Characteristics and Diurnal Variations of Nmhcs at Urban, Suburban, and Rural Sites in the Pearl River Delta and a Remote Site in South China

ARTICLE IN PRESS

Atmospheric Environment 41 (2007) 8620–8632 www.elsevier.com/locate/atmosenv

Characteristics and diurnal variations of NMHCs at urban, suburban, and rural sites in the Pearl River Delta and a remote site in South China

J.H. Tanga,1, L.Y. Chana, C.Y. Chana, Y.S. Lia,Ã, C.C. Changb, S.C. Liub, D. Wuc, Y.D. Lid

aDepartment of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China bResearch Center for Environmental Changes, Academia Sinica, Taipei, Taiwan cInstitute of Tropical and Marine Meteorology, CMA, Guangzhou, China dThe Research Institute of Tropical Forestry, CAF, Guangzhou, China

Received 21 November 2006; received in revised form 28 June 2007; accepted 12 July 2007

Abstract

The Pearl River Delta (PRD) is one of the most industrialized and urbanized regions in China. With rapid growth of the economy, it is suffering from deteriorating air quality. Non-methane hydrocarbons (NMHCs) were investigated at urban and suburban sites in Guangzhou (GZ), a rural site in PRD and a clean remote site in South China, in April 2005. Additional roadside samples in GZ and Qingxi (QX, a small industrial town in PRD), ambient air samples at the rooftop of a printing factory in QX and exhaust samples from liquefied petroleum gas (LPG)—fueled taxis in GZ were collected to help identify the source signatures of NMHCs. A large fraction of propane (47%) was found in exhaust samples from LPG-fueled taxis in GZ and extremely high levels of toluene (2.0–3.1 ppmv) were found at the rooftop of the printing factory in QX. Vehicular and industrial emissions were the main sources of NMHCs. The effect of vehicular emission on the ambient air varied among the three PRD sites. The impact of industrial emissions was widespread and they contributed greatly to the high levels of aromatic hydrocarbons, especially toluene, at the three PRD sites investigated. Leakage from vehicles fueled by LPG contributed mainly to the high levels of propane and n-butane at the urban GZ site. Ethane and ethyne from long-range transport and isoprene from local biogenic emission were the main contributors to the total hydrocarbons at the remote site. Diurnal variations of NMHCs showed that the contribution from vehicular emissions varied with traffic conditions and were more influenced by fresh emissions at the urban site and by aged air at the suburban and rural sites. Isoprene from biogenic emission contributed largely to the ozone formation potential (OFP) at the remote site. Ethene, toluene and m/p-xylene were the main contributors to the OFP at the three PRD sites. r 2007 Elsevier Ltd. All rights reserved.

Keywords: Non-methane hydrocarbons; Source signature; LPG; PRD; South China

1. Introduction

ÃCorresponding author. Tel.: +852 2766 6069. E-mail address: [email protected] (Y.S. Li). The Pearl River Delta (PRD) is one of the most 1Now at Yantai Institute of Coastal Zone Research for urbanized and industrialized regions in South China. Sustainable Development, CAS. Accompanying the substantial economic development,

1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.07.029 ARTICLE IN PRESS J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632 8621 large amounts of air pollutants are discharged, ruralsiteinthePRD;aswellasataremotesitein resulting in a rapid deterioration of the overall air South China in April 2005. quality. High levels of carbon monoxide, sulfur dioxide, nitrogen oxides (NOx), ozone (O3) and 2. Experiment volatile organic compounds (VOCs) are frequently found in the PRD (Wang et al., 2005; Zhao et al., 2.1. Sampling sites 2004). Elevated aromatic hydrocarbons were reported at roadsides of the PRD cities (Chan et al., Four sites were selected for this study in 2005: 2002a; Wang et al., 2002). The frequency of ozone Guangzhou (GZ), Panyu (PY), Dinghu mountain episodes in the downwind regions, such as Hong (DM) and Jianfeng mountain (JM). Fig. 1 shows Kong has increased significantly (Chan and Chan, their geographical locations and Table 1 describes 2000; Chan et al., 2002b; Wang et al., 2005). the characteristics of the sampling sites. Non-methane hydrocarbons (NMHCs) are key GZ is the capital city of Guangdong Province ozone precursors and play a very important role in with a total population of 7.5 million. It is the tropospheric chemistry. Photochemical reactions of economic and cultural center of the PRD. The NMHCs with NOx result in the formation of ozone sampling site is located in the urban center, and other atmospheric oxidants, which affect the the Dongshan district, and is surrounded by resi- global distribution of hydroxyl radical (OH), and dential buildings and business offices. At 100 m consequently influence the lifetime of other trace distance away from the sampling site to the north species in the troposphere (Poisson et al., 2000). In and the south directions, there are two main roads urban areas, such as Guangzhou and Hong Kong, with very heavy traffic. This site represents a typical ozone formation is VOC limited, as there is GZ urban site, which is influenced by residential sufficient NOx from vehicular exhausts (Wang and traffic emission sources. et al., 2005). Therefore, an effective strategy for PY is a satellite city of GZ with a total population mitigating ozone pollution in urban areas can be of one million in an area of 1300 km2. The sampling obtained by controlling the emission of NMHCs site was located in a suburban environment. This (a subset of VOCs). site is about 20 km from the urban center of GZ to Several studies on NMHCs distribution had been the north, 25 km from Dongguan (a famous conducted in China, especially in South China. A industrial manufactory center of China with about study in 43 Chinese cities indicated that vehicular 6.6 million inhabitants) to the west, 30 km from emissions were the major source of NMHCs in 10 Zhongshan (an important industrial city in PRD cities, while coal and biofuel combustions were the with 2.4 millions inhabitants) to the south. There is major sources in 15 other cities (Barletta et al., no obvious anthropogenic source nearby except for 2005). Another study of two urban sites in Hong a road about 50 m to the south. Air samples were Kong showed that vehicular emission, solvent use collected at the rooftop of an environmental and liquid petroleum gas (LPG) or natural gas (NG) monitoring chamber (about 4 m above ground). leakage were the major sources of NMHCs (Guo DM is located in the middle of Guangdong et al., 2004). However, at rural sites in Hong Kong, Province in South China and is surrounded by the NMHC sources were highly variable. For air tropical forest. It is about 85 km to the west of GZ masses transported through the Hong Kong urban and 18 km to the northeast of Zhaoqing (a less area, local vehicular emissions were still the domin- industrialized city compared with other cities in ant source, while for air masses transported through PRD with about 3.7 millions inhabitants). There is a the PRD area, local industrial emissions were the small town (the Guicheng district, with a population major source (Guo et al., 2006). A study of NMHCs of 20,000) and a highway near the sampling site in the PRD industrial, industrial-urban, and in- about 3 km to the south. The sampling site is dustrial-suburban atmospheres showed that indus- located at the hilltop in the Dinghushan Biosphere trial emissions had great influence on the ambient Reserve, which is a station of the Man and the levels of NMHCs (Chan et al., 2006). Biosphere Programme of the United Nations In this study, we have presented a snapshot of Educational, Scientific and Cultural Organization. atmospheric NMHCs in the fast urbanizing and JM is situated on the southeast coast of Hainan industrializing PRD region. Air samples were collected Island and is about 120 km from Sanya (the second at an urban and a suburban site of Guangzhou and a largest city in Hainan Province, with a population ARTICLE IN PRESS 8622 J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632

Fig. 1. Locations of the four sampling sites in the PRD and South China.

Table 1 Characteristics of the four sampling sites

Site Abbreviation Location Altitude, Sampling periods Site characteristics m asl

Guangzhou GZ 23.081N, 88 16–22 April 2005 Urban site, at the rooftop of 113.181E a 21-storey building Panyu PY 22.561N, 16 16–22 April 2005 Suburban site, 4 m above the 113.191E ground Dinghu mountain DM 23.101N, 320 16–22 April 2005 Rural site, at the top of a hill 112.321E Jianfeng mountain JM 18.401N, 820 7–13 and 16–18 Remote site, at the rooftop 108.491E April 2005 of a 2-storey building

of 0.5 million) and 315 km from Haikou, the capital also collected at the rooftop of a printing factory city of Hainan province. The mountain faces the (a three-storey building) between 14:45 and 15:00 local South China Sea in the south and west directions. time in QX to obtain the source signature of NMHCs The sampling site was situated within the Jianfen- from a particular industry. QX is a small industrial gling National Reserve, which is the second largest town with many factories. Later, to study the impacts tropical rain forest in China with a total area of of LPG-fueled vehicles, two exhaust samples from 475 km2. The sampling site was surrounded by LPG-fueled taxis were collected on 23 June 2006. several hills with elevations exceeding 1000 m asl. To study the impacts of NMHCs from vehicular 2.2. Sampling procedure emissions, six roadside samples were collected at Xingang Road in GZ, on 8 September 2006 and three Ambient samples in 2005 were collected using roadside samples were collected at a roundabout of pre-evacuated 2-L stainless steel canisters provided Qingxi Township (QX), Dongguan, between 11:40 by the Research Center for Environmental Changes and 11:55 local time from 7 to 9 January 2006. (RCEC), Academia Sinica, Taiwan. A ﬂow-controlling During the same period, three ambient samples were device was used to collect1 h integrated samples. ARTICLE IN PRESS J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632 8623

At the three PRD sites, air samples were collected roadside samples are listed in Table 2. The re- from 16 to 22 April while at JM, they were collected maining hydrocarbon mixing ratios for the samples from 7 to 13 and 16 to 18 April 2005. Five canister collected in JM were at or below the detection limit, samples were collected at 8:00, 11:00, 14:00, 17:00 and hence are not shown in Table 2. However, the and 20:00 local time each day. Sampling canisters gases listed in Table 2 account for about 83%, 81% were shipped back to Taiwan for chemical analysis and 87% of the total amount of measured hydro- within 1 month of collection. carbons (total NMHC) in GZ, PY and DM, Additional roadside, exhaust and ambient sam- respectively. Both toluene and ethyne are among ples were obtained from a different project and the three most abundant hydrocarbons in all three analyzed by the University of California at Irvine PRD sites. Propane is the most abundant hydro- (UCI). The roadside samples were 2-minute inte- carbon in GZ, while ethene and ethane are the third grated air samples in GZ and 15-min integrated air abundant hydrocarbons in PY and DM, respec- samples in QX and the sampling flow was controlled tively. For the roadside samples, GZ had much by a mass flow controller. The sampling procedure higher level of propane and butanes, while QX had for ambient sample collected at the rooftop of the much higher level of aromatic hydrocarbons. We printing factory was the same as that for the will further examine these phenomena in a later roadside samples in QX. section. Exhaust samples were collected by directly In the samples collected at the rooftop of the sampling the exhaust streams of idling LPG-fueled printing factory, extremely high levels of toluene taxis. Each canister was used to collect exhaust from (2006.6–3128.1 ppbv) were observed. Toluene is an three taxis to obtain an average and, in addition, industrial solvent extensively used in various in- to save resource. A pressure gauge with a flow dustrial processes, and higher toluene levels were controller was used to regulate the sampling volume also found in other industrial sites of PRD (Chan from each taxi, and nearly the same volumes were et al., 2006). Besides toluene, elevated n-heptane collected from each taxi. All the taxis selected were (91.6–210.1 ppbv) and 1,2,4-trimethylbenzene (19.2– newly registered cars (around 1-year old). 206.2 ppbv) were also present in the rooftop of the printing factory. The high levels of these hydro- 2.3. Chemical analysis carbons measured on the rooftop of the printing factory can be attributed to emissions from the In the RCEC, 56 photochemical assessment printing process. monitoring stations (PAMS)-targeted hydrocarbons were quantified. An automated GC–MS/FID system 3.2. Comparison with other studies in Asian cities with two columns (a PLOT and a DB-1) was used to analyze C2–C10 hydrocarbons. The detection limit for A comparison of hydrocarbon mixing ratios from C2–C10 hydrocarbons was 15 pptv, and the precision various Asian cities is presented in Table 3. of the measurement was 5% (Chang et al., 2003). For Numerous factors can affect the hydrocarbon the UCI system, a 6-column multiple GC–MS/FID/ mixing ratios observed in different studies, such as ECD system was used to identify and quantify 48 the nature of the sampling sites, time periods of each C2–C9 NMHCs and other trace gases. The detection study, meteorological conditions, human activities limit for C2–C9 hydrocarbons was 5 pptv. The and fuel composition in the target cities (Hseih and precision of the measurement varied between com- Tsai, 2003). Therefore, a direct comparison of pounds and mixing ratios. For example, it was 1% hydrocarbon mixing ratios in different studies (or 1.5 pptv) for alkanes and alkyne, and 3% entails careful interpretation. Hence, in this study, (or 3 pptv) for alkenes (Colman et al., 2001). the comparison is just used for the illustration of the variation of NMHC levels in different Asian cities. 3. Results The mixing ratios of most hydrocarbons in GZ did fall within the ranges of hydrocarbons measured in 3.1. General characteristic of NMHCs at PRD the 43 Chinese cities (Table 3). Toluene (10.0 ppbv) urban, suburban and rural sites and i-butane (4.5 ppbv) in GZ were at the upper end of the ranges (0.4–11.2 ppbv and 0.4–4.6 ppbv, The average mixing ratios and standard devia- respectively) in 43 Chinese cities. Propane, toluene, tions of selected hydrocarbons in the ambient and ethyne, ethene and i-butane were observed to be ARTICLE IN PRESS 8624 J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632

Table 2 Mixing ratios of selected hydrocarbons at different sites (units: ppbv)

Hydrocarbon Ambient Roadside Industrial

GZ (n ¼ 28a)PY(n ¼ 33) DM (n ¼ 30) JM (n ¼ 52) GZ (n ¼ 6) QX (n ¼ 3) QX (n ¼ 3)

Ethane 3.9071.17 2.7170.95 2.7270.48 1.5170.40 3.6370.83 5.4270.69 8.2874.96 Propane 11.2975.69 4.2772.20 2.0370.94 0.3270.19 29.85712.31 5.3970.68 42.88762.76 i-Butane 4.4872.31 1.8771.01 0.7870.41 0.0770.05 5.7271.27 3.2271.63 13.04713.89 n-Butane 6.3173.21 2.9871.64 1.1770.64 0.1070.09 10.7172.59 7.1573.69 24.34723.67 i-Pentane 3.8171.93 2.2771.20 0.9070.46 0.0870.04 3.7371.06 4.2671.33 66.18766.69 n-Pentane 1.7670.87 1.3270.73 0.5270.29 0.0470.04 1.6370.45 1.7570.72 23.27723.68 n-Hexane 1.2470.59 0.9370.56 0.3270.18 0.0570.03 0.6670.27 1.7170.51 25.89728.59 n-Heptane 1.1270.65 1.0470.67 0.3070.23 0.0270.01 0.4970.22 0.7670.28 138.75762.81 Ethene 8.6074.28 5.4273.42 2.4571.17 0.5270.43 12.6775.29 11.9071.96 44.01753.50 Propene 2.3671.34 1.1070.76 0.4370.25 0.1370.07 2.9171.28 21.970.41 8.95711.80 i-Butene 1.0470.64 0.6470.44 0.1870.07 0.1470.21 1.1170.50 0.8070.13 3.1471.21 Isoprene 0.2770.14 0.1870.10 0.1270.80 0.4870.47 1.1370.88 0.1870.02 0.9870.63 a-Pinene 0.3270.29 0.2370.19 0.3270.16 0.0470.02 N.A.c N.A. N.A. Ethyne 9.8073.95 7.7174.13 4.3071.38 0.9470.46 10.2075.14 16.9872.89 48.61744.85 Benzene 2.7571.19 2.6771.58 1.1770.54 0.2170.11 2.0770.52 3.6270.46 19.34718.30 Toulene 10.0274.69 9.3975.59 3.0971.79 0.0970.06 4.0172.34 18.9474.19 2564.377560.78 Ethylbenzene 1.1971.04 1.8771.07 0.4870.28 0.0270.01 0.9170.36 3.1670.33 11.2872.36 m/p-Xylene 3.0371.69 3.1072.11 0.6670.48 0.0470.02 1.9270.64 8.1770.93 46.79716.09 o-Xylene 1.1670.62 1.1670.75 0.2770.18 0.0270.01 0.6370.19 2.2270.46 13.1673.19 1,2,4-TMBb 0.4570.26 0.3570.24 0.0870.05 0.0370.02 0.6270.29 1.3470.14 86.977103.56 SNMHC 88.79738.56 60.79731.47 23.4079.84 4.7871.85

aNumber in parentheses indicates the total number of samples at each site. b1,2,4-TMB, 1,2,4-trimethylbenzene. cN.A., not available. higher (1.1–10 times) in GZ than in most other ethane levels (93 ppbv) (mainly from NG leakage) Asian cities (Ulsan, Korea, Na et al., 2001; were approximately 23 times higher than those in Kathmandu, Nepal, Sharma et al., 2000; Hong GZ. Hydrocarbons from vehicular emissions such Kong, So and Wang, 2004; Ahmedabad, India, as ethene and ethyne also showed about two times Sahu and Lai, 2006; and Kaohsiung, Taiwan, higher levels in Karachi than in GZ, which indicates Chang et al., 2005), except for Taipei, Taiwan the poor efﬁciency in emission control in Karachi (Wu et al., 2006) and Karachi, Pakistan (Barletta (Table 3). et al., 2002). The high levels of NMHCs indicated the higher emission strength and lower efﬁciency of 3.3. Source signatures of NMHCs at the four sites emission control in GZ. In Taipei, air samples were collected in the LPG-fueled vehicles (taxis and buses) have been afternoon rush hour (6:00 pm). Vehicular emissions promoted in GZ since 2003. In November 2005, there contributed greatly to the high levels of most were about 5060 and 9200 LPG-fueled buses and NMHCs, such as ethene, ethyne, i-pentane, and taxis in GZ, which accounted for 67% and 58% of toluene, while leakage from natural gas (NG) the total numbers of buses and taxis, respectively contributed to the high levels of ethane. These gases (New Express, 18/11/2005). Fig. 2 shows the distribu- are about 1.3–2.7 times higher in levels in Taipei tion of NMHCs in the exhaust of LPG-fueled taxis. than in GZ. Although LPG leakage also contrib- Propane was the dominant hydrocarbon in exhaust uted greatly to propane and n-butane levels in samples (47%). Ethene (19%) and n-butane (11%) Taipei, these two compounds are about half and also contributed greatly to the total amount of two-thirds of those in GZ, respectively (Wu et al., NMHCs (Fig. 2). The slopes of propane to n-butane 2006). However, in Karachi, propane (41 ppbv) and were 1.6670.12, 1.3070.05 and 1.4270.07 at GZ, i/n-butane (11.0/19.8 ppbv) levels (mainly from LPG PY and DM, respectively (not shown). At leakage) were approximately three times higher and the roadside of GZ, the slope was 3.6670.12. ARTICLE IN PRESS J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632 8625

Table 3 Comparison of selected NMHCs in Guangzhou with other Asian cities (units: ppbv)

Hydrocarbon Ulsana Taipeib Kathmanduc Karachid Hong Konge 43 Citiesf China Ahmedabadg Kaohsiungh Guangzhoui Korea Taiwan Nepal Pakistan China January–February India April Taiwan China April June 97 August 98 November 98 December November 2001 2002 October 2005 98–January 99 2000–October 2002 2001

Ethane 1.6 6.03 1.86 93 3.7–17.0 2.59 4.5 3.98 Propane 5.0 5.65 1.14 41 4.01 1.5–2.08 2.93 3.1 11.29 i-Butane 1.4 1.10 11.0 2.87 0.4–4.6 0.97 0.7 4.48 n-Butane 3.5 4.26 3.32 19.8 6.53 0.6–18.8 2.15 2.3 6.31 i-Pentane 2.0 10.30 1.15 12.1 4.48 0.3–18.8 1.43 3.8 3.81 n-Pentane 1.5 5.38 1.13 13.4 1.71 0.2–7.7 0.54 1.3 1.76 Ethene 6.9 13.80 3.27 19.0 2.1–34.8 2.34 7.5 8.6 Propene 2.1 3.92 0.63 5.5 2.8 0.2–8.2 0.77 2.2 2.36 Isoprene 0.02 0.8 0.65 0.04–1.7 0.6 0.27 Ethyne 1.9 12.80 1.25 18.0 2.9–58.3 1.90 4.9 9.8 Benzene 1.1 3.22 5.2 1.58 0.7–10.4 1.1 2.75 Toulene 3.9 23.50 7.1 8.24 0.4–11.2 8.2 10.02 Ethylbenzene 2.84 0.1–2.7 0.7 1.91 m/p-Xylene 2.1 8.87 3.1 0.4–15.3 1.2 3.03 o-Xylene 0.9 4.45 1.1 0.1–6.9 0.6 1.16

aNa et al., 2001. bWu et al., 2006. cSharma et al., 2000. dBarletta et al., 2002. eSo and Wang, 2004. fBarletta et al., 2005. gSahu and Lai, 2006. hChang et al., 2005. iThis study.

There were very limited LPG-fueled vehicles in use at 50 50 PY and DM areas. Hence, the large contribution of LPG Taxi propane in the urban area of GZ may be attributed to 40 40 leakage from LPG-fueled vehicles. This ﬁnding is further supported by the high abundance of propane (29.8 ppbv) in the GZ roadside atmosphere and the 30 30 highest mixing ratio of propane (11.3 ppbv) in the GZ atmosphere among the four sites. 20 20 Percent (%) Ethyne had been used extensively as a tracer for vehicular exhausts (e.g., Barletta et al., 2002, 2005). 10 10 To assess the impact of vehicular emission on the ambient NMHC level, NMHC/ethyne ratios were calculated. Fig. 3 shows the NMHC/ethyne ratios in 0 0 the samples at the two roadside sites, the four ethyne ethene ambient sites and at the rooftop of the printing ethane toluene i-butane propene propane o-xylene isoprene benzene n-butane i-pentane n-hexane isobutene factory. Among the four sites, the lowest individual n-pentane n-heptane 1,2,4-TMB m/p-xylene thylbenzene

NMHC/ethyne ratios were found at JM, except for e ethane and isoprene. This is due to the aged air Fig. 2. Distribution of NMHCs in the exhaust of LPG-fueled transported to this site and the longer lifetime of taxis. ethane than ethyne in the atmosphere. Moreover, the atmospheric lifetime of isoprene is very short (about several hours) and it could not be trans- For the roadside samples, propane/ethyne, n-butane/ ported from long-range source regions. Hence, its ethyne, i-butane/ethyne and ethene/ethyne showed major source should be the local biogenic emission. higher values at GZ, while toluene/ethyne and ARTICLE IN PRESS 8626 J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632

3.0 3.0 Rd QX Rd GZ 2.5 2.5 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 1.8 1.8 GZ PY DM JM 1.2 1.2

0.6 0.6

0.0 0.0

hydrocarbon/ethyne ratio 55 55 QX ambient 50 50 5 5

0 0 ethyne ethane ethene toluene i-butane propane propene o-xylene isoprene benzene n-butane i-pentane n-hexane isobutene n-pentane n-heptane 1,2,4-TMB m/p-xylene ethylbenzene

Fig. 3. Comparison of hydrocarbon/ethyne ratios at different sites, including roadside of Qingxi (Rd QX) and roadside of Guangzhou (Rd GZ). m/p-xylene/ethyne showed higher values at QX. 3.4. Diurnal variations of NMHCs These are also in line with the fact that LPG-fueled buses and taxis were used in GZ, but not in QX. In Average diurnal variations of several hydrocarbons the samples collected at the rooftop of the printing together with the total amount of NMHC measured factory, a very high toluene/ethyne ratio (about 53) atthefoursitesareshowninFig. 4. The diurnal was found (Fig. 3). The much higher toluene/ethyne variation patterns of hydrocarbons and total NMHC ratios at QX roadside and the rooftop of the were quite different from each other at the four sites. printing factor reflect the influence of industrial In GZ, high levels were observed in the morning emissions. Moderately high n-heptane/ethyne ratio and evening for most hydrocarbons and total (2.9) was found at the rooftop of the printing NMHC. They showed high mixing ratios in the factory. It was related to industrial emission from morning (8:00–9:00 local time), decreased to the the printing factory. lowest at noon (14:00–15:00), and later, increased The profile of NMHC/ethyne ratios in the gradually to the highest value in the evening ambient air of GZ matched well with that of (20:00–21:00). For example, the levels of total roadside air in GZ, except for the toluene/ethyne NMHC decreased by about 28% from 8:00–9:00 ratios (1.0), which were close to those at the to 14:00–15:00, but increased by about 46% from roadside samples of QX (1.1). This indicates that 14:00–15:00 to 20:00–21:00. This two-peak pattern although the major source of NMHCs at GZ was had also been observed at many other urban sites, roadside vehicular emission, industrial emission such as Changchun, northeast China (Liu et al., contribution was still important. The profiles of 2000); Taichung, central Taiwan (Yang et al., 2005); NMHC/ethyne at PY and DM were closer to those and Bilbao, Spain (Durana et al., 2006). The two- at the QX roadside, where vehicular emissions were peak pattern indicates that the major sources of mixed with industrial emission contribution. This these hydrocarbons were traffic emissions, as traffic observation suggests mixed contributions from volumes in the urban area reached their highest vehicular and industrial emission influenced more levels at rush hours in the morning (8:00–10:00) and on the PY suburban area and DM rural area. afternoon (17:00–19:00). ARTICLE IN PRESS J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632 8627

6 28 20 DM GZ 100 5 16 21 80 4 12 60 3 14 8 2 40 7 1 4 20 Hydrocarbon mixing ratio (ppbv) total NMHC mixing ratio (ppbv) total NMHCmixing ratio (ppbv) 0 0 Hydrocarbon mixing ratio (ppbv) 0 0 08:00 10:00 12:00 14:00 16:00 18:00 20:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00

2.4 6 15 PY 80 JM 2.0 12 60 1.6 4 9 40 1.2 6 0.8 2 20 3 0.4 total NMHC mixing ratio (ppbv) total NMHC mixing ratio (ppbv) Hydrocarbon mixing ratio (ppbv) 0 0 Hydrocarbon mixing ratio (ppbv) 0.0 0 08:00 10:00 12:00 14:00 16:00 18:00 20:00 08:0010:00 12:00 14:00 16:00 18:00 20:00 Local Time Local Time ethane ethene ethyne propane total NMHC n-butane i-pentane benzene toluene

Fig. 4. Diurnal variations of hydrocarbons at the four sites.

The lowest hydrocarbon levels at 14:00–15:00 at all at PY rather than in the morning rush hour the three PRD sites are likely the result of both (8:00–9:00). PY is located in the GZ suburban area, reaction with OH and increased mixing height of the and there is no observed morning rush hour trafﬁc planetary boundary layer (PBL). The concentrations (8:00–9:00). This phenomenon leads us to further of OH as well as the height of PBL reach its peak at examine the diurnal variation of hydrocarbons. It noon due to the highest solar radiation and tempera- was found that hydrocarbons were more inﬂuenced ture at noon. However, toluene exhibited a different by aged air masses from the urban areas than from diurnal pattern compared with those observed for fresh local vehicular emission. the other hydrocarbons, with peak levels occurring The major sink of hydrocarbons in the atmo- between 11:00–12:00 and 17:00–18:00. Toluene exhib- sphere is the reaction with the OH radical, and the ited a decreasing trend in the evening (20:00–21:00) reactivity of individual hydrocarbons varies greatly. rather than in the afternoon (17:00–18:00), indicating The concentration ratio of two hydrocarbons with that its primary sources were different from most other common sources but different reaction rates in the hydrocarbons. As mentioned above, industrial emis- atmosphere can be used to assess the photochemical sion was the major contributor to the high levels of ages of air masses (Nelson and Quigley, 1983). toluene in the PRD region. Most industrial activities Although toluene to benzene ratio was widely used were still in full operation in the afternoon and only to assess the ages of air masses, it is not applicable stopped in the evening. in this study due to the different sources of these The major difference in the hydrocarbon diurnal two hydrocarbons. Ethene and propene are both patterns between PY and GZ was that a higher total derived from vehicle exhaust in urban area (Wadden NMHC mixing ratio was observed at 11:00–12:00 et al., 1988), but have differing rates of reaction with ARTICLE IN PRESS 8628 J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632

12 3 1 1 OH (KOH ¼ 8.5 and 26.3 10 cm mol s at a maximum at 17:00–18:00, and then decreased (by 298 K for ethene and propene, respectively, Atkin- 17% for total NMHC) to 20:00–21:00. The sampling son and Arey, 2003). After being emitted into the site was located at a remote tropical rain forest with atmosphere, propene will decay more quickly than very limited local anthropogenic sources. In a ethene, making their ratio a useful indicator of an previous study, the major sources of hydrocarbons air masses’s photochemical age. Fig. 5 shows the at this remote site were identified as long-range correlation of propene with ethene at the four sites. transport from the upwind source regions (Tang The correlation of propene with ethene (R40.87) et al., 2007). In this study, backward air mass indicates that they were from the same or similar trajectory analysis was used to determine the possible sources. The corresponding slopes of the propene/ transport pathways of air masses. Using the NOAA ethene correlations for the four sites in descending HYSPLIT model (Rolph, 2003), 10-day backward order are the following: GZ (0.3070.01), PY air trajectories were calculated for the air masses (0.2170.01), DM (0.1970.02), and JM (0.137 reaching JM. Fig. 6 shows two typical trajectories at 0.01). Likewise, decreasing slope values are asso- two different heights (at 800 and 1400 m above mean ciated with an increase in photochemical air mass sea level) during the sampling periods. Air masses age. Compared with GZ, the air masses are more originated from North China, traveled through the aged in PY, as indicated by the low propene/ethene western Pacific Ocean, and passed over the Philip- ratio. This also coincided with the later appearance pines before reaching JM (Fig. 6). This site was far of the first hydrocarbon peak in PY than that in away from the source regions, and it would need at GZ. The diurnal variations of hydrocarbons at the least several hours to transport the air masses from rural DM site had different characteristics from the immediate upwind source regions, such as those at the urban GZ site and suburban PY site. Southeast Asia. Hence, the lowest propene to ethene Most hydrocarbons and total NMHC peaked at ratio (0.13) was found there among the four sites. In 11:00–12:00 like those that happened in PY, but a the evening (20:00–21:00), the sea–land breeze might high plateau also existed from 17:00 to 21:00. bring the fresh air from the South China Sea, and Compared with the continuing increase anthropo- dilute the air pollutants at this site. genic emissions at the GZ urban site and PY In this study, we also noted that the average suburban site, anthropogenic emissions at DM rural mixing ratio of isoprene at JM was much higher site weakened at night (20:00–21:00). This may level than that at the other three sites (Table 2). Fig. 7 off or even slightly decrease the hydrocarbon mixing ratio at night. At the remote JM site, most hydrocarbons showed a one-peak pattern. The mixing ratios of most hydrocarbons and total NMHC were similar between 8:00–9:00 and 11:00–12:00. They both increased (by 23% for total NMHC, for example) to

2 propene (ppbv) Slope Stnd err R JM 0.13 0.01 0.87 DM 0.19 0.02 0.93 PY 0.21 0.01 0.95 GZ 0.30 0.01 0.99 0 048121620 ethene (ppbv) Fig. 6. Typical backward air trajectories in Jianfeng Mountain Fig. 5. Correlation of propene and ethene at the four sites. during sampling periods. ARTICLE IN PRESS J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632 8629 shows the diurnal variations of isoprene at the four hydrocarbon: sites. At the mountainous sites of JM and DM, PropyequivðiÞ¼concðiÞk ðiÞ=k ðC H Þ, isoprene showed similar diurnal patterns, which OH OH 3 6 increased between morning (8:00–9:00) and after- where propy-equiv(i) is a measure of species i on noon (14:00–15:00) and then steadily decreased until an OH reactivity-based scale, normalized to the 20:00–21:00. These patterns were opposite those of reactivity of propylene, kOH(i) is the rate constant other NMHC patterns shown in Fig. 4, and were between species i and OH radical (cited from mainly driven by the local biogenic emission of Atkinson and Arey, 2003), and kOH(C3H6) is the isoprene, which increased with the solar radiation rate constant between C3H6 and OH radical. The and temperature (Kesselmeier and Staudt, 1999). photochemical formation of ozone is inﬂuenced by The levels of isoprene at JM (0.4870.47 ppbv) were many factors besides the reactivity of hydrocarbon, much higher than those at DM (0.1270.08 ppbv), such as the NOx concentration, solar radiation resulting from the tropical forest in JM. The diurnal intensity and meteorological conditions. MIR is a variations of isoprene at the urban GZ site and the good indicator for comparing the ozone formation suburban PY site were negligible. The higher levels potential (OFP) of individual hydrocarbons. The of isoprene at night in GZ (0.3370.22 ppbv) than following equation was used to calculate the the other three sites maybe attributed to vehicular contribution of ozone formation by each hydro- emission of isoprene at GZ urban area, as vehicular carbon under optimal conditions: emission contributes to isoprene levels in urban area Ozone formation potentialðiÞ (Durana et al., 2006). ¼ concentrationðiÞMIR coefficientðiÞ. 3.5. Comparison of the reactivity and ozone MIR coefﬁcients are taken from Carter (1994). formation potential of NMHCs The top 10 hydrocarbons ranked by their propylene-equivalent concentrations and ozone forma- Photochemical reactions of NMHCs are mainly tion potentials were listed in Tables 4 and 5, initiated by the OH radical, and the mechanism of respectively. These NMHCs accounted for about ozone formation for each hydrocarbon varies greatly. 55–65% of the total propylene-equivalent concen- To compare the reactivity and the contribution to tration and about 70% of the total OFP at the three photochemical ozone formation of individual hydro- PRD sites, and 95% of total propylene-equivalent carbons, a propylene-equivalent concentration meth- concentration and 80% of total OFP at the remote od proposed by Chameides et al. (1992) and the JM site. Toluene and m/p-xylene were among the maximum incremental reactivity (MIR) method top three hydrocarbons based on propylene-equiva- proposed by Carter (1994) were used. lent concentrations in GZ, PY and DM, and they The following equation was used to calculate the accounted for 21–27% of the total propylene- propylene-equivalent concentration for individual equivalent concentration. The remaining top 10 hydrocarbons were alkenes and aromatics in the three PRD sites. Toluene, m/p-xylene and ethene 1.6 were the top 3 contributors to the OFP at the three GZ PY DM JM PRD sites, which accounted for 45–52% of the total 1.2 OFP. The high contributions of toluene, m/p-xylene and ethene to ozone formation had also been 0.8 reported in other Asian cities, such as Karachi, Pakistan (Barletta et al., 2002), Hong Kong (So and Wang, 2004) and Kaohsiung, Taiwan (Chang et al., Mixing ratio (ppbv) 0.4 2005). Almost all the remaining top 10 hydrocarbons were also alkenes and aromatics at the three PRD sites. The major source of alkenes was motor 0.0 vehicular exhaust, and the major sources of aro- 08:00 10:0012:00 14:00 16:00 18:00 20:00 matics were industrial emissions and vehicular Local Time emissions in the PRD region (Chan et al., 2006; Fig. 7. Diurnal variations of isoprene at the four sites. Tsai et al., 2006). Therefore, hydrocarbons from ARTICLE IN PRESS 8630 J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632

Table 4 Top 10 NMHCs ranked according to propylene-equivalent concentration (units: ppbC)

Rank Guangzhou Panyu Dinghu Mountain Jianfeng Mountain

Species Conc. (%) Species Conc. (%) Species Conc. (%) Species Conc. (%)

1 m/p-Xylene 17.2 11.2 m/p-Xylene 17.7 14.9 a-Pinene 6.4 16.6 Isoprene 9.1 71.0 2 Toulene 15.0 9.8 Toulene 14.1 11.9 Toulene 4.6 12.0 i-Butene 1.1 8.2 3 Isobutene 8.1 5.3 Styrene 12.6 10.6 m/p-Xylene 3.8 9.7 a-Pinene 0.8 6.3 4 Propene 7.7 4.6 i-Butene 5.0 4.2 Isoprene 2.2 5.7 Propene 0.4 3.0 5 a-Pinene 6.3 4.1 o-Xylene 4.8 4.1 Styrene 1.9 4.8 Ethene 0.3 2.6 6 Styrene 6.3 4.1 a-Pinene 4.6 3.9 Ethene 1.6 4.1 m/p-Xylene 0.2 1.6 7 cis-2-Butene 6.1 4.0 Ehylbenzene 4.0 3.4 Isobutene 1.4 3.6 Toulene 0.1 1.0 8 trans-2-Butene 6.1 4.0 1,2,4-TCM 3.8 3.2 Propene 1.3 3.3 o-Xylene 0.1 0.6 9 2-Methyl-1-2-Butene 6.0 3.9 Ethene 3.5 3.0 o-Xylene 1.1 2.8 Ethyne 0.1 0.5 10 Ethene 5.7 3.6 Isoprene 3.5 2.9 Ethylbenzene 1.0 2.6 Benzene 0.1 0.5

Table 5 Top 10 NMHCs ranked according to ozone formation potential (units: ppbv)

Rank Guangzhou Panyu Dinghu Mountain Jianfeng Mountain

Species OFP (%) Species OFP (%) Species OFP (%) Species OFP (%)

1 Toulene 51.8 16.7 m/p-Xylene 50.8 21.4 Toulene 16.0 20.8 Isoprene 6.2 46.1 2 m/p-Xylene 49.5 15.9 Toulene 48.7 20.4 m/p-Xylene 10.8 14.0 Ethene 2.3 16.7 3 Ethene 37.2 12.0 Ehene 23.5 9.9 Ethene 10.6 13.8 Propene 1.0 7.8 4 Propene 19.4 6.2 o-Xylene 16.7 7.0 o-Xylene 3.8 5.0 i-Butene 0.8 6.2 5 o-Xylene 16.7 5.4 Ethylbenzene 11.2 4.7 Propene 3.5 4.6 1,2,4-TMB 0.6 4.8 6 Ethylbenzene 11.4 3.7 Propene 9.0 3.8 a-Pinene 3.0 3.9 m/p-Xylene 0.6 4.3 7 1,2,4-TMB 9.9 3.2 1,2,4-TMB 7.6 3.2 Ethylbenzene 2.9 3.7 Toulene 0.5 3.5 8 cis-2-Butene 8.3 2.7 i-Prentane 4.7 2.0 i-Pentane 1.9 2.4 a-Pinene 0.4 2.8 9 1-Butene 8.1 2.6 i-Butene 4.0 1.7 1,2,4-TMB 1.7 2.3 o-Xylene 0.3 2.0 10 i-Pentane 7.9 2.5 n-Butane 3.7 1.5 Isoprene 1.5 1.9 Ethane 0.2 1.8 vehicular emissions and industrial emissions played greatly to the levels of aromatic hydrocarbons and a dominant role in ozone pollution in this region. At in particular toluene at the three PRD sites. The the remote JM site, isoprene was the dominant remote JM site was affected more by long-range species while the level of a-pinene is much lower transported air masses and isoprene was the only (0.04 ppbv) and hence isoprene accounted for 71% highly contributing local NMHC. of the total propylene-equivalent concentration and Our measurements have revealed that the diurnal about 40% of the total OFP. So, biogenic emission patterns of the hydrocarbons differed between each was the major contributor to the ozone formation at of the sampling locations. The slopes of propene to this remote site. ethene showed the increasing photochemical age of air masses from urban to suburban, rural and to 4. Conclusions remote sites, and thus in turn indicated the degree of contributions from vehicular emissions. At the Vehicular and industrial emissions were the urban GZ site, the levels of NMHCs were affected primary sources of NMHCs at the GZ urban and by fresh vehicular exhausts, whereas at the sub- suburban sites and the DM rural site, although the urban PY site and rural DM site, transport from magnitudes of the effects differed. In addition, LPG nearby urban areas was the major source of leakage from LPG-fueled vehicles contributed sig- NMHCs. Long-range transport from the upwind nificantly to the high levels of propane and n-butane source regions was the major source of NMHCs at in the urban GZ area. The impact of industrial the remote JM site. Toluene showed a different emissions was widespread and they contributed diurnal pattern from those of other hydrocarbons ARTICLE IN PRESS J.H. Tang et al. / Atmospheric Environment 41 (2007) 8620–8632 8631 from vehicular exhausts in urban GZ, as its major Chan, C.Y., Chan, L.Y., Wang, X.M.etal., 2002a. 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