Diurnal cycle of air pollution in the Valley, : Observations

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Citation Panday, Arnico K., and Ronald G. Prinn. “Diurnal Cycle of Air Pollution in the , Nepal: Observations.” Journal of Geophysical Research 114, no. D9 (2009).

As Published http://dx.doi.org/10.1029/2008jd009777

Publisher American Geophysical Union

Version Final published version

Citable link http://hdl.handle.net/1721.1/85838

Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D09305, doi:10.1029/2008JD009777, 2009

Diurnal cycle of air pollution in the Kathmandu Valley, Nepal: Observations Arnico K. Panday1,2 and Ronald G. Prinn2 Received 1 January 2008; revised 11 November 2008; accepted 28 January 2009; published 9 May 2009.

[1] During the dry season of 2004–2005 we carried out field measurements of air pollution and meteorology in the Kathmandu Valley, Nepal, a bowl-shaped urban basin in the Himalayan foothills of Nepal. We measured the trace gases carbon monoxide (CO) and ozone (O3) and particulates (PM10), as well as meteorological variables. In our field observations we noted a very regular pattern of morning and evening peaks in CO and PM10 occurring daily in the valley bottom, interspersed with low values in the afternoons and at night. This pattern occurred even on days with unusual timing of emissions and was influenced by the timing of ventilation from the valley. Meteorological variables showed great day-to-day similarity, with a strong westerly wind blowing through the valley from late morning until dusk. We found that the air mass on nearby mountaintops was disconnected from pollution within the valley during the night, but received significant pollution during the morning, when up-slope flows began. At a pass on the western edge of the valley we found a diurnal switch in wind direction, with an inflow from late morning until late evening, and an outflow during the rest of the time. We found that part of the morning peak in pollution was caused by recirculation of pollutants emitted the night before, which spend the night in elevated layers over the valley. Citation: Panday, A. K., and R. G. Prinn (2009), Diurnal cycle of air pollution in the Kathmandu Valley, Nepal: Observations, J. Geophys. Res., 114, D09305, doi:10.1029/2008JD009777.

1. Introduction et al., 2001; Shirley et al., 2006; Whiteman et al., 2000b], and Santiago de Chile [Rappenglu¨ck et al., 2000, 2005; [2] Cities that are located in basins surrounded by tall Romero et al., 1999; Schmitz, 2005]. Other urban mountain mountains often face severe air pollution problems. The basins in developing countries have seen much less diurnal cycle of air pollution in those cities is often strongly research. We present here results from a study designed to influenced by the mountains. Mountains block horizontal understand the processes responsible for the accumulation winds that would otherwise blow locally emitted emissions and ventilation of air pollution in the Kathmandu Valley, a away. Nighttime cooling of the mountain sides creates down- mountain basin in the Himalayan foothills of Nepal. slope flows of denser cold air; in wide basins this cold air [4] We carried out field measurements in the Kathmandu can pool, suppressing vertical mixing of pollutants. Daytime Valley during the 9-month dry season from September 2004 heating of mountain slopes, on the other hand, creates ver- to June 2005. Our principal goal was to understand how tical transport of pollutants up the slopes. Meanwhile hori- surface emissions and valley ventilation together shaped the zontal flows through a basin are strongly affected by the observed diurnal cycles in air pollutants. We focused on location and orientation of mountain passes and gaps, as well continuous measurements of two primary pollutants (CO, as by the shape of the topography outside of the basin. particulates) and one secondary pollutant (O3), as well as [3] Mountain-enclosed cities that are growing rapidly, several meteorological variables. and that are located in developing countries, are particularly [5] CO is a particularly useful indicator of urban air vulnerable to air pollution problems. In recent years the pollution. It shares many combustion sources with particu- international scientific community has extensively studied late matter and with other urban primary pollutants. For several such cities, including Mexico City [Bossert, 1997; example it is well correlated with NMHC from combustion Fast and Zhong, 1998; de Foy et al., 2006; McKinley et al., sources [Kuebler et al., 1996; Mo¨llman-Coers et al., 2002]. 2005; Molina et al., 2007; Molina and Molina, 2002; Raga It tends to show greater spatial homogeneity [Holmes et al., 2005] than many shorter-lived pollutants whose concentra- tion is strongly affected by local sources. In fact, compared 1Atmospheric and Oceanic Sciences Program, Princeton University, to most urban air pollutants, carbon monoxide has a long Princeton, New Jersey, USA. atmospheric lifetime, from 1.1 to 2.4 months in the tropical 2Center for Global Change Science, Massachusetts Institute of boundary layer, and from 2.2 to 3 months in the midlatitude Technology, Cambridge, Massachusetts, USA. lower troposphere [Martin et al., 2002]. The long lifetime has several implications. The gas has often been utilized as Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JD009777 a tracer of long distance and intercontinental pollution

D09305 1of19 D09305 PANDAY AND PRINN: KATHMANDU VALLEY FIELD OBSERVATIONS D09305 transport [Bertschi and Jaffe, 2005; Guttikunda et al., 2005; industries (especially brick kilns), do not have emissions that Lobert and Harris, 2002; Stohl et al., 2002]. More impor- vary as distinctly over the course of the day. tantly, it meant that, on the timescale of its emission and ventilation from urban areas (hours to a day), CO could be 3. Past Research in the Kathmandu Valley treated effectively as an inert tracer. 10 [6] We measured PM10 even though it shares many of the [ ] Air pollution in the Kathmandu Valley has become a same combustions sources with CO, because its shorter visible problem that catches much attention, with widely lifetime could help us distinguish spikes in the data from publicized statistics showing a doubling in hospital admis- local pollution sources. In addition, the portability of the sions due to respiratory illnesses over a 5-year period, with PM10 instrument allowed it to be occasionally lent to local especially large numbers recorded in the winter months university students for their class projects on pollution [Clean Energy Nepal, 2005]. Less attention has been paid to exposure, in exchange for help with simultaneous bag the physical processes responsible for the accumulation and sampling of CO at multiple locations. ventilation of air pollutants within the valley. Research [7] Section 2 of this paper introduces the Kathmandu conducted prior to our study provided only partial glimpses Valley. Section 3 describes past field research in Kath- into what appeared to be a complex system of human- mandu. Section 4 describes our field experiment, Section environmental interactions. 5 presents results from the field observations. Section 6 [11] Records kept at the valley-center airport show a large discusses these results, distilling out key questions needing decrease in visibility between the late 1960s and the mid- further investigation. These questions are addressed through 1990s [Larssen et al., 1997]. The first published measure- theoretical investigations using a numerical model, which ments of CO in Kathmandu found concentrations in the city are presented in a companion paper (A. J. K. Panday et al., center averaging 2 ppm, and ranging from less than 1 ppm Diurnal cycle of air pollution in the Kathmandu Valley, to 2.5 ppm in winter 1982–1983 [Davidson et al., 1986]. Nepal: 2. Modeling results, submitted to Journal of Geo- These concentrations are not too different from what we physical Research, 2009). found in 2004–2005. In 1982–1983, the vehicle fleet was very small compared to today, but a much larger fraction of households used biofuels. 2. Kathmandu Valley [12] Several passive sampling campaigns have collected [8] The Kathmandu Valley is located in the midhills of NO2 and SO2 data around the valley over several weeks Nepal, halfway between the Ganges Plains and the Tibetan each [Larssen et al., 1997; Leaders Nepal, 1999; Nepal Plateau, at a latitude of 27.7°N. It is shaped like a circular Environmental and Scientific Services, 1999; Regmi and bowl with a flat floor area of 340 km2, averaging 1300 m Kitada, 2003] quantifying the average pollution exposure above sea level (masl). Its total watershed area is 625 km2. in different regions of the valley. They generally found Surrounding the valley is a ring of mountains ranging from polluted conditions in the city and in the brick-kiln region 2000 to 2800 m above sea level, with five mountain passes of the valley’s southeast. One study quantified 28 non- dipping down to 1500–1550 masl. There is no river inlet methane hydrocarbons in 38 flask samples collected around into the valley, and only one narrow winding outlet (the Kathmandu [Sharma et al., 2000] and found significant ) at the basin floor level. Neighboring valleys weekday to weekend differences in species emitted by to the west, north, northeast, and south have substantially automobiles. High-frequency measurements of atmospheric lower elevations. The Kathmandu Valley’s climate is influ- turbidity over 21 days [Sapkota and Dhaubadel, 2002] and enced by the South Asian summer monsoon. It receives up of condensation nuclei count over 9 days [Hindman and to 90% of its annual rainfall during the three summer Upadhyay, 2002] provided some observations of the aero- months. During that time intense convection and heavy sol diurnal cycle. Both aerosol studies showed that the rains keep the valley’s air relatively clean. Air pollution is a Kathmandu valley generally experienced morning and even- concern largely during the remaining nine months’ dry ing peaks in pollution. Hindman and Upadhyay [2002] also season. A more detailed description of the Kathmandu operated two automated weather stations in the valley for Valley’s geography can be found in a doctoral dissertation nine days, finding a regular pattern of calm nights and strong [Panday, 2006]. afternoon westerly winds. A team from MIT and Chalmers [9] Over the past quarter century the Kathmandu Valley’s University ran a DOAS instrument in Kathmandu in January population has quadrupled to more than 2.5 million. 2003, finding morning and evening peaks in NOx [Yu et al., Between 1990 and 2003 the total vehicle fleet grew from 2008]. 45,871 to 249,282 with the number of motorcycles growing [13] As part of the Atmospheric Brown Clouds project from 22,359 to 173,646 [Faiz et al., 2006]. Today the [Ramanathan et al., 2005], two nephelometers were number of motorcycles exceeds three hundred thousand, while installed near Kathmandu in winter 2003. During their setup larger vehicles total more than a hundred thousand. Traffic phase, a high-resolution micropulse lidar was run from 9 to is the biggest source of CO within the Kathmandu Valley, 17 February 2003 to measure the vertical aerosol extinction accounting for 60% of the total emissions [Kitada and profile over Kathmandu [Ramana et al., 2004]. The lidar Regmi, 2003; Shrestha and Malla, 1996]. Rush hours are found a layer of local pollutants near the surface, as well as responsible for distinct morning and evening peaks in traffic an elevated layer of regional pollutants centered at 1.3 km and in congestion. Most of the valley’s population cooks above the surface. The same project also measured aerosol one large meal in the morning, and one in the evening, optical depth, and broadband global radiative fluxes mostly using fuel sources that produce CO (LPG, kerosene, [Ramanathan and Ramana, 2005]. The authors found aero- and firewood). The third major source of CO, namely sol optical depths in Kathmandu in the range of 0.3–0.5, and

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Figure 1. The Kathmandu Valley, viewed from the south in Google Earth. The high Himalaya and Tibetan Plateau are visible at the top edge of the image. Kathmandu city is labeled in red. Yellow labels show important measurement sites. a diurnally averaged surface aerosol forcing of 25 Wm2. ical parameters with an automated weather station and an They speculate that the layer of aerosols observed at 1.3 km acoustic sounder (SODAR) around the Kathmandu Valley. above the surface might have been lofted by orographic The temperature profile of the boundary layer was recorded flows. More recently published data of ozone and NOx by loggers installed at the top and bottom of a mountain, at measurements at Pulchowk near Kathmandu between four different heights on a 72-m TV tower in the valley November 2003 and October 2004 found the highest ozone center, near the top of another 50-m tower, and on a hill levels in the premonsoon period, and the lowest in winter overlooking our main laboratory site. Figure 1 and Table 1 [Pudasainee et al., 2006]. The authors also found that ozone show the locations of individual sites. Table 2 provides was higher on weekends and days with general strikes than details about the instrument deployment. Bag sampling on regular weekdays. An increase in ozone with a drop in provided information about the spatial distribution of CO. NOx emissions suggests a VOC limited regime [Sillman et In this section we describe briefly the set up at each site. al., 1990]. [16] A main laboratory was set up in Kathmandu, to [14] The only other routine air quality measurements in the house the continuous monitoring instruments and to serve valley prior to our field experiment have been gravimetric as a base of operations. Selection criteria for the main daily average PM10 measurements carried out by the Ministry laboratory included that it be downwind of Kathmandu city, of Population and Environment at six locations since late and that it have good security, generator back-up power 2002 (http://www.most.gov.np/pollution/pollution.php). supply, and public transportation access. The site also had to be surrounded by open space and thus be some distance 4. Experimental Setup from local pollution sources. Available space at the Hyatt Regency Hotel, which occupies 37 acres of gardens in [15] From September 2004 until June 2005 we measured Bouddha, met all these criteria. The southern end of the carbon monoxide, PM10, and ozone, as well as meteorolog- hotel building had a free-standing fire escape tower

Table 1. Locations of Our Instrument and Sampling Sites in and Around the Kathmandu Valley Location Description Longitude Latitude Altitude Bouddha laboratory Enclosure on Hyatt fire escape, Bouddha 27°43.290N85°21.420E 1350 m Hyatt roof Weather station above top gable 27°43.270N85°21.420E 1363 m Nagarkot Pipe from roof of Hotel Viewpoint 27°43.420N85°31.520E 1972 m Kharipati Tree among fields 27°42.450N85°28.220E 1372 m TV tower 72-m-tall metal tower 27°41.770N85°19.600E 1300 m (base) Dharahara City-center 52-m tower with park at base 27°42.010N85°18.700E 1294 m (base) SODAR Roof of CTEVT building 27°40.920N85°22.470E 1321 m Hattiban 10-m view tower overlooking valley 27°37.730N85°16.410E 1784 m Pullahari Pole on seven-storey tall monastery roof 27°44.740N85°22.490E 1476 m Grassy ridge, 50 m from pass 27°43.580N85°13.830E 1496 m

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Table 2. Details of the Instrument Deployment at Each Site Location Instrument Parameters Measured Time Period of Deployment Bouddha laboratory Teledyne API 300EU CO Sep 2004 to Jun 2005 2B Technology model 202 Ozone Sept 2004 to Jun 2005 TSI Dusttrak PM10 Sep 2004 to May 2005 Hyatt roof Rainwise Portlog Temperature Sep 2004 and Dec Humidity 2004 to Jun 2005 Solar radiation Pressure Rainfall Nagarkot Onset Hobo H8 Pro Temperature Jan 2005 to Jun 2005 2B Technology model 202 Ozone Dec 2004, Jan and Apr 2005 Kharipati Onset Hobo H8 Pro Temperature Jan 2005 to Jun 2005 TV tower 4 Onset Hobo H8 Pro Temp. at 4 levels Jan 2005 to Jun 2005 Dharahara Gemini Tinitag Temperature Jan 2005 to Apr 2005 SODAR Remtech PA-0 Wind profiles Sep 2005 and Apr Mixed layer height 2005 to Jun 2005 Hattiban Gemini Tinitag Temperature Apr 2005 to May 2005 Pullahari Gemini Tinitag Temperature Feb 2005 to Jun 2005 Bhimdhunga Rainwise Portlog Temperature 3 times in April 2005 Humidity Solar radiation Pressure Rainfall connected to the main building via bridges. A metal-frame- zenith angles to avoid beam reflection off nearby objects. and-wood-panel enclosure was built on the fifth-floor land- Ambient noise also had to be low, while the sodar’s loud ing of this tower. It housed our gas filtercorrelation CO beeping sounds should not disturb nearby residents. After instrument (API 300EU, Teledyne Advanced Pollution evaluating 12 sites with a digital sound meter (Model 322, Instrumentation, San Diego, CA), a UV-absorption ozone Center Technology, Taipei, Taiwan), we installed the sodar instrument (Model 202, 2B Technology, Boulder, CO) zero on the fourth-floor roof of the Council for Technical air and calibration gas cylinders (Scott Marrin and BOC Education and Vocational Training (CTEVT) building in Gases), computers, spare parts and tools, field sampling Sano Thimi, due east of the airport, and close to valley’s equipment, and, after each bag sampling campaign, filled geographic center. sampling bags prior to their analysis. The optical PM10 [19] Generally sodars perform best during the day when a instrument (Dusttrak, TSI Inc, Shoreview, Minnesota) sat convective boundary layer is topped by an inversion; they nearby in its own environmental enclosure. Our air sample have more difficulty detecting the nighttime boundary layer inlet funnel was suspended 1.8 m south of the edge of the height when an inversion extends down close to the ground building, 12.5 m above ground, overlooking the hotel’s [Beyrich, 1997; Venkatesan et al., 1995]. We also needed grounds. The nearest road and parking lot were 60 m away. detailed measurements of the nighttime temperature inver- The sampling inlet was located at a point which had direct sions. Since tethersondes were incompatible with the day- views of the valley-rim mountains to the northeast, east, time air traffic in the valley and helium was not readily south, west, and northwest of the Kathmandu Valley, and available, we chose 1-min measurements of temperature on where it was fully exposed to ambient winds. Additional existing towers and on the valley rim mountains. This details about the available instrument choices, instrument approach has successfully been compared with tethersondes measurement techniques, inlet and sample line plumbing, as by Whiteman et al. [2004]; inexpensive temperature log- well as calibration procedures, are given by Panday [2006, gers on basin sidewalls were found to provide good proxies section 2.4]. for temperature soundings in a basin’s free air during times [17] An automated weather station (Portlog, Rainwise of low wind speeds and stable nighttime conditions. We Inc., Bangor, Maine) was installed on the main roof of the used seven Hobo H8 Pro temperature loggers (Onset Com- Hyatt building, approximately 20 m above the ground, on puters, Bourne, Massachusetts), with external thermistors in its own tripod mast, at a corner of the north-south running an epoxy-potted cylinders installed inside radiation shields. gable of the hotel’s roof. During several days in April, May These sensors have been extensively tested [Whiteman et and June 2005, the weather station was alternatively al., 2000a], and have been used to study cold pools in the deployed at Bhimdhunga Pass, on the western entrance to Columbia basin over horizontal scales of tens of kilometers the valley for a few days at a time. There it was set up on a [Whiteman et al., 2001] as well as in a tower study of grassy ridge at the water divide of the valley, 50 m south of drainage flows in gullies [Mahrt et al., 2001]. In addition and 15 m higher than the road. we used two Gemini Tinytag loggers with temperature [18] Choosing a site for our sodar (acoustic sounder, shields (Gemini Data Loggers, Chichester, U.K.). After model Pa-0, Remtech S.A., Velizy Cedex, France) required several days of intercomparison at the laboratory in Kath- special considerations. A sodar works by emitting discrete mandu, one of the nine loggers was set aside for mobile acoustic frequencies into the atmosphere, and measuring the measurements on the roof of a GPS-equipped automobile. timing and frequency shift of the return signal to calculate Four loggers were installed 62, 48, 26, and 5 m above the mixed layer height and vertical wind profile. The sodar ground on the tallest structure at the center of the valley: the antenna needed to have a clear sky view down to large

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Figure 2. Two weeks each of CO measurements at the Bouddha laboratory on the eastern edge of Kathmandu City in (a) October 2004 and (b) January 2005. Weekends are marked in dark shading, while the medium shading between 20 and 25 October marks the holidays associated with the Dashain festival.

72-m-tall Nepal Television (NTV) Metro transmission tower versity. A typical bag sampling day saw teams of 2–3 inside the Singhadurbar government secretariat compound. students camping at five or six locations within the Kath- [20] Another logger was first installed 48 m above ground mandu Valley and on surrounding passes and peaks, filling on the Dharahara tower in central Kathmandu, and then bags every 30–60 min at preset times while recording local moved to Hattiban ridge (1785 masl) in early April 2005. meteorological conditions. Successful bag sampling cam- Other temperature loggers were installed at Nagarkot Peak’s paigns were carried out eight times between February and Hotel Viewpoint (1975 masl), on a tree at Kharipati (ele- May 2005. On several bag sampling days a digital camera vation 1375 masl) at the base of Nagarkot, and at Pullahari (Nikon D70, Nikon, Japan) was set up on the Hattiban view Monastery (1500 masl), on a hill overlooking the Hyatt tower for time-lapse photography of the valley fog and haze hotel. Details of these sites are described by Panday [2006]. layers to help inform our meteorological analysis. Studies of [21] We only had one real-time CO instrument in Kath- the dynamics of cold air pools by photographing the heights mandu, yet were interested in understanding the spatial of fog and air pollution layers viewed from mountaintops heterogeneity of this trace gas, especially its concentrations have been carried out extensively in the former Yugoslavia upwind of the city, and on mountaintops surrounding the in the 1970s [Rakovec et al., 2002]. valley. Spatial heterogeneity of CO has previously been studied in Santiago de Chile with the help of 30 investi- 5. Results gators spread around the city, who simultaneously filled flasks that were then analyzed at a central laboratory to [23] Throughout the nine month study, we observed a provide contour maps of CO around the city [Chen et al., very regular diurnal pattern in the air quality and meteorol- 1999]. To emulate this approach, we used six model 222– ogy of the Kathmandu Valley. Figure 2 shows measure- 2301 Grab Air bag sampling pumps (SKC Inc., Pennsylvania) ments of CO at the Bouddha laboratory during 2 weeks each to fill a set of up to 190 5-L foil bags with polypropylene in October 2004 and January 2005. Several patterns stand fittings (SKC model 245–25). We collected at least hourly out immediately. First, every single day showed a morning samples at a number of locations around the Kathmandu and an evening peak. The low values were similar on most Valley, over the full 24-h diurnal cycle, and then brought the days (between 350 and 500 ppb), but the morning and samples to our main laboratory for analysis. evening peak values showed greater variation. Second, peak [22] For this task, we collaborated with the Central values were significantly higher in January than in October. Department of Environmental Science at Tribhuvan Uni- While they seldom reached 1.5 ppm in October, they

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Figure 3. Bag sampling results for CO on 9–11 February 2005. The yellow line shows CO measurements at the Bouddha laboratory during the same time period. Bag sample data are displayed for Nagarkot (red), Hattiban (blue), the base of Dharahara tower (black), the top of Dharahara tower (50 m above ground, purple), and the sodar site at CTEVT (green).

regularly approached 3 ppm in January. Third, nighttime 48-h period. It is evident that CO and PM10 were very well and afternoon low values still far exceeded typical northern correlated (R = 0.88) and that they both showed the distinct hemispheric clean air values; at Nepal’s latitude, clean-air morning and evening peaks already seen for CO. Ozone had tropospheric CO mixing ratios would be closer to 150 ppb very low values at night, typical of urban areas where it is (http://agage.eas.gatech.edu/images/Data_figures/gcmd_ titrated out by abundant NO, and also lost owing to dry month/co_AmonS5.pdf). During our entire measurement deposition. In the late morning ozone rose rapidly to a period, CO rarely dropped below 350 ppb, with low values narrow peak, before dropping to 50% lower values for the more frequently hovering around 500 ppb. rest of the afternoon. Two processes can cause ground-level [24] Figures 3–8 show data from a 48-h period in ozone in urban areas to rise in the morning: photochemical February when all instruments except the sodar functioned formation after sunrise, and downward mixing of ozone-rich reliably,andwhenwealsohada24-hperiodofbag air from aloft displacing air depleted in ozone owing to sampling, shown in Figure 3. Other locations on the valley nighttime surface loss. The rapid growth from minimum to floor, Dharahara and CTEVT, also showed the twin peak maximum daily ozone levels suggests a strong contribution diurnal pattern found at the Bouddha laboratory. The by the second process; this is supported by the rapid mountaintop locations, Nagarkot and Hattiban, meanwhile, decrease in CO concentrations at the same time. The low showed steady nighttime CO values around 450 ppb. At values of CO and PM10 found during the afternoons and late night these mountaintop sites were far above the valley floor at night imply the existence of processes that remove boundary layer, so their CO measurements reflected regional pollutants from the near-surface air during those times. background levels similar to what was found arriving in [26] Our meteorological observations in the Kathmandu Kathmandu during the afternoon when westerly winds were valley help explain the observed diurnal cycle of pollutants. sweeping into the valley. During the daytime, the moun- Figure 5 shows wind and solar radiation data from the taintop sites experienced higher, fluctuating CO values, with rooftop weather station at the Bouddha laboratory. Nights a very pronounced increase in the morning (presumably were characterized by low wind speeds and varying wind when the first polluted air parcels from the city started directions. The highest wind speeds were reached during the arriving). These patterns were repeated during subsequent afternoon, when there were consistent westerlies. Mountain bag sampling campaigns at these same sites and at other basins often experience a sharp increase in wind speed at the similar locations. time when the nocturnal inversion has completely dissipated [25] Figure 4 shows simultaneous CO, O3, and PM10 [Banta and Cotton, 1981; Whiteman, 1982]. In the Bolivian measurements at the Bouddha laboratory during the same Altiplano, strong inflows through the western passes (facing

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Figure 4. (a) CO, (b) PM10, and (c) ozone measurements at the Bouddha laboratory from 1200 Nepal Standard Time (NST) on 9 February 2004 through 1200 NST on 11 February 2004. toward the Pacific lowlands) have been observed a few relative humidity data (Figures 6), shows saturated condi- hours after sunrise, coinciding with the transformation of a tions during early morning hours, matching our observation stable nocturnal boundary layer over the Altiplano into a of fog that lasted several hours. In Figures 6b and 6c, we see convective boundary layer [Egger et al., 2005]. Figure 6 daytime fluctuations in dew point temperature, suggesting shows relative humidity, temperature, dew point, and baro- arrival of different air masses. There is an afternoon dip in metric pressure at the same rooftop weather station. The barometric pressure.

Figure 5. (a) Wind direction, (b) wind speed (1-min average in black, maximum gusts in green), and (c) solar radiation measured by the roof-top weather station at the Bouddha laboratory from 1200 NST on 9 February 2004 through 1200 NST on 11 February 2004.

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Figure 6. (a) Relative humidity, (b) temperature and dew point (DP) in °C, and (c) barometric pressure at the weather station on the Hyatt roof during the same 48-h period.

[27] Figure 7 gives a visual overview of the Kathmandu sunset, Kharipati’s temperature was 3°–5° colder than the Valley’s morning fog as seen from Hattiban view tower on temperature at valley center locations (Bouddha and TV the morning of 4 February, while collecting bag samples tower). This is consistent with the formation of a cold air (Figure 4). It illustrates the formation of a thin veil of fog pool driven by the arrival of katabatic flows from surround- over the city before dawn, and its gradual thickening and ing slopes. Observations of smoke plumes on the valley rim lifting. mountains (not displayed) also consistently indicated down- [28] Figure 8a shows the temperature on Nagarkot Peak slope flows after sunset. (1975m), and at Kharipati (1375m), at the base of Nagarkot. [29] The nighttime vertical stratification within the Kath- Nagarkot was cooler than Kharipati during the day, but mandu Valley, and its isolation from air masses above, can warmer at night. The lapse rate, computed from the tem- be further seen in ozone measurements made alternately at perature difference between the two sites (Figure 8b), shows the Bouddha laboratory and on Nagarkot Peak (Figure 9). a temperature inversion at night, while from 1200 to 1500 Ozone levels in Nagarkot did not go to zero at night. This is Nepal Standard Time (NST) the lapse rate approached the consistent with ozone observations on remote mountaintops dry adiabatic lapse rate, indicating a vertically well mixed in Arizona [Fast et al., 2000], China [Gao et al., 2005] and atmosphere. Figure 8c shows temperature at the Bouddha Thailand [Pochanart et al., 2001]. At mountaintop locations rooftop weather station (1363 m) and Pullahari monastery at night there is insufficient NO to titrate ozone. Often there (1476 m), while Figure 8d shows the lapse rate between the are additional inputs of ozone from the free troposphere, in two. It is evident that the 1363–1476 m altitude layer also contrast to the valleys below, which are isolated at night by experienced a temperature inversion at night; in fact our TV stable air [Baltensperger et al., 1997]. tower temperature measurements showed stable conditions [30] Unfortunately between November 2004 and April at night down to the level of 5–25 m above ground, 2005 the sodar was at the manufacturer’s for repairs. Figure 10 suggesting that vertical mixing was suppressed at the shows sodar data for 11 and 12 May 2007. Other data elevations where most of the pollutant emissions took place. collected in May (not shown) indicates that, except for During afternoons, the TV tower loggers showed rapid warmer temperatures, lack of morning fog, and longer fluctuations including unstable thermals (lapse rate exceed- daylight hours, these days in May were very similar to the ing 40 K/km), while the Kharipati and Nagarkot loggers February days discussed above. While the sodar was capa- showed an observed lapse rate approaching the dry adia- ble of estimating the mixed layer height to an altitude batic lapse rate of 9.8 K/km, suggesting a mixed layer of at exceeding 1000 m, its measurement of wind speed and least 600 m depth. An earlier modeling study had found the direction was usually confined to the bottom few hundred Kathmandu Valley’s mixed layer height peaking at 700– meters. Nevertheless, the sodar data shows several patterns 900 m above the basin bottom [Regmi et al., 2003]. that augment what we observed with the weather station on Additional comparisons between loggers at the same alti- the Hyatt roof in Bouddha. The afternoon westerlies are tude (not shown), found that during the first 2 hours after confined close to the surface. After 2200 NST much weaker

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Figure 7. View from Hattiban on 10 February 2005 looking toward the northwest (city is in the fog in the right middle ground), and toward the east. All times in NST. easterlies push underneath the westerlies, producing a rising Pa-0 sodar and its inability to capture the full height of the region of wind shear. These weak easterlies are seen through- mixed layer. An alternative explanation will be discussed out the night. In the mornings the wind direction changes in a companion paper (Panday et al., submitted manuscript, several times, with some distinctly visible southerlies. 2009), where model results show the afternoon westerly [31] Figure 11 shows 48 h of sodar observations of the winds bringing cooler air into the Kathmandu Valley, creating mixed layer height above central Kathmandu Valley in May renewed stratification. 2005. The sodar-derived mixed layer height was found to be [32] Figures 12a and 12b show wind direction and wind very regular from day to day, peaking between 700 and speed at Bhimdhunga Pass on the western valley rim in 1000 m on most days when we had observations, consistent April 2005. The wind switched back and forth between with the temperature logger findings of a mixed layer reach- daytime westerlies (from about 1000 to 2200 NST), and ing at least the height of Nagarkot Peak. This is relatively nighttime easterlies. We hypothesize that the nighttime low compared to the height of more than 2 km that is easterlies at the pass began at the time when the valley’s often found over the United States [Berman et al., 1999] cold air pool filled up to the level of the pass. Our time- and India [Krishnan and Kuhnhikrishnan, 2004], and also lapse photography from Hattiban ridge indicated that the top significantly lower than what is often found over Mexico of the Kathmandu Valley’s winter fog layer matched the City [de Foy et al., 2006]. It is possible that the surprisingly height of the passes. The fog has also been observed low sodar observations of afternoon mixed layer height in flowing out across the western passes in the morning and the Kathmandu Valley were due to the limited range of the dissipating while descending into the neighboring western

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Figure 8. (a) Temperature at Kharipati (black) and Nagarkot (green); (b) lapse rate between the Kharipati and Nagarkot station altitudes; (c) temperature at the Bouddha laboratory weather station (black) and at Pullahari monastery (green); and (d) lapse rate between the Bouddha and Pullahari station altitudes. valleys (Figure 13). The descending motion outside the Bagmati valley outflow, which is lower than the western pass indicates that ‘‘overflow’’ of the Kathmandu Valley passes, has too small a cross section to allow the cold air pool cold pool across the passes occurs. In several studies to drain off. elsewhere, cold pools in basins have been found to fill [33] Figure 12c shows CO measurements made simulta- up to the elevation of the height of the lowest mountain neously at the main laboratory in Bouddha, and (with bag barrier [Allwine and Lamb, 1992], from where they can sampling) at Bhimdhunga Pass and Nagarkot Peak. Shortly overflow [Neff and King, 1989]. As we shall see later, the after midnight on 20 April, the main laboratory in the basin

Figure 9. Ozone measurements before and after the instrument was moved from Bouddha (blue) to Nagarkot (black) in (a) December 2004, (b) January 2005, and (c) April 2005.

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Figure 10. Vertical profiles of (a and c) wind direction and (b and d) wind speed on 11 May 2005 (Figures 10a and 10b) and 12 May 2005 (Figures 10c and 10d) as measured by the sodar. Wind speeds are coded in gray scale, with darker shades indicating stronger winds. Wind directions are coded in color, with reddish colors indicating westerly and bluish colors indicating easterly flows. All times in NST. bottom, Bhimdhunga Pass, and Nagarkot Peak all had sites. Given the lack of local sources, there is only one similar CO concentrations. However, CO concentrations at plausible explanation for this rise in CO observed at the Bhimdhunga Pass rose by almost 50% (to 1.8 ppm) from pass, namely, that the easterly winds arriving at the pass 0000 to 0400 NST while they remained low at the other two were bringing air that had been at the surface within the

Figure 11. The mixed layer height measured by the SODAR in Sano Thimi during a typical 48-h time period in May 2005. The sodar’s lower detection limit was 50 m above ground.

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Figure 12. (a) Wind direction on Bhimdhunga Pass in April 2005. (b) Minute average wind speed (black) and maximum gusts (green) on Bhimdhunga Pass during the same days. (c) CO measurements at the laboratory in Bouddha, and CO measured with bag sampling on Bhimdhunga Pass and Nagarkot Peak during the same days.

Figure 13. Air photo of fog at Bhimdhunga (BP) and Nagdhunga (NP) passes from the northwest. The fog-covered Kathmandu Valley is in the left half of the middle ground. The Bagmati outflow valley (BV) is in the upper right, and the fog-covered Ganges Plains are in the background.

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Figure 14. CO at the Bouddha laboratory on selected sunny days in January. The color codes are as follows: blue, Tuesday, 11 January 2005; red, Wednesday, 12 January 2005; yellow, Thursday, 13 January 2005; and orange, Friday, 14 January 2005.

Kathmandu Valley and polluted by the city during the times did not vary substantially from one day to the next, evening before. and neither did the rush-hour traffic patterns. We also see that the evening CO peaks began around 1800 NST, even 6. Discussion though the heaviest evening rush-hour traffic took place just after 1700 NST. It is also worth noting that the morning [34] The results presented in the previous section indicate peak often began shortly after 0600 NST (at dawn), even that the Kathmandu Valley’s air quality and meteorology though morning traffic and cooking activities remained light show a lot of day-to-day similarity. There is a morning and until at least 0800 NST. It is thus clear that the twin peak an evening peak in CO and PM10 every day, while ozone pattern was not just simply result of morning and evening decreases to zero in the valley bottom at night. Afternoons peaks in emission. are marked by strong westerly winds, while at night there [37] Looking back at Figure 2a, we see additional evi- appears to be a pooling of cold air in the basin bottom up to dence that the CO peaks were not simply caused by traffic the level of the western passes. Here we consider several or cooking. Days 20 through 25 October 2004 were the questions in order to understand this pattern better. main days of the Dashain festival, when hundreds of [35] First, why is there such a regular twin peak pattern in thousands of people left the Kathmandu Valley to return CO and PM10 in the valley bottom in Kathmandu? Morning to their ancestral villages, while offices remained closed. and evening peaks in urban CO have been observed in some People stayed home or visited relatives, and cooked the other cities as well, including in Bangkok [Zhang and morning meal later than usual. There were therefore no Oanh, 2002], Santiago de Chile [Rappenglu¨ck et al., morning or evening rush hours. Yet the morning and 2000, 2005; Schmitz, 2005], Buenos Aires [Bogo et al., evening peaks in CO are still visible on these days. The 2001], Athens [Gros et al., 2002], and at times in Hong existence of the peaks was thus not a result of diurnal Kong [Wang and Kwok, 2003]. In southern California [Qin emissions patterns alone. We see, however, that the peak et al., 2004] and in Brisbane [Morawska et al., 2002] values were lower on the holidays, indicating that emissions morning and evening CO peaks were found to be stronger volumes might have had an effect on the peak amplitude on weekdays than on weekends. Often the observed peaks observed among different days in October. However, because were found to be a direct result of local rush-hour traffic there was no seasonal difference in traffic between a regular emissions; in Buenos Aires and Santiago de Chile the peaks weekday in October and a regular weekday in December, correlated very well with traffic volumes. emission volumes do not explain the difference between peak [36] Figure 14 compares daily CO variations on four amplitudes in these 2 months. consecutive days in January 2005. If the morning and [38] The diurnal cycle of atmospheric CO concentrations evening peaks were directly correlated with traffic or near the ground can be affected by factors other than local cooking, they would be found at the same time each day. CO emissions. For example, CO from biomass combustion Instead, on Tuesday and Wednesday, 11 and 12 January, the in rural Thailand was found to have nighttime highs and morning peak dropped off sharply around 0900 NST, while daytime lows due to reduced nighttime ventilation caused on Thursday and Friday, 13 and 14 January, the morning by inversions [Pochanart et al., 2003]. During a year of peak reached a maximum shortly after 0900 NST and did street-side measurements in Buenos Aires, Argentina [Bogo not drop off until 1000 NST. Office and school start and end et al., 2001], days with no wind had higher nighttime values

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Figure 15. Close-up look at (a) ozone, (b) CO, (c) wind speed, and (d) wind direction at midday on 10 February 2005 at the Bouddha laboratory and rooftop weather station. of CO, and higher morning peaks. Downwind of Bangkok, ozone and CO peaks. We thus have evidence that the Thailand, the heights of the morning CO peaks are strongly observed peaks in pollution corresponded to rapid transport dependent upon wind direction [Zhang and Oanh, 2002]. In past our Bouddha sensors of the air mass that had been spring 2002, CO canister sampling upwind, within, and almost stagnant over the city during the morning. The drop downwind of Santiago de Chile [Rappenglu¨ck et al., 2005] in pollutant concentrations after 1252 NST happened once found distinct morning and evening peaks within the city, the tail end of that polluted air mass had gone past the with minimum values at noon, like we found in Kathmandu. measurement station. From that time onward, throughout An earlier study in Santiago de Chile indicated that there the afternoon, the sensors appeared to be measuring air that were no evening CO peaks during the summer, because had initially been upwind of the city (including outside of there was still strong ventilation at the time of rush hour the western passes), and that had then traversed the city [Rappenglu¨ck et al., 2000]. It is likely that the winter rapidly, picking up less CO and other ozone precursors over evening peaks in Kathmandu were taller because of an the city, while having less time to form ozone in situ by the earlier suppression of ventilation. The pattern observed in time of its arrival at the Bouddha laboratory. Kathmandu appears to be strongly affected by the timing [40] The afternoon dip in CO and PM10 concentrations in and mechanism of ventilation. Kathmandu is clearly a result of westerly winds sweeping [39] Returning to Figure 4, we see that the prominent through the valley. These winds bring cleaner air through noontime spike in ozone on 10 February was accompanied passes such as Bhimdhunga on the west, and sweep the by concurrent small peaks in CO and PM10. Figure 15 valleys’ pollutants out toward the east. But why do pollutant zooms in to show that the increase in westerly wind speed at levels drop at night? Figure 16 shows PM10 at the time of 1218 NST on 10 February apparently initiated a small peak the Shivaratri festival on 8 March, when hundreds of bon- each in ozone and CO that was then followed by larger broader fires were lit at dusk. Unfortunately we don’t have reliable peaks in both gases from 1230 to about 1252 NST. The CO data from those days. In the evening of 8 March, we see average wind speed from 1130 to 1217 NST was 2.0 m/s, much higher PM10 values than on adjacent evenings. By but from 1218 to 1252 NST it was 4.9 m/s. After 1252 NST 2100 NST most of the fires had been extinguished. We see it was even faster. On the basis of the wind data and PM10 values decrease during the night; however record- Kathmandu Valley geography, we hypothesize that when high PM10 values we measured again the next morning the stronger westerly winds began, they carried the polluted despite the absence of fire emissions. The only plausible air mass that had previously been stationary over the city explanation is that aerosols emitted by the fires the previous toward the east and past the instruments in Bouddha. The night had returned to the surface observation site after initial peak marked the arrival of polluted air from the initially having been transported away. This recirculation nearby Chabahil Ring Road intersection. This was followed of pollutants also helps explain why we see the morning by a trough indicating cleaner air from the residential area peak in pollutants appearing shortly after dawn, even immediately west of there, and then by a broad peak though the peak in cooking and traffic emissions does not marking the arrival of polluted air from the main city farther happen until several hours later. But where do the pollutants west. The distance from the Bouddha site to the western go during the nighttime? edge of the city is 10 km. At 4.9 m/s, it would take 34 min [41] Let us consider the hypothesis that the city’s polluted to cross the city: that was almost exactly the duration of the air mass might follow nighttime drainage flows down the

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Figure 16. PM10 at the Bouddha laboratory. After sunset on 8 March 2005, bonfires were lit around the city on the occasion of Shivaratri festival. These fires were mostly extinguished before 2100 NST.

Bagmati River Valley, and then return to the city when Seeing in Figure 17 that CO levels started rising first at the the wind direction in the Bagmati River Valley reversed in lower-elevation site in Pullahari, let us assume for the the morning. This hypothesis can easily be rejected: drain- moment that we are indeed seeing a nocturnal lifting of age flows leaving the Kathmandu Valley down the Bagmati pollutants originating in the Kathmandu Valley bottom. River must flow through the very narrow Chobhar Gorge, What mechanism could lift these during the night, in the and simple dimensional analysis shows that unrealistic presence of a stable cold air pool? supersonic wind speeds would be needed to drain the entire [44] Could pollutants from the valley bottom be lifted by polluted air mass of the valley through the Chobhar Gorge mechanical turbulence? Turbulence in stable nocturnal outlet over the course of one night. boundary layers is generally not caused by surface heating, [42] What we did observe, and frequently photograph at but by shear in the nocturnal jet. A field study involving a dawn, were elevated polluted layers over the Kathmandu 60-m-high instrumented tower near Wichita, Kansas, found Valley. Nighttime elevated layers of pollutants have been turbulence in the stable nocturnal boundary layer that was measured at the top of skyscrapers in Phoenix [Berkowitz et created by wind shear and that was increasing with height al., 2006] and Milan [Rubino et al., 1998], and by sampling [Mahrt and Vickers, 2002]. Substantial vertical mixing air with a customized tethersonde in Japan [Sahashi et al., within stable layers is also possible owing to internal gravity 1996]. Elevated layers of aerosols have also been observed waves [Monti et al., 2002; Stull, 1988]. Turbulent transport, over the Rhine Valley [Frioud et al., 2003]. In Figure 12, like any diffusive process, only happens if there is a we saw that at Bhimdhunga Pass, 200 m above the valley gradient in the tracer being transported. For turbulence to floor, CO levels rose after midnight. That was after the be able to transport nighttime pollutants away from the evening peak at the valley bottom had subsided, and two surface, CO concentrations must decrease with altitude. hours after an easterly wind had started transporting Kath- Although we did not have the capability to carry out vertical mandu Valley air out of the pass. Could this rise in CO at soundings of CO, we did collect bag samples at several the pass indicate the arrival of a lifted layer of CO from the locations near the top of the Kathmandu Valley’s nighttime valley bottom? cold air pool. A look at Figures 12 and 17 shows that CO [43] Figure 17 shows CO bag sampling results at 3 concentrations at higher locations (Pullahari, Bhimdhunga, different elevations during the night of 4–5 April at the Hattiban) exceeded CO concentrations at the valley bottom Bouddha laboratory on the valley bottom, at Pullahari (as measured at the Bouddha laboratory) for extended monastery (150 m higher), and at Hattiban (600 m above periods. In fact concentrations rose at Pullahari, while they the valley bottom). During the night, shortly after the continued to drop in Bouddha. Vertical transport appeared evening CO peak subsided in Bouddha around 2300 NST, to continue for several hours beyond the time when the CO started to rise in Pullahari, and after 0200 NST it started gradient was reversed. We can thus exclude turbulent trans- to rise in Hattiban as well. Since Pullahari and Hattiban port as the primary agent of nighttime CO removal from the have no local sources of CO, the observation indicates the valley bottom. arrival of a polluted air mass. This could have been [45] A more likely explanation of the observed nighttime advected horizontally from elsewhere, or it could be pollu- lifting in pollutants as suggested by the observed low values tants originating in the Kathmandu Valley that are lifted up. in the valley bottom and the rising values at higher

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Figure 17. Bag sampling CO data from 4–5 April 2005 at Pullahari and Hattiban, along with CO measurements at the Bouddha laboratory during the same time period. The horizontal axis spans from 0448 NST on 4 April through until 1912 NST on 5 April. elevations is based upon the mechanism that forms the cold the latter up along with the pollutants they contain. The air air pool in the valley bottom in the first place. This is parcels that were heavily polluted in the evening rise higher illustrated in Figure 18. Radiative cooling of mountains and higher as colder cleaner airflows underneath that is less generates downslope (katabatic winds) which lead to a polluted by the few emissions sources that remain on all pooling of cold air in the basin bottom, as seen in the night. In the morning, the elevated pollutants mix down temperature data (Figure 8a). While doing our field experi- again as the growing mixed layer entrains elevated layers of ments, we observed plumes of smoke on valley rim slopes polluted air. We see in Figure 17 that around 0500 NST in heading downhill during the evening, indicating such kat- Pullahari, and 0600 NST in Hattiban, concentrations of CO abatic winds. Parcels of relatively clean cold air arriving in dropped suddenly to similar levels found at the Bouddha the city from the valley rim mountains flow underneath the laboratory, where the CO was starting to rise to a morning less cold, less dense air parcels that are already there, lifting peak. This is consistent with the dilution that takes place

Figure 18. Idealized time sequence illustration of deduced circulation, showing the (top) nighttime lifting and (bottom) morning downward mixing of pollutants. The cities of Kathmandu, Bhaktapur, and Dhulikhel are marked by the letters K, B, and D, respectively. At the eastern and western ends of the valley, both passes and peaks are shown.

16 of 19 D09305 PANDAY AND PRINN: KATHMANDU VALLEY FIELD OBSERVATIONS D09305 when elevated pollutant layers are mixed downward as they are investigated using the numerical model MM5 in a com- are entrained into a growing mixed layer in the morning. panion paper (Panday et al., submitted manuscript, 2009). [46] To be viable, this explanation does need to pass one [49] Although local emissions within the valley play an check: If the katabatic winds were transporting cold air from important role in creating pollution peaks, our CO measure- the valley-rim mountains to the basin bottom, they would ments on mountaintops and Bhimdhunga Pass, as well as also be bringing in a fresh supply of ozone. Achieving the the high minimum CO levels ever recorded at the Bouddha observed near-zero ozone levels found at the Bouddha laboratory make it clear that there is significant background laboratory at night would require an efficient ozone removal air pollution present that comes to the Kathmandu Valley mechanism along the way. Since NO emissions are most from elsewhere. Part of it may be from biofuel combustion likely small late at night (and hence the reaction of NO with in rural Nepal, while part of it is likely to originate in the ozone is small), we would need ozone to be removed by dry Ganges Plains. deposition before the cold air arrived at the Bouddha measurement location. Even with the low nighttime wind [50] Acknowledgments. The research described in this paper were speeds, converging katabatic winds would arrive at the made possible through generous support from an MIT Presidential Fellow- ship, the Martin Fellowship for Sustainability, a pilot research project grant Hyatt sensors from the valley rim base in, at most, a few from the Alliance for Global Sustainability, the PAOC Houghton Fund, the hours. The katabatic flows would be following the surface, TEPCO Chair account, NSF grant ATM-0120468 to MIT, NASA grants traveling underneath layers of air from which ozone would NAG5-12099 and NAG5-12669 to MIT, and the federal and industrial already have been largely removed. Thus they would not sponsors of the MIT Joint Program on the Science and Policy of Global Change. have any en-route replenishment. Existing literature [Lagzi et al., 2004; Matsuda et al., 2005; Sorimachi et al., 2003; References Zhang et al., 2002] suggests that agricultural lands (as Allwine, K. J., and B. K. Lamb (1992), Wintertime dispersion in a moun- found between the valley-rim slope and the Hyatt sensors) tainous basin at Roanoke, Virginia: Tracer study, J. Appl. Meteorol., 31, have ozone dry-deposition velocities on the order of 0.2 cm/s, 1295–1311, doi:10.1175/1520-0450(1992)031<1295:WDIAMB>2.0. CO;2. or 7.2 m/hour. Given that katabatic flows are only a few Baltensperger, U., H. W. Ga¨ggeler, D. T. Jost, M. Lugauer, M. Schwikows- meters thick, this implies an ozone deposition lifetime of ki, P. Seibert, and E. Weingartner (1997), Aerosol climatology at the less than 1 h, thus providing a reasonable mechanism for the high-alpine site Jungfraujoch, Switzerland, J. Geophys. Res., 102(D16), 19,707–19,715, doi:10.1029/97JD00928. removal of ozone from air masses arriving at the Hyatt Banta, R., and W. R. Cotton (1981), An analysis of the structure of local measurement station at night. We conclude that the cold air wind systems in a broad mountain basin, J. Appl. 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Meteorol., 36, 119–140, doi:10.1175/1520- ventilated by strong westerly winds entering through the 0450(1997)036<0119:AIOFRA>2.0.CO;2. western passes. In the evening emissions in the valley Chen, T.-Y., D. R. Blake, J. P. Lopez, and S. Rowland (1999), Estimation of accumulated in the city and its environs. Late at night, the global vehicular methyl bromide emissions: Extrapolation from a case study in Santiago, Chile, Geophys. Res. Lett., 26(3), 283–286, city air quality improved again, as polluted surface layers doi:10.1029/1998GL900214. appeared to be lifted up owing to the convergence of cleaner Clean Energy Nepal (2005), Nepalma Vayu Pradushan: Samasya ra colder air into the city driven by katabatic flows on the Samadhan, Kathmandu. valley rim slopes. In the early morning, as the mixed layer Davidson, C. I., S.-F. Lin, and J. F. Osborn (1986), Indoor and outdoor air pollution in the Himalayas, Environ. Sci. Technol., 20(6), 561–566, started to grow, the elevated pollutants were mixed back doi:10.1021/es00148a003. down into the city. de Foy, B., J. R. Varela, L. T. Molina, and M. J. Molina (2006), Rapid [48] Analysis of our observations indicates that the twin ventilation of the Mexico City Basin and regional fate of the urban plume, Atmos. Chem. Phys., 6, 2321–2335. peak pattern existed owing to an interplay between the Egger, J., et al. (2005), Diurnal circulation of the Bolivian Altiplano. Part I: timing of emissions and ventilation. The morning peak Observations, Mon. Weather Rev., 133, 911–924, doi:10.1175/ decreased when the strong westerly winds removed pollu- MWR2894.1. Faiz, S., B. B. Ale, and R. K. Nagarkoti (2006), The role of inspection and tants that had accumulated in the city over the course of the maintenance in controlling vehicular emissions in Kathmandu Valley, mornings. Our observations do not tell us the pathways of Nepal, Atmos. 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