1016 2025 30 20 −12 5860 −15 5860 1014 30 5820 −13 −7 5860 20 30 20 1026 1012 5860 30 −145860 1018 25 −10 −7 5880 20 1006 −11 −9 −9 1014 35 1020 25 5880 −6 5820 1014 15 1028 1018 −9 5840 1024 −7 30 1016 −8 5820 −6−12 −105880 −6 5880 1020 1018 −7 ° 10° 30 °30 30 25 30 ° 5840 20 20 40 −10 −11 5840 5900 −6 N 30 25 1018 20 N N 35 20 N −5 25 1022 5880 −11 −5 1014 25 5860 −13 1016 25 1016 2025 −9 −12 59005860 1014 35 20 −8 5900 −10 5860 −4 35 25 30 1010 −11 −9 −5 1012 40 −7 5900 −8 −5 1014 35 −6 5880 1012 1008 −9 5840 −3 −6 25 35 10161016 1014 1010 −8 −7 35 30 1012 30 −7 −5 ° 25 5880 ° −4 −10 −3 2030 N 25 30 20 N 30 35 5880 −4 25 1014 5860 −9 1014 1016 30 −6 35 5900 Atmos. Chem. Phys.,30 5, 1–40, 2005 www.atmos-chem-phys.org/acp/5/1/1012 5900 30 −4 −8 35 25 30 −5 1012 1012 40 −8 −7 −6 −4 1012 −3 25 35 −5 35 30 1012 30 5900 −3 30 35 1014 −7 −3 −5 30 5880 −4 25 −4 5900 1014 −6 5900 −3 1010 5900 1012 30 −3 ° 1012 ° 30 ° ° 10 N 10 N 10 N −2 −5 10 N −4 5900 1012 1012 5900 −5 30 35 −3 −3 5900 Sea−Level Pressure (hPa) Height (gpm)1012 5880 ° ° Sea−Level Pressure (hPa) ° ° Height (gpm) 120 W o ° 120 120 W o ° −4 ° 120 o ° ° W W o ° W W W W Isotherm ( C) ° 90 Isotherm ( C) ° 1010 ° Isotherm ( C) 90 ° 90 Isotherm ( C) ° ° 90 110 W ° 1001012 W 1012 110 W 100 W 110 W ° 100 W 110 W 100 W Wind (0.1−14.0 m/s) 10 N Wind (0.4−12.4 m/s) Wind (1.0−38.7 m/s) 10 N Wind (0.4−33.9 m/s)−2 1012 5900

−3 ° Sea−Level Pressure (hPa) ° Height (gpm) 5880 120 W o ° 120 W o ° Isotherm ( C) ° ° 90 W Isotherm ( C) ° ° 90 W 110 W 100 W 110 W 100 W (b) 8 April (Cold Surge)Wind (0.1−14.0 m/s) (c) 24 April (O3-North) (b) 8 April (Cold Surge)Wind (1.0−38.7 m/s) (c) 24 April (O3-North)

Fig. 6. Continued. Fig. 6. Surface weather maps for (a) 14 April (O3-South),(b) 8(b) April8 April (ColdFig. (Cold 6. Continued. Surge) Surge) and (c) 24 April (O3-North) at 13:00 CDT. Thick blue (b) 8 April (Cold Surge) lines for sea level pressure contours, thin red lines for surface isotherms and wind vectors are shown. 20 Fig. 6. Surface weather maps for (a) 14 April (O3-South), (b) 8 Fig. 7. Weather maps at 500 hPa for (a) 14 April (O3-South), (b) 1002 5680 1010 −14

15 1012 15 5700 5640 5520 998 1004 5560−20 −16 1006 1000 1008 5540−19 1012 20 1014 −17 −13 −15 5580−18 5600 56405660 1016 1008 −12 25 April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT.5660 Thick5620 8 April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT. 1014 5700 −14 20 20 5720 −16 −17 1010 5740 25 1018 20 20 5720 5740 1004 1002 5680 15 1010 −13 −14 −15 25 15 1012 5680 15 −13 5700 5700 5640 5520 blue lines for sea998 level1004 pressure contours, thin red lines for surface−14 Thick blue lines5560−20 for 500 hPa−16 height, thin red lines for isotherms 25 1006 1000 1008 5720 5540−19 ° 1012 1012 20 1014 ° −17 1006 5760 −13 −15 5580−18 5600 56405660 30 N 1016 1008 30 N −12 5660 25 25 5760 5740 5620 1014 5760 5700 −14 30 25 20 25 20 5720 30isotherms and wind vectors are shown. −12 −13 and wind vectors−16 are−17 shown. 30 1010 5740 25 5780 −11 5780−12 1018 20 5720 5740 15 1004 −12 5780 −13 −15 35 25 −13 5700 5680 30 5800 −14 5720 1006 25 −11 1008 −10 ° 1010 1012 ° −11 40 1006 5800 5800−10 5760 30 N 1012 −10 30 N 30 25 5820 5740 1014 1008 5820 5760 5760 20 30 25 25 1016 −12 30 30 −9 −9 −13 30 5820 5780 5780−12 1010 −9 5840 −12 5840 −11 25 35 5840 −8 5780 25 30 −8 5860 −8 5800 Atmos. Chem. Phys., 5, 2267–2288,30 20051006 www.atmos-chem-phys.org/acp/5/2267/ −7 5860 −11 1008 −10 1010 −11 1012 40 5800 5800−10 25 1012 5860 −7 5880 −10 1006 30 1008 5820 25 1014 5820 1016 20 Atmos. Chem. Phys., 0000, 0001–24, 2005 www.atmos-chem-phys.org/acp/0000/0001/ www.atmos-chem-phys.org/acp/5/1/1018 Atmos. Chem. Phys., 5, 1–40, 2005 5880 −9 5880 30 Atmos. Chem. Phys., 5, 1–40, 2005−6 −6 www.atmos-chem-phys.org/acp/5/1/5820 −7 −9 ° 30 30 30 25 30 ° 40 1010 5900 −6 −9 5840 5840 20 25 20 N 35 N −5 −5 5840 −8 25 −8 5860 −8 30 5900 −7 5860 35 1012 25 −4 5860 −7 5880 1010 1006 5900 −5 −5 35 1018 1008 25 −3 5880 5880 1016 1014 1010 −6 −6 −7 ° 30 30 30 25 30 ° 40 −3 5900 25 2030 N 35 20 N −5 −6 35 −4 −5 30 35 5900 1012 35 −4 1010 5900 −5 −5 35 1008 −3 −4 1016 1014 1010 −3 30 −3 −3 1014 25 30 35 5900 −4 1012 30 35 −3 5900 −3 ° ° 10 N 10 N 5900 −4 −3

30 −3 1014 1012 ° Sea−Level Pressure (hPa) ° Height (gpm) 120 ° 120 ° 5900 W o W o −3 5900 Isotherm ( C) ° ° 90 W Isotherm ( C) ° ° 90 W −3 110 W ° 100 W 110 W ° 100 W Wind (0.4−12.4 m/s) 10 N Wind (0.4−33.9 m/s) 10 N 5900

1012 ° Sea−Level Pressure (hPa) ° Height (gpm) 120 W o ° 120 W o ° Isotherm ( C) ° ° 90 W Isotherm ( C) ° ° 90 W 110 W 100 W 110 W 100 W (c) 24 April (O3-North)Wind (0.4−12.4 m/s) (c) 24 April (O3-North)Wind (0.4−33.9 m/s)

(c) 24 April (O3-North) (c) 24 April (O3-North) Fig. 6. Surface weather maps for (a) 14 April (O3-South), (b) 8 Fig. 7. Weather maps at 500 hPa for (a) 14 April (O3-South), (b) April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT. Thick 8 April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT. blue lines for sea level pressureFig. 6. contours,Surface weather thin red maps lines for surface(a) 14 AprilThick (O3-South), blue lines (b) 8for 500Fig.hPa 7. height,Weather thin maps red at lines 500 forhPa isothermsfor (a) 14 April (O3-South), (b) isotherms and wind vectorsApril are (Cold shown. Surge) and (c) 24 April (O3-North) atand 13:00 wind CDT. vectors Thick are shown.8 April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT. blue lines for sea level pressure contours, thin red lines for surface Thick blue lines for 500 hPa height, thin red lines for isotherms isotherms and wind vectors are shown. and wind vectors are shown. Atmos. Chem. Phys., 0000, 0001–24, 2005 www.atmos-chem-phys.org/acp/0000/0001/ Atmos. Chem. Phys., 0000, 0001–24, 2005 www.atmos-chem-phys.org/acp/0000/0001/ 6 B. de Foy et al.: Mexico City wind circulation

° 30 ° 20 N 20 N 1020 5900 5920 −8 1020 35 −7 −7 1018 25 −7 −6 25 1018 30 30 35 5920 −6 30 5920 1018 −5 −6 5920 25 1018 30 −5 30

5920 −5 ° ° −5 10 N 1016 10 N 1016 −5 1016 5920 ° Sea−Level Pressure (hPa) ° Height (gpm) 120 W o ° 120 W o ° Isotherm ( C) ° ° 90 W Isotherm ( C) ° ° 90 W 110 W 100 W B. de Foy et al.: Mexico City wind circulation 110 W 100 W 21 Wind (0.2−9.5 m/s) Wind (0.8−39.8 m/s) 6B. de Foy B. et al.: de Mexico Foy et City al.: wind Mexico circulation City wind circulation 2273 (a) 14 April (O3-South) (a) 14 April (O3-South) 6 B. de Foy et al.: Mexico City wind circulation

5 20 15 −14 20 10 5800 5660−26 1022 −24 −16 10 1024 1026 5640 5580 −23 −18 15 10 101220 1010 −23 5 1018 −23 1034 5560−215600 −18 5820 −22 −22−21 15 1012 1022 20 1014 1016 5580 5620 5600 −20 −15 −13 1018 1020 −13 −19 −20 −22 5640−17 −161024 −14 −16 5760 −20 5680 1020 1024 25 5740 5760 −21 56605680 5720 −14 15 5620 1014 1030−19 5800 −15 5800 −15 −25 5700−16 1020 5780 5700 −24 −19 5720 5 30 1030 5840 30 20 5780 15 1028 −13 −17 −14 570015 −17 57405720 −15 25 20 20 1018 1032 1028 1016 −24 −18 5740 ° 25 10 ° 5660 30 101220 1010 ° 1022 30 −23 −15 5820 ° N N 5680 5560−215600 −18 5820 −11 15 30 1012 251022 30 −13 20 1014 N 1016 5580 5620 N 35 25 25 5820 5840 −20 −15 −13 25 1018 1022 −13 −22 5640 −16 −14 −14 20 −17 −16 1018 1020 1024 −13 5740 5760 30 5720 −14 −16 5760 1014 −19 −14 5800 −15 1026 −13 −12 5780 40 1016 1024 1016 5800 20 30 15 5840 5780 30 5860 5800 1016 1022 25 30 35 20 5700 −12 −15 1018 5760 −13 5860 −13 30 20 −14 5820 25 1016 ° 1020 1026 5800 ° 5660 102425 30 5780 −12 −14 1018 −11 −15 5820 1020 30 N 20 30 N 5680 5840 −9 20 35 −111020 5880 5820 1018 30 35 251014 15 1028 5820 5840 1024 −12 25 25 5820 −12 −10 1018 1018 −13 30 1020 10 −10 −14 5840 35 40 1018 5840 −13 −12 −11 5840 20 20 25 1022 −11 25 25 20 5860 −12 −13 1016 1022 25 1016 25 5860 30 30 1018 5860−8 −9 1014 5900 5760 −10 −13 5860 20 −14 25 20 25 5860 1020 5880 −10 5780−1158005880 −10 35 1024 −7 −9 −11 −125840 −11 −9 1020 1022 1014 −9 −11 5880 −9 5840 5880 −6 25 1018 30 1016 5900 −8 −7 25 30 −8 −12

° 30 ° 20 N ° 20 N −10 ° −10 20 N 25 20 N 35 30 5920 5840 −8 5880 1020 20 1014 5860 −9 35 25 1016 5900 −7 −7 1018 1020 25 30 30 5860−8 −9 5900−7 −8 −10 5900 25 20 35 25 30 −10 −11 5880 35 1012 40 −6 5880 −8 −7 −6 1012 −7 25 1018 30 30 1022 −9 35 25 25 35 −9 5920 35 30 1012 −8 5900 −7 −5 ° 30 −6 ° 5880 −4 20 N 30 30 5920 20 N 1018 25 −5 −6 1020 5920 5920 −8 1014 35 5900 −7 −7 −6 5900 30 25 1020 30 1018 25 30 −7 1018 −5 −4 1012 −5 −6 3025 1018 30 30 −5 35 30 35 5920 −3 5900 −6 30 225920 5920 B. de Foy et al.: Mexico City wind circulation 1018 −5 B. de Foy et−5 al.: Mexico−6 City−4 wind circulation 23 1010 5920 ° ° −5 1016 ° 1012 25 1012 ° 10 N 1018 10 N 30 10 N 10 N −2 1016 1012 −5 −5 5900 1016 30 5920 −3 ° Sea−Level Pressure (hPa) Sea−Level Pressure (hPa) ° Height (gpm) Height (gpm) 5880 ° ° 120 ° ° 120 W o 120 W o W o ° 5920 120 W o ° Isotherm ( C) ° ° 90 W ° Isotherm ( C) ° W −5° 90 W ° W 110 W W Isotherm ( C) 110 ° 110 W 90 W Isotherm ( C) 110 ° 90 ° 100 W 100 W ° 100 −5 W 100 W Wind (0.2−9.5 m/s) 10 N Wind (0.1−14.0 m/s) 1016 Wind (0.8−39.8 m/s) 10 N Wind (1.0−38.7 m/s) 1016 −5 1016 5920 ° Sea−Level Pressure (hPa) ° Height (gpm) 120 W o ° 120 W o ° Isotherm ( C) ° ° 90 W Isotherm ( C) ° ° 90 W 110 W 100 W 110 W 100 W (a) 14 April (O3-South)Wind (0.2−9.5 m/s) (b) 8 April (Cold Surge) (a) 14 April (O3-South)Wind (0.8−39.8 m/s) (b) 8 April (Cold Surge)

(a) 14 April (O3-South) (a) 14 April (O3-South)

15 20 −14 1002 5680 1010 5800 −14 10 15 1012 15 Fig.5660 7.−26 Weather maps at 500 hPa for (a) 14 April (O3-South), (b) 1022 −16 5700 5640 5520 1024 998 1004 5640 5580 −23 −18 5560−20 −16 1026 1006 1008 15 1034 5 10 1018 1000 −23 −22−21 5540−19 20 1012 20 1014 −22 5600 −17 8 April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT. −13 −15 5580−18 5600 56405660 1020 1016 1008 −19 −12 1024 5680 −20 5660 25 5760 −20−21 56605680 5620 25 1014 5700 −14 15 20 20 5620 1030 5800 −15 Thick blue lines−25 for 500 hPa height,5700−16 thin red lines for isotherms and5720 1020 5700 −19 −16 −17 1010 −24 5720 5740 1030 25 1018 20 5780 −14 5720 15 1028 wind vectors are shown. 5740 −13 −14 15 15 1004 −17 −13 15 25 −17 57405720 −15 −15 20 10 5800 −13 5700 5680 1018 1032 1028 5660−26 −14 1022 5740 −16 1024 1026 25 −18 5640 5580 −23 −18 5720 ° 15 1034 5 10 1018 ° −23 −22−21 1022 20 ° 1012 1006 −11 −22 ° 5600 5760 30 N 25 30 N 30 N −13 30 N 25 25 1020 −19 1022 1024 5680 −20 5740 −14 5760 −20−21 5760 5760 20 25 30 25 25 56605680 15 30 5620 −12 −13 1030 30 −16 5800 5760 −15 −25 5700−16 1026 1020 1016 5700 −24 5780−19 5720 −11 5780−12 1016 1024 1030 5800 15 5780 5780 −12 5780 15 1028 35 −13 5800 −17 −14 35 15 −12 −17 57405720 −15 30 20 20 1018 1032 30 −15 5800 1028 1006 5820 −13 −18 5740−11 30 20 1008 −10 1026 1010 −11 30 ° 40 ° −14 5800 5800−10 1018 2520 1022 1012 −10 −11 30 N 30 30 N −13 −9 35 1020 1008 5820 5820 1014 1028 25 1022 1014 5820 15 1016 20 1024 −14 20 30 5820 −12 −10 −9 −9 1020 1018 5840 −16 5820 5760 10 1026 −11 5840 −9 5840 5840 1018 1024 1016 1010 5800 1016 15 25 −11 57805840 −8 25 25 1022 25 −13 −8 5800 −8 25 1016 2025 30 35 20 −12 −12 5860 −15 5860 1014 30 5820 −13 −7 5860 20 30 20 1026 1012 5860 5860 30 −10 −14 −7 5880 1018 25 20 1006 −11 −9 1014 5880 −9 35 1020 25 −9 5840 −6 5820 1014 15 1028 1018 −7 5880 5880 30 1016 1024 −8 5820 −6−12 −10 −6 1018 −7 ° 1020 ° 30 ° 30 30 25 30 ° 10 20 40 −10 5840 5840 5900 −6 20 N 25 1018 20 N N 35 −11 20 N −5 30 5880 −11 −5 1014 25 25 1022 5860 −9 −13 1016 25 1016 2025 −12 59005860 1014 35 5900 20 −8 5860 −4 35 25 30 1010 −10 5900 −5 1012 40 −11 −9 −7 −5 35 −8 −6 1012 1014 1008 5880 1014 1010 −9 5840 −3 −6 25 35 10161016 −7 35 30 1012 30 −8 25 −75880 −5 −3 30° 30 ° −4 −10 20 N 25 20 N 25 30 35 www.atmos-chem-phys.org/acp/5/1/5880 Atmos.−4 Chem. Phys., 5, 1–40, 2005 1016 1014 35 5900 5860 −9 1014 30 1012 30 −6 30 −4 −8 5900 35 25 30 −5 −4 1012 1012 40 −8 −7 −6 −3 1012 25 35 −5 30 −3 30 5900 30 35 351014 1012 −7 −3 −5 30 5880 −4 25 −4 5900 −3 5900 −3 1014 1010 −6 5900 ° 1012 ° 30 ° ° 1012 30 10 N 10 N 10 N −2 −5 10 N −4 5900 1012 1012 5900 −5 30 35 −3 −3 5900 Sea−Level Pressure (hPa) 1012 Height (gpm) Sea−Level Pressure (hPa) ° ° Height (gpm) 5880 ° ° 120 ° 120 ° 120 W o ° W o 120 W o −4 ° W o ° ° 90 W Isotherm ( C) ° ° ° 90 W ° 90 W Isotherm ( C) ° ° 90 W Isotherm ( C) 110 W 110 W 1010 WIsotherm ( C) 110 W 110 W W ° 1001012 W 1012 100 ° 100 W 100 Wind (0.1−14.0 m/s) 10 N Wind (0.4−12.4 m/s) Wind (1.0−38.7 m/s) 10 N Wind (0.4−33.9 m/s)−2 1012 5900

(b) 8 April (Cold Surge) Fig.Fig. 7. 7.WeatherContinued. maps at 500 hPa for (a) 14 April (O3-South),(b) 8 April(b) 8 (Cold AprilFig. 7. Continued. (Cold Surge) Surge) and (c) 24 April (O3-North) at 13:00 CDT. Thick blue lines for 500 hPa height, thin red lines for isotherms and wind vectors are shown. 20 Fig. 6. Surface weather maps for (a) 14 April (O3-South), (b) 8 Fig. 7. Weather maps at 500 hPa for (a) 14 April (O3-South), (b) 1002 5680 1010 −14

15 1012 15 5700 5640 5520 998 1004 5560−20 −16 1006 1000 1008 5540−19 1012 20 1014 −17 −13 −15 5580−18 5600 56405660 1016 1008 April (Cold Surge) and (c) 24 April (O3-North) at 13:00−12 CDT. Thick 25 5660 5620 8 April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT. 1014 5700 −14 20 20 5720 −16 −17 1010 5740 25 1018 20 1004 20 5720 5740 15 1002 −13 −15 5680 25 1010 −14 blue lines for15 sea level pressure1012 contours, thin red lines for−13 5700 surface5680 Thick blue lines for 500 hPa height, thin red lines for isotherms 15 5700 5640 5520 998 1004 −14 5720 5560−20 −16 25 1006 1000 1008 5540−19 ° 1012 10121006 20 1014 ° 5760 −17 30 30 −13 −15 5580−18 5600 56405660 N 1016 1008 N −12 25 25 5760 5740 5760 5660 5620 30 25 1014 25 5700 −14 20 30 isotherms and wind vectors20 are shown. −12 5720−13 and wind vectors are shown. 30 −16 −17 1010 5740 25 5780 −11 5780−12 1018 20 −12 5720 5740 35 15 1004 5780 −13 −15 25 −13 5700 5680 30 5800 −14 1006 25 −11 5720 1008 −10 1010 −11 40 ° 1012 1006 5800 5800−10° 30 N 1012 −10 30 N 5760 30 25 5820 1014 1008 5820 5760 5740 5760 1016 20 30 25 25 −9 −12 30 30 5820 −9 −13 30 5780−12 1010 −9 5840 5780 5840 −11 25 35 5840 −8 −12 5780 25 −8 30 5860 −8 5800 30 1006 www.atmos-chem-phys.org/acp/5/2267/−7 5860 −11 Atmos. Chem. Phys., 5, 2267–2288, 2005 1008 −10 1010 −11 1012 40 5860 5800 5800−10 25 1012 −7 5880 −10 1006 30 Atmos. Chem. Phys., 0000, 0001–24, 2005 www.atmos-chem-phys.org/acp/0000/0001/5820 25 1014 1008 5820 1018 20 1016 −6 5880 −6 −9 5880 www.atmos-chem-phys.org/acp/5/1/ Atmos. Chem. Phys., 5, 1–40, 2005 30 Atmos. Chem. Phys., 5, 1–40, 20055820 −7 www.atmos-chem-phys.org/acp/5/1/−9 ° 30 30 30 25 30 ° 20 35 40 1010 20 −5 5900 −6 −9 5840 5840 N 25 N −5 5840 −8 25 −8 5860 −8 30 5900 −7 5860 35 1012 −4 5860 −7 5880 1010 25 5900 −5 1006 −5 35 1008 25 1014 1018 1010 −3 5880 5880 1016 −6 −6 −7 ° 30 30 30 25 30 −3 ° 25 40 2030 N 35 20 N −5 5900 −6 35 −4 −5 1012 30 35 5900 35 −4 1010 −4 5900 −5 −5 35 1008 −3 −3 1016 1014 1010 30 −3 1014 −3 25 30 35 5900 −4 1012 30 35 −3 5900 −3 ° ° 10 N 10 N 5900 −4 −3

30 −3 1014 1012 ° Sea−Level Pressure (hPa) ° Height (gpm) 120 W o ° 120 W o ° 5900 Isotherm ( C) ° ° 90 W Isotherm ( C) ° −3 ° 5900 90 W −3 110 W ° 100 W 110 W ° 100 W Wind (0.4−12.4 m/s) 10 N Wind (0.4−33.9 m/s) 10 N 5900

(c) 24 April (O3-North) (c) 24 April (O3-North) Fig. 6. Surface weather maps for (a) 14 April (O3-South), (b) 8 Fig. 7. Weather maps at 500 hPa for (a) 14 April (O3-South), (b) April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT. Thick 8 April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT. blue lines for sea level pressureFig. 6. contours,Surface weather thin red maps lines for for (a)surface 14 AprilThick (O3-South), blue lines (b) 8 for 500Fig.hPa 7. height,Weather thin maps red at lines 500 forhPa isothermsfor (a) 14 April (O3-South), (b) isotherms and wind vectorsApril are (Cold shown. Surge) and (c) 24 April (O3-North) atand 13:00 wind CDT. vectors Thick are shown.8 April (Cold Surge) and (c) 24 April (O3-North) at 13:00 CDT. blue lines for sea level pressure contours, thin red lines for surface Thick blue lines for 500 hPa height, thin red lines for isotherms isotherms and wind vectors are shown. and wind vectors are shown. Atmos. Chem. Phys., 0000, 0001–24, 2005 www.atmos-chem-phys.org/acp/0000/0001/ Atmos. Chem. Phys., 0000, 0001–24, 2005 www.atmos-chem-phys.org/acp/0000/0001/ 2274 B. de Foy et al.: Mexico City wind circulation

Table 1. Summary of MCMA-2003 ﬁeld campaign days with classiﬁcation by episode type.

Date Type of Day Cloud Cover1 Rain2 Category Episode Max O3 (RAMA) Max NOx (RAMA) Max CO (RAMA) Morning Afternoon mm ppb Site Sec. ppb Site Sec. ppm Site Sec. 01/04/2003 Broken Overcast Cold Surge Cold0 154 SUR SW 379 EAC NW 8.8 TAX SE 02/04/2003 Broken Overcast Cold Surge 117 TAH SE 395 EAC NW 7.1 EAC NW 03/04/2003 Few Broken O3-North North1 243 AZC NW 368 EAC NW 9.5 VAL NW 04/04/2003 Few Broken 0.7 O3-North 134 AZC NW 365 HAN CR 9.4 ARA NE 05/04/2003 Saturday Few Broken O3-North 157 TLA NW 352 MER CR 7.5 MER CR 06/04/2003 Sunday Scattered Broken O3-North 171 AZC NW 298 XAL NE 10.1 TAX SE 07/04/2003 Broken Overcast 0.4 O3-North 167 AZC NW 436 HAN CR 11.6 ARA NE 08/04/2003 Broken Overcast 4.9 Cold Surge Cold1 147 TPN SW 287 XAL NE 7.8 MIN CR 09/04/2003 Overcast Overcast 0.1 Cold Surge 122 TAH SE 238 TAX SE 6.4 MIN CR 10/04/2003 Broken Overcast 0.7 Cold Surge 148 TPN SW 337 EAC NW 7.8 MIN CR 11/04/2003 Overcast Overcast 2.0 Cold Surge 150 CHA NE 439 HAN CR 12.5 ARA NE 12/04/2003 Saturday Clear Broken O3-North Transition1 173 CHA NE 374 HAN CR 6.8 MIN CR 13/04/2003 Sunday Clear Broken 0.2 O3-North 145 LAG CR 251 TAX SE 4.5 MIN CR 14/04/2003 School vacation Scattered Broken O3-South South1 164 TAH SE 387 XAL NE 9.9 VAL NW 15/04/2003 School vacation Clear Broken O3-South 146 PED SW 419 HAN CR 6.9 HAN CR 16/04/2003 School vacation Clear Scattered O3-South 180 TPN SW 352 HAN CR 7.2 MIN CR 17/04/2003 M. Thursday Clear Broken O3-South 160 BJU CR 328 MER CR 6 MER CR 18/04/2003 Good Friday Clear Broken 0.5 O3-North Transition2 121 AZC NW 269 TAX SE 4.8 TAX SE 19/04/2003 Holy Saturday Clear Overcast 0.1 Cold Surge Cold2 153 BJU CR 257 TAX SE 4.7 MIN CR 20/04/2003 Easter Clear Overcast 0.3 Cold Surge 145 PED SW 173 MER CR 4.4 MIN CR 21/04/2003 School vacation Few Overcast 0.4 Cold Surge 175 AZC NW 305 SUR SW 6 SUR SW 22/04/2003 School vacation Broken Overcast 0.5 Cold Surge 148 SAG NE 352 TAX SE 6.1 TAX SE 23/04/2003 School vacation Clear Overcast 0.3 O3-North North2 209 LAG CR 426 XAL NE 7.2 MIN CR 24/04/2003 School vacation Clear Clear O3-North 210 SAG NE 382 AZC NW 10.1 ARA NE 25/04/2003 School vacation Clear Broken O3-North 163 XAL NE 342 TLA NW 8.4 CES SE 26/04/2003 Saturday Clear Broken O3-North 184 AZC NW 459 XAL NE 10.3 XAL NE 27/04/2003 Sunday Clear Broken O3-North 178 AZC NW 316 TAX SE 5.9 TAX SE 28/04/2003 Clear Overcast 0.2 O3-North 179 AZC NW 352 BJU CR 10.2 VAL NW 29/04/2003 Clear Broken 0.1 O3-North 185 BJU CR 474 MER CR 11 ARA NE 30/04/2003 Clear Scattered O3-North 175 AZC NW 436 HAN CR 8.4 HAN CR 01/05/2003 Labor Day Clear Broken O3-South South2 177 TAH SE 319 XAL NE 6.2 TAC NW 02/05/2003 Clear Scattered O3-South 185 TPN SW 373 HAN CR 6.8 HAN CR 03/05/2003 Saturday Clear Broken O3-South 165 BJU CR 404 XAL NE 7.2 XAL NE 04/05/2003 Sunday Few Broken O3-North North3 152 TAC NW 292 TAX SE 5.2 TAX SE

1 Average of AERO METAR reports 2 Average of afternoon (12:00–00:00) accumulated rain at GSMN, ENCB, AERO, TEZO and PIME

5 MCMA classification tween 12:00–18:00. These are arranged by episode type and show clearly high ozone levels in the south of the city for O3- The classification into 3 episode types was carried out sub- South days, in the north for O3-North, and lower levels with jectively from an analysis of the synoptic flow conditions. a more even geographical distribution for Cold Surge. The Table 1 lists all the days of the campaign with their group, variation within the episode types can be clearly seen. The cloudiness, rain, O3 and CO levels, showing the relevance Cold Surge category contains more variation than the others. of the episodes to the basin atmospheric chemistry. There Each Cold Surge event can be of different strength and in are 7 O3-South days, 10 Cold Surge days and 17 O3-North addition, each day of the Cold Surge has different features. days making each episode type well represented during the O3-South events are most similar to each other, in particular campaign. Each of the three episode types lead to high pollu- because they all have low level of cloudiness. O3-North days tion events: O3-South days have a peak around 150 ppb O3, have different levels of clouds and of wind strengths, leading with one day above 200 ppb, Cold Surge days have a broad to substantial variations in the ozone levels. They also in- distribution from 140 to 175 ppb and O3-North days have clude transition days (e.g. 12, 13 and 18 April) that are less a narrower distribution around 170 ppb but with three days clearly defined than the others. above 200 ppb. Figure 8 shows the wind rose for CENICA during the en- Two-dimensional maps of the maximum ozone for every- tire campaign. The preferred directions are from the north- day of the campaign are shown in Fig. 18. The timing of the east and south, but there is no clear relationship between the maximum differs based on the station and the day, and is be- wind speed and direction, and no way of seeing the variation

Atmos. Chem. Phys., 5, 2267–2288, 2005 www.atmos-chem-phys.org/acp/5/2267/ 24 B. de Foy et al.: Mexico City wind circulation

B. de Foy et al.: Mexico City wind circulation 2275

(b) Wind Speed and Direction 6 O3−South 5 Cold Surge O3−North 4

(a) Wind Rose 3

Wind Speed (m/s) 1 N 15% 0 0 3 6 9 12 15 18 21 24 10% 5% E N W W 5% E S B. de Foy et al.: Mexico City wind circulation 25 E

Wind Direction N W S S 0 3 6 9 12 15 18 21 24 0.50 2 5 1 3.5 m/s Time of Day

Fig. 8. (a) Wind rose for CENICA during MCMA-2003. Dominant wind direction is from the south with both stronger and calmer winds, secondary preferred directionFig. 8. (a) fromWind therose north-north-east for CENICA with during medium MCMA-2003. winds. Diurnal Domi- variation of wind speed and direction (b) at CENICA segregated by episodenant type: wind red direction for O3-South, is from theblue south for Coldwith both Surge stronger and green and calmer for O3-North. Thick line indicates median value, thin lines winds, secondary preferred direction from the north-north-east with show the 25 and 75 percentile (inter-quartile range), dotted lines show minimum and maximum values. Arrows show timing of wind shift medium winds. Diurnal variation of wind speed and direction (b) for O3-North (15:00) and O3-South (20:00). at CENICA segregated by episode type: red for O3-South, blue for Cold Surge and green for O3-North. Thick line indicates median value, thin lines show the 25 and 75 percentile (inter-quartile range), of the winds by timedotted of day. lines Also show shown minimum in Fig. and8 maximumare the diur- values. ArrowsO3−South show Cold Surge O3−North nal profile of wind speedtiming and of wind wind shift direction for O3-North as median, (15:00) upper and O3-South (20:00). N 15% N 15% N 15% 10% 10% 10% and lower quartile and data range for each episode. For wind 5% 5% 5% speed, there is a clear pattern that differs little between the W E W E W E episodes. Winds are below 2 m/s in the early morning, increase steadily from sunrise to speeds of 4 to 6 m/s at sunset and then decrease until about midnight. The wind direction S S S is southerly in the early morning, shifting to northerly during the day and back to southerly at night. There is a sharp dif- 0 9 15 Time of Day ference between the episodes in the timing of the wind shift 6 12 18 from northerly to southerly, highlighted by the arrows in the Fig.Fig. 9. 9.“Time“Time rose” rose” for CENICA for CENICA for O3-South, for Cold O3-South, Surge and O3- Cold Surge and O3- figure. O3-South days have a late shift to southerly between North. Rose segments correspond to time of day as follows: green 18:00–21:00. The same shift exists on O3-North days but North.00:00–6:00, Rose blue 06:00–9:00,segments pink correspond 09:00–12:00, to red time 12:00–15:00, of day as follows: green 00:00–6:00,brown 15:00–18:00 blue and tan06:00–9:00, 18:00–24:00. pink 09:00–12:00, red 12:00–15:00, 3–6 h earlier, starting at 15:00. For Cold Surge days winds brown 15:00–18:00 and tan 18:00–24:00. are more consistentlyAtmos. from Chem. the north Phys., and 5, north-west, 1–40, 2005 with a www.atmos-chem-phys.org/acp/5/1/ gradual shift limited to the early hours of the morning. Be- cause traditional wind roses fail to capture the differences in these patterns, Fig. 9 shows a “Time rose” for each episode. Wind speed categories have been replaced by time of day categories, showing the dominant wind direction for each time These sharp differences suggest that this classification ac- segment. These were selected to be 0–6, 6–9, 9–12, 12–15, counts for the main variability in basin observations. It 15–18 and 18–24: 3 h resolution during the day and 6 h at will therefore be used in understanding the circulation in the night when the variations are smaller. The distinctive feature MCMA and the manner it which it is influenced by the syn- of each episode can be clearly seen: northerly to southerly optic flow. Once the circulation patterns associated with each wind shift between 18:00–24:00 for O3-South, same shift but episodes are established, they will help in the analysis of at 15:00 for O3-North and northerly winds for Cold Surge. chemical measurements in the basin and provide a basis for organizing and averaging field campaign results. www.atmos-chem-phys.org/acp/5/1/ Atmos. Chem. Phys., 5, 1–40, 2005 www.atmos-chem-phys.org/acp/5/2267/ Atmos. Chem. Phys., 5, 2267–2288, 2005 B. de Foy et al.: Mexico City wind circulation 9 B. de Foy et al.: Mexico City wind circulation 9 with distance towards the south-west. There is, however, a B. de Foy et al.: Mexico City wind circulationclear line of influence that extends from Zacatecas to Presa 9 26 B. de Foy et al.: Mexico City windwith circulation distance towards the south-west. There is, however, a Allende, Nevado de Toluca and Izucar´ de Matamoros. withclear distance line of influence towards the that south-west. extends from There Zacatecas is, however, to Presa a clearAllende, line Nevado of influence de Toluca that extends and Izucar´ from de Zacatecas Matamoros. to Presa Allende,6 Basin Nevado Surface de Observations Toluca and Izucar´ de Matamoros. 6 Basin Surface Observations ° 30 N The winds at CENICA have a strong diurnal pattern in wind 6 Basin Surface Observations 2276° 10 m/s speed B. de Foy and et al.: sharp Mexico differences City wind circulation in wind direction between the 30 N The winds at CENICA have a strong diurnal pattern in wind ° 25 N episodes. This suggests that the basin circulation is a com- ° 10 m/s speed and sharp differences in wind direction between the 30 N 6 RegionalThebination surface winds observations of at strong CENICA thermally-induced have a strong diurnal flows andpattern the in line wind of ° 25 N episodes. This suggests that the basin circulation is a com- 10 m/s speed and sharp differences in wind direction between the ° convergence described above that passes roughly through the 20 N Figure 10 showsbination the surface of strong winds over thermally-induced the sea measured flows and the line of ° 25 N by SeaWindsepisodes.basin. on QuikSCAT Fig. This (12Perry shows suggests, 2001), maps averaged that of forthe “Time both basin roses” circulation for selected is a com- ob- ° convergence described above that passes roughly through the 20 N ascending andbination descending of passes strong by episode. thermally-induced On the Pacific flows and the line of B. de° Foy et al.: Mexico City wind circulationservations 27 in and around the basin. 15 N side, the Hawaiianbasin. high Fig. leads 12 to north-westerly shows maps winds of sweep- “Time roses” for selected ob- ° convergence described above that passes roughly through the 20 N ing down theservations PacificFor coast O3-South infrom and Baja around days, California night-time the and basin. turning drainage flows into the ° 15 N easterly intobasin.basin the intertropical give Fig. way 12 convergence shows to morning maps zone below of northerly “Time Aca- roses” winds. for As selected thermally ob- ° 10 N ° ° For O3-South days, night-time drainage flows into the 115 W ° ° W pulco. On the Gulf of Mexico side, the Bermuda-Azores high 110 ° ° ° 85 ° W 105 W 95 W 90 W servations in and around the basin. 15 N 100 W induced winds develop both in the basin and on the regional leads to south-easterlybasin give winds way moving to up morning towards Texas. northerly O3- winds. As thermally ° 10 N For O3-South days, night-time drainage flows into the ° ° 115 W ° ° 85 W North days followscale, this winds pattern turn very closely. to north-easterly O3-South days across the whole basin. 110 ° ° ° W W 105 W 100 W 95 W 90 induced winds develop both in the basin and on the regional (a) O3-South are similar exceptbasin for give a weakening way to of the morning flow along northerly the Pa- winds. As thermally ° The passage of the regional scale convergence zone over the 10 N ° ° 115 W ° ° W 110 ° ° ° W 85 cific coast andscale, easterly, winds rather than turn south-easterly, to north-easterly winds over across the whole basin. W 105 W 100 W 95 W 90 induced winds develop both in the basin and on the regional (a) O3-South the Gulf of Mexico.basin On can Cold be Surge seen days, by the the strong presence northerly of north-easterly winds at scale,The passage winds of turn the to regional north-easterly scale convergence across the zone whole over basin. the ° current is clearlyboth visible Cerro over Catedral the Gulf, leading and to Nevado a gap flow de Toluca, to the west of 30 N basin can be seen by the presence of north-easterly winds at (a) O3-South over the IsthmusThetheof basin. passage Tehuantepec It of is theand only into regional after the Pacific. sunset scale Note convergence that there is zone a wind over shift the ° 10 m/s that averagingboth blurs Cerro this feature Catedral as the strength and Nevado of the flow de Toluca, to the west of Fig.10. 30 NAverage Quikscat winds for (a) O3-South, (b) Cold Surge basin can be seen by the presence of north-easterly winds at and (c) O3-North. All available data from both ascending and de- to southerly as a gap flow develops in the Chalco passage. 25° varies strongly both during and between Cold Surge events. scending N passes is combined to make the average. the basin. It is only after sunset that there is a wind shift ° both Cerro Catedral and Nevado de Toluca, to the west of 30 N 10 m/s Surface windsThisto southerly along reinforces the coasts as the ofa gapMexico drainage flow are determined develops at the basin in edge the Chalco leading passage. to basin ° 25 N by the interactionthe basin. of the sea It is breezes only with after these sunset station- that there is a wind shift 10 m/s venting towards the northeast. 20° ary highs. FigureThis reinforces11 shows “Time the roses” drainage for representa- at the basin edge leading to basin N to southerly as a gap flow develops in the Chalco passage. ° 25 N tive stations onOn the regional O3-North scale. days, On the the Pacific situation coast, at is very similar with a cru- 28 B. de Foy et al.: Mexico City windventing circulation towards the northeast. ° This reinforces the drainage at the basin edge leading to basin 20 N the stations ofcial Acapulco difference (ACAP) inand timing. Puerto Angel The (PUER) drainage flows are equally ° On O3-North days, the situation is very similar with a cru- 15 N the winds areventing predominantly towards westerly the with northeast. a secondary sea ° present and the morning winds are northerly. The afternoon 20 N breeze turningcial the wind difference towards the in ocean timing. before sunrise The and drainage flows are equally ° On O3-North days, the situation is very similar with a cru- 15 N towards thewindpresent land in shift the and afternoon. starts the morning at R´ıo 15:00 Tomatlan winds in the (TOMA) aresouth-east northerly. with The the afternoon gap flow 10° N ° ° and Chinatucial (CHIN) difference have winds determined in timing. by the The local drainage to- flows are equally 115 W ° ° W ° ° ° 85 through Chalco. It is sufficiently strong to reverse the up- ° 110 W 105 90 W 15 N W 100 W 95 W pography. Onpresentwind the Gulf shift coast,and starts the to the morning at north, 15:00 the strength winds in the of are south-east the northerly. with The the afternoon gap flow ° slope flow at Chapingo (CHA) and to sweep through the 10 N ° ° 115 W ° ° W surface easterlies prevents a flow reversal and winds are con- 110 ° ° ° W 85 through Chalco. It is sufficiently strong to reverse the up- Atmos. Chem. Phys., W 5, 1–40 105, 2005 W 100 W 95 W 90 www.atmos-chem-phys.org/acp/5/1/wind shift starts at 15:00 in the south-east with the gap flow (b) Cold Surge sistently inlandwhole (easterly). city To in the the south mid-afternoon. of Tampico (TAMP), With a slight delay, there is ° 10 N slope flow at Chapingo (CHA) and to sweep through the ° ° 115 W ° ° W 110 ° ° ° W 85 starting withthrough Tuxpan, (TUXP) Chalco. there It is ais reversal, sufficiently however, strong to reverse the up- W 105 W 100 W 95 W 90 an additional flow over the basin rim in the south-west lead- (b) Cold Surge with westerlyslopewhole flows in flow city the early in at the morning. Chapingo mid-afternoon. (CHA) With and to a slight sweep delay, through there the is In the interioring to of Mexico,wind reversals the surface at winds Tlalpan are deter- (TPN) and Pedregal (PED) ° an additional flow over the basin rim in the south-west lead- Fig.10. 30 NContinued. (b) Cold Surge whole city in the mid-afternoon. With a slight delay, there is mined by thethating competing to actually wind inland reversals begin flows which before at create Tlalpan sunset. a conver- (TPN) These winds and Pedregal are reinforced (PED) ° 10 m/s gence zone thatan runsadditional down the flow Mexican over Plateau the towards basin the rim in the south-west lead- 30 N by the winds at Cerro Catedral and Nevado de Toluca which MCMA. Onthat certain actually days, this begin is visible before as cumulus sunset. clouds These winds are reinforced ° 25 N ing to wind reversals at Tlalpan (TPN) and Pedregal (PED) ° forming in theare afternoon now entirely over the Mexican from thePlateau. west, Thesuggesting po- that the regional 30 N 10 m/s thatby the actually winds begin at Cerro before Catedral sunset. and These Nevado winds de are Toluca reinforced which ° sition of the convergence line is influenced by the strength 25 N convergence zone is being pushed to the north-east. 10 m/s are now entirely from the west, suggesting that the regional ° and direction of the westerlies aloft. At Zacatecas (ZACA) 20 by the winds at Cerro Catedral and Nevado de Toluca which N Cold Surge days have predominant northerly winds which 25° and Los Colomos (LOSC), on the Pacific side of the Mexican N areconvergence now entirely zone from is being the west, pushed suggesting to the north-east. that the regional ° Plateau, the windsare superimposed are predominantly onwesterly the andthermal the differ- basin winds. Despite being 20 N Cold Surge days have predominant northerly winds which ° ence in episodesconvergence is limited to an zone increasing is being northerly pushed compo- to the north-east. 15 N weakened by cloudiness and reduced solar heating, these are ° are superimposed on the thermal basin winds. Despite being 20 N nent for O3-SouthstillCold strong days. Surge To enough the days south-east tohave cause of predominant the morning MCMA, drainage northerly flows winds into which the 15° surface stations are very influenced by local topography. For N areweakened superimposed by cloudiness on the andthermal reduced basin solar winds. heating, Despite these being are 10° these stations,basin windin patterns the south follow diurnal and west mountain/valley of the city and afternoon westerly N ° ° www.atmos-chem-phys.org/acp/5/1/ 115 W ° ° W Atmos. Chem. Phys., 5, 1–40, 2005 110 ° ° ° W 85 still strong enough to cause morning drainage flows into the ° W 90 15 N 105 W 100 W 95 W flow patternsweakenedand with south-westerly little variation by cloudiness between gap the flows. episode and reduced types. solar heating, these are ° 10 N basin in the south and west of the city and afternoon westerly 115° ° The real difference among episodes exists in the Mexico W ° ° ° ° 85 W still strong enough to cause morning drainage flows into the 110 W 105 ° 90 W (c) W O3-North 100 W 95 W City basin andand the neighboringsouth-westerly part of the gap Mexican flows. Plateau to 10° basin in the south and west of the city and afternoon westerly N ° ° 115 W ° ° W the north. On O3-South days, the stations west of the MCMA 110 ° ° ° W 85 W 105 W 100 W 95 W 90 Fig.Fig. 10.Continued.Average Quikscat(c) winds O3-North for (a) O3-South, (b) Cold Surge have a south-westerlyand7 Radiosonde south-westerly flow in the morning Observations gap but flows. strong afternoon turning with winds from the north-west and north-east. Fig. 10. Averageand (c) O3-North. Quikscat All available winds data for from (a) O3-South,both ascending (b) and Cold de- Surge scending passes is combined(c) O3-North to make the average. At Pachuca,7 the afternoon Radiosonde winds are Observations north-westerly instead and (c) O3-North. All available data from both ascending and de- Vertical profiles of humidity, wind speed and wind direction Fig. 10. Average Quikscat winds for (a) O3-South, (b) Cold Surgeof south-westerly. The north-westerly winds are due to lo- scending passes is combined to make the average. cal heating and7for terrain RadiosondeAcapulco, turning the Mexico Observationsnorth-easterly City Gulf and flow. Veracruz are shown in Fig. and (c) O3-North. All available data from both ascending and de- Vertical profiles of humidity, wind speed and wind direction Fig. 10. Average Quikscat winds for (a) O3-South, (b) Cold SurgeOverall, this13 suggests and a 14, convergence segregated line that by passes episode through type. This is a compos- scending passes is combined to make the average. Verticalfor Acapulco, profiles Mexico of humidity, City and wind Veracruz speed and are wind shown direction in Fig. and (c) O3-North. All available data from both ascending and de- ite13 ofand all 14, the segregated morning (07:00) by episode soundings type. This available is a compos-showing scendingAtmos. passes Chem. is combined Phys., 5, 2267– to make2288 the, 2005 average. for Acapulco, www.atmos-chem-phys.org/acp/5/2267/ Mexico City and Veracruz are shown in Fig. inite greater of all the detail morning the difference (07:00) soundings along a coast-to-coast available showing tran- 13sect and through 14, segregated Mexico City. by episode The weak type. winds This at Acapulcois a compos- are itein greater of all the detail morning the difference (07:00) soundings along a coast-to-coast available showing tran- asect very through robust Mexico feature due City. to The the sub-tropical weak winds high, at Acapulco with west- are Atmos. Chem. Phys., 5, 1–40, 2005 www.atmos-chem-phys.org/acp/5/1/inerly greater winds detail in the the boundary difference layer along but a north-easterly coast-to-coast above. tran- secta very through robust Mexico feature dueCity. to The the sub-tropicalweak winds high,at Acapulco with west- are aerly very winds robust in feature the boundary due to the layer sub-tropical but north-easterly high, with above. west- www.atmos-chem-phys.org/acp/0000/0001/erly winds in the Atmos. boundary Chem. layer Phys., but north-easterly0000, 0001–24, above. 2005 www.atmos-chem-phys.org/acp/0000/0001/ Atmos. Chem. Phys., 0000, 0001–24, 2005 www.atmos-chem-phys.org/acp/0000/0001/ Atmos. Chem. Phys., 0000, 0001–24, 2005 B. de Foy et al.: Mexico City wind circulation 2277 the Mexico City basin and moves west as the day progresses. On O3-North days, the stations west of the MCMA have south-westerly flow in the morning and north-westerly in the afternoon. On the opposite side of the basin, at Pachuca (PACH), there is north-easterly flow in the morning turning to south-westerly in the afternoon. This suggests that in the morning the convergence line passes through the MCMA and is then shifted to the north-east as the incoming flow from the Pacific is reinforced by the westerlies aloft. Cold Surge days have the northerly flow superimposed on local thermal flows with the greatest influence along the Gulf coast, decreasing with distance towards the south-west. There is, however, a clear line of influence that extends from Zacatecas (ZACA) to Presa Allende (ALLE), Nevado de Toluca (NEVA) and Izucar´ de Matamoros (IZUC) as shown in Fig. 11.

7 Basin surface observations

The winds at CENICA have a strong diurnal pattern in wind speed and sharp differences in wind direction between the episodes. This suggests that the basin circulation is a combination of strong thermally-induced flows and the line of convergence described above that passes roughly through the basin. Figure 12 shows maps of “Time roses” for selected observations in and around the basin. For O3-South days, night-time drainage flows into the basin give way to morning northerly winds. As thermally induced winds develop both in the basin and on the regional scale, winds turn to north-easterly across the whole basin. The passage of the regional scale convergence zone over the basin can be seen by the presence of north-easterly winds at both Cerro Catedral and Nevado de Toluca, to the west of the basin. It is only after sunset that there is a wind shift to southerly as a gap flow develops in the Chalco passage. This reinforces the drainage at the basin edge leading to basin venting towards the northeast. On O3-North days, the situation is very similar with a cru- cial difference in timing. The drainage flows are equally present and the morning winds are northerly. The afternoon wind shift starts at 15:00 in the south-east with the gap flow through Chalco. It is sufficiently strong to reverse the upslope flow at Chapingo (CHA) and to sweep through the whole city in the mid-afternoon. With a slight delay, there is an additional flow over the basin rim in the south-west leading to wind reversals at Tlalpan (TPN) and Pedregal (PED) that actually begin before sunset. These winds are reinforced by the winds at Cerro Catedral and Nevado de Toluca which Fig. 11. “Time roses” for selected SMN surface stations on the na- are now entirely from the west, suggesting that the regional tional scale for O3-South, Cold Surge and O3-North. Topography is convergence zone is being pushed to the north-east. shown by colored contours. Dashed red lines show estimated loca- Cold Surge days have predominant northerly winds which tion of convergence zones, with black arrow showing its estimated are superimposed on the thermal basin winds. Despite being displacement as the day progresses. weakened by cloudiness and reduced solar heating, these are still strong enough to cause morning drainage flows into the www.atmos-chem-phys.org/acp/5/2267/ Atmos. Chem. Phys., 5, 2267–2288, 2005 32 B. de Foy et al.: Mexico City wind circulation

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basin in the south and west of the city and afternoon westerly O3−South PACH and south-westerly gap ﬂows.

VIF 8 Radiosonde observations

CATE SAG MADI XAL TLA Vertical proﬁles of humidity, wind speed and wind direc- B. de Foy et al.: Mexico City wind circulation 33 CHA tion for Acapulco, Mexico City and Veracruz are shown in TAC AERO MER Figs. 13 and 14, segregated by episode type. This is a com- posite of all the morning (07:00) soundings available show- CUA CENI ing in greater detail the difference along a coast-to-coast tran- PED NEVA sect through Mexico City. The weak winds at Acapulco are a TPN very common feature due to the sub-tropical high, with west- TAH erly winds in the boundary layer but north-easterly above. There is a humid boundary layer below 1000 m with lit- 0 6 9 121518 Time of Day tle variation of wind speeds. Veracruz, in comparison, has

Fig. 12. “Time roses” for selected stations in the Mexico City basin more humid conditions and a boundary layer extending up to for O3-South, Cold Surge and O3-North. Topography is shown by colored contours.Cold Surge 2000 m. Due to the Bermuda high, the winds are weak and PACH southerly below 3000 m, except for the Cold Surge where

VIF a clear easterly jet can be seen below 1000 m. In Mexico City, the boundary layer is much drier and extends up to 3000 m. There is a slight but signiﬁcant trend of increas- CATE ing humidity from O3-South to O3-North and Cold Surge. XAL MADI SAG TLA 34 B. de Foy et al.: MexicoWinds City wind are circulation below 6 m/s below 3000 m and increase steadily

TAC CHA above this, suggesting the impact of mountain shielding on AERO MER the basin. Wind directions aloft are clearly segregated by episode, with north-westerly flow for O3-South, westerly Atmos. Chem. Phys., 5,CUA 1–40, 2005CENI www.atmos-chem-phys.org/acp/5/1/ flow for O3-North and south-westerly for Cold Surge. It is PED NEVA only in a shallow layer that Cold Surge days have northerly TPN TAH and easterly winds. Due to the intense solar radiation heating, high altitude and low humidity in the basin there is intense vertical mix- 0 9 15 Time of Day 6 12 18 ing with mixing heights up to 4000 m. Maximum mixing Fig. 12. Continued. heights for each day of the campaign were calculated by find- O3−North PACH ing the height at which the gradient of potential temperature first exceeds 2.5 K/km, following Fast and Zhong (1998). VIF The radiosonde data was collected at 2 s intervals leading to high resolution profiles. For Mexico City, the top of the CATE mixing height is a sharp feature, with different estimation SAG MADI XAL TLA methods yielding similar results and an accuracy estimated

CHA TAC to be within that of the uncertainty of the feature itself (100– AERO MER 200 m). O3-South days have mixing heights in the range of

CUA CENI 3000 to 4000 m. O3-North are in the range of 1000 to 3500 m www.atmos-chem-phys.org/acp/5/1/ Atmos. Chem. Phys., 5, 1–40, 2005 PED and Cold Surge 900 to 3000 m. For clear days, the maximum NEVA is at the 19:00 sounding. On cloudy days, it is usually at the TPN TAH 13:00 sounding. Morning thermal inversions are important in determining the concentration of primary pollutants dur-

0 6 9 121518 Time of Day ing rush hour. These existed on nearly half of the days of the campaign, with intensities reaching 2.5 K. On a further Fig. 12. Continued. Fig. 12. “Time roses” for selected stations in the Mexico City basin 6 days there were inversions above the surface between 500 for O3-South, Cold Surge and O3-North. Topography is shown by and 1000 m. These inversions occur on all types of days as colored contours. skies tend to be free of clouds in the morning for most meteorological conditions. Further aloft, sharp capping inversions occur on 75% of days with intensities up to 3.3 K. These are strongest and most persistent for the O3-South days but

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B. de Foy et al.: Mexico City wind circulation 2279

ACAP MEX VER 5000 O3−South Cold Surge O3−North 4000

3000

362000 B. de Foy et al.: Mexico City wind circulation

Height Above Ground (m) 1000

0 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 Water Mixing Ratio (g/kg) Water Mixing Ratio (g/kg) Water Mixing Ratio (g/kg)

Fig. 13. Radiosonde proﬁlesFig. 13. ofRadiosonde water mixing proﬁles ratio for of Acapulco water mixing (ACAP, ratio 3 m for a.s Acapulco.l.), Mexico City (MEX, 2309 m a.s.l.) and Veracruz (VER, 13 m a.s.l.) by episode(ACAP, for all available 3 masl), 07:00 Mexico CDT City soundings. (MEX, As 2309 for masl)the diurnal and Veracruzplots, the bold line indicates the median, the thin line the 25 and 75 percentile (inter-quartile(VER, 13 range) masl) and by episode the dotted for lines all availablethe data range. 07:00 Red CDT for sound- O3-South, blue for Cold Surge and green for O3-North. The height of the 500 hPaings. level Asis for 5850–5900 the diurnal m plots, a.s.l. the bold line indicates the median, the thin line the 25 and 75 percentile (inter-quartile range) and the

dotted lines the dataACAP range. Red for O3-South, blueMEX for Cold Surge VER and5000 green for O3-North. The height of the 500 hPa level is 5850– 5900 masl O3−South Cold Surge O3−North 4000

3000

2000

Height Above Ground (m) 1000

0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Wind Speed (m/s) Wind Speed (m/s) Wind Speed (m/s)

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2000

Height Above Ground (m) 1000

0 E S W N E S E S W N E S W N E S W N Wind Direction Wind Direction Wind Direction

Fig. 14. Radiosonde proﬁlesFig. 14. of windRadiosonde speed and proﬁles direction of windfor Acapulco speed and (ACAP), direction Mexico for City (MEX) and Veracruz (VER), see Fig. 13 for explanation. Acapulco (ACAP), Mexico City (MEX) and Veracruz (VER), see Fig. 13 for explanation.

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B. de Foy et al.: Mexico City wind circulation 15

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O3−South Cold Surge O3−North O3−South Cold Surge O3−North 5000 5000 01 01 07 07 13 13 4000 19 4000 19

3000 3000

2000 2000 Height Above Ground (m)

Height Above Ground (m) 1000 1000

0 00 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 0 2 Wind4 6 Speed8 10 (m/s)12 14 16 0 2 Wind4 6 Speed8 10 (m/s)12 14 16 0 2 Wind4 6 Speed8 10 (m/s)12 14 16 Wind Speed (m/s) Wind Speed (m/s) Wind Speed (m/s)

O3−South Cold Surge O3−North 5000 01 07 13 4000 19

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0 E S W N E S E S W N E S E S W N E S Wind Direction Wind Direction Wind Direction

Fig. 15. Radiosonde profiles of wind speed and direction at GSMN (MEX) by time of day for each episode. As for the previous profiles, the Fig. 15. Radiosonde profilesboldFig. line 15. indicates ofRadiosonde wind the median, speed theprofiles and thin line direction of the wind25 and at 75 speed GSMNpercentile and (inter-quartile (MEX) direction by range) at time GSMN and of the daydotted for lines each the data episode. range. Blue forAs 01:00 for the previous profiles, the bold line indicatesCDT,(MEX) the green median, forby 07:00 time the CDT, of thin red day forline 13:00for the each CDT 25 and andepisode. black 75 for percentile 19:00 As CDT. for theAll (inter-quartile available previous soundings pro- range) included. and the dotted lines the data range. Blue for 01:00 CDT, green for 07:00files, the CDT, bold red line for indicates 13:00 CDT the and median, black the for thin 19:00 line CDT. the 25All and available 75 soundings included. percentile (inter-quartile range) and the dotted lines the data range. Blue for 01:00 CDT, green for 07:00 CDT, red for 13:00 CDT and there is no clear patternblack otherwise, for 19:00 CDT. with All base available heights soundings rang- included.events and due to the Westerlies aloft. Appendix A2 lists ing between 2070 andwww.atmos-chem-phys.org/acp/0000/0001/ 4500 m. A table of mixing heights the jets identified Atmos. Chem. during Phys., the 0000, campaign, 0001–24, 2005 with maximum wind and temperature inversions for each day of the campaign is speed, average wind direction and vertical extent. The ver- shown in Appendix A1. tical extent can reach up to 3000 m. The maximum wind speed is usually located at mid-height, although it can also The inversions aloft mark the transition from the weak occur near the surface. At 13:00, the jet is weak and shal- winds sheltered by the mountains surrounding Mexico City low, coming from the east or north-east irrespective of the and the dominant westerly winds aloft. Figure 15 shows ag- episode type. By 19:00 the jets have strongly increased and gregate profiles of wind speeds and directions by time of day are differentiated by episode type. For O3-South, the jet is for each episode. Wind speeds below 3000 m are in the range the weakest of the three types and north-easterly. For Cold of 0 to 6 m/s and rarely exceed 8 m/s. Above the boundary Surge the jet is westerly and extends to 1000 to 1500 m. For layer, wind speeds increase steadily from around 5 m/s at O3-North the jet is strongest. It is south-westerly and can 3000 m to a maximum of 20 to 35 m/s at 10 000 m depending extend above 2000 m. At 01:00, the jet is strongly reduced. on the episodes. O3-Southwww.atmos-chem-phys.org/acp/5/1/ days have north-westerly winds Atmos. Chem. Phys., 5, 1–40, 2005 It is westerly for O3-South, north-westerly for O3-North and with a median maximum of around 20 to 25 m/s. Cold Surge northerly for Cold Surge. By 07:00, the jet has increased and days have west-south-westerly winds reaching up to 35 m/s becomes more consistently westerly across the episodes. and O3-North days have westerly winds around 25 to 30 m/s. In the boundary layer a jet forms on most days, as de- In terms of humidity, O3-South shows substantial subsi- scribed by Doran and Zhong (2000). During MCMA-2003, dence surface drying during the day due to vertical mixing jets were observed due to plateau winds, due to “El Norte” with drier air aloft. O3-North has an influx of humidity

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B. de Foy et al.: Mexico City wind circulation 39

B. de Foy et al.: Mexico City wind circulation 2281

CENICA 30 O3−South Cold Surge 180 O3−South O3−North 160 Cold Surge C)

o 25 140 O3−North 120 20 100 80 Temperature (

15 Ozone (ppbv) 60 40

10 20 0 3 6 9 12 15 18 21 24 0 Time of Day 0 3 6 9 12 15 18 21 24 Time of Day 15 O3−South Cold Surge Azcapotzalco 180 O3−North O3−South 160 10 Cold Surge 140 O3−North 120 100 5 80 Ozone (ppbv)

Water Mixing Ratio (g/kg) 60 40 0 0 3 6 9 12 15 18 21 24 20 Time of Day 0 0 3 6 9 12 15 18 21 24 Time of Day Fig.Fig. 16. 16.DiurnalDiurnal pattern pattern of of temperature temperature and and water water mixing mixing ratio ratio at at CENICACENICA by by episode, episode, as as for for Fig. Fig. 8. For temperature, temperature, arrows arrows show show thethe delay delay in in the the maximum maximum temperature temperature for for O3-South O3-South relative relative to to O3- O3- Tlalpan North. For water mixing ratio, arrows indicate the start of influx of 180 North. For water mixing ratio, arrows indicate the start of influx of O3−South moistmoist air air into into the the basin. basin. 160 Cold Surge 140 O3−North accompanying the south-westerly flow. Cold Surge days 120 have a more gradual and more humid profile, with drying 100 during the day and a combination of influx and mixing in the 80

afternoon. Ozone (ppbv) 60 40 9 Diurnal profiles and surface pollution levels 20 0 0 3 6 9 12 15 18 21 24 The diurnal profiles of wind speed and direction at CENICA Time of Day were shown in Fig. 8 by episode type. Similar plots for temperature and water mixing ratio, shown in Fig. 16, contribute Fig. 17.Fig.Diurnal 17. Diurnal pattern pattern of of ozone ozone at at CENICA, CENICA, Azcapotzalco Azcapotzalco and and to the classification of episode types. O3-South days have a Tlalpan by episode, as for Fig. 8. later peak in temperature and slower decrease than O3-NorthTlalpan by episode, as for Fig. 8. Atmos.days. TheChem. humidity Phys., contrasts 5, 1–40, 2005are greater on O3-South days www.atmos-chem-phys.org/acp/5/1/ as dry air is both transported from the Mexican Plateau to the north and mixed down due to subsidence aloft. The O3-North influx of southerly air similar to that described by Doran and days have a reduced diurnal range with an increase in hu- Zhong (2000). midity corresponding to the jet coming from the south-west. Diurnal profiles of ozone are shown for 3 individual Cold Surge days are cooler and more humid, as would be ex- stations in Fig. 17: CENICA, Azcapotzalco and Tlalpan. pected. The temperature and humidity trends act as tracers CENICA, while being in the south-east sector of the city of air mass movements that indicate basin cooling due to an is close to the geographical middle of the basin. The

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Fig. 18. Two-dimensional maps of peak ozone measured by RAMA for each day of the campaign over MCMA, arranged by episode. Bars are colored by episode type: red for O3-South (O3S), blue for Cold Surge (CS) and green for O3-North (O3N). The range of ozone values in ppbv is printed in the lower-left corner of each plot. differences between episodes are smaller here, with higher the evening. For O3-North days there is a sharp drop at 15:00 levels for O3-South and a slightly sharper drop for Cold as the lower-level jet comes over the mountains from the Surge associated with clouds and rain. Ozone levels between south blowing clean air into the basin. For Cold Surge days, midnight and sunrise are noticeably higher for O3-South due there is a bigger spread in the data with a later drop. In con- to lower concentrations of CO and NOx. As there is no dif- trast to CENICA and Azcapotzalco, where the dop is due to ference in wind direction for this time period, the cleaner air cloud and rain, the drop here is due to a drainage flow similar must be due to the higher wind speeds (Fig. 8) leading to to O3-North that forms just on the southern rim of the basin. greater dispersion. This flow is weaker than for both O3-South and O3-North In Azcapotzalco, to the north-west of the city, the diurnal and the reduced winds lead to a more polluted air mass after profiles illustrate the main features of the basin circulation. midnight, signalled by lower ozone concentrations. There is a rapid rise and equally rapid fall of O3 for O3-North days due to the strong afternoon jet transporting pollutants from the whole city over the area after 15:00 and then venting 10 Discussion them out of the basin. For O3-South days, the levels rise slowly as the area receives cleaner air from the north, and The schematic representation of the circulation patterns in then stay high as the weaker jet blows from the city through the basin is shown in Fig. 5, summarizing the conclusions the early evening. Cold Surge days have the same O3 rise as from the data analysis presented. On O3-South days there O3-South days followed by a drop starting at 13:00. This is is weak synoptic forcing and thermal flows with reduced in- faster than the decrease at CENICA due to the stronger winds fluence from outside the basin. Southerly drainage flows at bringing clouds and rain from the north. night give way to north-westerly influx during the day from In Tlalpan, to the south of the city, the rise in O3 is the the Mexican Plateau. This changes to north-easterly Gulf same for all episodes as the wind blows from the north. O3- flow in the afternoon, transporting ozone to the south, before South days have a peak at 16:00 with a gradual decline into switching back to southerly in the early evening. The gap

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flow is reduced in strength and does not influence the west- The gap flows however are clearly separated between O3- ern edge of the basin as for O3-North days. On Cold Surge South and O3-North episodes. On O3-South days there are days, there is a weak component of basin winds with a strong later, weaker jets driven by thermal effects. On O3-North influence from the synoptic conditions. North-easterly flows days, the strong, early jets are due to the influence of the from the Gulf of Mexico are found during the day and give westerlies and extend over the entire basin rim beyond the way to north-westerly Plateau/Pacific air at night. Despite gap itself. The gap flows during MCMA-2003 correlate with cloudiness, high O3 levels take place on these days and tend wind direction aloft, suggesting that the westerlies could be to be centered on the city. These are in part due to the much a clearer predictor of the extent of the jet. higher morning concentrations of primary pollutants due to The presence of a convergence line between surface flows reduced vertical mixing. The cloudiness tends to be patchy from the Pacific and the Gulf of Mexico was suggested by in both space and time, with large clouds forming in the basin surface as well as upper air observations. This regional fea- or moving through. There is therefore sufficient radiation to ture was found to pass through the Mexico City basin, and cause significant photochemistry. O3-North days are domi- to move either to the south-west or north-east based on vari- nated by the westerlies. Early morning drainage flows from ations in regional forcings. This leads to strong differences the south give way to north-westerly inflow from the Mexi- in transport of moisture in and out of the basin. The Mexico can plateau. In the afternoon, the winds shift with the begin- City basin has its own mountain-basin flow with an afternoon ning of the gap flow through the Chalco passage. This domi- wind shift and corresponding convergence line. This feature nates the thermal flows and increases to the point where it is a is very sensitive to the regional flow and leads to large dif- general south-westerly flow coming over the basin rim with ferences in the location and peak concentration of the ozone channeling around the Ajusco mountain through the passes plume. Future work using models and more extensive ver- towards Cuernavaca and Toluca. tical data sets (such as are available from the IMADA cam- This picture is consistent with radiosonde observations. paign) will explore the combination of the two convergence On O3-South days, inflow from the north-east reinforces the lines and the superposition of the regional flow with the local up-slope flow at GSMN. This increases during the afternoon basin flow. The factors affecting the timing and strength of and then decreases, reversing into a westerly drainage flow at the afternoon wind shift will need to be analyzed given its night. For Cold Surge days, the upslope flow is still present, large impact on urban pollution levels. but by late afternoon the northerly influx of cold and humid air leads to a flow reversal and to westerly flow at GSMN. At night, with less heating and cooling the flow is northerly and then also turns to a westerly drainage flow with a northerly 11 Catalogue of events component. O3-North days have weaker near-surface upslope flows during the day due to the stronger westerly com- While the meteorological conditions during the field cam- ponent of the winds aloft. In the late afternoon, the ups- paigns can be shown to belong to three broad categories, lope flow is eliminated by the incoming south-westerly flow. there were specific variations that are of interest for evalu- From the surface observations, this was seen to originate as a ating chemical episodes. The highest O3 level occurred on 3 gap flow at Chalco around 15:00 but then strengthens and be- April. With a maximum of 243 ppb, this is above the trigger comes a generalized south-westerly flow over the basin rim. level for the government’s pre-contingency plan (Comision´ The meteorological conditions of Cold Surge episodes Ambiental Metropolitana, CAM). This was a O3-North day, were analyzed by Schultz et al. (1998). Despite the potential with the O3 peak sharply focussed in the north-west of the for high primary pollutant concentrations there is no pub- city. There were very weak and variable winds in the basin, lished analysis of the air pollution on these episodes. Pre- with neighboring stations often reporting winds of opposite vious modeling work described in the literature review has directions. The mixing height was below 3000 m and the ver- focussed on O3-South events. The episodes described in tical winds were particularly weak: less than 2 m/s for most Williams et al. (1995) and Whiteman et al. (2000) can be of the day and below 3 m/s at 19:00. There was, however, a classified as O3-South with Fast and Zhong (1998) looking stronger than usual jet at 07:00 with westerly winds reaching at both O3-South and O3-North. Bossert (1997) looked at 6 m/s between 750 and 1500 m. In addition to meteoro- three days. His synoptic flow corresponds to O3-North with logical conditions, one of the Metro lines to that section of a particularly strong gap flow. Both the thermal and regional the city was out of operation, causing increased traffic con- flows correspond to O3-South, albeit with different intensity gestion. This day followed the cleanest day in the campaign of forcing aloft. with peak O3 of 117 ppb and was followed by the fourth The gap flow was analyzed by Doran and Zhong (2000) cleanest day with 134 ppb. It therefore poses an interesting who found a correlation with thermal gradients between ra- case where very high photochemistry can take place in a sin- diosonde observations over the southern edge of the basin gle day. In addition, further information on the traffic levels rim and within the basin. During MCMA-2003, no such ob- could be used to analyze this day as a natural experiment of servations were available, precluding a direct comparison. increased emissions. www.atmos-chem-phys.org/acp/5/2267/ Atmos. Chem. Phys., 5, 2267–2288, 2005 2284 B. de Foy et al.: Mexico City wind circulation

2 and 9 April were both Cold Surge days with O3 maxima ing concentrations. While there is no perfect match for this in the south-east, with levels of 117 and 122 ppb. Whereas 2 day, the high O3 levels of the close relatives suggest that it April had very little photochemistry over the city, 9 April had can be used as a natural experiment for analyzing the impact a relatively uniform O3 levels over most of the eastern part of reduced emissions. of the basin. Winds were consistently from the north-north- 22 April was an interesting day at the campaign super-site. west until 14:00 at which point a weakening in the south- There was a broad ozone plume, with a relatively low peak east was followed by convergence of winds from the north- of 148 ppb in the north-east and levels of 110 to 140 ppb at west and south-east and followed by westerly winds. 9 April most of the monitoring sites. Winds were from the north- shows that significant photochemistry can take place even on west before sunrise, shifting to easterly from sunrise until cloudy days with steady winds. 13:00. They then shifted to northerly and at 15:00 formed a On 10 April, a large SO2 plume covered the northern half very sharp convergence zone over CENICA. Strong north- of the city, with maximum concentrations of 277 ppb, start- westerly flow in the north met with strong south-westerly ing around midnight and lasting until around 10 a.m. The flow to the south in a clear east-west line across the basin, mixing heights for the whole day were below 900 m and causing strong convection and rainfall in the process. After winds were weak and from the north in the early morning. this, the winds became more westerly and variable for the Similar but smaller events were recorded on 1, 2, 27 and 30 rest of the day as the skies cleared. April. Finally, 1 May was a O3-South day with clear skies and 11 April had the highest CO levels of the campaign. This peak O3 of 177 ppb in the south-east despite being a national is due to one of the strongest surface inversions with an in- holiday. 2 May had a slightly higher O3 peak at 185 ppb tensity of 1.7 K for a depth of 170 m. The winds in the in the south-west. Both of these days are comparable to the inversion were low but had a 4 m/s jet in the surface layer first 3 days of Holy week where peak O3 was 164 to 180 ppb. and started rising rapidly above 500 m, reaching 10 m/s by There is no clear signal of lower emissions. Nevertheless, 1 1000 m – the strongest vertical winds observed by the morn- and 2 May present an informative case on the importance of ing radiosondes. This was accompanied by a second weak carry-over effects as the O3 peak on 1 May is quite broad inversion at 960 m. The strength of the surface inversions whereas on 2 May high O3 is limited to the south-western and the presence of the second one suggest that the surface edge of the basin. On 1 May, three quarters of stations re- layer was isolated from the flow aloft. At 13:00 the winds ported higher O3 than the median of 2 May. were very similar to 07:00, but by 19:00 they had increased to 6 m/s almost uniformly below 2000 m. Good Friday, had CO concentrations around 50% lower 12 Conclusions than normal and O3 around 20% lower, with a peak value of 121 ppb in the north-west. In addition, it followed a day Three types of episodes have been identified that account with emissions estimated to be 30% lower. Maximum to- for the meteorological conditions occurring during the cam- tal solar radiation at different sites in the basin is around paign. They have been designated “O3-South”, “Cold Surge” 50 W/m2 higher than on the previous day, in the range of and “O3-North” and can be diagnosed from a combination of 1050 to 1100 W/m2. This is similar to the difference ob- synoptic and basin observations. These episodes serve as an served on certain days between city sites and Presa Mad´ın organizing axis for measurements made during the field cam- (MADI), suggesting that the effect of air pollution on sur- paign that is more meaningful than the time axis. By group- face radiation heating is of the order of 50 W/m2. This was a ing or averaging the data according to episodes, significant O3-North day with weak surface winds and clear skies sim- trends and differences become apparent. ilar in some respects to 3 and 25 April which had O3 peaks Diurnal patterns of winds on both the basin and regional of 243 and 163 ppb. The surface winds started off north- scale were displayed using “Time roses”. These figures suc- westerly with an afternoon shift to south-easterly leading to cintly capture a large amount of information and enable the the ideal condition for a repeat of 3 April. There was one competing effects of regional and basin thermal flows to be of the strongest surface inversions of the campaign at 07:00, analyzed. In particular, the extent of the sea breezes from the with an intensity of 1.6 K for a depth of 210 m, but this Pacific and from the Gulf can be clearly seen as a conver- was accompanied by an unusually strong jet above, reaching gence line over the Mexican plateau that interacts with the 9 m/s between 500 and 1000 m. In this respect, it differed convergence line in the basin due to wind shifts. markedly from 3 April, which had very weak winds through- The existence of a wind jet through the pass in the south- out the mixed layer, and resembles more closely 25 April, east of the basin has been well established in previous field which had a 07:00 jet reaching 8 m/s at 700 m. 21 April campaigns. Its strength was found to correlate with thermal was also similar with a 07:00 jet of 8 m/s between 1000 and gradients across the basin rim. In this study, the strength 1500 m. While this is above the early morning mixed layer, of the jet was found to be a function of episode type and CO concentrations were very low on all three of those days, more closely correlated with wind direction aloft. Further suggesting that the jet had an important impact on the morn- analysis will be needed to establish the forcing mechanisms

Atmos. Chem. Phys., 5, 2267–2288, 2005 www.atmos-chem-phys.org/acp/5/2267/ B. de Foy et al.: Mexico City wind circulation 2285 responsible for this flow feature. Because no thermal gradi- Acknowledgements. The analysis contained in this paper was made ent measurements exist for MCMA-2003, modeling studies possible by the collaborative efforts of many people involved in might be able to address this question and allow for a better field measurements, both during the campaign and over longer pe- comparison with the IMADA events. riods of time. We are indebted to the staff of CENICA who hosted Previous analysis of the circulation in the Mexico City the campaign. We would like to thank S. Blanco, A. Sanchez, basin has covered a large range of conditions but focussed O. Fentanes, J. Zaragoza, A. P. Ocampo, C. Cruz, C. Aguirre, R. Romo, A. Pino, R. Castaneda,˜ R. Rodr´ıguez, P. Escamilla and on days found to be similar with the O3-South episode type. the operators and analyst personnel of the “Red Automatica´ de Because high pollutant concentrations occur during each of Monitoreo Atmosferico´ del Gobierno del Distrito Federal” for their the episodes, it is recommended that analysis of O3-North contribution in administering and gathering the data used in this and Cold Surge days be included in air quality studies in manuscript. We are grateful to M. Rosengaus, J. L. Razo, J. Olalde the future. In addition, the climatological relevance of the and P. Garc´ıa of the Mexican National Meteorological Service for 3 episode types will need to be established in order evalu- providing the EHCA and Radiosonde data. We thank A. Garc´ıa and ate their applicability for other time periods and the possible A. Jazcilevich of the Universidad Nacional Autonoma´ de Mexico,´ need to establish new episode types. A. Soler and F. Hernandez of the Secretar´ıa del Medio Ambiente, The meteorological goals of the field campaign were to Gobierno del Distrito Federal, Mexico´ for helpful discussions, analyze the mixed layer, the day-to-day carry-over and the and F. San Martini, R. Volkamer, J. L. Jimenez, B. Lamb and performance of mesoscale models. By identifying the basic D. Worsnop for valuable comments on the manuscript. The financial support of the Comision´ Ambiental Metropolitana of circulation patterns in the basin, this paper provides a frame- Mexico and of the US National Science Foundation (Award ATM work for analyzing these questions and for making use of the 308748) for this work is gratefully acknowledged, as well as specialized observations made during the campaign but not support from Pemex-Refinacion´ to the Instituto Mexicano del included in this paper (see Sect. 3). In addition, models can Petroleo´ for additional radiosonde observations. now be evaluated in terms of the specific flow features found in the basin and in terms of how well they reproduce the three Edited by: U. Poschl¨ episode types. Finally, scenario analysis will need to take into account the three episode types in order to evaluate policies. The air quality response of alternate strategies will be a function of the wind patterns. Identifying the optimal pollution con- trol mechanisms requires careful consideration of the types of wind circulation occuring in the Mexico City basin.

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Appendix: supplementary material

Table A1. Maximum mixing heights and morning (07:00 CDT) thermal inversions from radiosondes at SMN. Mixing heights are calculated from the potential temperature gradient and inversions are calculated from temperature proﬁles.

Date Category Max Mixing Height Surface Inversion Shallow Inversion Upper Inversion m Sounding base depth intensity base depth intensity base depth intensity a.g.l. CDT m m K m m K m m K 01/04/2003 Cold Surge 3090 19 0 500 2.5 02/04/2003 Cold Surge 690 13 03/04/2003 O3-North 2680 19 3410 80 1.4 04/04/2003 O3-North 3480 19 830 80 0.8 3050 200 1.0 05/04/2003 O3-North 3080 19 0 80 0.5 06/04/2003 O3-North 3280 19 4500 100 0.7 07/04/2003 O3-North 3280 19 3760 110 0.6 08/04/2003 Cold Surge 1690 13 300 60 0.4 2740 90 0.7 09/04/2003 Cold Surge 890 13 920 100 0.8 10/04/2003 Cold Surge 890 13 4480 120 1.8 11/04/2003 Cold Surge 2300 13 0 170 1.7 960 40 0.2 3150 110 0.5 12/04/2003 O3-North N.A. 13/04/2003 O3-North 3300 19 2840 260 2.2 14/04/2003 O3-South 2890 19 3310 300 2.5 15/04/2003 O3-South 3490 19 0 110 0.4 3090 270 1.1 16/04/2003 O3-South 3480 19 0 70 0.2 690 50 0.3 2810 200 1.1 17/04/2003 O3-South 3280 19 3040 80 0.5 18/04/2003 O3-North 3090 19 0 210 1.6 3650 120 1.7 19/04/2003 Cold Surge 1490 13 3300 190 3.3 20/04/2003 Cold Surge 2480 13 0 340 0.6 3800 120 1.8 21/04/2003 Cold Surge 2890 19 510 130 1.1 4000 110 0.9 22/04/2003 Cold Surge 890 13 3420 110 0.6 23/04/2003 O3-North N.A. 0 270 1.4 3140 100 0.5 24/04/2003 O3-North 3500 19 1130 60 0.4 2070 90 0.6 25/04/2003 O3-North 3490 19 110 80 0.3 26/04/2003 O3-North N.A. 0 100 1.0 2740 120 0.2 27/04/2003 O3-North 1290 19 0 220 0.4 590 60 0.2 28/04/2003 O3-North 2090 13 29/04/2003 O3-North 3890 19 0 260 0.8 30/04/2003 O3-North 1090 13 650 40 0.6 3270 250 0.7 01/05/2003 O3-South 4090 19 0 190 1.2 3490 90 0.3 02/05/2003 O3-South 3680 19 0 410 0.4 03/05/2003 O3-South 3680 19 0 240 1.2 4450 80 1.1 04/05/2003 O3-North 3490 19 3840 110 0.8

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Table A2. Wind jets identiﬁed from radiosonde proﬁles for each day of the campaign. (Surf indicates maximum wind speed at the surface.)

Date Category Wind Jet 13:00 Wind Jet 19:00 Wind Jet 01:00 Wind Jet 07:00 Wsmax WD Htmin Htmax Wsmax WD Htmin Htmax Wsmax WD Htmin Htmax Wsmax WD Htmin Htmax m/s dir m m m/s dir m m m/s dir m m m/s dir m m 01/04/2003 Cold Surge 4 N 0 1500 6 N Surf 1000 4 N Surf 500 5 N 500 1500 02/04/2003 Cold Surge 0 7 W Surf 1000 0 0 03/04/2003 O3-North 2 E 0 500 3 S Surf 500 0 0 04/04/2003 O3-North 0 9 W Surf 1500 No Sounding 5 SW 0 1000 05/04/2003 O3-North 2 E 0 1000 10 SW Surf 2500 No Sounding 4 SW 0 500 06/04/2003 O3-North 4 E 0 750 7 SW Surf 1000 6 NW Surf 750 5 NW 500 2500 07/04/2003 O3-North 0 6 WNW 1500 3500 4 NW Surf 500 0 08/04/2003 Cold Surge 7 S 0 750 No Sounding 5 NE Surf 1000 5 W 0 750 09/04/2003 Cold Surge 3 W 0 500 6 S 0 1500 3 S Surf 1000 6 W 0 1500 10/04/2003 Cold Surge No Sounding 8 W Surf 1500 0 0 11/04/2003 Cold Surge 0 0 0 5 NW 0 1500 12/04/2003 O3-North No Sounding 5 W Surf 1500 6 W Surf 1000 5 S 0 1250 13/04/2003 O3-North No Sounding 4 S 0 1250 6 W 0 2500 7 W 0 2500 14/04/2003 O3-South 0 0 No Sounding 6 W 0 1000 15/04/2003 O3-South 3 SE Surf 500 3 N 0 2000 No Sounding 5 W 0 750 16/04/2003 O3-South 5 NW 500 2500 8 N 0 2500 No Sounding 6 N 1500 3500 17/04/2003 O3-South No Sounding No Sounding No Sounding 9 S 0 1500 18/04/2003 O3-North 3 E 0 1500 No Sounding No Sounding 6 N 500 2500 19/04/2003 Cold Surge 3 N Surf 750 0 No Sounding 4 WSW 0 1000 20/04/2003 Cold Surge 4 E 0 1500 7 NW 0 1500 0 8 S 0 2000 21/04/2003 Cold Surge 0 0 No Sounding 7 W Surf 1500 22/04/2003 Cold Surge 0 5 NW 0 1000 0 0 23/04/2003 O3-North No Sounding 8 W 0 3000 0 7 W 0 1500 24/04/2003 O3-North 0 4 SE 0 1250 8 N 0 2500 8 W 0 1500 25/04/2003 O3-North 0 0 0 4 W 0 2000 26/04/2003 O3-North No Sounding No Sounding 10 W 0 1250 5 SW 0 500 27/04/2003 O3-North No Sounding 0 5 W 0 1500 3 S 500 2000 28/04/2003 O3-North 3 NE 0 750 12 E Surf 1250 No Sounding 7 W 0 2000 29/04/2003 O3-North 3 NE 0 750 10 W 0 3000 0 0 30/04/2003 O3-North 0 7 W 0 1500 No Sounding 0 01/05/2003 O3-South No Sounding 0 No Sounding 8 N 0 2000 02/05/2003 O3-South No Sounding 0 No Sounding 7 WSW 0 1250 03/05/2003 O3-South No Sounding 10 NE 0 2500 No Sounding 5 SE 0 1250 04/05/2003 O3-North No Sounding 0 No Sounding

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