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Atmospheric Environment 43 (2009) 584–593

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Atmospheric Environment

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Aircraft measurements of O3, NOx, CO, VOCs, and SO2 in the Yangtze RiverDelta region

Fuhai Geng a, Qiang Zhang b, Xuexi Tie c,*, Mengyu Huang b, Xincheng Ma b, Zhaoze Deng d,Qiong Yu a, Jiannong Quan b, Chunsheng Zhao d

a Shanghai Meteorological Bureau, Shanghai, Chinab Beijing Weather Modification Office, Beijing, Chinac National Center for Atmospheric Research, Boulder, CO, USAd Department of Atmospheric Science, School of Physics, Peking University, Beijing, China

a r t i c l e i n f o

Article history:Received 14 July 2008Received in revised form26 September 2008Accepted 6 October 2008

Keywords:Air pollutants in the YRD regionAircraft measurements

* Corresponding author. National Center for AtmosUSA. Tel.: þ1 303 497 1470; fax: þ1 303 497 1400.

E-mail address: xxtie@ucar.edu (X. Tie).

1352-2310/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.atmosenv.2008.10.021

a b s t r a c t

In this study, air pollutants, including ozone (O3), nitrogen oxides (NOx¼NOþNO2), carbon monoxides(CO), sulfur dioxide (SO2), and volatile organic compounds (VOCs) measured in the Yangtze River Delta(YRD) region during several air flights between September/30 and October/11 are analyzed. Thismeasurement provides horizontal and vertical distributions of air pollutants in the YRD region. Theanalysis of the result shows that the measured O3 concentrations range from 20 to 60 ppbv. These valuesare generally below the US national standard (84 ppbv), suggesting that at the present, the O3 pollutionsare modest in this region. The NOx concentrations have strong spatial and temporal variations, rangingfrom 3 to 40 ppbv. The SO2 concentrations also have large spatial and temporal variations, ranging from 1to 35 ppbv. The high concentrations of CO are measured with small variations, ranging from 3 to 7 ppmv.The concentrations of VOCs are relatively low, with the total VOC concentrations of less than 6 ppbv. Therelative small VOC concentrations and the relative large NOx concentrations suggest that the O3 chemicalformation is under a strong VOC-limited regime in the YRD region. The measured O3 and NOx concen-trations are strongly anti-correlated, indicating that enhancement in NOx concentrations leads todecrease in O3 concentrations. Moreover, the O3 concentrations are more sensitive to NOx concentrationsin the rural region than in the city region. The ratios of D[O3]/D[NOx] are �2.3 and �0.25 in the rural andin the city region, respectively. In addition, the measured NOx and SO2 concentrations are stronglycorrelated, highlighting that the NOx and SO2 are probably originated from same emission sources.Because SO2 emissions are significantly originated from coal burnings, the strong correlation betweenSO2 and NOx concentrations suggests that the NOx emission sources are mostly from coal burned sources.As a result, the future automobile increases could lead to rapid enhancements in O3 concentrations in theYRD region.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The Yangtze River Delta (YRD) region located in east China (withlongitude from 120�E to 121�E, and latitude from 30.5�N to 31.5�N)is highly urbanized with a cluster of large cities (includingShanghai, Hangzhou, Shuzhou, Wuxi, etc.). Among these largecities, Shanghai has the highest population in China (about 20millions). During the past 20 years, this region is undergoing a rapidincrease in economical development. For example, in Shanghai, theareas of buildings have expanded from 172.56 to 702.82 million m2

pheric Research, Boulder, CO,

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from 1990 to 2006, and the numbers of automobiles have increasedfrom 0.47 to 2.38 million between 1996 and 2006. The GDP (grossdomestic production) has increased by 500% from 1996 to 2006,accounting for more than 5% of the total GDP in China (SMSB,2007). The rapid urbanization causes wide-ranging potentialconsequences for weather and climate related to urban environ-ments, such as air pollutions. Like other large cities in east China,Shanghai is suffering severe air pollution problems, such as highparticular matter (PM) concentrations and poor visibility (Tie et al.,2006a; Zhang et al., 2006; Bian et al., 2007; Deng et al., 2008;Streets et al., 2008). As industrial activities and the number ofautomobiles increase together with the changes in natural activi-ties (such as vegetation and forest), emission of VOCs (volatileorganic compounds) and NOx (NOþNO2) will significantly increasein the YRD region (Tie et al., 2006b; Geng et al., 2008). Both VOCs

F. Geng et al. / Atmospheric Environment 43 (2009) 584–593 585

and NOx play critical roles during O3 formation in the troposphere(Sillman, 1995). The variations in the concentrations and theensuing effects on the O3 production rate can be characterized aseither NOx-sensitive or VOC-sensitive (Sillman, 1995; Lei et al.,2004; Zhang et al., 2004; Tie et al., 2007). A better understanding ofthe relationship between O3 precursors (VOCs, NOx) and O3

formations in the city of Shanghai (as well as many large cities inChina) is one of the critical pre-required information to developeffective O3 control strategies (Lei et al., 2007; Tie et al., 2007).

Several studies regarding the air quality in the YRD region havebeen conducted. For example, Geng et al. (2007, 2008) studied theozone formation and the effects of VOCs on ozone formation inShanghai. Their study suggested that meteorological conditions,such as clouds, have strong impacts on the ozone concentrations inShanghai. The measurements of VOCs suggested that alkanes andaromatics were among the highest VOC concentrations observed inShanghai, and the aromatics had largest contribution for the O3

chemical production. Yang et al. (2005) measured black and organiccarbons in Shanghai. They found that the concentrations of organiccarbons were generally about twice higher than the concentrationsof black carbon. Xiu et al. (2005) measured mercury concentrationin Shanghai, and showed that coal burning was estimated tocontribute approximately 80% of total atmospheric mercury inShanghai. Zhao et al. (2004) studied the O3 concentrations in theShanghai region, suggesting that small scale dynamical processeshad important influences on the distribution of O3 concentrationsin this region. However, O3 and its precursors (NOx, VOCs, etc.) arenot systemically measured and the relationship between O3 and itsprecursors (NOx, VOCs, etc.) has not been analyzed. To preventpotential high ozone pollution in this region, those measurementsare urgently needed.

In this study, we investigate the O3, NOx (NOþNO2), CO, SO2,and VOCs measured in the YRD region during several air flightsbetween September/30 and October/11. The advantages of in-situaircraft measurements can provide information for the horizontaland vertical distributions of air pollutants in a large spatial area,and for the gradients between cities and rural regions. Thesemeasurements are necessary in order to better understand thecharacterizations of air pollutants not only in the city of Shanghai,but also in its surrounding areas. In Section 2, we will describe theinstruments and measurements, including the flight information.In Section 3, the measured results are analyzed to show thecharacterizations of air pollutants in the YRD region.

2. Description of the measurements

2.1. Flight information

Fig. 1 displays the horizontal routes for several representativeflights. It shows that the flight routes are located in west side of themegacity (Shanghai) with several large cities, such as Suzhou (withpopulation of 8.4 millions), Wuxi (with population of 5.5 millions)and Jiading (with population of 3.4 millions). In addition, there areseveral large power plants in this region. For example, the Chang-shou power plant generates 1200 MW per year (SSB, 2006). Thereare also several inter-state high ways, and high density of smalltowns and villages. As a result, the local emissions of air pollutantsfrom automobiles, and area and point sources are intensive. Thereare totally five flights between September/30 and October/11(flight-1 on September/30, flight-2 on October/2, flight-3 onOctober/5, flight-4 on October/9, and flight-5 on October/11), whichare analyzed in this study. The flight-1 (on 30/September) route isin southeast direction from Wuxi (the location of airport) withapproximate 120 km in horizontal distance. The flight-2 (onOctober/2) has a similar flight route with the flight-3 (on October/5), which is a short flight to southeast of Wuxi. The flight-4 (on

October/9) has a similar flight route with the flight-5 (on October/11) which has a longer flight route to southeast of Wuxi, reaching tothe East Ocean in south of Shanghai. All the flight routes are oversome large cities, especially at the take-off and landing area, wherethe measurements are strongly under the influences of the city ofWuxi.

The flight altitudes are displayed in Fig. 2. It shows that the airplane takes off at Wuxi, and quickly reaches 500–2500 m in alti-tude. The maximum flight height is below 3000 m. In order toestimate whether the flights are inside or outside the PBL (plane-tary boundary layer) height, we show the correlations between theflight heights and the in-situ measured dew points. Because thedew points are determined by air temperature and relativehumidity, there are large changes in the dew points at the top of thePBL, where temperature is at coldest and humidity is rapidlyreduced (Wilczak et al., 1996). As a result, the flights change frominside to outside of the PBL can be roughly estimated at the altitudewhere there is a rapid decrease in dew points along the flightroutes. For example, during flight-2 (2/October), the dew points arerapidly decreased at 10:30 a.m. when flight height reaches2500 km. During flight-4 (on 9/October) there are a rapid decreasein dew points, when the flight height is about 1500 m around10:20 a.m. and 2500 m around 13:00 p.m., suggesting that the PBLheights are increased between 10:20 a.m. and 13:00 p.m. Theseresults suggest that the daytime PBL height is approximately 1500–2500 m, and the maximum PBL height could be higher than 2500 min this region. Fig. 2 also indicates that during the most flight routes,the flights are located inside the PBL. During take-off and landingperiods, the air plane moves upward or downward rapidly, whichprovide useful information of vertical gradient of air pollutantsnearby the city of Wuxi.

2.2. Instrumentation on the aircraft

Several commercial instruments were mounted on the aircraft(Y-12) to measure the concentrations of O3, NOx (NOþNO2), CO,SO2, and VOCs. Standard meteorological devices are also installedon board, including 3-D graphical position, temperature, humidity,and winds. Ozone was measured using a commercial UV photo-metric analyzer (Thermo Environmental Instruments (TEI) Inc.;Model 49iTL) with a detection limit of 0.5 ppbv and a precision of�1 ppbv. During flight the range was set from 0.5 to 200 ppbv withtemperature and pressure correction. NO–NO2–NOx was measuredwith a chemiluminescent trace level analyzer (TEI; Model 42iTL).The analyzer had a detection limit of 0.025 ppbv. The Model 42iTLoperates on the principle that nitric oxide (NO) and ozone (O3)react to produce a characteristic luminescence which is linearlyproportional to the NO concentration. The NO and NOx concen-trations calculated in the NO and NOx modes are stored in memory.The difference between the concentrations is used to calculate theNO2 concentration. CO was measured by the Model 48iTL EnhancedCO analyzer, using gas filter correlation technology. The instrumentis based on the principle that carbon monoxide (CO) absorbsinfrared radiation at a wavelength of 4.6 microns. Because infraredabsorption is a nonlinear measurement technique, it is necessaryfor the instrument electronics to transform the basic analyzersignal into a linear output. The detection limit is 0.04 ppmv. Sulfurdioxide (SO2) was detected with a pulsed UV fluorescence analyzer(TEI; Model 43 i-TLE). The detection limit for this analyzer is0.05 ppbv for 2-min integration with a precision of about 0.20 ppbv.

These analyzers were calibrated before the field campaign byinjecting a span gas mixture in scrubbed ambient air generated bya TEI model 111. A NIST traceable standard in a cylinder (ScottSpecialty Gases) containing 797.0 ppmv CO (�2%), 10.1 ppmv NO(�2%), and 10.1 SO2 ppmv (�2%) was diluted using a dynamiccalibrator (TEI; Model 146.). The zero checks were also conducted.

Fig. 1. The horizontal flight routes for the flights on September/30, October/2, and October/11. The red dot represents the location of Shanghai, and the brown dots represent severallarge size cities, such as Suzhou (with population of 8.4 millions), Wuxi (with population of 5.5 millions), Jiading (with population of 3.4 millions). The green cylinder shows thelocation of Changshou power plant. The colors in flight routes indicate the flight heights (meters). (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

F. Geng et al. / Atmospheric Environment 43 (2009) 584–593586

Fig. 2. The flight altitudes (black lines; meters) and the corresponding dew points (green dots; 0.01� �C) for the five flights. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

F. Geng et al. / Atmospheric Environment 43 (2009) 584–593 587

VOC concentrations were sampled during each flight with a 6 Lsilonite canister with silonite coated valve (Model 29-10622) andanalyzed in Shanghai Atmospheric Chemistry laboratory. Theinternal silonite coating improves long-term VOC storage. Theinstrument has a large volume to detect volatile chemicals down tolow pptv range. Absorption is eliminated by using nupro packlessvalves and by eliminating teflon tape in the valve stem. Thesecanisters are recognized to meet or exceed the technical specifica-tions required for EPA methods TO14-A and TO15. Gas samples werepre-processed using Model 7100 VOC preconcentrator (EntechInstruments Inc., USA). Samples are analyzed for VOCs using a gaschromatography system (Agilent GC6890) coupled with mass-selective detection (Agilent MSD5975 N) with a length of 60 m,diameter of 0.32 mm, and film thickness of 1.0 mm. The columntemperature is controlled by an initial temperature of �50 �C. Theprogrammed temperature was used with helium as carrier gas and

flow gas at 1.5 ml /min. The initial temperature program with 3 minhold time was 4 �C/min to 170 �C switching to 14 �C/min to 220 �C.This measurement system can detect VOC concentrations down tolow pptv range.

3. Analysis of the result

3.1. Result

Fig. 3a shows the measured O3 concentrations along the flightroutes during the five flights. The result shows that after take-offand before landing periods, the O3 concentrations are relativelyhigh, ranging from 40 to 60 ppbv with small fluctuations. Bycontrast, the O3 concentrations have very strong variability duringtake-off and landing periods (nearby the city of Wuxi). The O3

concentrations exhibit a negative vertical gradient, with the lowest

Fig. 3. (a) The flight altitudes (black line; meters) and the measured O3 concentrations (0.05� ppbv) along the flight routes in the five flights. The red and brown dots represent themeasured O3 concentrations in the NCITY and RPBL cases, respectively. (b) Same as (a) except for NOx concentrations (0.02� ppbv). (For interpretation of the references to colour inthis figure legend, the reader is referred to the web version of this article.)

F. Geng et al. / Atmospheric Environment 43 (2009) 584–593 589

O3 concentrations at the surface. This measured O3 distributions arestrongly anti-correlated with the measured NOx concentrations.Fig. 3b shows the measured NOx concentrations along the flightroutes in the five flights. The result shows that after take-off andbefore landing periods, the NOx concentrations are generally low,ranging from a few ppbv to 10 ppbv with relatively small fluctua-tions during the flights. However, during take-off and landingperiods (nearby the city of Wuxi), the concentrations of NOx arerapidly enhanced. Contrasting to the negative vertical gradient ofO3, the NOx concentrations have a strong positive vertical gradient,with highest NOx concentrations at the surface. Comparing O3 toNOx, especially during take-off and landing periods (as indicatedred dots in Fig. 3a and blue dots in Fig. 3b; for interpretation of thereferences to colour, the reader is referred to the web version of thisarticle), the low O3 concentrations correspond to the high NOx

concentrations, especially nearby the surface, suggesting that NOx

plays key roles in depressing O3 in this region. This result isconsistent with previous studies of the characterizations of O3, NOx

and VOCs in the Shanghai region (Geng et al., 2007, 2008). In theirstudies, the model calculated ratio of CH2O/NOY is much less than0.28, which suggests that in the Shanghai region, the O3 chemicalproduction is under VOC-sensitive regime. Under the VOC-sensitiveregime, O3 concentrations decrease with enhancement of NOx

concentrations.In general, there were two very different cases regarding the

measured distributions of O3 and NOx concentrations during thefive flights. The first case occurred when the measurements madenearby the city of Wuxi, and another case happened when themeasurements made in the altitudes between 500 and 2500 m andaway from cities. In the following discussions, we define the firstcase as ‘‘near city case’’ (NCITY), and the second case is defined as‘‘rural PBL case’’ (RPBL). In the NCITY case, the flight routes covera large vertical distance, ranging from surface to about 1500 m.

In order to analyze the characterizations of air pollutantsbetween the two cases, we calculate the averaged values of O3, NOx,SO2, and CO in these two cases (see Fig. 4), respectively. The meanvalues highlight that in the NCITY case, the averaged O3 concen-trations vary from 15 to 40 ppbv; while in the RPBL cases, the O3

concentrations range from 45 to 60 ppbv during the five flights.Both the O3 mean values are generally lower than the US nationalstandard values (84 ppbv), suggesting that the O3 values are modestin the YRD region. The NOx concentrations have large variability,ranging from 3 to 40 ppbv. The highest concentrations of NOx areobserved normally in the NCITY case, ranging from 10 to 40 ppbv. Bycontrast, in the RPBL case the NOx concentrations are normally lessthan 5 ppbv. The SO2 concentrations also have a large variability,ranging from 1 to 35 ppbv. The highest concentrations of SO2 aremeasured in the NCITY case, ranging from 7 to 35 ppbv. By contrast,in the RPBL case the SO2 concentrations are generally less than5 ppbv. The CO concentrations are relatively high, and have smallervariability compared to NOx and SO2, ranging from 3 to 7 ppmv.There is an indication that the CO concentrations are higher in theNCITY case than in the RPBL case. The mean values for all flights are5 ppmv and 4 ppmv for the NCITY case and the RPBL case, respec-tively. The high CO concentrations suggest that there are large COsources in this region, and the less variability of CO is mainly due tothe long chemical resident time of CO (a few months).

3.2. O3 sensitivity to NOx and VOCs

Another important aspect regarding the air pollutant charac-terizations in this region is that the O3 concentrations are lower inthe NCITY case than in the RPBL case. The averaged O3 concentra-tions in the five flights are 34 and 52 ppbv in the NCITY case and theRPBL case, respectively. The difference (O3(NCITY)�O3(RPBL)) is�18 ppbv. By contrast, NOx, SO2, and CO concentrations are all

higher in the NCITY case than in the RPBL case. Because NOx, SO2,and CO are directly emitted from the surface, it is understandablethat the concentrations of NOx, SO2, and CO are higher in the sourceregion (NCITY). However, in the RPBL case the concentrations ofNOx, SO2, and CO are strongly depended upon the rates of chemicaldestructions of these pollutants. The main gas-phase chemicaldestructions for NOx, SO2, and CO are due to the following reactions:

CO D OH // products D HO2 (R-1)

SO2 D OH / Sulfate (R-2)

NO2 D OH / HNO3 (R-3)

The reactions coefficients at temperature 298 K (k1, k2, and k3)for (R-1)–(R-3) are 2.4�10�13, 8.9�10�13, and 1.1�10�11, respec-tively (Stockwell et al., 1990). Because the reaction coefficients ofSO2 and NO2 are significantly larger than the coefficients of CO, thedecay from source regions (NCITY) to remote regions (RPBL) of SO2

and NOx concentrations is much quicker than the CO concentra-tions. For example, the averaged NOx concentrations are 22 and5 ppbv in NCITY and RPBL, respectively. The difference betweenNOx concentrations in NCITY (NOx(NCITY)) and in RPBL (NOx(RPBL)) is17 ppbv, accounting for 72% higher in NCITY than in RPBL. Thedifference between SO2 concentrations in NCITY (SO2(NCITY)) and inRPBL (SO2(RPBL)) is 12 ppbv, accounting for 80% higher in NCITY thanin RPBL. The difference between CO concentrations in NCITY(CO(NCITY)) and in RPBL (CO(RPBL)) is 0.7 ppmv, accounting for 18%higher in NCITY than in RPBL. The differences of NOx and SO2 aresimilar, and are much larger than the difference of CO. This resultsuggests that the smaller gradient of CO between NCITY and RPBL ismainly due to the slower chemical reaction.

The above analysis indicates that the higher NOx concentrationsare corresponsive to the lower O3 concentrations in the NCITY case,suggesting that the high NOx concentrations intend to reduce theO3 concentrations in the NCITY case. The anti-correlation betweenO3 and NOx suggests that the chemical O3 formation is under ‘‘VOC-limited’’ regime. A simple O3 chemical formation process canexplain this mechanism as shown by the following two differentreaction chains:

(case-a) Under high VOC and low NOx condition (low NOx/VOCratio)

OH D VOC / RO2 D products (R-4)

RO2 D NO / NO2 D products (R-5)

NO2 D hv // O3 (R-6)

(case-b) Under low VOC and high NOx condition (high NOx/VOCratio)

O3 D NO / NO2 D O2 (R-7)

NO2 D OH / HNO3 (R-8)

Fig. 4. The calculated the averaged values of O3, NOx, SO2, and CO in the NCITY (red columns) and RPBL (blue columns) cases, respectively. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

F. Geng et al. / Atmospheric Environment 43 (2009) 584–593590

F. Geng et al. / Atmospheric Environment 43 (2009) 584–593 591

when VOC concentrations are high, the oxidation of VOCs (R-4)produces high RO2 concentrations. In this case, increase in NOconcentrations (or NO emissions) leads to enhancement in thereaction of (R-5). As a result, the O3 chemical production isincreased with the increase in NOx concentrations. This situation isnormally defined as a ‘‘NOx-limited’’ condition for the O3 chemicalproduction. However, if VOC concentrations are low not enoughRO2 are produced throughout the reaction of (R-4). As a result, thereaction of NOþO3 is faster than the reaction of NOþ RO2, and theprocess can be explained by the case-b). In this case, increase in NOconcentrations (or NO emissions) results in decrease in O3

concentrations. This situation is normally defined as O3 chemicalproduction is under the ‘‘VOC-limited’’ condition (Sillman, 1995).

From the above analysis, the O3 chemical formation is stronglyrelated to NOx and VOC concentrations. Thus, it is important tounderstand not only NOx conditions, but also the VOC conditions inthe YRD region. Fig. 5 shows the in-situ measurements of VOCconcentrations. Because the VOC measurements use canisters, onlya few measurements can be taken during each flight, and the mostmeasurements were taken in the rural PBL region. The measuredresult indicates that the concentrations of VOCs are relatively small.The total VOC concentration is about 6 ppbv. Among the total VOCs,alkanes have highest values (2.8 ppbv). Aromatics and alkenes havethe concentrations of 0.9 and 0.8 ppbv, respectively. The percent-ages of alkanes, alkenes, aromatics, ketones, and other VOCs in thetotal VOCs are 52, 16, 17, 3, and 5%, respectively. As we described in

Fig. 5. The in-situ measurements of averaged values (upper panel) for alkenes, 1; aromaticsmeasured alkenes, 1; aromatics, 2; alkanes, 3; ketones, 4; and others, 5 to the total VOCs (

the above sections, under small VOC concentrations, the O3

chemical formation is normally under ‘‘VOC-limited’’ regime, underwhich O3 concentrations are reduced when NOx concentrations areenhanced.

Fig. 6 shows the correlation between O3 and NOx concentrationsfor all measured results collected from all flights. The result showsthat O3 and NOx are anti-correlated in both the NCITY and the RPBLcases. In the RPBL case, the O3 concentrations have a stronger anti-correlation than in the NCITY case. For example, in the RPBL case,when the NOx concentrations decreased from 20 to 5 ppbv, the O3

concentrations increased from 35 to 70 ppbv. The ratio of D[O3]/D[NOx] is about �2.3. By contrast, in the NCITY case, when the NOx

concentrations decreased from 60 to 20 ppbv, the O3 concentra-tions increased from 25 to 35 ppbv. The ratio of D[O3]/D[NOx] isabout �0.25. This result suggests that in both the rural PBL regionand the nearby city region, the O3 chemical formation is under the‘‘VOC-limited’’ regime. In addition, the O3 concentrations are moresensitive to NOx concentrations in the rural PBL region than in thecity region, which is mainly due to the small VOC concentrations ashighlighted in Fig. 5.

The complex relationship among NOx, VOC, and O3 concentra-tions can be seen from O3-isopleth diagram (Fig. 7). The O3-isoplethdiagram is constructed by the U.S. Environmental ProtectionAgency’s EKMA (empirical kinetic modeling approach) model(Dodge, 1977). The model calculates isopleths of peak O3 concen-trations under different NOx and VOC concentrations. When VOCconcentrations are relatively high and NOx concentrations are

, 2; alkanes, 3; ketones, 4; others, 5; and total VOCs, 6 (in ppbv). The percentages of thelower panel).

Fig. 6. The correlation between O3 (ppbv) and NOx (ppbv) concentrations for all flights.The blue-circled values highlight the measured O3 (ppbv) and NOx (ppbv) in the NCITYcase, and the red-circled values highlight the measured O3 (ppbv) and NOx (ppbv) inthe RPBL case. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

F. Geng et al. / Atmospheric Environment 43 (2009) 584–593592

relatively low (in the lower right portion of the figure; under thedot-dash line), the isopleths are oriented horizontally, and O3

chemical formation is limited by the availability of NOx (NOx-limited). In this case, changes in VOC concentrations have littleeffect on O3 concentrations, and increase in NOx concentrationslead to increase in O3 concentrations. By contrast, when VOC

Fig. 7. The O3-isopleth diagram. The red dot indicates the measured values of O3

(ppbv), NOx (ppbv), and total VOCs (ppbC), which are positioned in the O3-isoplethdiagram. (For interpretation of the references to colour in this figure legend, the readeris referred to the web version of this article.)

concentrations are relatively low and NOx concentrations arerelatively high (in the upper left portion of the figure; above thedot-dash line), O3 chemical formation is limited by the availabilityof VOCs (VOC-limited). In this case, changes in VOC concentrationslead to enhancement in O3 concentrations, and increase in NOx

concentrations lead to decrease in O3 concentrations. The red dot(for interpretation of the references to colour, the reader is referredto the web version of this article) represents the measured meanVOC and NOx concentrations during all flights, and it is clearlypositioned in the ‘‘VOC-limited’’ region. This information is usefulfor air pollution control strategy in the YRD region. For example, ifthe VOC concentrations increase to 75 ppbC in the future, O3

concentrations will exceed 80 ppbv, which is normally consideredto exceed the national standard for human’s health. However, ifonly NOx concentrations increase, O3 concentrations will bedecreased in the YRD region.

3.3. NOx and SO2 relation

As we described above, O3 is very sensitive to NOx concentra-tions in the YRD region, and it is very important to understand theorigination of NOx emissions in the YRD region. Fig. 8 shows thecorrelation between measured SO2 and NOx concentrations duringall flights. The result shows that NOx and SO2 are correlated to eachother, suggesting that NOx and SO2 are probably originated ina same emission source. As suggested by USEPA (1995), the SO2

emissions are mostly related to coal burning from area sources(including the usages by residences and small factories) and powerplants, and have small contributions from automobile exhausts.The strong correlation between SO2 and NOx concentrationssuggests that the NOx emission sources are mostly from area andpower plant sources. As suggested by Geng et al. (2007), thenumbers of automobile are rapidly increased in the PRD region,which will lead to strong impacts on the enhancement on VOCconcentrations. As a result, the O3 concentrations in the PRD regioncould rapidly increase in the future as indicted by the red-arrow(for interpretation of the references to colour, the reader is referredto the web version of this article) in Fig. 7.

Fig. 8. The correlation between SO2 (ppbv) and NOx (ppbv) concentrations for allflights.

F. Geng et al. / Atmospheric Environment 43 (2009) 584–593 593

4. Summary

The Yangtze River Delta (YRD) region located in east China ishighly urbanized with a cluster of large cities (including Shanghai,Hangzhou, Shuzhou, Wuxi, etc.). During the past 20 years, thisregion is undergoing a rapid increase in economical development.The rapid growing of urbanization could cause wide-rangingpotential consequences for weather and climate related urbanenvironments, such as air pollutions. This rapid change in urbanenvironment has important impact on human’s health and naturalecosystems. Thus, the consequences of this change in urban envi-ronment need to be assessed. However, during the past, onlylimited studies are conducted, and there are urgently needs for airpollutant studies in this region. In this study, in-situ aircraftmeasurements of O3, NOx (NOþNO2), CO, SO2, and VOCs in severalaircraft flights between 30/September and 11/October are analyzed.The measurements provide information of horizontal and verticaldistributions of air pollutants in the YRD region. These measure-ments are necessary in order to better understand the character-izations of air pollutants not only in cities, but also in itssurrounding areas. The results are summarized in the follows:

(1) The measured O3 concentrations range from 20 to 60 ppbv.These values are normally below the US national standard(84 ppbv), suggesting that at the present, the O3 pollutions aremodest in this region. The NOx concentrations have a largevariability, ranging from 3 to 40 ppbv. The SO2 concentrationshave very large variability, ranging from 1 to 35 ppbv. The COconcentrations are relatively high, and have less variability,ranging from 3 to 7 ppmv. The concentrations of VOCs arerelatively small, with the total VOC concentration of 6 ppbv.Among the total VOCs, alkanes have highest values (2.8 ppbv).Aromatics and alkenes have the concentrations of 0.9 and0.8 ppbv, respectively. The percentages of alkanes, alkenes,aromatics, ketones, and other VOCs in the total VOCs are 52, 16,17, 3, and 5%, respectively.

(2) The relatively small VOC concentrations and large NOx

concentrations indicate that the O3 chemical formation isunder ‘‘VOC-limited’’ regime in the YRD region. The measuredO3 and NOx concentrations are strongly anti-correlated, sug-gesting that enhancements in NOx concentrations lead todecrease in O3 concentrations.

(3) The O3 concentrations are more sensitive to NOx concentra-tions in the rural PBL region than in the city region. Forexample, the ratios of D[O3]/D[NOx] are �2.3 and �0.25 in therural PBL and in the city region, respectively. This resultsuggests that changes in NOx emissions could have strongeffects on O3 concentrations in rural areas of the YRD region.

(4) The O3-isopleth diagram study shows that the O3 concentra-tions could increase rapidly with increase in VOC concentra-tions in the YRD region. For example, if the VOC concentrationsincrease to 75 ppbC in the future, O3 concentrations couldexceed to 84 ppbv, which is normally considered as thenational standard for human’s health.

Acknowledgments

The authors are grateful to Dr. Xu Tang at Shanghai meteoro-logical Bureau for his support for the measurement. This research ispartially supported by China Meteorological Administration (CMA)

under Grant No. GYHY(QX)2007-6-36; Science and TechnologyAdministration of China under Grant No. 2006BAC12B00; Scienceand Technology Administration of Beijing City under Grant No.80710002; China Meteorological Administration (CMA) underGrant No. GYHY(QX)2007-6-19; Science and Technology Adminis-tration of Shanghai City under Grant No. 08230705200; NationalNatural Science Foundation of China (NSFC) under Grant No.40705046; Shanghai Meteorological Bureau (SMB) under GrantNos. YJ200702 and MS200707. The National Center for AtmosphericResearch is sponsored by the National Science Foundation andoperated by UCAR.

References

Bian, H., Han, S.Q., Tie, X., Shun, M.L., Liu, A.X., 2007. Evidence of impact of aerosolson surface ozone concentration: a case study in Tianjin, China. Atmos. Environ.41, 4672–4681.

Deng, X.J., Tie, X., Wu, D., Zhou, X.J., Tan, H.B., Li, F., Jiang, C., 2008. Long-term trendof visibility and its characterizations in the Pearl River Delta Region (PRD),China, Atmos. Environ. 42 (7), 1424–1435.

Dodge, M.C., 1977. Combined use of modeling techniques and smog chamber data toderive ozonedprecursor relationships. In: Dimitriades, B. (Ed.), InternationalConference on Photochemical Oxidant Pollution and its Control: Proceedings,vol. II. U.S. Environmental Protection Agency, Environmental Sciences ResearchLaboratory, Research Triangle Park, NC, pp. 881–889. EPA/600/3-77-001b.

Geng, F.H., Zhao, C.S., Tang, X., Lu, G.L., Tie, X., 2007. Analysis of ozone and VOCsmeasured in Shanghai: a case study. Atmos. Environ. 41, 989–1001.

Geng, F.H., Tie, X., Xu, J., Zhou, G., Peng, L., Gao, W., Tang, X., Zhao, C., 2008. Char-acterization of O3, NOx and VOCs measured in Shanghai, China. Atmos. Environ.42, 6873–6883.

Lei, W., Zhang, R., Tie, X., Hess, P., 2004. Chemical characterization of ozoneformation in the Houston–Galveston area. J. Geophys. Res. 109. doi:10.1029/2003JD004219.

Lei, W., de Foy, B., Zavala, M., Volkamer, R., Molina, L.T., 2007. Characterizing ozoneproduction in the Mexico City Metropolitan area, a case study using a chemicaltransport model. Atmos. Chem. Phys. 7, 1347–1366.

Sillman, S., 1995. The use of NOy, H2O2, and HNO3 as indicators for ozone–NOx-hydrocarbon sensitivity in urban locations. J. Geophys. Res. 100, 14175–14188.

Shanghai Municipal Statistics Bureau (SMSB), 2007. Shanghai Statistical Yearbook.China Statistical Press (in Chinese).

Stockwell, W.R., Middleton, P., Chang, J.S., Tang, X., 1990. The RAMD2.0 chemicalmechanisms for regional air quality modeling. J. Geophys. Res. 95, 16,343–16,367.

Streets, D.G., Yu, C., Wu, Y., Chin, M., Zhao, Z., Hayasaka, T., Shi, G., 2008. Aerosoltrends over China, 1980–2000. Atmos. Res. 88, 174–182.

Suzhou Statistics Bureau (SSB), 2006. Suzhou Statistical Yearbook. China StatisticalPress (in Chinese).

Tie, X., Brasseur, G., Zhao, C., Granier, C., Massie, S., Qin, Y., Wang, P.C., Wang, G.L.,Yang, P.C., 2006a. Chemical characterization of air pollution in Eastern Chinaand the Eastern United States. Atmos. Environ. 40, 2607–2625.

Tie, X., Li, G., Ying, Z., Guenther, A., Madronich, S., 2006b. Biogenic emissions ofVOCs and NO in China and comparison to anthropogenic emissions. Sci. TotalEnviron. 371, 238–251.

Tie, X., Madronich, S., Li, G.H., Ying, Z.M., Zhang, R., Garcia, A., Lee-Taylor, J., Liu, Y.,2007. Characterizations of chemical oxidants in Mexico City: a regional chem-ical/dynamical model (WRF-Chem) study. Atmos. Environ. 41, 1989–2008.

United States Environmental Protection Agency, 1995. Compilation of EmissionFactors, AP-42, fifth ed., vol. 1. USEPA.

Wilczak, J.M., Gossard, E.E., Neff, W.D., Eberhard, W.L., 1996. Ground-based remotesensing of the atmospheric boundary layer: 25 years of progress. In:Garratt, J.R., Taylor, P.A. (Eds.), Boundary-Layer Meteorology. Kluwer AcademicPublishers 25th Anniversary vol., 1970–1995.

Xiu, G.L., Jin, Q., Zhang, D., Shi, S., Huang, X., Zhang, W., Bao, L., Gao, P., Chen, B.,2005. Characterization of size-fractionated particulate mercury in Shanghaiambient air. Atmos. Environ. 39 (3), 419–427.

Yang, F., He, K., Ye, B., Chen, X., Cha, L., Cadle, S.H., Chan, T., Mulawa, P.A., 2005. One-year record of organic and elemental carbon in fine particles in downtownBeijing and Shanghai. Atmos. Chem. Phys. Discuss. 5 (1), 217–241.

Zhang, R., Lei, W., Tie, X., Hess, P., 2004. Industrial emissions cause extreme diurnalurban ozone variability. Proc. Natl. Acad. Sci. USA 101, 6346–6350.

Zhang, Q., Zhao, C., Tie, X., Wei, Q., Li, G., Li, C., 2006. Characterizations of aerosolsover the Beijing region: a case study of aircraft measurements. Atmos. Environ.40, 4513–4527.

Zhao, C., Peng, L., Sun, A., et al., 2004. Numerical modeling of tropospheric ozoneover Yangtze Delta region. Acta Sci. Circum. 24 (3), 525–533.