nonmethane hydrocarbon measurements at a suburban (1)

Nonmethane hydrocarbon measurements at a suburban site in Changsha City, China Jungang Zhang a , Yuesi Wang a, ⁎ , Fangkun Wu a , Hong Lin a , Weidong Wang b a LAPC, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China b Institute of Substropical Agriculture, Chinese Academy of Sciences, Changsha, 410125, China abstractarticle info Article history: Received 10 December 2008 Received in revised form 8 July 2009 Accepted 9 July 2009 Available online 24 October 2009 Keywords: GC–MS Air quality Propane Wind influence Carbon monoxide South-central China The concentration, composition, and variability of nonmethane hydrocarbons (NMHCs) and carbon monoxide (CO) were characterized in a suburban region of south-central China. Weekly samples were collected in 2007 in the Changsha suburban area and analyzed with a three-stage preconcentration method coupled with GC–MS. A time series of NMHC measurements showed seasonal variation, with a higher level occurring in winter and a lower level in summer.Toluene was the most abundant species with an average concentration of 2.51± 1.87 ppbv, followed by benzene (2.04±1.30 pptv). According to the level of identified NMHCs, vehicular exhaust appears to be the main source of NMHCs in Changsha. Among alkanes, the highest level is propane with a concentration of 1.31± 0.71 ppbv, it indicated an extensive use and leakage of liquefied petroleum gas (LPG) in Changsha. The concentrations of NMHCs were influenced by the wind direction; a high level of NMHCs was carried by winds from southern China. Significant biogenic isoprene emissions were observed, with good correlation between isoprene level and temperature. Finally, when the typical individual NMHC species and CO in the morning and afternoon were compared, the shorter lifetimeof NMHC species relative to CO could explain the poorer correlation observed in the afternoon. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Nonmethane hydrocarbons (NMHCs) refer to light hydrocarbons containing 2–12 carbon atoms (Watson et al., 2001). NMHCs in urban areas can deteriorate the air quality and adversely affect human health, even at trace levels. They are important precursors of ozone and other photochemical oxidants in the atmosphere. Aromatic compounds comprise an important NMHC fraction that participates in the photochemical reactions leading to formation of secondary organic aerosols (Odum et al., 1997). NMHCs a r e well kno wn to arise from bo th mobile and stationary sources. Although anthropog enic sour ces ar e d ominant in u rban ar eas, biogenic sources such as v egetation, soil, and the ocean ar e also significant contribut ors (Guenther et al., 1 995). For e xample, isoprene is primarily considered t o be an emission from veg etation sour ces. In Bei jing,11% o f the isoprene in VOCs accounted for 23% of the total potential for ozone formation (Xie et al., 2008). Anthropogenic sources include vehicle exhaust, gasoline ev aporation, sol v ent usage , fuel combusti on, and industrial processes. V ehicular activity emissions, such as vehicle exhaust nd gasoline evaporation, are the important sources of NMHCs in urban areas (Bar letta et al., 2005; Song et al., 2007; Wang et al., 2005). With its rapidly growing economy, China now faces severe envi- ronmental issues, especially regarding air quality degradation in metropolitan areas. Much research on air pollution has been conducted in the cities in China, including those in the Pearl River Delta (PRD), the Yangtze River Delta (YRD), and Northern China (Barletta et al., 2008; Chan and Yao, 2008; Chan et al., 2006; Geng et al., 2007; Liu et al., 2007; Liu et al., 2005; Tang et al., 2007; Wang et al., 2006; Xie et al., 2008). These three regions are the fastest growing industrial regions in China; however, relatively few studies have been carried out in other regions, such as central China. Changsha is the capital city of Hunan province, locat ed i n S outh-central China. T he other two important cities, Zhuzhou and Xiangtan, are located to the south of Changsha. These three cities are the most developed cities in Hunan province. About 50 km apart, Changsha, Zhuzhou, and Xiangtan haveacombinedpopulationof3millionintheurbanareas,withatotal population of 1 2million in all three regions. The gross domestic product (GDP) of the three cities accounted for about 38% of the total of Hunan province (Hunan Statistical Y earbook 2007, 2008). These cities were officially designated as “national pilot zones for developing an environment-friend ly and a res ourc e-sa ving society” in December 2007. With the rapid urbanization and industrialization of cities, pollutant emissions have increased significantly, resulting in severe degradation of air quality in Chinese cities. In Changsha, vehicular exhaust is the major emission source of air pollutants. For the past few years, the vehicular fleet in Changsha has grown by about 10% per year, which is registered at about 230,000 private cars, 221,000 motorcycles, and 30,000 other vehicles, as of 2006. In general, most cars are gasoline fueled, while 45% of the buses use diesel. Since 2006, the government has implemented LPG as the fuel for taxis and buses. In China, coal use is very common for cooking and as a heating supply, especially in winter. Coal combustion can emit a substantial amount of NMHCs, including manyaromatic compounds (Moreiar dos Santos et al., 2004; Garcia et al., Science of the Total Environment 408 (2009) 312–31 7 ⁎ Corresponding author. Tel.: +86 10 82080530; fax: +86 10 82028726. E-mail address: wys@dq.cern.ac.cn (Y. Wang). 0048-9697/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2009.07.010 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv 1992). Coal is the major energy source, accounting for 60% of the total energy used in Changsha. Statistical data show that 4.5million tons of coal, 0.26 billion cubic meters of natural gas, 9.3thousand tons of LPG, and501 thousand tons of oil products were consumed in 2006. Beginning in 2001, Changsha initiated coal combustion control in the urban areas, and promoted the use of clean energy. In recent years, the air quality in Changsha city has shown a large improvement. The daily average of SO 2 has decreased from 135μgm − 3 in 2000 to 65μgm − 3 in 2007. The national air quality grade II standard attainment days reached 302 days in 2007. Changsha was appointed as one of the first batch of pilot cities for the Clean Air Initiative for Asian Cities (CAI-Asia) China Project in 2006 (CAI-Asia China Project, 2005). In this study, we present the results of NMHC and carbon monoxide (CO) measurements in a suburban area of Changsha. Measurements were determined on whole air samples collected in Changsha in 2007. 2. Experimental 2.1. Site descriptions Changsha is located in eastern Hunan province, in South-central China. The Xiangjiang river, a branch of the Yangtze River, flows through the city. Changsha is surrounded by mountains to the south, west, and east. It covers an area of 11,800km and has a population of about 6 million. Over the past 10 years, the mean temperature in Changsha was 17.0 °C, with the lowest monthly temperature at 4.4 °C in January and the highest temperature at 28.6 °C in July (Hunan Statistical Yearbook 2007, 20 08 ). The sampling site was locat ed on the roof of a three-story building (about 10 m above gr ound level) at the Institute of Subtropical Agriculture, Chinese Academy of Sciences, which is in a suburban site in the eastern section of Changsha. The site w as locat ed in a sparsely populat ed ar ea that has light local emissions, far away from t he most commercial and industrialized areas of Changsha. The Bejing–Zhuhai Highway is about 2.5 km to the west. T he climatic char acteristics c orrespond t o a continental subtropical monsoon climate, with a northwestern wind in winter and a summer s outheastern wind. A detailed geographical description of Changsha city and the sampling sit e is shown in Fig. 1. 2.2. Sampling and analysis Ambient air samples were drawn from the gas inlet, using a pump, through a PFA-Teflon tube (OD: 1/4in.) and collected in pre-evacuated 1 L electropolished canisters. Air samples were collected twice daily every Tuesday, from 2nd January to 25th December 2007. A 1 hour integrated sampling for each canister sample was taken from 8:00 to 9:00 in the morning and 13:00–14:00 in the afternoon. Ambient air was drawn in at a flow rate of 1L min − 1 . After the canisters were pressurized to 60psig, the valve was closed and the pump turned off. A total of 52 samples were collected. The canister samples were shipped in cardboard containers at an ambient condition to the Institute of Atmospheric Physics (IAP) and analyzed within one week after collection. Meteor- ological parameters, such as wind direction, wind speed, temperature, relative humidity, and dew point, and the weather conditions were also recorded during the sampling period. Volatile organic compound (VOC) measurements were conducted using a 7100A preco ncentrator ( Entech In c.) followed by gas chromatography and mass selective detection. The details of the VOC analysis procedures have been described previously by Mao et al. (2008) and here will be given a brief description. Fig. 1. The sampling site of Changsha, central-south of China. The grey line represents the arterial road; the blue regions refer to the river. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 1 The concentrations of NMHCs compounds in Changsha in 2007 (n=96). Compounds (all values in pptv) Formula Median Mean IQR Propene C 3 H 6 775 1051 469–1506 Propane C 3 H 8 1208 1313 795–1550 Isobutane C 4 H 10 51 7 714 367–1121 1-Butene C 4 H 8 323 405 242–541 1,3-Butadiene C 4 H 6 238 402 138–571 1-Butane C 4 H 10 273 354 201–472 t-2-Butene C 4 H 8 132 164 82–237 c-2-Butene C 4 H 8 103 134 56–188 2-Methyl-2-butene C 5 H 10 17 21 10–26 Isopentane C 5 H 12 938 1140 703–1517 1-Pentene C 5 H 10 53 72 35–84 Pentane C 5 H 12 373 437 227–598 Isoprene C 5 H 8 72 1 46 32–160 t-2-Pentene C 5 H 10 48 58 34–67 c-2-Pentene C 5 H 10 25 43 16–36 3-Methyl-1-butene C 5 H 10 56 64 40–81 2,2-Dimethyl-butane C 6 H 14 19 28 13–39 Cyclopentene C 5 H 8 NA NA NA 4-Methyl-1-pentene C 6 H 12 58 92 31–85 Cyclopentane C 5 H 10 36 56 25–78 2,3-Dimethyl-butane C 6 H 14 47 65 35–89 2-Methyl-pentane C 6 H 14 143 208 109–286 3-Methyl-pentane C 6 H 14 494 814 123–1389 Hexane C 6 H 14 125 206 88–282 t-2-Hexene C 6 H 12 NA NA NA c-2-Hexene C 6 H 12 12 41 10–20 Methyl-cyclopentane C 6 H 12 47 66 37–95 2,4-Dimethyl-pentane C 7 H 16 15 29 10–33 Benzene C 6 H 6 1652 2044 1131–3213 Cyclohexane C 6 H 12 85 264 25–232 2-Methyl-hexane C 7 H 16 69 84 45–130 2,3-Dimethyl-pentane C 7 H 16 22 27 15–38 3-Methyl-hexane C 7 H 16 85 100 61–140 2,2,4-Trimethyl-pentane C 8 H 18 NA NA NA Heptane C 7 H 16 79 99 59–128 Methyl-cyclohexane C 7 H 14 44 61 26–94 2,3,4-Trimethylpentane C 8 H 18 NA NA NA 2-Methyl-heptane C 8 H 18 22 32 17–47 Toluene C 7 H 8 1804 2511 1021–3053 3-Methyl-heptane C 8 H 18 39 38 17–54 Octane C 8 H 18 44 60 27–70 Ethylbenzene C 8 H 10 531 786 327–954 m/p-Xylene C 8 H 10 544 687 325–887 Styrene C 8 H 8 497 843 296–977 o-Xylene C 8 H 10 454 592 290–733 Nonane C 9 H 20 178 366 101–451 Isopropyl-benzene C 9 H 12 62 114 39–127 Propyl-benzene C 9 H 12 218 401 139–441 1.3.5-Trimethyl-benzene C 9 H 12 139 284 106–355 1.2.4-Trimethyl-benzene C 9 H 12 378 800 280–682 The median, mean and IQR (interquartile ranges, 25–75) for the NMHCs are listed above. NA: not available. 313J. Zhang et al. / Science of the Total Environment 408 (2009) 312–317 NMHCs were measured on a preconcentrator followed by the GC–MS system. 500 mL sample was concentrated in Entech 7100A preconcentrator (Entech Inc.USA), a three-stage preconcentration was utilized to remove the water, carbon dioxide, nitrogen and oxygen in air samples. VOCs will be further focused on a capillary focusing trap for rapid injection prior to the analytical column. A DB-5MS fused-silica capillary column (60 m×0.25 mm×0.25 µm, Agilent Technologies Inc.) coupled with a quadrupole mass spectrometer detector (Finnigan Trace 2000/ DSQ. Thermofisher Inc. USA) was used for qualification and quantifica- tion. In this study, 48 NMHCs were identified and quantified. The pro- cedure of QC/QAwas followed by the USEPATO-14A method. For most of the samples, the NMHCs species were above the detection limit of 10 pptv. The RSD of NMHCs was within 10%. A custom modified system with HP 5890 gas chromatograph equipped with flame ionization detection (FID) was used for CO (carbon monoxide) measurements. 5 mL air sample was injected to the packed column (1 m) filled with carbon sieve. CO was separated with other trace gases in the column at isothermal 50 °C. After eluted from the column, CO was catalyzed by hydr ogen in a nickel catalyzer at 300 °C, where CO would be converted to CH 4 and subsequent detection by FID. The detection limit was 5 ppbv and the precision was 5% for CO. 3. Results and discussion 3.1. General characteristics Table 1 summarizes the statistical concentrations of NMHCs found in Changsha in 2007. The time series of the NMHC mixing ratio is illustrated in Fig. 2. Of the weekly concentrations in 2007 in Changsha, the highest concentration of NMHCs was approximately seven times that of the lowest concentration observed. In this study, the seasonal variation of the total NMHCs showed a higher level in winter and a lower level in summer. The concentration in winter was 25.4 ppbv and only 17.8 ppbv in summer. Several factors influenced the NMHC levels in ambient air, including source emissions, meteorological conditions, and photochemical activity (Na and Kim, 2001). Meteorological conditions are also very important factors affecting the NMHC concentrations, as the higher urban boundary layer can dilute air pollutants within the troposphere in the summer. In addition, photochemical activity is very weak in winter; thus, more NMHCs can accumulate, leading to increased concentrations in the troposphere. The NMHCs quantified in this study were classified into three major groups: alkenes, alkanes, and aromatic compounds. Among the three groups, aromatic compounds showed the highest levels, representing 53% of the total. The most important aromatic compounds were toluene and benzene (a detailed discussion of these two compounds will be presented below). As abundant aromatic compounds can be identified in vehicular exhaust and fumes from gasoline and other solvents, the ratio of benzene to toluene (B/T) is usually used to identify VOC sources. In the current study, Changsha had a B/T ratio of 0.81, which was close to that of other studies (Barletta et al., 2005; Song et al., 2008), and which suggested vehicular emissions as the main source of VOCs. Alkanes were the second important group of NMHCs, accounting for 33% of the total. Alkene compounds accounted for only 14% of all of the identified NMHCs. This proportionality is very similar to that reported in other recent studies around the world (Moschonas et al., 2001; Guo et al., 2006; Parra et al., 2006). Propane (1.31±0.71 ppbv) was the most abundant species of the alkane group, followed by isopentane (1.14± 0.73 ppbv). Propane has a long lifetime in the atmosphere (8.5 days for an OH concentration of 1.5×10 6 molecules cm − 3 ) and most likely arises from emissions from liquid petroleum gas (LPG). The level of propane in Changsha was comparable to that in other cities in China, while it was much lower than that in certain specific cities, such as those in Guangdong province. Chan et al. (2006) measured propane levels in industrial/urban (2.5±0.3 ppbv) and industrial/rural (2.1±0.3 ppbv) locations. In contrast, Barletta et al. (2008) reported that propane was the most abundant NMHC measured in Guangzhou, with an average mixing ratio of 6.8±0.7 ppbv. From their analysis, they attributed the enhanced propane in PRD cities to the wide usage of liquefied petroleum gas (LPG) in taxis and other vehicles. In order to investigate the source of propane in Changsha, the cross correlation between the relative compounds and propane was exam- ined in the current study. CO has a long lifetime (usually 30 days) and is emitted from vehicular exhaust in urban areas. In contrast, n-butane is found in LPG in China and also has a relatively long lifetime (4 days) in ambient air, corresponding to [OH] =1.5× 10 6 molecules cm − 3 (Atkinson and Arey, 2003). Therefore, CO and n-butane represent two different emission sources. The correlations of these three compounds are shown in Fig. 3. Good correlation between n-butane and propane Fig. 2. The time series of NMHCs in Changsha in 2007. The averaged concentrations between morning and afternoon each week represent the weekly concentrations, and the blue refers to the river. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 3. Correlation between CO, butane and propane of the ambient air in Changsha. 314 J. Zhang et al. / Science of the Total Environment 408 (2009) 312–317 was observed (R 2 =0.75); however, the relationship between CO and propane was poorer. These observations suggest that n-butane and propane come from the same sources, perhaps LPG leakage, rather than being products of incomplete combustion. The high levels of propane among the alkanes indicated that LPG is now being widely used in Changsha City, consistent with the imple- mentation of cleaner fuel usage by the government in order to improve air quality. The total LPG supply was 9.4 ×10 4 t in Changsha in 2006, with 8.0×10 4 t LPG consumed for domestic uses, such as heating, cooking, and only 9.3 ×10 3 t used for industrial purposes. This usage was much lower than that for Guangzhou (1.2×10 6 t in 2006), a relatively more developed area of China. AscanbeseenfromTable 1, toluene is the most abundant NMHC in Changsha, which is similar to the situation in the majority of cities in China (Barletta et al., 2005; Guo et al., 2007; Liu et al., 2005). Toluene is primarily emitted from vehicular exhaust (Parra et al., 2006), but industrial processes and solvent usage are also very important sources. In Changsha, the mean concentration of toluene was 2.51± 1.87 ppbv, followed by benzene with a concentration of 2.04±1.30 pptv. The average concentrations of toluene and benzene measured during our studies were lower than the concentrations measured by Barletta et al., where toluene levels were 7.9±12.3 ppbv and benzene concentrations as were 3.4±4.2 ppbv (Barletta et al., 2005). 3.2. Influences of wind on NMHCs The mainland of China is located at the east of the Euro-Asian continent, and the climate is impacted by the typical monsoon seasons during summer and winter. The southeast wind originates from the Indian Ocean and the Pacific Ocean during the summer season. However, in winter, the mainland is primarily affected by strong north/northeast winter monsoons coming from the Siberia over the Asian continent. For a short specific period, NMHC concentrations could be influenced by meteorological conditions such as wind direction and wind speed. Fig. 4 illustrates the wind conditions and NMHC concentrations in the Changsha suburban area in 2007. Fig. 5 shows the frequency of wind directions in 2007, and indicates that the wind from the north and northwest direction accounted for about 40%, where the north and south directions were defined as 0 ±22.5° and 180±22.5°, respectively. As can be seen from Fig. 4, the NMHC concentrations had the highest levels when the winds came from the west and southeast directions. This is reasonable because urban areas are located to the west. Therefore, a wind direction from the west would transport anthropogenic emissions of NMHCs from these urban areas eastward to Changsha. To better illustrate the enhancement of NMHCs with the west winds, we extracted the BTEX data for periods when west winds were experienced. BTEX is emitted almost exclusively from anthropogenic activity sources. The west wind direction dependence of BTEX concentration was 7.02±2.17 ppbv and was higher compared to the other wind directions (5.61±1.89 ppbv) except for the calm. The statistical tests (t tests) showed that statistically significant differences existed between the different wind directions and the BTEX concentration (pb 0.01). This difference in BTEX concentration from separate directions could also be used to calculate the influences from the emissions from Changsha's urban area. Another important feature is that the observed NMHCs showed higher levels in south and southeast wind directions. The south and southeast winds make up only 15%of the total sampling periods, and the average wind speed from the south and southeast winds was 2.25m s − 1 . Because of the monsoon effects, the observed south and southeast winds during the sampling period occurred only in the spring and summer, which is a period of the strong outflow of pollution from the PRD and southern China. The enhancements of NMHC levels with a south wind direction can be attributed to the transport of pollutants from the south of China. Fig. 4. Influence of wind on the observed concentrations of NMHCs in 2007. Data was excluded when wind speed was zero. Fig. 5. The frequency of wind direction was observed in 2007. The wind direction includes eight directions and calm air. Fig. 6. The relationship between isoprene concentrations and temperature. 315J. Zhang et al. / Science of the Total Environment 408 (2009) 312–317 Adjacent to the Hunan province, the PRD region is located in the south of China and is the most populated, urbanized, and industrialized region in China. Many enterprises, such as shoemaking, cement, and petrochemical industries, operate there and emit large amounts of NMHCs (Chan et al., 2006). Moreover, these high levels of NMHCs can be transported to the other sites with the prevailing winds (Tang et al., 2007; Zhang et al., 2007). 3.3. Temperature dependence of isoprene emissions Isoprene is a very important compound due to its atmospheric reactivity. Although isoprene can be oxidized by a reaction with OH, Lelieveld et al. (2008) recently observed high OH concentrations due to a recycl ing of OH by reactions of organic peroxy radicals. Consequently, this represents a potential source of ozone generation. As listed in Table 1 , the isoprene levels in ambient air ranged from several to a few hundreds pptv over the whole year. In particular, the isoprene concentrations can exceed 500 pptv in the summer. High levels of isoprene were observed in the afternoon, indicating that its emission was larger than in the morning, despite the strong photochemical reactivity in the afternoon. It is well known that isoprene in ambient air is primarily emitted from the biogenic sources, although vehicle exhaust in urban areas also may contribute to its levels to a lesser extent (Durana et al., 2006). As indicated from the model developed by Guenther, isoprene emission changes with a non-linear relationship to light and temperature (Guenther et al., 2006, 1993). The relationship between the ambient air temperature and the isoprene concentration of afternoon air in Changsha is plotted in Fig. 6. Regression analysis revealed a good exponential relationship between the isoprene concentration and the temperature (R 2 =0.65). High temperatures increased the isoprene concentrations; however, the isoprene concentration remained constant, at only several pptv, when the ambient air temperature was below 20 °C. At low temperatures, photosynthesis by vegetation is very weak (Fig. 6). This suggests that other emission sources exist, and that mobile vehicular exhaust may be the main source of isoprene emission at low temperatures. Periods of temperatures lower than 20 °C are typical for the winter and early spring months in the Changsha region. 3.4. Correlations of NMHCs and CO CO is one of the products of incomplete combustion of fossil fuels (Wang et al., 2008). Consequently, CO is useful as a reference compound to identify anthropogenic sources of pollutants. The correlation and the slopes of the regression lines of NMHCs and CO can be used to gain a better understanding of the sources and can be used as emission ratios (Shirai et al., 2007). Table 2 shows the correlation between the typical NMHC species with CO in the mornings and in the afternoons during 2007 for the Changsha suburban site. The correlation coefficients, both in the morning and in the afternoon, indicate that combustion was an important emission source of NMHCs in Changsha. In many cities in the USA, the C2–C4 alkane mixing ratios are poorly correlated with CO (Baker et al., 2008). In the afternoon, the correlation coefficients were not very high for these compounds, because the concentrations of the NMHC species were affected by photochemical reactions, dilution, and transport. CO has a longer lifetime than the NMHC compounds in the atmosphere. The photochemical reactivity and the dilution effects occurred 1 h after sunrise, and the NMHC species emitted in the morning were destroyed and diluted, thus the mixing ratio of NMHCs would decrease. In contrast, the atmospheric lifetime of CO was estimated to be about twenty days when the OH concentration was considered to be 2.0 ×10 6 molecules cm − 3 . A decrease in the NMHC species to the CO ratio would therefore occur and the ratio would show little correlation with CO in the afternoon. 4. Conclusions The ambient atmospheric concentrations of NMHCs and CO were measured weekly in a Changsha suburban area in 2007. The NMHC measurements system demonstrates the feasibility of studying regional distributions of NMHC in southern China, similar to NMHC measurements measured previously in northern China. An analysis of the identified NMHCs and CO suggested that vehicular exhaust is the dominant source of pollutants in Changsha. The large portion of propane was related to the wide usage of LPG as fuel for vehicles. Wind influences and biogenic emissions were also important factors for NMHC pollution. There were different sources for NMHC species and CO, and NMHCs were destroyed more rapidly in the daytime compared to CO. A higher reaction rate of NMHCs with OH radical in the atmosphere can explain the poor correlation between the typical NMHCs species and CO. Acknowledgements This work was supported by the National Basic Research Program of China (Grant No. 2007CB407303), the grant of the Knowledge Innovation Program of the Chinese Academy of Sciences (approved # KZCX1-YW-06-01) and the Hi-tech Research and Deve lopmen t Program of China (Grant No. 20 06AA0 6A1). T he anonymous reviewers provided useful comments for improving the manuscript. References Atkinson R, Arey J. Atmospheric degradation of volatile organic compounds. Chem Rev 2003;103:4605–38. 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Nonmethane hydrocarbon measurements at a suburban site in Changsha City, China Jungang Zhang a , Yuesi Wang a, ⁎ , Fangkun Wu a , Hong Lin a , Weidong Wang b a LAPC, Institute of Atmospheric. air quality grade II standard attainment days reached 302 days in 2007. Changsha was appointed as one of the first batch of pilot cities for the Clean Air Initiative for Asian Cities (CAI-Asia). suburban site in the eastern section of Changsha. The site w as locat ed in a sparsely populat ed ar ea that has light local emissions, far away from t he most commercial and industrialized areas

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