Biogenic isoprene in subtropical urban settings and implications for air quality

Atmospheric Environment 79 (2013) 369e379

Contents lists available at SciVerse ScienceDirect

Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Biogenic isoprene in subtropical urban settings and implications for air quality Jia-Lin Wang a, Clock Chew b, Chih-Yuan Chang b, Wei-Cheng Liao a, Shih-Chun Candice Lung b, Wei-Nai Chen b, Po-Ju Lee b, Po-Hsiung Lin c, Chih-Chung Chang b, * a b c

Department of Chemistry, National Central University, Chungli 320, Taiwan Research Center for Environmental Changes, Academia Sinica, P.O. Box 1-55, Nankang, Taipei 11529, Taiwan Atmospheric Science, National Taiwan University, Taipei 10617, Taiwan

h i g h l i g h t s
Characteristics of biogenic isoprene in a subtropical metropolis were investigated.
Summertime isoprene ranks highest in OH reactivity among 66 measured VOCs.
Daytime biogenic isoprene overwhelmed anthropogenic isoprene in the summer and autumn.
Daytime residual isoprene contributed a large fraction to nighttime isoprene.
Coherence with OH diurnal cycles underscores the significance of biogenic isoprene.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 March 2013 Received in revised form 27 June 2013 Accepted 28 June 2013

Isoprene has potentially a large impact on secondary oxidant formation, particularly in the polluted urban atmospheres. The environmental conditions in tropical and subtropical cities with high temperatures and light flux are conducive to the production of large amounts of biogenic isoprene. Measurements of speciated volatile organic compounds (VOCs) were conducted in Taipei, a subtropical metropolis, to investigate the characteristics of biogenic and anthropogenic isoprene during the hot seasons (summer and autumn) and to assess their significance in secondary pollutant formation. The daily and daytime average concentrations of isoprene at the subtropical urban site in summer were 0.72 and 1.26 ppbv, respectively, which were considerably higher than the concentrations of isoprene in most temperate cities. Furthermore, summertime isoprene ranks highest in OH reactivity and second highest in terms of ozone formation potential (OFP) among 66 measured VOCs. The ratios of isoprene to 1,3-butadiene, an exhaust tracer, were used to estimate the fractions of biogenic and anthropogenic isoprene in the urban area. The results reveal that the biogenic contribution apparently overwhelmed the anthropogenic contribution in summertime, although traffic in the city is heavy. Furthermore, the residual isoprene (mostly biogenic) after daytime photochemical loss persisted into the nighttime and contributed a large fraction to nighttime isoprene. In autumn, daytime isoprene was also predominantly from biogenic sources because the hot and sunny conditions persist into the autumn months. The high biogenic isoprene levels in subtropical urban settings and its coherence with OH diurnal cycles accentuate the significance of biogenic isoprene and its potentially great impact on atmospheric oxidant capacity, urban air quality, and even regional climate. Ó 2013 Published by Elsevier Ltd.

Keywords: Secondary pollutant Isoprene Volatile organic compounds (VOCs) Secondary organic aerosols (SOA) Vehicular tracer

1. Introduction

* Corresponding author. E-mail address: [email protected] (C.-C. Chang). 1352-2310/$ e see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.atmosenv.2013.06.055

Isoprene has drawn attention because of its pervasive sources and extremely high reactivity that leads to the production of secondary oxidants, e.g., organic peroxy radicals (RO2), ozone, and secondary organic aerosols (SOA), which are of particular

370

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379

importance for atmospheric oxidant capacity, urban air quality and even regional climates (Atkinson, 2000; Chameides et al., 1988; Ryerson et al., 2001; Hallquist et al., 2009; Rollins et al., 2009; Pacifico et al., 2009). As opposed to most other precursors of secondary oxidants, which are almost exclusively from anthropogenic sources, a significant fraction of isoprene is biogenic in origin. In terms of global isoprene emission estimations, approximately 90% of the isoprene released into the atmosphere is produced by terrestrial plants with half from tropical broadleaf trees and the remainder primarily coming from shrubs (Guenther et al., 2006; Arneth et al., 2008). With respect to the investigation of isoprene, most of the areas chosen for study have been located in temperate, tropical, boreal forests and in lands containing scrub, steppe and boggy terrain, and the objective has been to provide information on the temporal variability of biogenic isoprene under ambient conditions. These types of studies are of particular importance for model validation and further estimates of the potential magnitude of associated climate feedbacks (Pacifico et al., 2009). Recently, more and more studies on urban air quality indicated the significant influence of biogenic isoprene on secondary pollutant formation in urban areas (Chameides et al., 1988; Li et al., 2007; Han et al., 2005; Lee and Wang, 2006; Xie et al., 2008; Hellén et al., 2012). In a polluted urban atmosphere with high levels of nitrogen oxides (NOx ¼ NO þ NO2) and other complex pollutants, the addition of biogenic isoprene emissions might substantially enhance the levels of secondary pollutants. The oxidation of isoprene by hydroxyl radical (OH) leads to the production of organic peroxy radical (RO2), which can react with NO to recycle OH and form NO2. Ozone would be produced upon photolysis of NO2. The oxidant products might further react with themselves or with other species to introduce additional functional groups, which may then form diverse oxidant products (e.g., SOA, peroxyacetyl nitrate and organic nitrates) through various reactions (Shirley et al., 2006; Ryerson et al., 2001; Hallquist et al., 2009; Fan and Zhang, 2004; Rollins et al., 2009). By contrast, isoprene emitted into unpolluted atmospheres by terrestrial vegetation is much less harmful compared to its release into polluted urban areas (Lelieveld et al., 2008). The primary environmental controls on biogenic isoprene emission are light and temperature (Guenther et al., 1993; Shao et al., 2001; Sharkey and Yeh, 2001). Biogenic isoprene increases dramatically as the temperature rises and maximizes at approximately 40  C (Guenther et al., 1993). Cities in tropical and subtropical zones with high temperature and light flux provide the conditions that are conducive for biogenic isoprene emissions, and abundant isoprene may have a greater impact on photochemistry. Modeling studies also have indicated the significance of biogenic isoprene emissions on air quality in subtropical urban areas (Li et al., 2007; Chameides et al., 1988; Han et al., 2005). In forest and rural atmospheres, the primary source of isoprene is plants. Nevertheless, isoprene has both biogenic and anthropogenic sources in urban areas (Borbon et al., 2001; McLaren et al., 1996). Several studies have quantified isoprene in vehicle exhausts (Duffy et al., 1999; Borbon et al., 2001), and ambient measurements have shown an anthropogenic origin for isoprene based on its strong correlation with common anthropogenic tracers, such as 1,3-butadiene and carbon monoxide (Reimann et al., 2000; Borbon et al., 2001; McLaren et al., 1996) in cold seasons. Studies in temperate urban areas have reported its relatively strong anthropogenic contribution in winter months and its dual nature of biogenic and anthropogenic origins during summer (Borbon et al., 2001). In crafting control strategies for the key precursors of secondary pollutants in urban areas, investigation of the biogenic and anthropogenic contributions to isoprene is essential. Furthermore, biogenic isoprene flux depends primarily on temperature, solar radiation and species of plants common to the area. Future changes

in climate and land use might lead to substantial changes in the biogenic isoprene budget and in the constituents of urban atmospheres, which would further influence the air quality in urban areas and even the radiative balance in the regional climate. In view of these effects, this study aims to provide a clearer understanding of the characteristics of biogenic and anthropogenic isoprene emissions and to assess their significance in secondary pollutant formation in a subtropical metropolis, in which high levels of biogenic isoprene are expected because of high temperature and light flux, particularly in hot seasons. 2. Methods 2.1. Site description and sampling periods The study was conducted in Taipei, a subtropical metropolis (the capital of Taiwan) (25 000 N/121320 E, 20 m a.s.l.). As with numerous urban settings around the world, smog including ground level ozone and fine particulate matter continues to be a public health issue in Taipei, particularly during the lengthy hot seasons (Taiwan EPA, 2012). Taipei is a typical business-oriented metropolis with a population of greater than six million, with four million registered vehicles. The high temperatures and strong solar intensity in summer render it an ideal location to assess the importance of biogenic isoprene in the air quality of subtropical cities. Fig. 1 shows the sampling sites and the surrounding environments. Ambient air collection was undertaken at an urban site and a road site in the city center for the studies of two typical urban conditions. Furthermore, as a contrast to these urban sites, air sampling was also conducted at a rural/forest site covered by broad-leaved trees, shrubs and grass. 2.1.1. Urban air samples The urban air sampling was conducted over two hot seasons, summer (4e11 July and 15e22 August 2011) and autumn (15e29 October 2012). The sampling inlet was set up on the roof of a 5story building (w25 m above ground) at National Taiwan

Fig. 1. Sampling sites and the surrounding environment in northern Taiwan. Mountainous areas are in darker green, and the gray areas are highly populated, urbanized or developed areas. The circle is the area of metropolitan Taipei. A and B denote the investigated urban and road sites, and C is the rural site, Hualin Weather Station. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379

University (NTU), which is in the southeast of the city and surrounded by business buildings, busy streets, shops and residential apartments. The species of plants near the site within a 3 km  3 km area consist primarily of almost tropical to subtropical broad-leaved trees, including Ficus microcarpa, Liquidambar formosana, Melaleuca leucadendra, Cinnamomum camphora and Koelreuteria henryi, which are common in the city. During the sampling periods, one-hour integrated air samples were collected every two hours to acquire time series data on isoprene and 105 other volatile organic compounds (VOCs). The 7 a.m., 9 a.m., 11 a.m., 1 p.m., 3 p.m. and 5 p.m. samples are defined as the daytime samples, and the 7 p.m., 9 p.m., 11 p.m., 1 a.m., 3 a.m. and 5 a.m. samples are defined as the nighttime samples. Ambient temperature, relative humidity, wind direction, wind speed and solar radiation were recorded every minute. During the investigation period over the summer and autumn, noontime temperatures reached as high as 37  C, and the low temperatures were approximately 22  C. The weather was hot and humid (RH of 55e85%) with mild southerly breezes (w1 m s1) in summer and northeasterly wind (w2 m s1) in autumn; these are typical weather conditions in Taipei in these two seasons. 2.1.2. Road samples To investigate anthropogenic isoprene from vehicular emissions and its correlation with exhaust tracers, flask samples were collected from a major roadway (a two-way, four-lane road carrying approximately 60,000 vehicles per day), which is typical in Taipei and represents a comprehensive mixture of various vehicle types. The sampling was conducted in two seasons, autumn (3e5 October 2012) and winter (19e21 December 2012). Samples collected in winter with weather conditions relatively unfavorable for biogenic emissions were intended to minimize the interference of biogenic isoprene. By contrast, road samples collected in autumn were intended to investigate the influence of ambient biogenic isoprene on the anthropogenic counterpart at the road site. During the sampling periods, one-hour integrated air samples were collected every two hours. Ambient temperature, relative humidity, wind direction, wind speed and solar radiation were also recorded every minute. 2.1.3. Rural/forest samples Rural samples were collected at the Hualin Weather Station (24 530 N/121340 E, 450 m a.s.l.), which is in a valley surrounded by broad-leaved trees, shrubs and grass. The station is located southeast of metropolitan Taipei; the straight distance between the station and the urban center of Taipei is approximately 15 km. Plants near the site within 3 km  3 km consisted primarily of Acacia confusa, Wendlandia formosana, Machilus thunbergii, Schefflera octophylla and Michelia compressa. Sampling was conducted in the summer (4e11 July and 15e22 August 2011) and the dates were identical to the urban sampling to draw a contrast between urban and rural/forest settings. The weather was hot (22e 34  C) and humid (RH of 60e95%) with mild breezes (<1 m s1) and there was no obvious prevailing wind during this summer study period. 2.2. Sampling method and sample analysis All air samples were collected in 2-L electropolished stainlesssteel canisters. Canisters were humidified and evacuated to <102 mmHg prior to sampling, and the cleaning procedures for canisters followed U.S. EPA compendium method TO-15 (1997). The canister was connected to a sampling apparatus with a mass flow controller (MFC) to collect a one-hour integrated air sample (Wang et al., 2012). Flask samples were analyzed by an automated GCeMS/

371

FID (Varian 450 GC and 240MS) system using two columns and two detectors to simultaneously analyze both low- and highboiling VOCs, i.e., isoprene and 105 other volatile VOCs (including selected alkanes, alkenes, aromatics, halocarbons, toxic chlorinated and brominated compounds, ethers, esters and ketones). The analytical system is an upgrade of the one described in the reference (Chang et al., 2003). In brief, a built-in cryo-trap, packed with fine glass beads, was cooled with liquid nitrogen to 170  C for trapping. The air sample collected in the canister was drawn through the cryo-trap at 40 mL min1 for three minutes to yield an aliquot of 120 mL. Subsequently, desorption was performed by flash heating the trap to 120  C; a stream of ultra-high purity helium (99.9999%) was then employed to back-flush the analytes from the trap onto the columns. A glass Y-splitter split the flow of thermal desorption into two columns, i.e., a PLOT column (Chrompack; 30 m  0.32 mm; df ¼ 5.0 mm) connected to FID for the separation and detection of the extremely volatile C2eC4 non-methane hydrocarbons (NMHCs), and a DB-1 column (J&W; 60 m  0.32 mm; df ¼ 1.0 mm) connected to MS for the separation and detection of the remaining heavier VOCs. The chromatograms of isoprene and 105 other volatile VOCs are as shown in Wang et al. (2012). Four internal standards (bromochloromethane, 1,4difluorobenzene, chlorobenzene-d5, and 1-bromo-4-fluorobenzene) were blended with each sample aliquot to check the stability of the MS and to ensure data quality. Two standard gas mixtures, 65C2eC11 NMHCs (Scott Marrin Inc., USA) and 55C1eC10 VOCs that consisted of hydrocarbons, halogenated hydrocarbons, ethers, esters, and ketones (Linde SPECTRA Environmental Gases, USA) were employed for concentration calibrations and quality control. Because there were 14 common compounds found in the two standard gas mixtures, a total of 106 VOCs were targeted with the method. Calibration curves were made by injecting the standard gas mixtures of six different concentrations in the range of 0.05e25 ppbv. The linearities (R2) of the calibration curves for all the target species were greater than 0.9995. Based on the repeated analyses of the standard gas mixtures, the repeatability for most of the target compounds was 0.5e2%, and the limits of detection for most of the species were 3e30 pptv. 2.3. Relative potentials of VOCs for secondary pollutant formation VOCs differ widely in reactivities (Carter and Atkinson, 1989; Carter, 1994), and several reactivity scales of VOCs have been proposed for the quick and convenient evaluation of ozone formation potentials (OFP) or their relative reactivities in the atmosphere. A common and useful reactivity scale is maximum incremental reactivity (MIR) developed by Carter (1994), which uses practical indices for quantifying ozone-forming impacts imposed by precursors in urban-suburban areas (Yu et al., 2000; Hsieh and Tsai, 2003; Xie et al., 2008). In addition, the kOH method is another means of referencing VOC’s reactivities with OH radical (Chameides et al., 1992; Dimitriades, 1996; Atkinson and Arey, 2003); this method is commonly used to measure a VOC’s reactivity relative to that of a reference VOC species. The two different methods address different purposes. After the initial reaction of isoprene with OH, peroxy radicals are formed, and the ensuing reactions with NOx generate photochemical smog in which ozone becomes abundant. Furthermore, these oxidation steps also lead to the generation of a set of secondary products in addition to ozone. Oxidation of isoprene and its products may introduce additional functional groups that tend to make the subsequent products less volatile and more water soluble or that lead to fragmentation of the carbon chains to form oxygenates of lower molecular weight (Hallquist et al., 2009). The MIR method is used to estimate the potentials of individual VOCs for ozone formation. Although the initial reactivity of a VOC with OH in the kOH method

372

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379

does not directly reflect OFP, it implies the potential for subsequent product formation after reaction with OH. In the study, both the MIR method and the kOH method were used for simple evaluation. The OFPs and OH reactivities of individual VOCs were calculated by multiplying individual VOC concentrations measured at the urban site by their corresponding MIR and kOH (reaction rate constants of VOCs with OH radical), as shown in Table 1. 2.4. Assessment of biogenic and anthropogenic contributions to urban isoprene In urban areas, isoprene has both biogenic and anthropogenic sources (Borbon et al., 2001). Several studies have quantified isoprene in vehicle exhaust (Duffy et al., 1999; Borbon et al., 2001), and measurements in temperate urban winter periods have revealed a good correlation between isoprene and common vehicle exhaust tracers, such as 1,3-butadiene, alkenes and carbon monoxide (Reimann et al., 2000; Borbon et al., 2001; McLaren et al., 1996). The results indicate that anthropogenic sources of isoprene in the investigated cities should be mainly attributed to vehicle exhaust, and thus simple regression analyses for the concentrations of isoprene and exhaust tracers can be used to estimate the contributions of vehicular and biogenic sources to ambient isoprene levels (Borbon et al., 2001; Reimann et al., 2000). The contribution of vehicle exhaust to ambient isoprene was assessed by comparing the ratios of ambient isoprene/tracers with the ratios of isoprene/ tracers characteristic of vehicle exhaust. The characteristic ratios of exhaust could be acquired via the following two ways: 1) the ratio of isoprene to exhaust tracer at locations under the dominant influence of vehicle exhaust; and 2) the minimum ratio values observed in an urban setting. In the second approach, the lower values of the isoprene/tracers in an urban setting are most likely to be dominated by vehicle exhaust; other data with higher isoprene/ tracer ratios may result from the additional contribution of biogenic isoprene. 3. Results and discussion 3.1. Overall characteristics of VOCs and their relative potentials for secondary oxidant formation Of the 106 measured VOCs, three categories of alkanes, alkenes and aromatics (66 VOCs totally) deemed to have significant potentials for secondary pollutant formation were chosen for discussion in the study because of their higher reactivities or atmospheric abundance. Fig. 2 shows the average daytime concentrations of the 66 VOCs in hot seasons (summer and autumn) at the urban site. Ethyne (acetylene), toluene and ethene, which come nearly exclusively from anthropogenic sources, were the most abundant species among the 66 VOCs. Ethyne and ethene in urban atmosphere are generated primarily from incomplete combustion, vehicle exhaust in particular. By contrast, toluene comes from multiple sources, including solvent use, gasoline evaporation, and vehicle exhaust. Fig. 2 shows that compounds mainly or partially used as solvents (e.g., toluene) and major gasoline constituents (C4eC6 alkanes) revealed higher concentrations in summer than in autumn in contrast to the species that are primarily emitted from incomplete combustion (e.g., ethyne, ethene, propene, isobutene). These results agree with the theory that the higher temperature in summer is favorable for evaporation of solvents and gasoline. Table 1 shows the daytime and nighttime average concentrations of the 66 VOCs. In the summer, concentrations of most VOCs were slightly higher at night than during the day. However, two species, toluene and isoprene, revealed notably higher daytime

concentrations, indicating that the two compounds apparently had higher daytime emissions. Higher daytime toluene may be mainly attributed to daytime higher temperatures, which are favorable to the evaporation of solvent toluene, whereas higher isoprene concentrations during the day than at night are usually associated with biogenic sources. Table 2 shows the ambient isoprene concentrations at different locations in temperate and subtropical zones. The daily and daytime average concentrations of isoprene in the summer at the urban site in Taipei were 0.72 and 1.26 ppbv, respectively. The values are comparable to the levels of isoprene in other subtropical metropolises, i.e., 0.63 ppbv (daily average concentrations) at an urban site in metropolitan Houston (29 470 N/95 210 W, Park et al., 2011) and 0.3e1.2 ppbv at four sites in Hong Kong (22 230 N/114 060 E, So and Wang, 2004). In contrast to concentrations in temperate zones, isoprene levels in these subtropical cities were much higher, implying that isoprene may play a more important role on photochemistry in subtropical cities than in temperate cities. To help further assess the importance of isoprene in the subtropical urban atmosphere in photochemical pollutant formation, the MIR method and kOH method (reactivity of VOC with OH radical) were used for quick and convenient evaluation. Fig. 3 shows the relative OFPs and OH reactivities of the 66 VOCs at the urban site for the daytime hours of summer. With respect to the OFPs of VOCs estimated by the MIR method, toluene was the top species with the highest OFP, which accounted for 19.2% of the total OFPs of the 66 VOCs. Isoprene was second, which accounted for 13.9% of the total OFPs. Following were m,p-xylene, ethene, o-xylene and propene, accounting for 10.6%, 7.5%, 4.3% and 4.2%, respectively. The top 10 reactive species assessed by the kOH method are shown in Fig. 3b. Comparing the results estimated by the MIR and kOH methods, the top 10 species with the largest OFPs and OH reactivities were nearly consistent, although their ranking was different. The top species with the highest OH reactivity was isoprene, which accounted for over 1/3 of the reactivity of the total reactivity of the 66 VOCs; its reactivity was approximately equal to the sum of the next nine species (see Fig. 3b), which indicates the vital significance of summertime isoprene to atmospheric chemistry in subtropical urban areas. Fig. 3 shows that the fraction of isoprene in total OH reactivities was much larger than the fraction of isoprene in total OFPs. The MIR value of isoprene listed in Table 1 is close to that of other reactive species, such as xylene, ethene and propene, which reveals their approximate potentials for ozone formation on a per-gram-of-VOC basis. Nevertheless, the reaction rate constant of isoprene with OH radical is much higher than the rate constants of other compounds by a factor of 5e10. The higher abundance of isoprene at the subtropical urban site, in addition to its high reactivity with OH, pushes isoprene to the top of the OH reactivity list. 3.2. Biogenic and anthropogenic contributions to isoprene Summertime isoprene ranked first in OH reactivity and was the second highest OFP species among 66 VOCs, indicating the significant potential of isoprene to influence urban atmospheric chemistry and air quality. Isoprene has both biogenic and anthropogenic sources in urban areas. Investigation on biogenic and anthropogenic contributions to ambient isoprene is essential to the understanding of their respective significance for urban air quality. Previous studies reported 1,3-butadiene as a suitable exhaust tracer for source apportionment of isoprene because of their robust correlation in traffic-related sources and the close chemical properties (Borbon et al., 2001; Reimann et al., 2000). Although the tracer method by employing the ratio of isoprene to 1,3-butadiene may not be applicable in the industrial type of metropolitan areas where

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379

373

Table 1 Photochemical properties of 66 VOCs and their average mixing ratios at the urban site in summer and autumn; units ppbv. Compound

Alkane Ethane Propane Isobutane n-Butane Isopentane n-Pentane 2,2-Dimethylbutane Cyclopentane 2-Methylpentane 3-Methylpentane n-Hexane Methylcyclopentane 2,4-Dimethylpentane Cyclohexane 2-Methylhexane 2,3-Dimethylpentane 3-Methylhexane 2,2,4-Trimethylpentane n-Heptane Methylcyclohexane 2,3,4-Trimethylpentane 2-Methylheptane 3-Methylheptane n-Octane n-Nonane n-Decane n-Undecane Alkenes Ethene Propene trans-2-Butene 1-Butene Isobutene cis-2-Butene 3-Methyl-1-butene 1-Pentene 1,3-Butadiene Isoprene trans-2-Pentene cis-2-Pentene 2-Methyl-2-butene Cyclopentene 4-Methyl-1-pentene 2-Methyl-1-pentene trans-2-Hexene cis-2-Hexene Alpha-pinene Beta-pinene Alkyne Ethyne Aromatics Benzene Styrene Toluene Ethylbenzene m,p-Xylene o-Xylene Isopropylbenzene n-Propylbenzene m-Ethyltoluene p-Ethyltoluene 1,3,5-Trimethylbenzene o-Ethyltoluene 1,2,4-Trimethylbenzene 1,2,3-Trimethylbenzene m-Diethylbenzene p-Diethylbenzene o-Diethylbenzene a b c

MIRa

kOH  1012b

Summer

Autumn

Daytime

Nighttime

Daytime

Nighttime

2.32 2.34 1.06 2.06 1.51 0.66 0.07 0.09 0.47 0.32 0.40 0.06 0.22 0.10 0.18 0.07 0.22 0.20 0.19 0.07 0.08 0.05 0.05 0.07 0.06 0.06 0.05

2.40 2.81 1.18 2.28 1.54 0.65 0.07 0.09 0.49 0.34 0.43 0.05 0.24 0.11 0.17 0.07 0.20 0.22 0.18 0.07 0.09 0.05 0.05 0.06 0.07 0.07 0.06

2.69 1.65 0.65 1.07 0.90 0.24 0.05 0.10 0.25 0.16 0.15 0.03 0.13 0.05 0.11 0.04 0.12 0.15 0.11 0.06 0.06 0.03 0.03 0.05 0.04 0.04 0.02

3.09 2.07 0.74 1.25 0.92 0.26 0.05 0.10 0.25 0.17 0.17 0.03 0.13 0.05 0.09 0.04 0.10 0.13 0.10 0.06 0.06 0.03 0.03 0.05 0.04 0.04 0.02

8.5 26.3 64 31.4 51.4 56.4 31.8 31.4 66.6 101 67 65 68.9 67 63 63 63 63 53.7 78.9

2.34 0.56 0.10 0.14 0.37 0.09 0.03 0.06 0.08 1.26 0.12 0.05 0.13 0.02 0.05 0.04 0.05 0.02 0.21 0.01

2.59 0.81 0.16 0.18 0.51 0.12 0.04 0.08 0.14 0.19 0.21 0.09 0.21 0.03 0.04 0.05 0.06 0.03 0.12 0.01

1.91 0.53 0.08 0.12 0.41 0.08 0.02 0.03 0.09 0.38 0.11 0.04 0.12 0.01 0.02 0.03 0.03 0.01 0.08 0.01

1.75 0.53 0.09 0.13 0.38 0.08 0.02 0.04 0.10 0.05 0.13 0.05 0.12 0.01 0.02 0.03 0.04 0.01 0.06 0.01

0.5

0.9

3.83

3.73

3.83

3.29

0.69 1.66 3.93 2.96 8.54 7.58 2.45 1.96 7.39 4.39 11.75 5.54 8.83 11.94 7.08 4.39 NRc

1.23 58 5.96 6.96 20.5 13.6 6.6 5.7 18.6 11.8 56.7 11.9 32.5 32.7 15 10 NRc

0.46 0.05 3.43 0.30 0.75 0.35 0.03 0.06 0.18 0.07 0.05 0.08 0.25 0.07 0.02 0.07 0.01

0.44 0.07 2.42 0.30 0.90 0.39 0.03 0.06 0.22 0.08 0.08 0.09 0.33 0.09 0.02 0.09 0.01

0.36 0.02 1.40 0.17 0.49 0.22 0.02 0.04 0.12 0.05 0.04 0.06 0.20 0.05 0.01 0.04 0.01

0.32 0.03 1.15 0.16 0.48 0.21 0.02 0.03 0.12 0.05 0.04 0.05 0.20 0.05 0.01 0.04 0.01

0.25 0.46 1.18 1.08 1.36 1.22 1.11 2.24 1.4 1.69 1.14 1.46 2.05 1.14 1.09 1.25 1.5 1.2 0.97 1.56 0.97 1.12 0.8 0.68 0.59 0.52 0.47 7.4 11.57 15.2 9.57 5.3 14.26 6.2 7.07 10.9 10.48 10.47 10.28 6.4 7.7 6.7 6.7 6.7 6.7 3.3 4.4

0.27 1.15 2.34 2.54 3.9 3.94 2.23 5.16 5.6 5.7 5.6 5.1 5.7 7.49 6.9 5.1 5.1 3.68 7.15 10.4 7 8.3 8.6 8.68 10.2 11.6 13.2

MIR denotes maximum incremental reactivity (g O3/g VOCs, Carter, 1994). kOH denotes rate constant of VOCs react with hydroxyl radicals at 298 K, (Atkinson and Arey, 2003). NR: not reported.

374

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379

5.0

summer

autumn

4.0

3.0

2.0

1.0

toluene

Alkenes

ethylbenzene m,p-xylene o-xylene isopropylbenzene n-propylbenzene m-ethyltoluene p-ethyltoluene 1,3,5-trimethylbenzene o-ethyltoluene 1,2,4-trimethylbenzene 1,2,3-trimethylbenzene m-diethylbenzene p-diethylbenzene o-diethylbenzene

isoprene

Alkanes

trans-2-pentene cis-2-pentene 2-methyl-2-butene cyclopentene 4-methyl-1-pentene 2-methyl-1-pentene trans-2-hexene cis-2-hexene alpha-pinene beta-pinene ethyne benzene styrene

ethane propane isobutane n-butane isopentane n-pentane 2,2-dimthylbutane cyclopentane 2-methylpentane 3-methylpentane n-hexane 2,4-dimethylpentane methylcyclopentane cyclohexane 2-methylhexane 2,3-dimethylpentane 3-methylhexane 2,2,4-trimethylpentane n-heptane methylcyclohexane 2,3,4-trimethylpentane 2-methylheptane 3-methylheptane n-octane n-nonane n-decane undecane ethene propene trans-2-butene 1-butene isobutene cis-2-butene 3-methyl-1-butene 1-pentene 1,3-butadiene

0.0

Aromatics

Fig. 2. Mean daytime average concentrations of 66 VOCs at the urban site (unit: ppbv). Error bars represent standard deviations (1s).

anthropogenic isoprene and 1,3-butadiene can be partially contributed by industrial sources whose constituents are usually complex and variable (Na et al., 2001; Karl et al., 2003), the approach is quite robust in business-oriented cities where vehicular exhaust is often the single most dominant source of anthropogenic isoprene and 1,3-butadiene. When employing 1,3-butadiene to assess vehicular and biogenic contributions to isoprene, the difference in chemical reaction rates between isoprene and 1,3-butadiene needs to be taken into consideration. In daytime, the chemical removal of VOCs in the atmosphere is mainly dominated by the reaction with OH radicals (Atkinson and Arey, 2003). In theory, the ratio of isoprene/tracer could vary during daytime due to their different reaction rates toward OH, which could lead to a bias in the estimate of vehicular

contribution to daytime isoprene. Because isoprene is highly reactive toward OH (Table 1), it is reacted away faster than most of common tracers, which would result in an overestimate of vehicular contribution. To reduce the influence on the ratio, 1,3butadiene with relatively close rate constant to isoprene was chosen as the tracer in several studies (Borbon et al., 2001; Reimann et al., 2000). Borbon et al. (2001) reported that isoprene was very well correlated with 1,3-butadiene (R2 ¼ 0.96) in temperate urban areas in wintertime when anthropogenic sources dominated atmospheric isoprene. The reported robust correlation suggested that the isoprene/1,3-butadiene ratios in the urban environment were well maintained. Other than the relatively close rate constants between isoprene and 1,3-butadiene, the continuous supply of fresh isoprene and 1,3-butadiene from motor vehicles in urban

Table 2 Isoprene ambient concentrations measured at different locations in temperate and subtropical zones. Site

Location

Year

Season

Mean concentration or range (ppbv)

Reference

SMEARIII (urban background site) Eltham (suburban site)

Helsinki (60 120 N/24 580 E, 26 m a.s.l.) London (51 270 N/0 030 E, 62 m a.s.l.)

2011 1998 to 2009

Summer All year

0.11 (310 ng m3) (daily) 0.02e0.33 (monthly)

Air Quality Monitoring Network of Lille Metropol stations (urban site)

Lille, France (50 370 N/3 20 E, 20 m a.s.l.)

1997 to 1999

Summer

0.29 (daily)

Hellén et al., 2012 von Schneidemesser et al., 2011 Borbon et al., 2001

Kasernenhof (urban background site)

Zurich (47 220 N/8 310 E, 441 m a.s.l.)

2005

Peking University (urban site) YUFA (rural site) 3e4 km north of downtown Houston (urban site) Four sites (rural, residential, industrial and roadside) Nine stations (urban site) National Taiwan University (urban site) Hualin Weather Station (rural site)

Beijing (39 590 N/116 180 E, 57 2m a.s.l.) Beijing (40 000 N/116 250 E, 45 m a.s.l.) Houston (29 470 N/95 210 W)

2006 2006 2008

Summer Autumn Summer Summer Summer

0.16 0.08 0.89 0.64 0.63

Hong Kong (22 230 N/114 060 E)

2000e2001

Summer

0.3e1.2 (monthly)

So and Wang, 2004

2002 2011 2011

Autumn Summer Summer

0.6 (daytime) 1.26 (daytime)/0.72 (daily) 1.31 (daytime)/0.70 (daily)

Chang et al., 2005 This study This study



0



0

Kaohsiung (22 37 N/120 34 E, 14 m a.s.l.) Taipei (25 000 N/121 320 E, 17 m a.s.l.) Northern Taiwan (24 530 N/121 340 E, 431 m a.s.l.)

(daily) (daily) (daytime) (daytime) (daily)

Legreid et al., 2007 Xie et al., 2008 Xie et al., 2008 Park et al., 2011

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379

(a)

(b)

toluene 19.2%

rest 56 VOCs 29.5%

isoprene 13.9%

n-butane 2.0% isopentane 2.3% methyltoluene 2.4% 1,2,4trimethylben propene o-xylene zene 4.1% 4.2% 4.3%

375

rest 56 VOCs 28.9% 1,2,4trimethylben zene 2.3%

isoprene 35.6%

trans-2pentene 2.3% 2-methyl-2butene 2.4% ethene 7.5%

m,p-xylene 10.6%

alpha-pinene 3.2%

toluene 5.8% propene m,p-xylene isobutene 4.3% 5.3% 4.2%

ethene 5.6%

Fig. 3. Fractional contributions of VOCs to (a) total OFP and (b) total OH reactivity at the urban site during the daytime in the summer.

settings and unceasing mixing with the existing portion was probably another cause to lower the influence on the ratio. As a result, 1,3-butadiene was employed as the exhaust tracer in the study. Its correlation with isoprene during daytime and nighttime in different seasons and environments are discussed as follows. 3.2.1. Road site To investigate the correlation between anthropogenic isoprene and exhaust tracers in the city, flask samples were collected from a major road near the investigated urban site in autumn and winter. Isoprene has no (or negligible) emissions from biogenic sources without illumination (Shao et al., 2001; Sanadze, 2004); thus, the isoprene/tracer ratio of the nighttime samples at the road site dominated by traffic emissions can generally be considered the value characteristic of anthropogenic emissions. During the nighttime of the two seasons, isoprene was well correlated with many exhaust tracers (e.g., 1-butene, isobutene, 1-pentene, 1,3-butadiene, trans-2-pentene and cis-2-pentene), with R2 > 0.95, which reveals nighttime isoprene at the road site was almost exclusively from vehicular emissions. Among these tracers, 1,3-butadiene exhibited the best correlation to isoprene (R2 ¼ 0.99). Fig. 4 shows the plots of isoprene vs. 1,3-butadiene at the road site in autumn and winter. During winter (Fig. 4a), the nighttime data represented by solid circles revealed a notably high correlation between isoprene and 1,3-butadiene, and the ratio of isoprene to 1,3-butadiene resulting from the nighttime regression slope (black line) was 0.42. To validate that the isoprene/1,3-butadiene ratio is typical, the autumn samples at the road site were also analyzed to

(a)

obtain nighttime regression slopes (the black line in Fig. 4b). The ratios of nighttime isoprene/1,3-butadiene for the winter and autumn samples are highly consistent, which suggests that the ratio of 0.42 is representative of the characteristic ratio vehicular emissions that can be exploited as a gauge to assess the excess concentration of isoprene contributed by biogenic sources. In comparison with the high consistence of isoprene/1,3butadiene ratios during the nighttime, the daytime data (the open circles in Fig. 4) show many outliers indicating that the daytime isoprene at the road site was not exclusively from vehicular emissions. Fig. 5a shows the temporal variation of temperature and solar radiation at the road site in winter. Although winter sampling was undertaken in order to minimize the interference of biogenic isoprene, the outliers of isoprene/1,3-butadiene in Fig. 4a that occurred around noon on 20 December were most likely caused by additional biogenic isoprene as a result of elevated temperatures (20e25  C) and radiation flux during that period (Fig. 5a). 3.2.2. Urban site The plots of isoprene versus 1,3-butadiene at the urban site are shown in Fig. 6. The superimposed red dashed line is the regression line fit to nighttime road samples to accentuate the baseline established by vehicular emissions. When additional sources of isoprene are present, it causes ratios to rise above the regression line. In summer, most of the daytime samples (the open circles in Fig. 6a) showed high isoprene/1,3-butadiene ratios well above the regression line of the road data (red dashed line), which indicates

(b)

1.2

1.2

winter

autumn 0.9

Isoprene

Isoprene

0.9

0.6

y = 0.421x + 0.008 R² = 0.996

0.3

0.6

y = 0.419x + 0.014 R² = 0.991

0.3

0.0

0.0

0.0

1.0

2.0

1,3-Butadiene

3.0

0.0

1.0

2.0

3.0

1,3-Butadiene

Fig. 4. Mixing ratios of isoprene versus 1,3-butadiene at the road site in (a) winter and (b) autumn. Solid circles denote the data for the nighttime, and open circles denote the data for the daytime. Black lines represent the regression lines fit to the nighttime samples (solid circles).

376

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379

97% of the daytime isoprene was released by biogenic sources, and only 3% could be attributed to vehicular emissions. Although the difference in the rate constants between isoprene and 1,3butadiene could lead to an overestimate of the vehicular contribution to daytime isoprene, this influence on this result is small since only about 3% daytime isoprene was attributed to vehicular emissions. Regarding the nighttime data in summer, initially, it was expected that a good correlation between isoprene and 1,3-butadiene would be observed because isoprene has no (or negligible) emissions from biogenic sources without illumination. Nevertheless, the results suggest otherwise with an R2 ¼ 0.38, and Fig. 6a shows that most of the nighttime data (solid circles) were above the regression line of the road samples. These data lying above the regression line indicate that there should be sources other than motor vehicles contributing to nighttime isoprene; perhaps some of the daytime air laden with isoprene was persisting into the nighttime. This possibility will be discussed in depth in the next section. In autumn, the daytime data scatter apparently (the open circles in Fig. 6b). The average ratio of measured isoprene/1,3-butadiene at the urban site is 7.8 times that of the vehicular sources; thus, approximately 89% of daytime isoprene is estimated to be from biogenic sources in the autumn. The result reveals that biogenic sources in autumn continued to have a significant contribution to ambient isoprene. In contrast to the daytime data, the autumn nighttime data revealed a high correlation (R2 ¼ 0.92) between isoprene and 1,3-butadiene (the solid circles in Fig. 6b), which indicates that nighttime isoprene at the urban site was mainly from vehicles. However, most of the nighttime urban data were above the regression line of the road data (red dashed line). The slope of the regression line (black line) fit to the nighttime urban samples was 0.48, which is slightly higher than the 0.42 of the road data regression line. The difference was likely caused by a small amount of non-vehicular isoprene added into the air; this will be discussed in the next section. Fig. 5. Time-series data of temperature, solar radiation, measured isoprene and calculated anthropogenic isoprene at the road site in (a) winter and (b) autumn.

that there were notable emissions of biogenic sources contributing to the daytime isoprene levels. The average ratio of measured isoprene/1,3-butadiene at the urban site during daytime is 16.1, which is on average 38.3 times that of the pure vehicular emissions (ratio ¼ 0.42). In other words, it is estimated that approximately

(a) 3.0

3.3. Diurnal patterns of temperature, solar radiation, ambient isoprene and calculated anthropogenic isoprene To investigate the diurnal emission characteristics of subtropical urban isoprene and assess the influence of temperature and solar radiation on isoprene during the day in different seasons, air sampling and meteorological observation were conducted synchronously in the urban site to obtain the diurnal variation of

(b) 1.6 summer

2.5

autumn 1.2

Isoprene

Isoprene

2.0 1.5 1.0

y = 1.242x + 0.021 R² = 0.394

0.8

y = 0.483x + 0.003 R² = 0.923

0.4

0.5 0.0

0.0 0.0

0.2

0.4

1,3-Butadiene

0.6

0.0

0.2

0.4

0.6

1,3-Butadiene

Fig. 6. Mixing ratios of isoprene versus 1,3-butadiene at the urban site in (a) summer and (b) autumn. Solid circles denote the data for the nighttime, and open circles denote the data for the daytime. Black lines represent the regression lines fit to the nighttime samples (solid circles). Superimposed red dashed line is the regression line (y ¼ 0.421  þ0.008) fit to the winter nighttime road samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379

377

ambient VOCs and meteorological factors. In addition to the urban sampling, ambient air at a rural/forest site (Hualin Weather Station) was collected to draw a contrast. 3.3.1. Rural/forest site Fig. 7 shows the diurnal characteristics of temperature, solar radiation and measured isoprene levels at the rural site in the summer. The rural site had minimal traffic and insignificant levels of anthropogenic VOCs; most of the 1,3-butadiene concentrations were below the detection limit of 10 pptv. The measured isoprene at this site showed a simple pattern, and the daily maximum isoprene occurred at approximately noon. After that, isoprene decreased gradually to its minimum level at late night. The distinct diurnal profile and the sensitivity to sunlight intensity and temperature indicate that the source of this isoprene to be primarily biogenic. 3.3.2. Urban site The diurnal variations of temperature, solar radiation, measured isoprene and calculated anthropogenic isoprene at the urban site are shown in Fig. 8. In the summertime, the temperature varied from 24  C to 37  C with regular diurnal patterns of maximum temperatures at approximately 1e2 p.m. and minima before dawn. Ambient isoprene also showed a diurnal pattern of maximum mixing ratios at approximately midday and minima at night (Fig. 8a), which reflects its biogenic nature that is in close relationship with temperature and radiation. However, the measured isoprene at the urban site should have an additional traffic contribution embedded in its data. Shown in Fig. 8a, the calculated anthropogenic isoprene was estimated based on the characteristic ratio of isoprene to 1,3-butadiene representative of vehicular emissions. The calculated anthropogenic isoprene shows two humps due to rush hour traffic, 6e8 a.m. and 5e9 p.m. Besides larger emissions during the rush hour, a lower boundary layer in the early morning was conducive to the elevation of anthropogenic isoprene level (sharp morning peak) whereas a deeper afternoon boundary layer would lead to dilution of isoprene around noon and a broad evening hump. Nevertheless, the calculated anthropogenic isoprene was overwhelmed by the biogenic isoprene during the daytime, although the traffic is heavy in the city. Furthermore, the average daily maximum isoprene concentration was 1.75 ppbv at the urban site, which was comparable to the level of 1.86 ppbv observed at the rural/forest site.

Fig. 7. Diurnal variations of temperature, solar radiation and ambient isoprene at the rural site (Hualin Weather Station) in summer.

Fig. 8. Diurnal characteristics of temperature, solar radiation, measured isoprene and calculated anthropogenic isoprene at the urban site in (a) summer and (b) autumn.

Although the emission strength of isoprene is important from the perspective of photochemistry, the timing of release is also critical in terms of shaping the daily ozone profile. The midday surge of biogenic isoprene could produce a much larger loss and a more efficient production of midday ozone because the midday peak of isoprene is concurrent in time with the peak of OH, a condition that could maximize photochemical loss (Lee and Wang, 2006). The high biogenic isoprene levels in subtropical urban settings and its coherence with OH diurnal cycles accentuate the significance of biogenic isoprene. Regarding the nighttime data in summer, Fig. 8a shows that the measured nighttime isoprene was higher than the calculated anthropogenic isoprene, which indicates that there should be sources contributing to nighttime isoprene in addition to vehicular exhaust. However, biogenic isoprene should fall to negligible rates without illumination (Shao et al., 2001; Sanadze, 2004), i.e., when night falls. The reason for these increased levels is likely that the weather in the subtropical city during the daytime in summer is rather favorable to high biogenic isoprene, and the residual isoprene (mostly biogenic) after daytime photochemical loss persists into the nighttime (Fig. 8a). At the urban site, the average ratio of isoprene/1,3-butadiene at night in the summer is 3.3 times that of vehicular emissions; thus, approximately 77% of the nighttime isoprene is estimated to be contributed from the daytime isoprene. Summing up the summertime results, biogenic isoprene dominated

378

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379

the isoprene level during the daytime, and the residual isoprene after daytime photochemical loss also contributed a non-negligible fraction to nighttime isoprene, and might have a large impact on nighttime chemistry. In autumn, the temperature varied from 20  C to 31  C, and solar intensity was slightly weaker than in the summer (Fig. 8b). Measured isoprene obviously showed the maximum mixing ratios at approximately midday, which indicates that ambient isoprene was predominated by biogenic sources in the autumn daytime. The calculated anthropogenic isoprene in nighttime lapped over the measured isoprene. This phenomenon reveals that vehicular isoprene accounted for most ambient isoprene during nighttime, which is different from the summer nighttime isoprene. Nevertheless, it is likely to have a little remainder of daytime isoprene persisting into the nighttime, which caused autumn nighttime data to be slightly above the regression line of road data (Fig. 6b). The importance of biogenic isoprene in autumn in the subtropical metropolis is apparent, and this may also be illustrated by the autumn data of the road site shown in Fig. 5b. The daytime isoprene was apparently influenced by biogenic sources although the road site was predominated by vehicular emissions and the weather during the sampling period was not ideal for the generation of biogenic isoprene (the maximum temperature during the sampling was only 27  C). These results reveal that biogenic isoprene in autumn still had a significant contribution to ambient isoprene at the urban and road sites. Isoprene in this city has more pronounced biogenic emissions than cities in temperate zones in autumn due to the fact that this city is located in the subtropical zone with abundant broad-leaved evergreen trees and its hot and sunny weather pattern persists into autumn months. Because of the high emission potential of biogenic isoprene in the subtropical urban areas in autumn, isoprene also plays a significant role in autumn photochemistry, in addition to its significant summer role. 4. Conclusions The daily and daytime average concentrations of isoprene in the summer at the subtropical urban site were 0.72 and 1.26 ppbv, respectively, which were higher than the measurements in many urban and rural areas in temperate zones. Summertime isoprene ranked first in OH reactivity and was the second highest OFP species among 66 VOCs, which indicates the significant potential of isoprene to influence urban atmospheric chemistry and secondary pollutant formation. The robust ratios of isoprene/1,3-butadiene obtained from traffic emissions served as a gauge to further assess the excess concentration of isoprene contributed by biogenic sources. The results reveal that biogenic isoprene in the summertime overwhelmed anthropogenic isoprene, although the traffic is usually heavy in the city. Furthermore, the residual isoprene (mostly biogenic) after daytime photochemical loss contributed a large fraction to nighttime isoprene and has potentially an impact on nighttime chemistry. In autumn, daytime isoprene was also predominated by biogenic sources because of the subtropical city’s hot and sunny weather pattern persisting into the autumn months. These results indicate the significance of biogenic isoprene in subtropical urban settings; the high levels of biogenic isoprene and its coherence with OH diurnal cycles accentuate its potentially great impact on urban air quality. Acknowledgment The authors would like to thank Prof. Charles C.-K. Chou and Mr. C.-Y. Tsai for logistic support. Discussions with Prof. Jen-Ping Chen and SOA group are gratefully acknowledged. This research was

supported in part by the National Science Council, Taiwan, under contract number NSC99-2111-M-001-006-MY3. References Arneth, A., Monson, R.K., Schurgers, G., Niinemets, U., Palmer, P.I., 2008. Why are estimates of global isoprene emissions so similar (and why is this not so for monoterpenes)? Atmospheric Chemistry and Physics 8, 4605e4620. Atkinson, R., 2000. Atmospheric chemistry of VOCs and NOx. Atmospheric Environment 34, 2063e3101. Atkinson, R., Arey, J., 2003. Atmospheric degradation of volatile organic compounds. Chemical Reviews 103, 4605e4638. Borbon, A., Fontaine, H., Veillerot, M., Locoge, N., Galloo, J.C., Guillermo, R., 2001. An investigation into the traffic-related fraction of isoprene at an urban location. Atmospheric Environment 35, 3749e3760. Carter, W.P.L., 1994. Development of ozone reactivity scales for volatile organic compounds. Journal of Air Waste Management Association 44, 881e899. Carter, W.P.L., Atkinson, R., 1989. Computer modeling study of incremental hydrocarbon reactivity. Environmental Science & Technology 23, 864e880. Chameides, W.L., Linsay, R., Richardson, R., Kiang, C., 1988. The role of biogenic hydrocarbons in urban photochemical smog: Atlanta as a case study. Science 241, 1473e1475. Chameides, W.L., Fehsenfeld, F., Fodgers, M.O., Cardelino, C., Martinez, J., Parrish, D., Lonneman, W., Lawson, D.R., Rasmussen, R.A., Zimmerman, P., Greenberg, J., Mlddleton, P., Wang, T., 1992. Ozone precursor relationships in the ambient atmospheric. Journal of Geophysical Research 97, 6037e6055. Chang, C.C., Lo, S.J., Lo, J.G., Wang, J.L., 2003. Analysis of methyl tert-butyl ether (MTBE) in the atmosphere and implications as an exclusive indicator of automobile exhaust. Atmospheric Environment 37, 4747e4755. Chang, C.C., Chen, T.Y., Lin, C.Y., Yuan, C.S., Liu, S.C., 2005. Effects of reactive hydrocarbons on ozone formation in southern Taiwan. Atmospheric Environment 39, 2867e2878. Dimitriades, B., 1996. Scientific basis for the VOC reactivity issues raised by section 183(e) of the Clean Air Act Amendments of 1990. Journal of Air Waste Management Association 46, 963e970. Duffy, B.L., Nelson, P.F., Ye, Y., Weeks, I.A., 1999. Speciated hydrocarbon profiles and calculated reactivities of exhaust and evaporative emissions from 82 in-use light-duty Australian vehicles. Atmospheric Environment 33, 291e307. Fan, J., Zhang, R., 2004. Atmospheric oxidation mechanism of isoprene. Environmental Chemistry 1, 140e149. Guenther, A., Zimmerman, P., Harley, P., Monson, R., Fall, R., 1993. Isoprene and monoterpene emission rate variability: model evaluation and sensitivity analysis. Journal of Geophysical Research 98, 12609e12617. Guenther, A., Karl, T., Harley, P., Wiedinmyer, C., Palmer, P.I., Geron, C., 2006. Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature). Atmospheric Chemistry and Physics 6, 3181e3210. Han, Z., Ueda, H., Matsuda, K., 2005. Model study of the impact of biogenic emission on regional ozone and the effectiveness of emission reduction scenarios over eastern China. Tellus B 57, 12e27. http://dx.doi.org/10.1111/j.16000889.2005.00132. Hallquist, M., Wenger, J.C., Baltensperger, U., Rudich, Y., Simpson, D., Claeys, M., Dommen, J., Donahue, N.M., George, C., Goldstein, A.H., Hamilton, J.F., Herrmann, H., Hoffmann, T., Iinuma, Y., Jang, M., Jenkin, M., Jimenez, J.L., Kiendler-Scharr, A., Maenhaut, W., McFiggans, G., Mentel, Th.F., Monod, A., Prévôt, S.H., Seinfeld, J.H., Surratt, J.D., Szmigielski, R., Wildt, J., 2009. The formation, properties and impact of secondary organic aerosol: current and emerging issues. Atmospheric Chemistry and Physics 9, 5155e5236. Hellén, H., Tykkä, T., Hakola, H., 2012. Importance of monoterpenes and isoprene in urban air in northern Europe. Atmospheric Environment 59, 59e66. Hsieh, C.C., Tsai, J.H., 2003. VOC concentration characteristics in Southern Taiwan. Chemosphere 50, 545e556. Karl, T., Jobson, T., Kuster, W.C., Williams, E., Stutz, J., Shetter, R., Hall, S.R., Goldan, P., Fehsenfeld, F., Lindinger, W., 2003. Use of proton-transfer-reaction mass spectrometry to characterize volatile organic compound sources at the La Porte super site during the Texas Air Quality Study 2000. Journal of Geophysical Research 108 (D16), 4508. http://dx.doi.org/10.1029/2002JD003333. Lee, B.S., Wang, J.L., 2006. Concentration variation of isoprene and its implications for peak ozone concentration. Atmospheric Environment 40, 5486e5495. Legreid, G., Lööv, J.B., Staehelin, J., Hüglin, C., Hill, M., Buchmann, B., Prevot, A.S.H., Reimann, S., 2007. Oxygenated Volatile Organic Compounds (OVOCs) at an urban background site in Zürich (Europe): seasonal variation and source allocation. Atmospheric Environment 41, 8409e8423. Lelieveld, J., Butler, T.M., Crowley, J.N., Dillon, T.J., Fischer, H., Ganzeveld, L., Harder, H., Lawrence, M.G., Martinez, M., Taraborrelli, D., Williams, J., 2008. Atmospheric oxidation capacity sustained by a tropical forest. Nature 452, 737. Li, G., Zhang, R., Fan, J., Tie, X., 2007. Impacts of biogenic emissions on photochemical ozone production in Houston, Texas. Journal of Geophysical Research 112, D10309. http://dx.doi.org/10.1029/2006JD007924. McLaren, R., Singleton, D.L., Lai, J.Y.K., Khouw, B., Singer, E., Wu, Z., Niki, H., 1996. Analysis of motor vehicle sources and their contribution to ambient hydrocarbon distributions at urban sites in Toronto during the Southern oxidants study. Atmospheric Environment 30, 2219e2232.

J.-L. Wang et al. / Atmospheric Environment 79 (2013) 369e379 Na, K., Kim, Y.P., Moon, K.C., Moon, I., Fung, K., 2001. Concentrations of volatile organic compounds in an industrial area of Korea. Atmospheric Environment 35, 2747e2756. Pacifico, F., Harrison, S., Jones, C., Sitch, S., 2009. Isoprene emissions and climate. Atmospheric Environment 43, 6121e6135. Park, C., Schade, G., Boedeker, I., 2011. Characteristics of the flux of isoprene and its oxidation products in an urban area. Journal of Geophysical Research 116, D21303. http://dx.doi.org/10.1029/2011JD015856. Reimann, S., Pierluigi, C., Hofer, P., 2000. The anthropogenic fraction contribution to isoprene concentrations in a rural atmosphere. Atmospheric Environment 34, 109e115. Rollins, A.W., Kiendler-Scharr, A., Fry, J.L., Brauers, T., Brown, S.S., Dorn, H.-P., Dube, W.P., Fuchs, H., Mensah, A., Mentel, T.F., Rohrer, F., Tillmann, R., Wegener, R., Wooldridge, P.J., Cohen, R.C., 2009. Isoprene oxidation by nitrate radical: alkyl nitrate and secondary organic aerosol yields. Atmospheric Chemistry and Physics 9, 6685e6703. Ryerson, T.B., Trainer, M., Holloway, J.S., Parrish, D.D., Huey, L.G., Sueper, D.T., Frost, G.J., Donnelly, S.G., Schauffler, S., Atlas, E.L., Kuster, W.C., Goldan, P.D., Hubler, G., Meagher, J.F., Fehsenfeld, F.C., 2001. Observations of ozone formation in power plant plumes and implications for ozone control strategies. Science 292, 719e723. Sanadze, G.A., 2004. Biogenic isoprene (a review). Russian Journal of Plant Physiology 51 (6), 729e741. Shao, M., Czapiewski, K.V., Heiden, A.C., Kobel, K., Komenda, M., Koppmann, R., Wildt, J., 2001. Volatile organic compound emissions from Scots pine: mechanisms and description by algorithms. Journal of Geophysical Research 106 (D17), 20483e20491.

379

Sharkey, T.D., Yeh, S., 2001. Isoprene emission from plants. Annual Review of Plant Physiology and Plant Molecular Biology 52, 407e436. Shirley, T.R., Brune, W.H., Ren, X., Mao, J., Lesher, R., Cardenas, B., Volkamer, R., Molina, L.T., Molina, M.J., Lamb, B., Velasco, E., Jobson, T., Alexander, M., 2006. Atmospheric oxidation in the Mexico City Metropolitan Area (MCMA) during April 2003. Atmospheric Chemistry and Physics 6, 2753e 2765. So, K.L., Wang, T., 2004. C3eC12 non-methane hydrocarbons in subtropical Hong Kong: spatial-temporal variations, source receptor relationships and photochemical reactivity. Science of the Total Environment 328, 161e174. Taiwan Environmental Protection Administration (Taiwan EPA), 2012. Air Quality Annual Report of Taiwan, 2011. Taipei, Taiwan (in Chinese). U.S. EPA, 1997. Compendium Method TO-15, the Determination of Volatile Organic Compounds (VOCs) in Air Collected in Specially Prepared Canisters and Analyzed by Gas Chromatography/mass Spectrometry (GC/MS). von Schneidemesser, E., Monks, P.S., Gros, V., Gauduin, J., Sanchez, O., 2011. How important is biogenic isoprene in an urban environment? A study in London and Paris. Geophysical Research Letters 38, L19804. Wang, J.L., Chang, C.C., Lee, K.Z., 2012. In-line sampling with gas chromatographye mass spectrometry to monitor ambient volatile organic compounds. Journal of Chromatography A 1248, 161e168. Xie, X., Shao, M., Liu, Y., Lu, S., Chang, C., Chen, Z., 2008. Estimate of initial isoprene contribution to ozone formation potential in Beijing, China. Atmospheric Environment 42, 6000e6010. Yu, T.Y., Lin, Y.C., Chang, L.F., 2000. Optimized combinations of abatement strategies for urban mobile sources. Chemosphere 41, 399e407.