Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , , ) x x x , 2 , ben- 2 40 ppbv NO ∼ ects on health. ff 10 m a.g.l. were , A. C. Lewis ± 2 mixing ratios by UV 3 15.2 ppbv for benzene, c densities were high- ± ffi ects on human health and veg- formation is generally well un- ff . 3 , R. M. Purvis 2 2 10 m a.g.l. over outer London were ± 7.3 and 34.3 ± 27336 27335 , A. Vaughan chemiluminescence and O 1 x B) concentration ratios were highest (average of 8.6, 21.0 / ± is directly emitted as NO 200 pptv benzene) and for ozone in the mid-afternoon respectively. Linear regression analysis between NO 10 m a.g.l. x ) is a secondary pollutant, produced from photochemi- x ∼ 3 ± , B. Davison 2 ) indicative of strong local sources. Daytime maxima in NO 1 0.07, 13.2 and some VOCs also have, themselves, direct e − benzene (T and NO ± / ). Ozone has significant detrimental e 2 1 by dual channel NO 2 2 x ) were measured in the atmospheric boundary layer above Greater Lon- ) all at 360 3 NO , J. D. Lee 3 1 + NO 0.05, 0.28 0.3 ppbv ppbv = ± x ± 40 ppbv O Average mixing ratios observed over inner London at 360 Average mixing ratios measured at 360 350 pptv toluene and This discussion paper is/has beenand under Physics review (ACP). for Please the refer journal to Atmospheric the Chemistry corresponding final paper in ACP if available. Lancaster Environment Centre, Lancaster University, Lancaster,National UK Centre for Atmospheric Science, University of York, York, UK ∼ etation while NO Whilst the basic atmospheric chemistry leading to O ∼ ( 1 Introduction Ground level ozonecal (O reactions(NO involving volatile organic compounds (VOCs) and nitrogen oxides always lowerest, than the toluene 1.8 over innerbenzene and London. toluene mixing Where ratios were tra observed in the morning ( (PTR-MS), NO absorption. 0.20 toluene, NO, NO zene and toluene mixing ratios yieldedpounds a predominantly trimodal distribution share indicating the thata these same significant com- or fraction of co-located NO sources within the city and that Highly spatially resolved mixingand ratios ozone of (O benzene anddon toluene, during nitrogen the oxides period (NO 24benzene June were to 9 determined July in-situ 2013 using using a a Dornier proton 228 transfer aircraft. reaction Toluene and mass spectrometer Abstract Atmos. Chem. Phys. Discuss., 14, 27335–27371,www.atmos-chem-phys-discuss.net/14/27335/2014/ 2014 doi:10.5194/acpd-14-27335-2014 © Author(s) 2014. CC Attribution 3.0 License. and C. N. Hewitt 1 2 Received: 24 September 2014 – Accepted: 15Correspondence October to: 2014 C. – N. Published: Hewitt 30 ([email protected]) OctoberPublished 2014 by Copernicus Publications on behalf of the European Geosciences Union. Airborne determination of the temporo-spatial distribution of benzene, toluene, nitrogen oxides and ozoneboundary in layer the across Greater London, UK M. D. Shaw 5 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and 2 is thought to and operated x 3 are combustion x and VOCs means x 50 % of the total (Na > respectively producing maxi- 1 − , benzene, toluene, NO, NO 3 species are currently measured at 130 resolution. Uncertainties in these emis- 2 x from urban areas, leading to uncertainties x mixing ratios measured in the national moni- and VOC emission rates are estimated by the 27338 27337 x x er from the limitations of being made at relatively ff ectiveness of control policies on both background and ff mixing ratios in the UK. There are therefore considerable eco- 2 and NO 3 airs). Hourly mixing ratios of NO ff across the Greater London region during several flights using the Natural Envi- and ozone mixing ratios across London from an airborne platform, with a view to x x The development of fast-response analytical instruments for NO In this study, we investigate the mixing ratios of O Validation of the NAEI emission estimates is provided, indirectly and in part, by the In the UK, spatially disaggregated NO In urban areas the dominant anthropogenic sources of VOCs are vehicular exhaust, The NERC Dornier 228 isoperated a twin-engine by turbo-prop the powered, noneter Airborne pressurised airport aircraft Research in and centralwith Survey England. a The Facility crew aircraft (ARSF) of hasa based 2 a minimum pilots cabin at and and volume Glouces- maximum 4 ofmum airspeed scientists range 14 m of of for 2400 the km 65 (5including duration and h fuel, of at 95 with m 500 the s kg). a flights. The maximum aircraft The operational has altitude aircraft a for has maximum science payload of of 4500 5970 m. kg made by the national monitoring networks. 2 Method 2.1 NERC Dornier 228 NO ronment Research Council (NERC) Atmospheric Research228 and aircraft Survey between Facility Dornier 24titatively June determine and the 9 vertical, JulyNO 2013. horizontal The spatial aim and ofidentify temporal this dominant distribution emission work of sources was VOCs, these in to fast the (i) response region quan- and airborne (ii) measurements wherever with possible, hourly compare ground-level measurements that the mixing ratioslution from of low-flying these aircraft. analytesthey The can advantages provide of now information in-situ be onover aircraft a measured the measurements large at is spatial horizontal area high that andacross allowing spatial cities continuous vertical and gradients reso- their distributions of surrounding mixing of rural ratios areas. air to be pollutants observed few sites and so may not be representative of mixing ratios over larger spatial scales. estimates. continuous hourly data of VOCtoring and networks NO (the Automatic Hydrocarbonand Network Rural (AHN) Network and (AURN), theand Automatic both Rural Urban operated A by thenetwork Department sites with of selected Environment,sites VOCs Food form measured part at of 4only sites. the measure Within London mixing Greater Air ratios London Quality and these su Network (LAQN). However, these networks for specific pollutants by source sectorsion at estimates 1 km propagate through intoto uncertainties uncertainties in in models the ofpeak likely air VOCs, O quality, e and leads nomic and societal pressures to ensure the precision and accuracy of these emissions the atmosphere in urbanet and al., suburban 2005; areas, Kansal, accountingdirectly 2009; for due Watson to etpartially fuel al., oxidized fuel evaporation 2001) components. The and with dominantprocesses, urban a from sources including wide of vehicular vehicles. NO range exhaust In ofbe the as derived VOCs UK unburnt from emitted as vehicles, fuel a although and whole, this percentage about as is 50 % larger in ofNational urban NO Atmospheric areas. Emission Inventory (NAEI), which provides emission estimates ciation of emissions of bothin VOCs the and detailed NO understanding of urban photochemistry and airfuel pollution. evaporation and emissions(Nelson from et the al., commercial 1983). and Vehicular emissions industrial are use the of predominant solvents source of VOCs to derstood, there are substantial uncertainties associated with the magnitude and spe- 5 5 20 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 . Each 2 (Beswick et al., 2008). ◦ to ensure the required fast response time. 1 − 27340 27339 27 h of research flights). Figure 1b shows a map of ∼ to NO using 395 nm blue light LED’s. They highlighted the 2 photolytic conversion to NO, giving high analyte selectivity was quantified in a second channel by photolytic conversion 2 2 10 m a.g.l. respectively (Table 1). ± c is heavily regulated and subject to financial charging. Airspeed ffi c regulations and to allow data analysis in both a temporal and spatial ffi nity for NO and 360 ffi 1 sampling chemiluminescence system built by Air Quality Design, Inc. The instrument − x x was measured from the aircraft using a fast time resolution (10 Hz), high sensi- 3 m s 50 km are plotted. Identical flight legs across Greater London were chosen due x nity for conversion of NO ± ∼ Boundary layer (BL) height determinations were made from a combination of air- The predominant wind directions observed during the flights were either north- ffi within the channel. Recent worka (Pollack et al., 2010) evaluated the relative high NO tivity NO has a dual channelchannel architecture for has independent a quantification sampleA of flow detailed NO review of and of 1.5 similar L NO channel min system instrument was which described operate by usingspheric (Lee the Observatory. et NO same al., principles 2009), at beingto the a NO Cape single Verde using Atmo- a blue specific light a LED diodes centred at 395 nm. The 395 nm wavelength has and altitude system at anacquisition accuracy of rate 0.05–0.3 of m. 20 All Hz. variables were acquired at a2.4 data NO NO Core equipment on the aircraftsurement consisted of System an (AIMMS-20) Aircraft Integrated turbulencein Meteorological an probe Mea- underwing (Aventech PMS Researchaircraft type altitude Inc.) pylon. and mounted The velocity to instrumentature within is and fractions capable of humidity of one measurementcombined precisely metre precision with per defining fully of second the compensated with approximately air-data aan 1 measurements %. temper- accuracy to This of compute information 0.5 windThe speed is knot 3-D with and position of wind the direction aircraft accuracy was measured of using 5–10 an IPAS 20 (Leica) inertial position directly north of Londonto (position determine BL the 2, height of Fig. the 1b). BL These before manoeuvres and2.3 after were the performed flights. Meteorological and GPS sampling after completing the city transects, a spiral ascent from 350 to 2500 m was performed was performed 70 km south of London (position BL 1, Fig. 1b). Similarly, immediately and their transport to suburbanthe and PBL rural above London. regions. RF 1 focussed on verticallyborne profiling observations andcloud ground base altitude based determinations were measurements.based made recorder from Approximately (LCBR) Heathrow observations. airport hourly The using lowestas laser lower observed BL cloud cloud height. base Where was cloud interpreted temporally based interpolated observations BL were height not determinations availableBriefly, from before (during aircraft commencing clear observations the skies), were city used. transects, a spiral descent from 2500 to 350 m a.g.l. westerly (RF 1–6), perpendicularparallel to to the the flight flight transectsviding (Table transects, 1). a Perpendicular or cross wind section north-easterly directions of areallow (RF pollutant useful us mixing 7–10), in to ratios pro- assess across the London. Parallel horizontal wind advection directions and dispersion of pollutants across the city Greater London on whichof typical repeated south-westerly toto north-easterly tight flight air legs tra domain. The grey areainner represents London the boundary Greater andin London the which boundary, blue the area road London’s black tra and congestion area altitude charging the were zone fixed (CCZ) 73 during the flights across Greater London with mean values of Eleven research flightshours (RF) of totalling 8:30–17:20 UTC (Table 45 1). h Figureproject, 1a overlaid in shows on all a duration flight transport legs were mapin conducted of during conducted SE transects the England. between Here across we the will London focus on ( data obtained 2.2 Flight description 5 5 20 25 15 10 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 21 m/z , the ff mixing ciency. derived m/z 2 er turbo- ffi 2 ff 32 were de- . NO 3 ciency interpo- ffi m/z at + 2 N ratio of approximately / by addition of O 2 39 and O converter was measured during . On the ground the PTR-MS in- 2 ff m/z was then quantified by ozonation of the . x . Typically calibrations are carried out at C) Teflon and stainless steel tubing, min- ff x ) at ◦ + O concentration. The measurement uncertainty 27342 27341 18 2 3 with approximate total errors at 1 ppbv being 10 2 OH ciency of the NO and NO mixing ratios. ffi 18 3 x respectively. 2 93 (toluene). Additionally, both the primary ion count ected by the 395 nm light, so in turn reducing possible non NO 0.8 ppbv. ff ± ects, inlet losses and the build-up of impurities in the inlet system. m/z ff orts were made to prevent VOC contamination of the PTR-MS inlet C and 480 V respectively, maintaining an E ◦ ff erence between NO ff ), its first water cluster (H + 2 Hz. The target protonated masses and likely contributing compounds were O ∼ 18 3 VOC measurements were obtained at a sampling rate of 5 Hz and a repetition rate The instrument was calibrated by adding a small a flow (5 sccm) of a known NO 75 pptv for NO and 100 pptv for NO at 2.0 mbar, 40 sample flow was instantaneously switchedica to coated stainless-steel dry can zero (Thames gradecontinuously Restek, air sampled UK) contained until within the the in aircraft, aircraft aPTR-MS which had 1 to was L reached be then sil- an fully altitude operational of during 2500 take-o m, allowing the of 79 (benzene) and (H termined. PTR-MS drift tube pressure, temperature and voltage were held constant let remained closed (andflight the all instrument sample voltages were tubingtion switched and capped). on instrument to Approximately allow calibration. 3 forthrough During h primary the this before zero ion time air count each dry generator stabilisa- andground zero PTR-MS and grade inlet air to in series (BOC) prevent to the was minimise purged build-up instrument of back- contaminants. Immediately prior to take o mountings (vibrachoc). A pressure controllerthe was inlet added flow (Bronkhorst) whichmaintained (50–500 regulates at STP a sccm), constantpendent such value. of that fluctuations As pressure in asample ambient upstream result air pressure of the was caused the only PTR-MS by exposedimizing varying drift controller to memory flight tube is heated altitude. e pressure (70 Ambient Considerable is e inde- during operation on the ground and during take-o fore only instrument setinstrument, up, normally operation and housed flightfacturers in into modifications one two are cabinet, racks outlinedvibration had suitable here. to been for The the mounting re-engineered PTR-MS intoMD4 by the during diaphragm the aircraft. flight, pump manu- To the were mitigate instrument individually shock racks, shock and mass mounted spectrometer using and stainless steel spring 2009; de Gouw and Warneke, 2007; Hewitt et al., 2003; Hayward et al., 2002). There- (PTR-MS) fitted with a stainlessmolecular steel ringed pumps. drift This tube instrument (9.6 has cm) and been three described Pfie in detail elsewhere (Karl et al., generating data every 4 s.tion at A 254 mercury nm lamp beingwas emitting estimated proportional to UV to be light O was used, with2.6 absorp- VOC sampling Benzene and toluenecon mixing (Innsbruck, ratios Austria) were high determined sensitivity simultaneously proton using transfer an reaction Ioni- mass spectrometer ∼ and 15 % for NO and NO 2.5 Ozone sampling Ozone was quantified in-situ, using a Thermo Scientific 49i UV absorption instrument concentration (5 ppmv –10 Air ppbv Liquide) of in NO. The theeach conversion calibration ambient e by sample gas flow, phaseratio titration resulting of data in the is around NO corrected toIn NO for flight using calibrations the wereing measured low always 90 and carried % stable photolytic outthe background conversion beginning above levels e and of the end NO boundarylated of between layer, a the thus flight, two ensur- with and sensitivities applied and to conversion all e data. Detection limits for the 1 Hz data were acid (HONO), being a species interfering with the measurement.subsequent NO total NO present infrom the the di reaction vessel after conversion with NO low probability of other species within the gaseous chemical matrices such as nitrous 5 5 25 20 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | N, 32 ◦ W). cps ◦ 6 m/z 10 at 15 min × 300 sccm) 3 ∼ N, 0.1319 ◦ . Ion counts of cps, which represented 7 6 10 10 × × 11 pptv for benzene and toluene ± 39 ranged between (1–5) E). The site is approximately 25 m from at hourly resolution and O ◦ x m/z 21 primary ion count ranged between (4– 8 and 18 . A portion of this ambient air ( 1 ± − 27344 27343 m/z and NO N, 0.0708 ◦ 2 C and 30 psi for the duration of the flights to maintain ◦ cps, with an average of 2 3 s. To determine blank VOC mixing ratios, the remain- 6 < , 6–8 normalised ion counts per second (ncps), and 400– Teflon (PFA) tube pumped by a stainless steel diaphragm cps, which represented 6 % of the primary ion signal. 10 1 9 % for benzene and toluene respectively, calculated using 00 − 6 ciently removes VOCs (de Gouw et al., 2004). The zero air × ± 4 ffi 360 m a.g.l., the / 10 ∼ × , 6–9 ncps, for benzene and toluene respectively. Instrument uncertain- 5 and 21 1 C) 5 m 1 − ◦ ± ion counts per second (cps) with an average of 6 W). stainless steel tube packed with 1 g of platinum coated quartz wool (Elemental ◦ 7 00 8 10 / × These sites all monitor NO, NO i. Marylebone Road (Westminster), an urban kerbside site in a streetii. canyon, Horseferry situ- Road (Westminster), an urban background monitoring station located iii. Greenwich–Eltham (Eltham), a suburban background site situated in Greenwich During flights, ambient air was sampled from the forward facing isokinetic inlet along Toluene and benzene calibrations were carried out approximately 2 h prior to each canister sampling (WAS) wassteel conducted cans twice (Thames per Restek, flight UK) using silica with coated subsequent stainless GC-FID analysis for benzene and shown in Fig. S1 in the Supplement. 3 Results 3.1 Intercomparison of WAS TD-GCFID andTo PTR-MS compare the volume mixingmeasured ratios by gas obtained chromatography with with the flame on-board ionisation PTR-MS detection with (GC-FID), those whole air the nearest road, the A210 Bexleyland, Road. The recreational surrounding areas area and consists suburban of housing. trees, grass- resolution. However, only the Westminster–Marylebonemonitoring Road sites and monitor Greenwich–Eltham benzene and toluene at hourly resolution. The locations are purposes. These were: ated 1.5 m from the0.1546 kerb of a frequently congested 6 lane road, thewithin A501 an (51.5225 area of mixedThe commercial nearest and road is residential the buildings B323 (51.4947 Horseferry Road approximately 17 mwithin north an education of centre the (51.4526 station. flushed through the inlet system to determine instrument2.7 background. LAQN ground monitoring sites Data were obtained from three LAQN ground level monitoring sites for comparative may depend upon sample air humidity. During flights, zero air was periodically back- from the sample stream, which is of particular importance as background impurities pump (Millipore) at a flow-ratewas of 22 diverted L min intothe the overall pressure delay controlled timeing inlet was ambient of air the wasa PTR-MS purged 3 instrument into a such customMicroanalysis) that built which zero e airgenerator generator was which operated consisted at of 350 optimal operating conditions. The catalytic converter does not remove water vapour limits of detection (LoDs) wereleagues determined (Taipale et by al., the 2008) and methodrespectively. were outlined 13 by Taipale and col- a heated (70 flight using andynamic in-house dilution built dynamic ofity dilution a controlled calibration 500 zero ppbv system.levels. grade certified This Typical air involved gas instrument the (BOCtween standard sensitivities gases) 380–480 icps (Apel, ppbv to observed Riemer)600 mixing icps during ppbv with ratios the humid- nearties campaign typically were observed 16 rangedthe be- standard deviation of linear regression (Sm) of pre-flight calibrations. Instrument 7) ranged between (0.8–3) 3 % of the primarywith an ion average signal. of 3 Ion counts of 110 Td. For flights at 5 5 20 25 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 c ffi ect on ff mixing ra- x respectively mixing ratios mixing ratios x x x , benzene and x 200 pptv benzene oxidation rate lead- ∼ x mixing ratios 10 m a.g.l. particularly directly x mixing ratios are highest when ± emission have a larger e x x by rapid titration with NO. As the day reaches its maximum (Langford et al., c related sources are highest and the 3 3 ratios from 27 individual flight-transects ffi 2 and VOC mixing ratios were observed in 350 pptv toluene and x (Pudasainee et al., 2010). Variations in O ∼ NO 3 / , 27346 27345 x morning and daytime mixing ratios over Greater 79 is assumed to be due to benzene alone. 3 and NO mixing ratios corresponding to lower NO m/z 3 3 and between 90–150 pptv benzene and toluene. During x 40 ppbv NO ,O x ∼ and increased O 2 mixing ratios were superficially anti-correlated to NO 3 10 m a.g.l., when emissions from tra ± mixing ratios across greater London during these flights were superficially con- c flow peaks, with higher O x ffi Figures 3 and 4 show 1 km averaged mixing ratios of VOCs and NO Recent studies of annually averaged daily VOC mixing ratios in London from the mixing ratios were observed within innerdownwind London of at 360 the London CCZ. Average mixing ratios observed within inner London the flight transects (Table 1). Researchof flights 2–6 pollutant were mixing used ratios toNO provide across a London. cross section The relativesistent, spatial with average distribution mixing of ratioshence VOC for only and each RF flight 5 leg is only shown changing here. temporally (Fig. 2), during RF 5 against latitudeshow across significantly Greater London. higher Both mixing VOCs ratios and in NO inner London. For all compounds the highest observed mixing ratios in the boundary layer than does3.3 boundary layer dynamics. Horizontal spatial distribution of VOCsThe and predominant NO wind directions observed during(RF the 2–6), flights perpendicular were to either north-westerly the flight transects, or north-easterly (RF 7–10), parallel to 191 m high BTdisplay Tower two have day-time maxima, shown oneoccurring that occurring between around 18:00 benzene and 9:00 and LT 21:00 and LT coincidingperiods toluene a with (Langford second morning mixing et larger and ratios evening peak al., peaktra 2010; typically tra Lee et al.,and 2014). vice versa NO during a 242005). h This period (Im indicates et that al., patterns 2013; Lu in and VOC Wang, 2004; and Mazzeo NO et al., London were attributed toprogresses, the sunlight destruction intensity of becomes O highering increasing to the decreased NO NO mixing ratio are generally attributedwell to as photochemical transport production from in the the upper mixing layer layer (Dueñas as et al., 2002). observed. Subsequently, the low O the same period, O tios, increasing from morning minima until an evening maximum of 40–45 ppbv was mum at approximately 16:00–18:00 LT when2010; O Marr et al.,the 2013). morning The at highest 10:30at LT, NO 360 mixing height relatively low. Mixingdue ratios to decreased a throughout combination the of morning,OH boundary probably oxidation layer leading development to leading enhanced toin chemical dilution removal, the and with day increasing mixing at ratios 10–20 stabilising ppbv NO later Mixing ratios of VOCs, NO of Greater London duringrespect RF to 2–6 time weretoluene over averaged mixing the to ratios assess 7ban followed how days areas the they of with typical changed measured flights. with diurnal maxima As pattern during shown morning previously in rush observed Fig. hours in and 2, ur- a NO measured mini- overestimation of benzene mixingmethod, ratios attributed determined to by theInter-comparison PTR-MS fragmentation of the compared of two higher to samplingcertainty alkyl-benzenes a methods and showed (eg GC in excellent ethyl-benzene). particular agreement within suggestthe un- that fragmentation the of PTR-MS higher demonstrated alkylthe minimal benzenes PTR-MS bias during signal due this to obtained study, at as shown Fig. S2. Hence 3.2 Interpretation of temporal trace gas profiles mation or analyte losssystem that is used may (Cao occur and Hewitt, oneral 1993, urban adsorbents 1994a, environments b). if have Previous shown a ground generallytoluene observations pre-concentration good in mixing agreement sampling sev- ratios between benzene obtained and duringet PTR-MS al., and GC-FID 2006; intercomparisons Warneke (Rogers et al., 2001). However (Jobson et al., 2010) suggest a 16 % toluene (Hopkins et al., 2009, 2003). The WAS system avoids possible artefact for- 5 5 25 20 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 1 2 k − 12 B) ra- 10 / × B concen- 7500) and / = and 1.8 (1.3– 0.06 n 1 ± − B ratios between B ratio, attributed / / 0.12, 10 m a.g.l. increased B ratio progressively = / ± 15.2 ppbv for benzene, 2 6500) and toluene and respectively. This could B concentration ratios in for benzene and toluene ± R = 1 / 1 − , as seen in Fig. 3. Toluene n − 1 s − c emissions. This secondary 1 − ffi B mixing ratios will be a product / 0.51, B ratios in Lambeth, this peak is ). This was observed during three , benzene and toluene mixing ra- ◦ x / = 2 in Greater London (Langford et al., NO 7.3 and 34.3 R B ratios of up to 8.5–12.5 ppbv ppbv x 2 ± 0.141 / molecule 0.3 ppbv ppbv − 3 ± , cm 27348 27347 B ratios are similar to the average T k ) ranging between 0.12 and 0.64. The weakest / ciency for benzene with respect to alkylated ben- 2 8.6, 21.0 12 observed within inner London in this study, where ffi R and 1.3 B ratios of 1.6 (1.3–2.0) ppbv ppbv ± 1 / 10 1 6500), not shown. The strongest linear regressions , longitude − ◦ − ) and south-western Greater London (latitude 51.35– × = ◦ have shared anthropogenic sources with very few bio- B ratio of up to 3 ppbv ppbv n / x cients ( respectively. Mixing ratios for benzene, toluene, NO, NO 0.17 ffi x ± 0.07, 13.2 0.14, ± (Heeb et al., 2000). The introduction of catalytic converters to = 2 1 7500) (Fig. 5). This trimodal distribution between benzene, toluene 0.3 ppbv ppbv ; 6.03 − 0.3 ppbv ppbv R and NO 1 = ± − ± respectively. These T 2 n NO concentration ratio at 360 m a.g.l. is likely to be dominated by pho- s 1 / 1 − 2 c sources, showed T − 0.05, 0.28 ffi 0.64, or benzene and hence is not related to tra ± indicates that these compounds potentially share the same or co-located for all flights are shown in Table 2. = 2 2 x 2 ) were 1.1 R ◦ ( molecule c sources are likely to be the highest (Fig. 4). Average T 2 3 ffi Figure 5 also suggests a secondary source contribution to toluene that is not shared Linear regression analysis between NO, NO Recent long-term VOC measurements made at two ground level sites in central Benzene, toluene and NO or some unidentified localised source of toluene. of the seven flight transects infrom which 0.25 toluene to mixing 0.6 ratios ppbv at withhas 360 a numerous T anthropogenic sources includingvents, paint evaporative fuel thinners losses, and the industrialfrom manufacturing sol- of industrialised ink areas and in paints.have Mexico Direct toluene previously city emissions with been T reportedindustrialised (Karl areas et upwind al., ofpossibly 2009). the due to region In the of horizontal the advection high absence of industrial T of emissions any from outside identifiable of London, tochemistry rather than emission sources (Atkinson et al., 2000). with NO source is localised to aough discrete of peak Lambeth in (latitude observed 51.455 toluene in SE London within the bor- tios yielded correlation coe linear regressions were observedbenzene between toluene and and NO NO ( were ( observed between tolueneNO and benzene ( and NO sources within the Greatermeasured London NO area, most likely vehicular emission. However the 51.42 be interpreted as increasing air massof age benzene from emission and and toluene suggestshorizontal that in advection the these from sources inner regions London. are likely the product of local emission and suburban (latitude 51.30–51.35 tra to the reduced catalytic conversion e zenes (Heeb et al., 2000). Henceof observed the ambient photo-chemical T age ofgasoline the composition air and mass the since ratio of emission, vehicles vehicular using fleet catalytic composition, converters. London (Marylebone Roadnated and by tra North Kensington;2.3) ppbv Valach ppbv et al.,tration ratio 2014) of 1.8 both domi- decreases as air islar transported exhaust over emission longer ratios distancespendent from on away combustion from gasoline during composition the transient but1.25–2.5 source. ppbv within ppbv engine Vehicu- Europe operation typically is yieldvehicular de- T exhausts significantly has been shown to decrease this T 2010; Lee etdue al., to 2014). faster Toluenecm photochemical has removal a by shorterrespectively OH at atmospheric (rate 298 K; lifetime constants Ohta andtio than of Ohyama, can 1985), benzene indicate 1.45 thus the the photochemicalet toluene age to of al., benzene the 2001; (T pollution Atkinson, carriedside), 2000). by the air Very ratio masses close of (Warneke to VOCselves. the mixing As source ratios toluene should of is be emissions similar more (e.g. to rapidly at those removed the in by kerb- the oxidation, emissions the them- T toluene, NO, NO and NO genic contributions into an be urban an important environment. source Vehicular for emissions VOCs are and considered NO were 0.20 5 5 25 20 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 20 km , typical ∼ 1 − mixing ratios x was previously 1 − were due to airport 2 , observed mixing ratios x and NO x 0.5 ppbv ppbv c sources. As LHA was mixing ratios are observed due ffi ∼ x erential optical absorption spectroscopy ratio also gradually increased across the ff 2 B ratio of / NO 10 m a.g.l. across the plume respectively, with 27350 27349 / , which is consistent with previous studies that ± 1 − indicative of vehicular exhaust emission as the dom- 1 − and VOC emission from London Heathrow Airport (LHA) dur- mixing ratios have been previously observed in jet engine ex- x latitude). On entering the LHA outflow plume, NO 20 km downwind distance (Fig. 7). x ◦ ∼ have also been studied using a combination of in-situ measurements emissions from airports are due to a combination of emissions from x x B ratios observed during engine ignition are up to 3.1 ppbv ppbv and NO / 10 m a.g.l. were observed to increase from 18 to 30 ppbv, suggesting a strong 2 close to the airport are dominated by road tra ± 7.2 m a.g.l. during RF 5. During RF 5 the transport time from LHA to the flight x source. As shown in Fig. 4, the NO ± x B ratio, since kerosene fuel tends to have an enhanced amount of aromatic com- B ratios of 1.5–1.7 ppbv ppbv VOC and NO Figure 6 shows the region of RF 5 flight transects which were influenced by LHA The influence of NO / / (DOAS) over several American2008; (Wang Hu et et al., al., 2012) 2003; and Stutz European et cities (Glaser al., et 2004; al., Velasco 2003). et To the al., author’s knowl- 3.4 Interpretation of vertical trace gasTo date profiles the influence of verticalboundary transport layer on the has distribution primarily ofwhich trace been gases are studied in typically the with urban et with respect al., to in-situ 1996; vertical Güsten instrumentsNO, et profiles NO mounted al., of on 1998; ozone, Newchurchfrom tethered et tethered balloons al., balloons 2003). (Beyrich and Vertical ground profiling based of di VOCs, the oxides of nitrogen. During aircraftfrom refuelling, the gaseous aircraft air-fuel tanks mixtures are throughT released fuel vents which canpounds. be VOC emissions discriminated during by the engine2.7 % observed refuelling of were the previously total found VOC to emissions account of for Zurich airport (Schürmann et al., 2007). shown T of kerosene fuel. Attend higher to engine crack temperatures, leading i.e.of to during benzene. a taxiing, Thus reduced higher for amountobserved aircraft aromatics of (Spicer taxiing, these a et species, T al.,to but higher 1985). increasing engine Similarly amounts combustion. higher Assions well NO from as the airport contribution environmentsGSE from occur aircraft, vehicles additional during are emis- mostly the diesel handling powered, of leading aircraft to with relatively GSE. high The emission rates of aircraft exhaust, ground support equipment (GSE)ing exhaust aircraft and refuelling. evaporative losses High dur- mixingbenzene, ratios and of low aromatic NO compounds,haust such immediately as after toluene ignition, and plete attributed to combustion low (Schürmann engine et temperature causing al., incom- 2007). Previous aircraft exhaust studies have enced by emissions during advectionsources from within LHA inner to London the and measurement as location such and conclusions local drawn from this data is tentative. inant VOC source. Previousgested ground that observations approximately 27 (Carslaw % etoperations of al., at the annual 2006) the mean airport at NO airport LHA boundary. they At sug- estimated background locations that15 %. 2–3 the km Our upper measurements downwind are of limiteven in though the of qualitative Heathrow is the agreement an with airport importantof this emission NO contribution study, source suggesting to of that NO beupwind less of than the flight transects, our observed mixing ratios are likely to be heavily influ- plume from 0.5 uphave to found 0.8 higher ppbv ppbv NOet mixing al., ratios 2000). in Toluene0.20–0.26 and aircraft and benzene exhaust 0.15–0.18 ppbv mixing (Spicer atT ratios et 360 showed al., a 1994; negligible Schäfer increase from persion layer were 300–400 m366 a.g.l., similar to thetransect measured was average flight approximately altitude 50 min,wind of speed calculated and from the the average observed horizontal outflow (51.39–51.45 at 360 NO ing this study was investigated withSingle-Particle plume Lagrangian dispersion Integrated modelling Trajectory using (HYSPLIT) the model. NOAAthe The Hybrid region model isolates of thethe flight track flight. influenced Four by hourfrom potential LHA averaged pollutant between forward outflow 9 from dispersion a.m.–12 p.m. LHA LT. forecasting The during was lower and modelled upper for limits RF5 of the averaged dis- 5 5 25 20 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 and 3 35 km by NO and its ∼ 3 2 ) were ver- 1 − (the sum of O x 0.01 ppbv) and toluene respectively. Hence, the ± 1 mixing ratios closer to the − and O 3 0.05 ppbv ppbv 3 ± measurements were lowest at 10 m a.g.l. The daytime vertical 3 ± depletion, with reduced surface O ,O and ozone dry deposition processes 10 m a.g.l. to assess how O 2 3 2 ± and 13.1 m s and lower O ◦ B ratio (1.1 2 above Greater London in this study was / 3 17.1 27352 27351 ects. The chemical production of NO ± ff are superficially anti-correlated with altitude. The 3 and O x destruction. NO 3 and O 10 m a.g.l. (43 and 4.4 ppbv for NO respectively and 28 are caused due to a combination of turbulent mixing and x ± 3 mixing ratios are substantially reduced at ground level likely 6 km downwind of the Marylebone Road measurement site. , causing higher NO exhibits a very uniform vertical concentration between 350– 2 x x ∼ show a large decrease in mixing ratios between measurements titration with NO, associated with the proximity of the Marylebone ). In contrast to NO and NO and O 2 2 3 2 and RO 3 profiles, with lower values close to the ground and higher values aloft, and subsequent O 3 2 species using research aircraft over a European city. x 0.01 ppbv) and their corresponding T . As a result, O 2 ± ) show the importance of NO emission for O 2 from NO The vertical profiles of NO Both NO and NO Figure 7 shows the comparison between the measured trace gas concentration pro- Vertical profiling of VOCs, NO 3 minster and Elthamdirection respectively (RF during 1, flights RF4–6, with Table a 1). prevailing RF north 2 and westerly 3 wind were not used in the comparison due Data obtained from three LAQNtypical air urban quality ground environments monitoringWestminster–Horseferry stations (urban Road; located kerbside: in and three Marylebone suburbancompared background: Road; against Greenwich–Eltham) airborne urban mixing were background: ratiosprecursors at are 360 distributed across thePLIT city. model Dispersion was modelling using used thetant to NOAA outflow highlight HYS- from regions of eachforward the of and flight reverse the dispersion track ground forecasting most was monitoring influenced modelled stations. by for Briefly, Marylebone pollu- four Road, West- hour averaged 650 m a.g.l. However, O due to enhanced O road site to the vehicular emission source. 3.5 Comparison of airborne measurements with LAQN ground sites which dominate closer to the surface (Wesely and Hicks, 2000). observed O agree in their generalet behaviour al., with 2003; other Güsten observations etNO (Beyrich al., 1998) et The al., vertical 1996; profilesmixing Glaser of ratios O closer toin the NO ground largely compensated by a corresponding increase profiles of NO, NO three main simultaneous competingtitration with e O surface due to higherO surface NO mixing ratios. Photochemical production of NO and the kerbside site, 17 ppb, increasing to 32 ppbv at 360 and 17 ppbv for NO (0.16 tically uniform with increasing altitude,tres. suggesting In this rapid case mixing VOC between lossesto 350–650 produce due observable me- to concentration reaction gradients with inthat the the vertical OH turbulence distribution. radical This mixes are suggests the evidentlydepletes too species slow them. up Over to urban 650thermal m areas a.g.l. forcing produced much turbulence by faster is the than urban promoted energy the throughout balance OH the (Velasco radical etat day al., the due 2008). kerbside to and at 360 vertical concentration profiles observed representshorizontal advection a of composite the of Greater local London emission region. and files over Greatermeasured London concentration and distributions the of both Marylebone benzene Road (0.15 kerbside measurements. The trace gas mixing ratios observed atflight each path altitude over were Greater then London. averaged along Thesehourly the concentration profiles averaged were upwind then comparedMarylebone LAQN with Road kerbside air measurements qualitytios made monitoring change at vertically station from 17:00 to LT thepoint, interpret from street flight how canyon the legs to trace were above gasDuring the RF mixing urban 1, ra- canopy. a At predominantlywind its north direction closest westerly and wind wind direction speed was of observed 285.1 with a mean and NO conducted during RF 1tical on profiling 24 consisted June of(17:00–17:20 2013 LT), three 510 between m sequentially a.g.l. the stacked (17:20–17:40 LT) hours flights and 650 of conducted m a.g.l. 17:00–18:00 at LT. (17:40–18:00 Ver- 350 LT). m The a.g.l. edge, the work herein represents the first vertical boundary layer profiling of both VOCs 5 5 25 20 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | in 3 mix- 4.28). 2 ± B ratios / 0.08 ppbv ± , which is in 3 concentration 2 0.09). Historically, NO ± / species at Westmin- 4.9 ppbv) were lower x ± 0.05 ; 0.21 ± species were significantly values ranging from 0.54– x 2 shown in Fig. 7. Mixing ratios R 3 emissions in the UK and Europe. . Strong positive correlations are 3 2 mixing ratios between Marylebone x 0.03 ppbv at Eltham, with T were dominated by emissions of NO 4.45 ppbv) and Eltham (10.02 ± x ± and O x 0.15) and Eltham (0.25 0.9)). However, recent developments in diesel . The lower and upper limits of the averaged ± 27354 27353 5 for NO 1 erence in dilution factors is largely due to firstly ≤ − ff , NO ∼ mixing ratios at these sites are anti-correlated to 2 3 0.01 ; 0.13 5 m s ratios were higher at the Marylebone Road site ± species observed at the roadside site at Marylebone < were consistently higher than those at ground level, con- ratios of x 2 3 2 NO NO / indicative of vehicular emissions as the dominant source at / 3.2 ppbv) whilst the lowest mixing ratios were at the Marylebone 1 ± − and being closest in proximity to the 6 lane frequently congested 8.42 ppbv). The O mixing ratios were also significantly higher at Marylebone Road x ± x ratios of (NO 45.28 ppbv) than Westminster (40 7). Ground mixing ratios of both VOCs and NO shown in Fig. 5, potentially indicating these species have common sources, ratios observed from all three ground air monitoring stations observed during 10 m a.g.l. during RF 1, RF4–6. Airborne mixing ratios for comparison were 2 ± 2 2 = 0.25) than at Westminster (0.50 erence is due entirely to mixing, this reduction in mixing ratio crudely indicates ± , the mean mixing ratios observed at Westminster (13.56 n ff 3 ± NO NO / / Airborne mixing ratios of O Also of interest, NO Figure 8 shows a linear regression analysis between airborne and ground mixing ventilated as with more open locations such as urban and suburban sites which tends which concluded a dilutionRoad and factor the of 190 m samplingthe point surrounding on building the height. BT Dilutionster tower factors (Harrison for and et VOC Eltham al., and 2012), NO rangedobserved well at between above Marylebone 0.46–2.34 Road. which Thisthe di are Marylebone significantly Road measurement lowerof site than being VOCs those in and central NO road. London, a Secondly very Marylebone large Roadserves source is to within maximise an mixing urban ratios street of canyon emissions whose therein. orientation Street canyons are not as well of the selective VOCRoad and were NO significantly higherthis than di those ofa the dilution airborne factor measurements. of355 Assuming m 2–6 sampling between point. the This agrees roadside well site with in comparisons the made Marylebone during REPARTEE Road I and the this study. This is inand good NO agreement with the trimodalmost distribution likely of from benzene, toluene vehicularratio emission. at 360 However m a.g.l. thesources is measured (Atkinson likely et NO al., to 2000). be dominated by photochemistrysistent rather with than the emission ground surfacegood in agreement London with acting the as measured a vertical chemical profile of sink O for O Current diesel emission control technologying deliberately ratios produces to enhanced NO gas oxidise (Carslaw and and reduce Rhys-Tyler,tral black 2013). London carbon Increasing with this numbers particulatesNO emission of in reduction diesel technology the vehicles could vehicular have in contributed exhaust Cen- to the low caused significant increases in direct vehicular NO (NO emission technology, specifically diesel oxidation catalysts and particulate filters, have For O than at Eltham (19.14 Road site (9.23 that of NO (Fig.proximity 8), to through busy enhanced road NO networks. emission and subsequent titration(0.62 of O vehicular diesel and petrol emissions of NO ground level for benzene andat toluene Marylebone respectively were Road 0.12 of and 1.7–1.8 ppbv 0.07 ppbv both sites. NO (121.96 and Eltham to thetively, calculated flight from transect the rangeddistance observed between for horizontal each 3–7, wind ground 7–15 speed station during and and each the 14–28 flight min downwind/upwind (Fig. respec- S1). ratios of benzene, toluene,observed NO, for NO all species0.97 at ( all threehigher ground at sites the with Maryleboneminster) Road and kerbside suburban site background relative (Eltham) to sites. the Average urban mixing background ratios (West- observed at to low observed winddispersion speeds, layer were 300–400 m a.g.l.,of similar 360 to the measuredgiven as average flight the altitude arithmeticpersion average and plume. The 1 approximate SD transport of times the from hourly Marylebone measurements Road, within Westminster the dis- 5 5 25 20 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 2 ) ob- 1 − and VOC chemilumi- x x mixing ratios x airs for funding. We 10 m a.g.l. was ob- ff 0.3 ppbv ppbv ± ± c and other pollutant sources ffi B ratios (1.8 , Atmos. Environ., 34, 2063–2101, 2000. / x . c emissions, with the highest mixing ra- ffi 7) suggesting that airborne mixing ratios 10 m a.g.l. agl due to a combination of its = ± n 27356 27355 in the boundary layer were made in transects 20 km upwind of the flight transects, these ob- 3 NO concentration ratio at 360 m a.g.l. is likely to even quite close to the airport are dominated by ∼ / x 2 10 m a.g.l. during the summer of 2013, with a view to , benzene and toluene species at all three ground sites and O ± x NO ratio in the vehicle exhaust (Carslaw and Rhys-Tyler, x / precursor sources within the region, and to better under- 2 3 species which showed a trimodal correlation between ben- , NO 2 x mixing ratios from 18 to 30 ppbv at 360 , potentially indicating that their dominant sources are the same 2 x M. D. Shaw and J. D. Lee redesigned the PTR-MS and NO We thank the UK Natural Environment Research Council (grant , NO, NO ects of chemical interactions between these pollutants and meteorolog- 3 ff 20 km downwind of LHA. Our measurements tentatively support previous c sources. Since LHA was values ranging from 0.54–0.97 ( ffi ∼ 2 R , observed mixing ratios of NO x An increase in NO Airborne mixing ratios were compared to kerbside data from three LAQN air quality Aventech AIMMS20AQ airborne probe for turbulence measurements during the Convective road tra served mixing ratios areadvection likely from to LHA be to the heavily measurement influenced location. by vehicular emissions during during vertical mixing. served studies that suggest thatNO even though Heathrow is an important emission source of proximity to the emission sources, photochemical aging and dilution of the air mass mixing ratios observed attimes the higher Marylebone than Road those air observed quality at monitoring 360 site were 2–6 which also enhance the NO 2013). However the measuredbe NO dominated by photochemistry rather than emission sources (Atkinsonground et al., monitoring 2000). stations withinobserved for Greater O London. Strongwith positive correlations were were characteristic of surface mixing ratios during the analysis period. NO analysis of VOC andzene, NO toluene and NO or are co-located throughout London.but The reason not these NO VOCs correlate isdiesel well possibly vehicles with because use NO emission of control the technology ubiquity to of reduce diesel black carbon vehicles emissions in but London. Modern Measurements of VOCs, NO across Greater London at 360 identifying the dominant O standing the e ical variables on urbanacross air Greater quality. London Observed were mostly benzene, duetios toluene to observed and tra over inner NO London, where theis density higher of tra than overserved outer within London. inner The London highest is T indicative of local vehicular sources. Linear regression 2013). 4 Conclusions to result in increased surface mixing ratios (Pugh et al., 2012; Carslaw and Rhys-Tyler, Atkinson, R.: Atmospheric chemistry of VOCsBeswick, and K. NO M., Gallagher, M. W., Webb, A. R., Norton, E. G., and Perry, F.: Application of the lard (ARSF) for their expert(National support Centre during for the flightsbenzene Atmospheric and and Science, James toluene Hopkins concentration University data. and of Shallini York, Punjabi UK) for the WASReferences TD-GC-FID NE/J00779X/1) and the Departmentthank of Captain Environment, Carl Food Joseph, and co-pilot Rural James A Johnson and instrumental engineer Thomas Mil- Author contributions. Acknowledgements. nescence instruments for the aircraft.experiment M. and carried D. Shaw, it J. out.C. D. M. Lee N. D. Hewitt Shaw, and J. were B. D. responsible Davison Lee, for A. designed analysis/interpretation Vaughan, the R. of field M. the Purvis, data. A. C. 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Environ., 35, 1567–1584,Environ., 2001. 34, 2261–2282, 2000. 10 9 Jul 13 14:00–16:00 52.8 9 9 Jul 13 9:30–11:30 52.1 78 8 Jul 13 8 Jul 13 11:30–13:00 16:30–18:00 56.4 54.3 6 4 Jul 13 15:20–16:55 240.7 5 3 Jul 13 10:40–13:00 280.7 4 27 Jun 13 14:20–17:50 275.1 3 27 Jun 13 9:40–12:15 277.6 2 26 Apr 13 16:00–18:00 287.5 RF Date1 Time (local) 24 Jun Mean 13 Wind 15:30–18:20 Mean Wind 285.9 Mean True Mean Flight Table 1. Valach, A., Langford, B., Nemitz, E., MacKenzie, A., and Hewitt, C.: Concentrations of selected Taipale, R., Ruuskanen, T. M., Rinne, J., Kajos, M. K., Hakola, H., Pohja, T., and Kulmala, M.: Velasco, E., Márquez, C., Bueno, E., Bernabé, R. M., Sánchez, A., Fentanes, O., Wöhrnschim- Warneke, C., Van der Veen, C., Luxembourg, S., De Gouw, J., and Kok, A.:Watson, Measurements J. G., Chow, J. C., andWesely, M. Fujita, and Hicks, E. B.: A M.: review of Review the of current status volatile of knowledge organic on dry compound deposition, source Atmos. Wang, S., Ackermann, R., Geyer, A., Doran, J. C., Shaw, W. J., Fast, J. D., Spicer, C. W., and 5 10 20 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 27364 27363 RF1 RF2 RF3 RF4 RF5 RF6 RF7 RF8 RF9 RF10 82 500 69 500 70 500 12 800 38 100 56 300 44 300 44 750 63 000 60 500 16 500 13 900 14 100 2560 7620 11 26016 8860 500 13 900 8950 14 100 12 600 2560 12 100 7620 11 26082 8860 500 69 500 8950 70 500 12 600 12 800 12 100 38 100 56 300 44 300 44 750 63 000 60 500 82 500 69 500 70 500 12 800 38 100 56 300 44 300 44 750 63 000 60 500 2 x Top: map showing all NERC Dornier-228 flights overlaid on UK transport map. Bot- Summary of mixing ratios (ppbv) observed over inner London during campaign. NO MeanMedianSD5th percentile95th percentile 15.19 13.59N 7.56 26.49 11.99 11.44 31.90 7.91 22.95 26.54 6.10 52.13 11.45 18.64 22.13 35.08 8.29 10.21 21.02 20.23 25.97 13.17 12.17 6.03 10.65 19.60 7.89 6.34 3.65 6.65 3.84 6.38 7.70 1.84 9.11 14.75 7.31 18.87 18.67 32.73 1.20 17.85 16.32 31.34 2.77 3.86 5.07 4.58 10.92 9.87 Benzene MeanMedianSD5th percentile95th percentile 0.08N 0.01 0.07 0.178Toluene 0.09 0.03 0.09 0.20 0.06Mean 0.22 0.06 0.15 0.27Median 0.05SD 0.10 0.03 0.09 0.175th percentile 0.0695th 0.20 percentile 0.09 0.19 0.22 0.17N 0.08 0.16 0.05 4.34 0.10 0.03 0.10 0.19NO 0.12 0.06 0.12 0.05 0.26 0.07 0.17Mean 0.09 0.17 0.26 0.28 0.05 0.27 0.05Median 0.39 0.08 0.18SD 0.08 0.18 0.30 0.15 0.05 0.15 0.055th 0.32 percentile 0.11 0.10 0.01 0.09 0.2595th 0.28 percentile 0.12 0.25 0.07 3.83N 0.38 1.06 3.46 0.07 0.07 7.82 0.01 0.07 0.16 0.12 0.01 0.11 0.08 2.06 0.28 0.70 2.44 0.07 7.94 2.16 0.20 0.09 0.20 17.46 0.05 16.19 0.32 3.49 0.09 25.71 2.50 0.14 8.81 0.01 0.13 7.76 0.32 1.91 7.44 0.07 7.42 13.20 0.29 12.43 0.01 0.27 21.33 0.61 1.98 0.11 4.80 1.88 0.14 3.41 12.17 0.01 0.13 0.28 1.23NO 0.19 2.20 8.60 2.38 3.84Mean 0.23 0.09 1.77 3.61Median 1.91 3.97SD 0.30 14.45 15.37 29.87 1.225th percentile 12.45 1.4495th 11.98 26.41 percentile 19.02 1.42 17.49N 8.71 33.62 1.96 16.05 10.21 17.91 39.36 9.00 40.41 36.02 7.96 8.99 76.08 15.53 27.45 26.78 10.39 42.69 12.16 34.3 32.40 19.87 44.54 9.35 15.95 16.97 30.04 9.67 5.20 9.07 8.54 13.61 15.20 11.27 18.57 2.21 9.48 8.50 38.79 61.67 34.74 1.33 3.94 45.67 28.9 30.3 5.77 6.44 8.41 20.39 8.66 Figure 1. tom: total flight legs acrossinner Greater London London. boundary, blue Grey area; area; London Greater CCZ. London boundary, black area; Table 2. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 3 B instan- / ratios and O 2 NO / 27366 27365 10 m a.g.l. across Greater London during RF 5. ± ) at 360 1 − Top: time series of averaged benzene and toluene concentrations observed at City cross section of 1 km averaged benzene, toluene mixing ratios and T 10 m a.g.l. during RF 2–6. Bottom: times series of averaged NO concentrations during RF 2–6. ± x Figure 3. taneous ratios (ppbv ppbv Figure 2. 360 NO Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ± mixing ratios across Greater against toluene mixing ratios 2 x and NO 2 27368 27367 10 m a.g.l. during RF 5. ± Top: linear regression analysis of benzene against toluene mixing ratios at 360 City cross section of 1 km averaged NO NO 10 m a.g.l. during RF 5. ± Figure 5. Figure 4. London at 360 10 m a.g.l. during RF 5.at Bottom: 360 linear regression analysis of NO Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 10 m a.g.l.) ± 10 m a.g.l.) during RF5 overlaid ± , benzene and toluene across London x error bars represent standard deviation X ,O 2 27370 27369 , NO, NO 3 concentration data (7 m resolved at 360 x Average vertical profiles of O Top: NO 10 m a.g.l.) during RF5 overlaid on UK transport map. Grey area; Greater London ± ) of mixing ratios observed during each flight leg. Mixing ratio at ground level is hourly σ Figure 7. during RF1, 17:00–18:00 LT on(1 the 24 Juneaverage 2013. from the LAQN Marylebone road air quality monitoring station. Figure 6. on UK transport map. Middle:during Benzene RF5 concentration overlaid data on (35at UK m transport resolved 360 map. at Bottom: 360 Toluene concentration data (35 m resolved boundary, black area; inner London boundary,averaged dark HYSPLIT blue dispersion area; trajectory. London CCZ, light blue area; 3 h Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 10 m a.g.l.) and hourly ground ± 27371 Linear regression analysis between airborne (at 360 Figure 8. measurements at Greenwich–Eltham,from Westminster–Horseferry the LAQN road monitoring network and during Marylebone RF1, 4, Road 5 and 6.