Regional ozone pollution and observation-based approach for analyzing ozone–precursor relationship during the PRIDE-PRD2004 campaign

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Atmospheric Environment 42 (2008) 6203–6218 www.elsevier.com/locate/atmosenv

Regional ozone pollution and observation-based approach for analyzing ozone–precursor relationship during the PRIDE-PRD2004 campaign Y.H. Zhanga,, H. Sua, L.J. Zhongb, Y.F. Chenga, L.M. Zenga, X.S. Wanga, Y.R. Xiangb, J.L. Wangc, D.F. Gaoa, M. Shaoa, S.J. Fand, S.C. Liue a

State Joint Key Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China b Guangdong Provincial Environmental Monitoring Center, Guangdong 510045, China c Department of Chemistry, National Central University, Chungli 320, Taipei, China d Department of Atmospheric Science, Sun Yat-sen University, Guangzhou 510275, China e Research Center for Environmental Changes, Academia Sinica, Taipei, China Received 5 October 2007; received in revised form 28 April 2008; accepted 8 May 2008

Abstract An intensive field campaign, Program of Regional Integrated Experiments of Air Quality over Pear River Delta (PRIDE-PRD2004), was carried out in Pearl River Delta (PRD) in October 2004 to provide an in-depth understanding and a comprehensive record of O3, PM2.5 and other air pollutants in this quickly developing region of China. Two super sites, one in Guangzhou urban center and the other at a downwind rural site Xinken, were extensively equipped with research-grade instruments. In addition, 16 stations of the PRD air quality monitoring network participated in the campaign to provide regional scale spatial coverage. We found high levels of O3 exceeding China’s National Standard throughout the PRD region and clear evidence of regional scale photochemical production of O3. Furthermore, the data suggested that in situ photochemical production was the primary source of high concentrations of O3 in the upwind area, and that the ozone photochemical production efficiency was highly non-linear with greater value at lower NOx. This nonlinear relationship between photochemical production of O3 and its precursors has an important implication for the O3 control strategy. The ozone–precursor relationship was examined by using an observation-based model (OBM). Large positive relative incremental reactivity (RIR) values for VOCs and negative values for NOx were found, suggesting that O3 production was usually in VOC-limited regime in both Guangzhou and Xinken sites, while reducing NOx could increase O3. However, we caution that this conclusion was reached in a campaign of limited time period and the OBM results could differ in other seasons. Finally, high concentrations of HONO were observed at the two super sites. Current known gas phase source of HONO could account for only a fraction of the observed values, particularly at the Guangzhou site. An unknown and presumed heterogeneous source of HONO would be required to sustain the observed concentrations.

Corresponding author. Tel.: +86 10 62756592; fax: +86 10 62751927.

E-mail address: [email protected] (Y.H. Zhang). 1352-2310/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.05.002

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At these high levels of HONO, the OH concentration and ozone production would be 2–4 times higher than the case with gas phase source only. r 2008 Elsevier Ltd. All rights reserved. Keywords: Regional ozone pollution; Ozone–precursor relationships; HONO; Observation-based model; PRD

1. Introduction Pearl River Delta (PRD) is one of the most populated city cluster areas in China. Fast economic development in this region has increased personal income, but also led to elevated concentrations of pollutants. The term air pollution complex is often used to describe the situation of severe air pollution that is characterized by complex interactions between primary pollutants from direct emissions and secondary pollutants, from atmospheric chemical–physical processes in both gaseous and aerosol phases (Shao et al., 2006). Ozone is one of the most severe air pollution problems in the world. It has serious adverse impacts on human health and ecosystems, and is very difficult to control (WHO, 2004). Large widespread emission sources in the PRD region (Streets et al., 2003; Richter et al., 2005) have resulted in high concentrations of ozone–precursors, namely NOx, VOCs and CO, and in turn led to high levels of oxidants (Zhang et al., 1998; Wang et al., 2003; Zhang, 2003; Shao et al., 2006). As early as October 1995, O3 pollution episodes were frequently observed in Guangzhou and its suburbs with concentrations over 100 ppbv during periods of stagnant conditions. Two halfmonth intensive experiments were conducted in 1998 to study the formation of high levels of ozone around Guangzhou. One was in June during the summer low ozone season, the other in October, the fall high ozone season. The design of the experiments was to have six measurement sites lined along the prevailing wind direction from north PRD, passing central Guangzhou, to south PRD (Zhang et al., 1999). The experiments revealed that Xinken, a rural site in south PRD, frequently had severe episodes of O3 pollution with concentrations as high as 150 ppbv in October. After that, more measurements aimed at O3 and its precursors were carried out to study the characteristics of the ozone pollution episodes in Hong Kong and PRD (Wang et al., 1998, 2001a, b, 2003; Zhang, 2004; Zhang et al., 2007). Their results reconfirmed that south PRD in general suffered the worst O3 pollution, particularly in autumn season when northerly winds and clear sky conditions prevailed.

A regional air quality model, comprehensive air quality model (CAMx) was used to simulate air quality for typical ozone episodes by Zhang (2003). NOx- and VOCs-limited regimes of O3 production were also studied by using ozone source apportionment technique (OSAT) in the CAMx model. The model results showed that ozone formation was controlled by ambient VOCs in the vast area between Guangzhou and Hong Kong. In addition, an observational-based model (OBM) was used to estimate the relative efficacy of reductions in NOx and VOC emissions for mitigating photochemical smog in Hong Kong and PRD (Zhang, 2004; Gao et al., 2007; Zhang et al., 2007). Again, O3 production was found to be in VOCs-limited regime at most sites in Hong Kong and southern PRD. Since OBMs use observed ambient concentrations of O3 and its precursors rather than relying on emission inventories, they tend to avoid some of the uncertainties inherent in emission-based models (EBMs) and thus provide a useful independent evaluation of results obtained by EBMs. However, large gaps and uncertainties remain in the knowledge of characteristics of regional O3 pollution and its mitigation strategies in PRD. Past investigations were fragmented and limited in scope and depth due to lack of human and instrument resources. Lack of adequate and accurate measurements of key species was a major concern. Measurements of VOCs were particularly lacking. For example, Zhang (2004) had to use estimated values of VOCs as an input parameter in the OBM. Inadequate spatial coverage in previous studies was also problematic as they focused in Hong Kong area. Since it is known that regional transport plays a key role in O3 episodes, it is essential to extend observations to Guangzhou and northern PRD region. Recently, high concentrations of HONO were observed in PRD (Su et al., 2008). HONO, depending how it is formed, could be a major source of OH radicals which play a pivotal role in the formation of high O3. Therefore, it is critically important that HONO is accurately measured in a comprehensive experiment to study regional O3 pollution and its mitigation strategies.

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Radiatively important aerosols can alter the radiation balance in the boundary layer and hence influence the ozone production (Dickerson et al., 1997). Concentrations of fine particles are known to be extremely high in the PRD (Wu et al., 2005). Their effect on the solar radiation can introduce substantial uncertainties in the photo-dissociation rates of O3, NO2, H2O2, CH2O, etc. Therefore, it is crucial to monitor the concentration, size distribution and optical properties of aerosols. In addition, the aerosol optical depth (AOD), solar UV flux and key photolysis rates (J values) also need to be measured to get a closure on the effect of aerosols on J values in the PRD. With the reduction of the above mentioned uncertainties in mind, the ‘‘Program of Regional Integrated Experiments of Air Quality over Pear River Delta (PRIDE-PRD2004)’’ was organized by Peking University to provide an in-depth understanding and a comprehensive record of O3, PM2.5 and other air pollutants in this quickly developing region of China. This campaign, collaborating with a number of European institutes, used many stateof-art instruments at two super sites. In addition, extensive spatial coverage was made possible by the participation of 16 stations of the PRD air quality monitoring network. For example, extensive highquality data of J values, VOCs and HONO were collected during the campaign. The wide spatial coverage of this campaign could also provide a clear picture of the ozone pollution on the regional scale. In this paper, we present major characteristics of air pollution observed at the two super sites. The regional distribution of oxidants in PRD is evaluated by using data from the PRD regional air quality monitoring network. Furthermore, based on high time resolution data of O3, NO, HONO and VOCs and photolysis rates of O3 and NO2 at the two super sites, the sensitivity of ozone production to its precursor is evaluated by using the OBM model (Georgia Institute of Technology (GIT)). Finally, the influence of aerosols on the reduction of solar UV flux and, in turn, the O3 photochemistry is estimated. 2. Measurements and methodology In October 2004, two super sites (Guangzhou and Xinken) and 16 routine monitoring sites were set up as parts of the PRIDE-PRD2004 campaign (Zhang et al., 2008). The Guangzhou site was located in the downtown area of Guangzhou city (23.131N,

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113.261E). The Xinken site was located in a rural area of large farmland near the Xinken village (22.611N, 113.591E) on the south coast of PRD. The routine monitoring sites were parts of the Guangdong Province and Hong Kong joint PRD regional air quality monitoring network. At the super sites and stations of PRD regional air quality monitoring network, O3, NO/NOy, NO/ NO2, SO2 and CO were measured by TECO commercial instruments TECO 49C, 42CY, 42C, 43C and 48C, respectively. Daily calibration was carried out at midnight for all these pollutants. The detection limits were 0.4 ppb and 50 ppt for NO/ NO2 and NO/NOy, respectively. Since the molybdenum converter was used in TECO 42C, the NO2 measurements would suffer interferences from other nitrogen-containing compounds such as PAN, HNO3, etc. So ‘NOn2 ’ will be used throughout the following text to indicate NO2 measured by TECO 42C. Fifty-five species of VOCs (in the range of C3–C12) were measured and identified by an online GC–FID (Wang et al., 2008). HCHO measurements in the Guangzhou site were performed from 26 to 30 October (by the methods developed by Ho and Yu, 2004). The sampling period ranged from 2 to 7 h and then interpolation was used to derive hourly HCHO concentrations. To get the HCHO concentration at Xinken, we then scaled the HCHO measurements in Guangzhou according to the concentrations of the total VOCs at both sites. Since the HCHO were measured on limited days, the averaged diurnal variations were used for all simulation days. HONO were measured by the coupled wet denuder sampling/ion chromatograph analysis system (WD/IC) developed by Peking University (PKU). The WD/IC used 25 mM Na2CO3 solutions as absorption solutions and had a detectable limit of 65 ppt (for details about this instrument, please see Su et al., 2008). An intercomparison between this WD/IC and a standard method long path absorption photometer (LOPAP) was carried in our PRIDE-PRD2006 campaign in July 2006. The inter-comparison showed that the difference between the two methods was about 18%, which might be caused by the interferences in a long inlet of our WD/IC. A linear transform was carried out to eliminate the interferences (details of which could be found in the work of Su et al., 2008). Photolysis rates of NO2 and O3 were measured at the Guangzhou site by filter radiometers (VolzThomas et al., 1996; Bohn et al., 2004). The

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radiometers were calibrated against reference spectroradiometer at Juelich before and after the campaign. The AOD, used in the J value simulation at Xinken, was measured by a Sun photometer (manufactured by Dr. Schulz and Partner GmbH, Buckow, Germany) at eight wavelengths from 380 to 1044 nm. Aerosol light absorption and scattering were measured by multi-angle absorption photometry, Thermo 5012 (MAAP) and nephelometer from TSI, respectively (Cheng et al., 2006). At stations of PRD regional air quality monitoring network, PM2.5 and PM10 were measured by using the R&P TEOM ambient particulate monitor. Performance of the PRD regional air quality monitoring network was evaluated by regular inter-comparisons between the stations and a wellequipped mobile station. Average accuracy and precision of data obtained in the network was better than 5%, which was well within the limits of 15% accuracy for gases and 10% for particulates required by the QA/QC manual. The high time resolution and quality-assured data provided a good opportunity for using the observational-based approach to analyze the relationship between ozone and ozone-precursors. The GITOBM was used to evaluate the effectiveness of various precursor control strategies in reducing ozone pollution. The GIT-OBM is a photochemical box model which uses observed data to determine

the sensitivity of ozone production to change in emissions of ozone-precursors. The sensitivity was expressed through the Relative Incremental Reactivity (RIR, Carter and Atkinson, 1989). RIR is the percent change in ozone produced per percent change in precursor emission, as shown in the following equation RIRðX Þ ¼

ðPO3 2NO ðX Þ  PO3 2NO ðX  DX ÞÞ=PO3 2NO ðX Þ DSðX Þ=SðX Þ

(1) where PO3 2NO is the net ozone formation and NO consumed, which was used as a measure of the ozone formation potential (Carter and Atkinson, 1987). X represented one or a group of specific VOCs species, NO or CO. S(X) was the integrated source function of species X (in ppb) emitted or transported to the measurement site. And DX was the change in the concentration of X as a result of change in S(X). PO3 2NO ðX Þ hence represented the ozone formation potential for different source functions over the evaluation period. GIT-OBM was designed to perform two-phase simulations. The first phase simulation assimilated the concentrations of specified species to calculate unspecified species and the source functions. With the source functions determined, the second phase simulation was carried out like an emission-based model. Then the RIR can be determined by changing

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source functions of certain ozone–precursors such as NOx, CO and VOCs. The input parameters in this study were similar to the work of Cardelino and Chameides (1995), except that we included HONO and J value measurements. Moreover, in order to reduce the large uncertainty in the photochemistry at low sun angles, we did not follow the 7:00–19:00 period in the simulation as Cardelino and Chameides (1995), but used 7:00–17:00 instead. 3. Results and discussion During the PRIDE-PRD2004 campaign, the PRD region encountered mainly stagnant weather conditions characterized by high-pressure systems with weak horizontal pressure gradients. The main direction of winds was from the north or northeast. October 2004 was relatively warm with less rain, which condition was favorable for photochemical smog formation. As a result, pollution episodes were observed frequently during the campaign. 3.1. Air pollution at two super sites Figs. 1 and 2 showed the time series of hourly concentrations of gaseous pollutants and PM2.5 at Guangzhou and Xinken, respectively. In first few days of October, air quality was generally good due to holiday week and the arrival of a cold front to

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PRD from the north on 30 September. However, a long continuous pollution episode starting from 6 October was observed at both super sites. Concentrations of primary pollutants SO2, NO and CO at the Guangzhou site were quite high even though they met national air quality standard. Two pollution episodes characterized by primary pollutants could be seen in Fig. 1, namely 10–12 October and 30 October–1 November. In the episode of 10–12 October, primary and secondary pollutants were high, peak hourly mean concentrations of NO, CO and SO2 were as high as 300 ppbv, 5 ppmv and 100 ppbv, respectively, in late evenings; and maximum hourly mean O3 concentration reached 110 ppbv in the afternoon of 11 October at the Guangzhou urban site. On the other hand, the episode of 30 October–1 November at Guangzhou was characterized mostly by primary pollution with rather low concentrations of O3. At the Xinken site, NO, SO2, CO and PM2.5 concentrations were lower than those at the Guangzhou urban site because there were few significant emission sources nearby. The most remarkable phenomenon at Xinken was persistent high concentrations of O3 over 100 ppbv in most of the days during the campaign with maximum hourly mean concentration of O3 up to 160 ppbv. In fact, at Xinken O3 concentration exceeded the national standard (200 mg m393 ppbv) in 76% of days (Fig. 2). PM2.5 at both the sites

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Fig. 2. Time series of observed O3, NOy, CO, SO2 and PM2.5 at Xinken station.

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Fig. 3. Average diurnal patterns of O3, PM2.5, HONO, NO, TVOC and CO at the Guangzhou and Xinken sites. Vertical bars are standard errors (for NO the negative errors are not plotted because of the limitation of log scale).

stayed high after 6 October, with average concentrations of 118 and 100 mg m3 at the Guangzhou and Xinken sites, respectively, during the campaign. Fig. 3 showed the average diurnal variations for O3, NO, HONO, CO, TVOC, SO2 and PM2.5 at the two super sites. Peak values of O3 hourly mean concentrations were 68 and 108 ppbv at Guangzhou and Xinken, respectively. NO, CO, TVOC, SO2 and PM2.5 had different diurnal variations at the two sites. In Guangzhou, NO, CO, TVOC and PM2.5 clearly had two peaks at 8:00 and 21:00, which was consistent with traffic flow and implied the impact of vehicular emissions. In Xinken, weak diurnal

variations were observed for NO, CO, TOVC, SO2 and PM2.5 with one peak at 8:00, but PM2.5 and CO had another peak value at 19:00, SO2 concentration was high throughout the daytime. HONO accumulated during the night with a peak in the early morning and decreased rapidly after sunrise. Its concentrations seemed scaled with the NOx level with a higher HONO in Guangzhou than in the Xinken site. Different diurnal variations of NO, CO, TVOC, SO2 and PM2.5 suggested that air pollution at Xinken might be caused by a combination of local emissions and regional transport. However, it was difficult to quantify their

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relative contributions based on ground observations alone. 3.2. Regional ozone pollution in PRD To examine the regional extent of ozone pollution, the data from the PRD regional air quality monitoring network and a few stations of Guangzhou municipality were used. After a careful data evaluation based on the QA/QC protocols and the equation of photochemistry stationary state defined by the fast photochemical reactions among NO, NOn2 and O3 measurements at CH (Cong Hua), HD (Hua Du), LH (Lu Hu), 86Z (86 Zhong), PY (Pan Yu), WQS (Wan Qing Sha), ZML (Zi Ma Ling), JM (Jiang Men), JGW (Jin Guo Wan), CW (Central West), TC (Tung Chung), YL (Yuen Long), TM (Tap Mun) and TW (Tsuen Wan) stations (Fig. 4) were determined to be reliable and accepted in the following analysis. A clear picture of the regional nature of the PRD O3 pollution emerged. In October, CH, HD, JGW and TM were in the upwind area of the prevailing northwest and northeast winds. Relatively high daytime mean O3 concentrations of 71–77 ppbv were found in the upwind area, suggesting that the entire PRD was severely polluted by anthropogenic emissions. At

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sites downwind of the mega-city Guangzhou, O3 concentrations were elevated substantially to 73–100 ppbv in WQS, ZML, JM and TC. Actually, ozone concentrations exceeding the national standard occurred at all 14 sites listed above. The frequency of days with ozone exceeding the national standard varied between 6% and 86% with an average value of 43%. This showed again that the ozone pollution in PRD was not limited to certain area but spread across the large PRD region. The results from the network was the first evidence to show that regional burden of O3 was high and southern PRD suffered the most serious photochemical smog pollution in PRD. Fig. 5 showed the 1-month average diurnal variations of O3, NOn2 and Ox (O3+NOn2 ) at 14 sites. Here we defined ‘‘total oxidant’’ as Ox following the suggestion of Liu (1977) and Levy II et al. (1985). Based on their definition, Ox ¼ O3+ O+O(1D)+0.5OH+0.5HO2+H2O2+NO2+2NO3 +3N2O5+1.5HNO3+1.5PAN. With the molybdenum NO2 converter, assuming Ox ¼ O3+NOn2 would mean Ox ¼ O3+NO2+ NO3+2N2O5+HNO3+PAN because the converter also converts NO3, N2O5, HNO3 and PAN. So the molybdenum NO2 would make Ox ¼ O3+NOn2 closer to the real Ox. In practical application, only

Fig. 4. Regional distribution of 8 h average O3 concentration and PM2.5 during the campaign. The data of different sites are given as O3 concentration in ppbv/PM2.5 concentration in mg m3. Abbreviated’ names of the sites are listed below: XK-Xinken and GZ-Guangzhou represented the two supersite and marked with solid rectangles. CH-ChongHua, HD-HuaDu, LH-LuHu, PY-PanYu, JM-JiangMen, ZML-ZiMaLing, YL -Yuen Long, TC-TungChung, TW-TsuenWan, TM-TapMun, CW-Central Western, JGW-JingGuoWan, WQSWanQingSha and 86Z—The 86th Middle School in Guangzhou.

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Fig. 5. Average daytime variations of NOn2 , O3 and Ox (O3+NOn2 ) (in ppbv) at the monitoring sites of PRD. CH, HD, JGW and TM sites are upwind sites; WQS, ZML, JM and TC sites are downwind sites; and the rests are urban sites. As elaborated in the text, the three groups are clearly separated by their values of Ox.

NO2 is the essential component of Ox because the photochemical exchange between O3 and NO2 is fast and the magnitude of NO2 can be significant compared to O3. The other terms of Ox are either small compared to O3+NO2 or they photochemically exchange with O3, significantly slower than NO2 such that they can be neglected from Ox. Therefore, Ox ¼ O3+NO2 is frequently used to study the O3 chemistry in local, regional and global scales (Logan, 1989; Jacob, 1999; Sillman, 1999; Finlayson-Pitts and Pitts, 2000; Kleinman et al., 2002; Molina and Molina, 2002). A major advantage of this definition is that its production and loss are independent of the rapid photochemical reactions which convert O3 to NO2 and vice versa. Since these reactions are fast in the urban and suburban atmosphere, bypassing them enable the Ox production to represent the real photochemical production of ozone (Shiu et al., 2007). In other words, Ox concentration is a better measure of the real production of O3 than O3 itself. This notion was nicely demonstrated in Figs. 5 and 6 which showed that observation sites could be clearly classified into three groups, namely northeast upwind area, urban area and downwind southern PRD according to the concentrations of Ox (Figs. 5c and 6c), but not O3

(Figs. 5b and 6b). One can see that Ox clearly displays an increasing trend from north to south PRD, with daytime average mixing ratios of 79717, 92723 and 116713 ppbv (mean7s of each measurement) in northeast upwind area, urban area and downwind southern PRD, respectively (Fig. 5c). This trend of increasing Ox along the direction of the prevailing wind is consistent with the fact that there is widespread regional scale in situ photochemical production of oxidant. This type of trend does not exist in the O3 distribution because some urban stations have lower O3 values than upwind stations due to the titration effect of NO in urban area that suppressed O3. Both Ox and O3 in downwind southern PRD were extremely high, with daytime averages of 116713 and 87722 ppbv, respectively. In this context, aircraft measurements also corroborate the ground measurements showing a high level of O3 pollution in the southern PRD (Wang W. et al., 2008). A remarkable feature in Figs. 5 and 6 was the high concentrations of O3 and Ox in the upwind area. Average daytime concentrations of O3 and Ox were 73711 and 79717 ppbv, respectively. As noted earlier, the value of upwind O3 was even higher than the average daytime level of O3 in urban

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Fig. 6. Average daytime variations of NOn2 , O3 and Ox (O3+NOn2 ) (in ppbv) at the upwind, urban and downwind sites.

area. Ox which is a good measure of the real production of ozone was only about 15% less in the upwind area than the urban area. The high concentrations of O3 and Ox in the upwind area can only come from two processes; one is in situ photochemical production and the other transport from the urban area. The latter is unlikely because the urban area is downwind in the prevailing wind system; and local circulations such as land–sea breeze usually does not reach more inland than Guangzhou (Dudhia et al., 2003; Zhou, 2007). This is substantiated by the distribution of NOn2 shown in Fig. 5, which shows upwind NOn2 to be the lowest while downwind NOn2 very close to the urban value. If transport from the urban area to the upwind area were effective, NOn2 at the upwind area would have been close to the urban value rather than a factor five lower. This leaves the in situ photochemical production as the only possible source of the high concentrations of O3 and Ox in the upwind area. However, the average daytime concentration of NOn2 in the upwind area was only about 5 ppbv, in order to produce the high concentrations of O3 and Ox, the ozone production efficiency (O3 produced per unit NO2 consumed) would have to be substantially higher than the urban area where NOn2 was about 30 ppbv. In other words, the ozone photochemical production efficiency was highly

non-linear and was greater at lower NOx. This conclusion is consistent with a number of previous investigations of the ozone photochemical production efficiency in urban and rural areas (Liu et al., 1987; Seinfeld and Pandis, 1998; Jacob, 1999; Sillman, 1999). The non-linear relationship between photochemical production of O3 and O3 precursors obviously has important implications to the O3 control strategy. A quantitative understanding of the nonlinear relationship is the key to formulating an effective O3 control strategy. In the following, we examine the ozone–precursor relationship with the help of the GIT-OBM. 3.3. Ozone– precursor relationships in PRD Days with complete high time resolution measurements of gaseous pollutants can be used to perform the OBM simulations to study the ozone– precursor relationship. These included 17 d at the Guangzhou site and 11 d at the Xinken site. The precursors of ozone were divided into four groups: anthropogenic hydrocarbons (AHC), natural hydrocarbons (NHC, isoprene only), NO and CO. For the input of OBM, gaseous species with 1 and 5 min time resolution were integrated to get hourly values. The photolysis rates (J) of NO2 and O3 were

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Fig. 7. J (NO2) values: shaded were measured at Guangzhou, solid circles simulated at Xinken and lines simulated for background site.

measured at the Guangzhou site, while AOD was measured at Xinken. A tropospheric ultraviolet– visible (TUV) radiative transfer model was used to calculate J values at Xinken based on measured AOD (550 nm) and single scattering albedo (SSA, 550 nm) (for TUV, see http://www.acd.ucar.edu/ TUV). As shown in Fig. 7, the TUV model results at Xinken were in good agreement with J measurements at Guangzhou with average differences o5% (some large differences in J values, e.g. on 21 October may be due to the differences in cloudiness for the two sites). The good agreement between the simulated and measured J values at two separate sites was consistent with the satellite observation of relative homogenous AOD distribution in PRD during autumn season (Wu et al., 2005). Thus, it was reasonable to assume that J values were the same at Guangzhou and Xinken. So J values measured at Guangzhou can be applied to OBM simulations at both sites. Most of the conditions and parameters in the OBM were the same as in the work of Cardelino and Chameides (1995), except that in our study the measured HONO and J values were used. Fig. 8 summarizes the RIRs for O3 precursors at the two super sites. O3 production in Guangzhou was always in VOC-limited regime, same as urban areas elsewhere. At Xinken, VOCs showed significant contributions to O3 production, while RIR of biogenic VOCs (NHC, only isoprene) and CO are positive but small. In most days, NOx was negatively correlated with O3 production in Xinken, indicating NO ‘‘dis-benefited’’ O3 production.

Although Xinken was far from large emission sources and local emissions were insignificant, relatively high NO concentrations were found in most days during the campaign. It was noted that NO concentration at Xinken on 21 October was much lower and resulted in a positive RIRNOx , i.e. in a mild NOx-limited regime. From Fig. 8, it was obvious that O3 production at both sites was within the VOC-limited regime in general since RIRAHC was much larger than other species. NOx disbenefiting effects on O3 production were found at both sites most of the time, indicating that reductions in NOx emissions in the area would actually lead to an increase in O3 concentration. This conclusion is supported by results of the regional air quality model simulated for 20–24 October, which showed that O3 formation was VOC-limited in the entire PRD, particularly in southern region (Cheng, 2007). Contributions of individual hydrocarbons were calculated based on the chemical mechanism of carbon bond-IV (CB-IV). Fig. 9 showed the top 10 AHC species with highest RIR. At Guangzhou, propylene concentration was quite high and thus had the highest RIR among all the VOCs species, while in Xinken its contribution was negligible compared to toluene or m+p-xylene. Propylene was one of the major VOCs species in vehicular emissions and it may also come from leakage of the liquid petroleum gas (LPG) vehicles as well as other LPG usages. Aromatic VOCs, e.g. toluene and m+p-xylene, were the important reactive species at both Guangzhou and Xinken according

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to their contribution to O3 production, particularly toluene, m+p-xylene, o-ethyltoluene and o-xylene were the dominant reactive species at Xinken. From the source apportionment of the VOCs species by receptor model (Liu et al., 2008a), we found that although aromatic species were important at both super sites, their origins were different. At Guangzhou site, most of toluene and m,p-xylene (40–60%) could be attributed to gasoline vehicular exhaust, 27–46% were associated with the application of solvents and coatings. At Xinken, about 30–40% toluene and m,p-xylene can be attributed to gasoline emissions, over half (57–69%) of them were related to the evaporative emissions from industrial coatings in upwind area (Liu et al., 2008a). Combining results from OBM and receptor model (Liu et al., 2008a), and OH loss rate of VOCs (Liu et al., 2008b), it can be concluded that traffic emission and solvent coatings were the major sources for reactive VOCs at the two super sites. An important finding in our OBM study is that both the Guangzhou urban site and Xinken downwind rural site are in VOC-limited regime for O3 formation, while reducing NOx can increase O3. This is remarkable given the large differences in concentrations of O3 precursors, namely NO, VOC and CO (Fig. 3), and in the compositions of VOC at the two sites (Figs. 8 and 9). Fig. 3 also shows that the diurnal variation of ozone at Guangzhou exhibit typical features of urban atmosphere with clear titration effect by NO at dusk and suppressed peak value in the early afternoon, while ozone at Xinken site stays around 20–30 ppb at night and has a much prominent peak ozone value in the day. Furthermore, as shown in the following two subsections, production rates of total oxidants at the two sites are also substantially different. In this context, the fact that Hong Kong which is farther downwind from Guangzhou during autumn (albeit with its own urban emissions), is also in VOClimited regime for O3 formation (Zhang et al., 2007) is even more remarkable. Is this coincident or the VOC-limited regime much more robust and widespread than previously believed? We think that the answer is the latter. The primary process contributing to the widespread VOC-limited regime is that the total oxidants in various parts of PRD (including Hong Kong) are closely coupled to one another in terms of their formation and transport. In other words, the PRD ozone problem is a regional phenomenon with scale around a few hundred kilometers or more.

The above finding is extremely important to the design of an effective O3 control strategy for the PRD. Our results suggest that VOC control would be most effective for controlling ozone in nearly the entire PRD, while NOx control would be counterproductive. In this context, as noted earlier, concentrations of O3 in the upwind region are high. They can contribute significantly to the O3 in both urban and downwind regions. Therefore, an effective O3 control strategy also needs to consider the O3 formation in the upwind region. 3.4. Impact of HONO on O3 production The importance of HONO in the atmosphere has been discussed for almost 30 yr since its first observation by differential optical absorption spectroscopy (DOAS) in 1979 (Perner and Platt, 1979). Recent observations found that the daytime HONO may be up to hundreds of pptv (Ren et al., 2003; Kleffmann et al., 2005; Acker et al., 2006). At these high levels of HONO, it is easy to see that HONO, depending on the nature of its sources which are poorly known, can play a significant role in the abundance of the pivotal OH radicals and thus the photochemistry in boundary layer (Su et al., 2008). In previous investigations, the GIT-OBM model did not include HONO because the known gas phase photochemical source of HONO could sustain only a trivial amount of HONO (Cardelino and Chameides, 1995, 2000; Zhang, 2004; Zhang et al., 2007). We studied the effect of HONO by specifying it at the observed value in the OBM. In Fig. 10, we compared the simulated PO3 2NO and OH with specified HONO (Scenario C) to the case without specified HONO (Scenario A). In Scenario A, only the gas phase source of HONO, namely the reaction of NO with OH, was considered, while Scenario C used the observed HONO as input. So the difference between the two scenarios can be interpreted as the contribution of heterogeneous source of HONO. As shown in Fig. 10, ignoring the heterogeneous source of HONO led to a large reduction in both simulated OH concentrations and PO3 2NO . For example, OH peak was reduced by 81% in Guangzhou and 57% in Xinken while the PO3 2NO peak decreased by 81% in Guangzhou and 59% in Xinken. This means the heterogeneous sources of HONO will lead to 2–4 times increase of OH and PO3 2NO . Results from both sites revealed the importance of heterogeneous sources of HONO to

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photochemical processes in this region. If these heterogeneous productions of HONO could be attributed to reactions on the aerosol surface, our results would suggest the importance of aerosols in photochemistry for a heavily polluted region like PRD. In addition, we investigated the impact of HONO on the results of OBM, especially the calculated RIRs. A comparison between the two scenarios with and without HONO as a specified species showed that the absolute RIR values for each O3 precursor were changed by different amounts. However, the relative contributions to the ozone production among species and the dominant limited species were about the same. Therefore, even though previous studies did not include the observed HONO in their OBM simulation (Zhang, 2004; Gao et al., 2007; Zhang et al., 2007), their conclusion regarding the NOx-or VOC-limited regime of O3 production in PRD should not differ from the conclusion reached here. It should be noted that the effects of HONO discussed above applies only to the surface layer where we have measurements of HONO. Above the surface layer, the effects could be significantly smaller if the HONO concentration is lower there. Clearly, it is important to have measurements of HONO above the surface layer. Of course, the real

solution to the HONO problem is to understand its source(s). 3.5. Impact of aerosols on O3 production The dense anthropogenic aerosol layer over PRD observed by MODIS showed an AOD higher than 0.6 (Wu et al., 2005). During PRIDE-PRD2004, the aerosol concentrations in both Guangzhou and Xinken sites were quite high with average value over 100 mg/m3, and the visibility was o10 km in most of days during the campaign. At Xinken, Raman lidar and Sun photometer measurements showed an average AOD of 0.92 (Ansmann et al., 2005). High aerosol loading can block solar radiation and reduce actinic flux at lower troposphere, and as a consequence, photochemical processes can be slowed down. Fig. 10 depicted the impact of aerosols on photochemistry at Xinken and Guangzhou. In Guangzhou sites, the J values were measured by filter radiometers, while at Xinken it was simulated with the TUV model driven by 550 nm AOD and SSA observed (Su et al., 2008). As shown in Fig. 7, on average surface J (NO2) and J (O1D) values had been reduced by 33% and 34% compared with the background atmosphere (AOD ¼ 0.235, from continental aerosol given by Elterman, 1968).

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A simple way to study aerosol effects is to replace the measured J values with the background ones and then compared the results of the two cases. Under background J values, more oxidants would be produced and their concentration would increase, however, simulations in the OBM with species concentration unchanged were not realistic in this case. So we kept the measured J values in the first simulation in OBM when calculating the actual source functions and just replaced the J values in the second simulation which was driven by initial conditions and source functions, and did not depend on observed concentrations. The OH variation and PO3 2NO in the second run were then used to do the comparison. Fig. 10 showed the OH and PO3 2NO with and without aerosol layers (Scenarios C and B). Both of OH and PO3 2NO increased with background J values. Maximum OH had been increased by 24% (Guangzhou) and 69% (Xinken), while the PO3 2NO peak increased by 29% (Guangzhou) and 51% (Xinken). This meant the change of radiation balance caused by aerosols led to 19% and 22% reduction in surface OH and PO3 2NO at Guangzhou, 41% and 34% reduction at Xinken, respectively, compared with the background atmosphere. Similar to the effects of HONO, the effects of aerosols on OH and other photochemical processes discussed above applies only to the surface layer where we have measurements of aerosols and their optical properties. Above the surface layer, aerosols could either accelerate or inhibit photochemical reactions and ozone production depending on the vertical distribution and single scattering albedo of aerosols (Dickerson et al., 1997). 4. Summary and conclusions After economic growth over nearly three decades, PRD is suffering from a serious air quality degradation problem, which is characterized by coexistence of high concentrations of O3 and PM2.5 as well as other airborne pollutants throughout the region. The PRIDE-PRD2004 campaign was designed to provide an in-depth understanding and a comprehensive record of O3, PM2.5 and other air pollutants in this quickly developing region of China. This campaign, lead by Peking University and collaborated with a number of European institutes, used many state-of-art instruments at two super sites, an urban Guangzhou site and a downwind

rural Xinken site. Extensive spatial coverage was made by the participation of 14 regular monitoring stations of Guangdong Province Environmental Protection Bureau (EPB) and Hong Kong Environmental Protection Division (EPD). In addition to O3, ‘‘total oxidant’’ Ox (O3+NOn2 ) of which the concentration is a good measure of the real photochemical production of O3 was used to characterize the spatial distribution of O3. We found the severe O3 pollution problem throughout the PRD region. O3 pollution event exceeding 93 ppbv (China’s Grade II Standard) occurred at all 14 sites during the campaign. Ox clearly displayed an increasing trend along the prevailing winds, with daytime average mixing ratios of 79.3, 91.8 and 116.4 ppbv in northeastern upwind area, urban area and downwind southern PRD, respectively. This trend of increasing Ox along the direction of the prevailing wind is consistent with the fact that there is regional scale photochemical production of O3. Furthermore, we found that in situ photochemical production was primarily responsible for the high concentrations of O3 and Ox in the upwind area, and that the ozone photochemical production efficiency is highly non-linear with greater value at lower NOx, consistent with previous findings (e.g. Liu et al., 1987; Seinfeld and Pandis, 1998). The non-linear relationship between photochemical production of O3 and its precursors has important implications to the O3 control strategy. The ozone–precursor relationship was explored by using the GIT-OBM. The precursors of ozone were divided into four groups: AHC, NHC (isoprene only), NO and CO. Large positive RIR values for VOCs and negative ones for NOx suggest that O3 photochemistry was usually in VOC-limited regime in both Guangzhou site and Xinken. Negative RIR for NOx at the two sites suggested that a reduction in NOx emissions in PRD would lead to O3 increase in entire PRD region. However, we caution that the conclusion was reached in a campaign of limited time period and the OBM results could differ in other seasons. According to the OBM, propylene, toluene and m+p-xylene, mainly from traffic emission, were the primary reactive VOCs species in Guangzhou urban, while toluene and m+p-xylene transported from sources upwind were the major reactive VOCs species at Xinken. High concentrations of HONO were observed at Guangzhou site and Xinken. Current known gas

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phase photochemical source of HONO could account for only a small fraction of the observed concentrations of HONO due to its fast photodissociation rate, particularly at Guangzhou site. An unknown and presumed heterogeneous source of HONO would be required to sustain the observed concentration. By specifying the concentration of HONO in the OBM to the observed value, we found that the OH concentration and ozone production were increased by 2–4 times at the two sites over the gas phase source only case. Therefore, the observation of unexpected high concentrations of HONO in the PRD might be an indication of an important missing source of OH radicals, the primary reagent of atmospheric oxidation capacity. Acknowledgment This study was mainly supported by China National Basic Research and Development Programs 2002CB410801 and China National Science Foundation (Project No. 40675072). Reference Acker, K., Moller, D., Wieprecht, W., Meixner, F.X., Bohn, B., Gilge, S., Plass-Dulmer, C., Berresheim, H., 2006. Strong daytime production of OH from HNO2 at a rural mountain site. Geophysical Research Letters 33 (L02809). Ansmann, A., Engelmann, R., Althausen, D., Wandinger, U., Hu, M., Zhang, Y.H., He, Q., 2005. High aerosol load over the Pearl River Delta, China, observed with Raman lidar and Sun photometer. Geophysical Research Letters 32 (L13815). Bohn, B., Kraus, A., Mueller, M., Hofzumahaus, A., 2004. Measurement of atmospheric O3-O (1D) photolysis frequencies using filterradiometry. Journal of Geophysical Research 109 (D10S90). Cardelino, C.A., Chameides, W.L., 1995. An observation-based model for analyzing ozone–precursor relationships in the urban atmosphere. The Journal of the Air and Waste Management Association 4S, 151–180. Cardelino, C.A., Chameides, W.L., 2000. The application of data from photochemical assessment monitoring stations to the observation-based model. Atmospheric Environment 34, 2325–2332. Carter, W.P.I., Atkinson, R., 1987. An experimental study of incremental hydrocarbon reactivity. Environmental Science and Technology 21, 670–679. Carter, W.L., Atkinson, R., 1989. Computer modeling study of incremental hydrocarbon reactivity. Environmental Science and Technology 23, 864. Cheng, Y.F., Eichler, H., Wiedensohler, A., Heintzenberg, J., Zhang, Y.H., Hu, M., Herrmann, H., Zeng, L.M., Liu, S., Gnauk, T., Brueggemann, E., He, L.Y., 2006. Mixing state of elemental carbon and non-light-absorbing aerosol components derived from in situ particle optical properties at Xinken

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