Organosulfates from Pinene and Isoprene over the Pearl River Delta

Article pubs.acs.org/est

Organosulfates from Pinene and Isoprene over the Pearl River Delta, South China: Seasonal Variation and Implication in Formation Mechanisms Quan-Fu He,† Xiang Ding,*,† Xin-Ming Wang,† Jian-Zhen Yu,‡ Xiao-Xin Fu,† Teng-Yu Liu,† Zhou Zhang,† Jian Xue,‡ Duo-Hong Chen,§ Liu-Ju Zhong,§ and Neil M. Donahue⊥ †

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ‡ Chemistry Department & Division of Environment, Hong Kong University of Science & Technology, Hong Kong, China § Environmental Monitoring Center of Guangdong Province, Guangzhou, 510045, China ⊥ Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *

ABSTRACT: Biogenic organosulfates (OSs) are important markers of secondary organic aerosol (SOA) formation involving cross reactions of biogenic precursors (terpenoids) with anthropogenic pollutants. Until now, there has been rare information about biogenic OSs in the air of highly polluted areas. In this study, fine particle (PM2.5) samples were separately collected in daytime and nighttime from summer to fall 2010 at a site in the central Pearl River Delta (PRD), South China. Pinene-derived nitrooxyorganosulfates (pNOSs) and isoprene-derived OSs (iOSs) were quantified using a liquid chromatograph (LC) coupled with a tandem mass spectrometer (MS/MS) operated in negative electrospray ionization (ESI) mode. The pNOSs with MW 295 exhibited higher levels in fall (151 ± 86.9 ng m−3) than summer (52.4 ± 34.0 ng m−3), probably owing to the elevated levels of NOx and sulfate in fall when air masses mainly passed through city clusters in the PRD and biomass burning was enhanced. In contrast to observations elsewhere where higher levels occurred at nighttime, pNOS levels in the PRD were higher during the daytime in both seasons, indicating that pNOS formation was likely driven by photochemistry over the PRD. This conclusion is supported by several lines of evidence: the specific pNOS which could be formed through both daytime photochemistry and nighttime NO3 chemistry exhibited no day− night variation in abundance relative to other pNOS isomers; the production of the hydroxynitrate that is the key precursor for this specific pNOS was found to be significant through photochemistry but negligible through NO3 chemistry based on the mechanisms in the Master Chemical Mechanism (MCM). For iOSs, 2-methyltetrol sulfate ester which could be formed from isoprene-derived epoxydiols (IEPOX) under low-NOx conditions showed low concentrations (below the detection limit to 2.09 ng m−3), largely due to the depression of IEPOX formation by the high NOx levels over the PRD.

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INTRODUCTION

oxidation and NO3 dark reactions can form pinene-derived nitrooxy-organosulfates (pNOSs) on acidic particles.9 The pNOSs were only detected in nighttime samples in northeastern Bavaria, Germany, suggesting a role for nighttime NO3 chemistry in pNOS formation.15 However, if acidic aerosols are required by the NO3 pathway,9 what is the explanation for the phenomena that neutralized aerosols coincided with the high overnight concentrations of pNOSs?26 On the other hand, Kristensen and Glasius suggested that photochemical reactions were the dominant source of pNOSs based on the positive

Biogenic volatile organic compounds (BVOCs) including isoprene and monoterpenes1 contribute significantly to the global secondary organic aerosol (SOA) budget.2 Recent studies have shown that the conversion of BVOCs to SOA can be significantly promoted in the presence of high anthropogenic emissions.3−6 As notable SOA products of BVOCs reacting with anthropogenic pollutants under acidic conditions, organosulfates (OSs) have been detected in both laboratory-generated SOA7−10 and ambient aerosols.11−18 Moreover, OSs could contribute a significant fraction of fine particles, accounting for up to 30% of organic matter (OM)9,19−23 and up to 10% of total sulfate.24,25 The formation mechanisms of OSs are still unclear. Previous chamber studies have demonstrated that both OH-photo© 2014 American Chemical Society

Received: Revised: Accepted: Published: 9236

March 16, 2014 July 4, 2014 July 14, 2014 July 14, 2014 dx.doi.org/10.1021/es501299v | Environ. Sci. Technol. 2014, 48, 9236−9245

Environmental Science & Technology

Article

shown in Figure S1 in the Supporting Information, WQS is located in the central Pearl River Delta (PRD) region, about 60 km away from the Pearl River estuary and surrounded by large city clusters (e.g., Hong Kong, Guangzhou, Shenzhen, Zhongshan, Dongguan). As the surrounding terrain is flat with extensive farmland nearby and little traffic, this site is an ideal place to monitor the regional background air pollution in the PRD. Previous studies were undertaken at this site to characterize regional air pollution, such as O3, PM2.5, SOA, and haze.32,41−44 A high volume sampler (Tisch Environmental Inc., Ohio, USA) was set up on the rooftop of a seven-floor building, about 30 m above ground on the campus of a middle school, to collect PM2.5 onto prebaked 8 in. × 10 in. quartz filters at a flow rate of 1.1 m3 min−1. All the filters were prebaked at 500 °C for 24 h, covered with aluminum foil, and stored in zipped antistatic bags containing silica gel at 4 °C before and −20 °C after collection. Field blanks were collected with the same manner as ambient sample for 5 min when the sampler was turned off. The summer and fall campaigns were undertaken in 2010 from September 9 to 30 and November 1 to 26, respectively. Daytime (6:00−18:00) and nighttime (18:00− 6:00) samples were separately collected during summer and November 17−26. Twenty-four hour sampling (18:00−18:00) was carried out from November 1 to 16, 2010. A total of 62 PM2.5 samples were collected, along with 4 field blanks. A previous study revealed that the emissions of both isoprene and monoterpenes were indeed high in September and dropped in November in the PRD.45 Thus, September and November could be regarded as typical summer and fall, respectively. Chemical Analysis. Detailed information about the extraction procedure was presented previously.9,21 Briefly, a punch (Φ = 47 mm) was taken of each filter and extracted twice for 30 min in 30 mL methanol by ultrasonication. Before extraction, ketopinic acid was spiked onto the filters as an internal standard. The extracts were combined and concentrated to approximately 5 mL using a rotary evaporator at 35 °C, centrifuged (Heraeus Multifuge X3R, Thermo Fisher, USA) for 10 min at 12000 rpm, and syringe filtered (0.22 μm, PTFE, JET.BIOFIL, China). The remaining liquid was then evaporated to near dryness under a gentle N2 stream and redissolved in 500 μL of a 1:1 (v/v) solvent mixture of 0.1% acetic acid in water (LC-MS-grade, Merk) and 0.1% acetic acid in methanol (LC-MS-grade, Merk). Blank filters were analyzed in the same manner as field samples. Sample extracts were analyzed in the full scan mode by an Agilent 6410 Triple Quadrupole liquid chromatography (LC) mass spectrometer equipped with an electrospray ionization (ESI) source operated in negative ion mode. A Waters Atlantic T3 column (3 μm; 2.1 mm × 250 mm, Waters, Milford, USA) was employed. The eluents were water with 0.1% acetic acid (eluent A) and methanol (eluent B) with a total flow rate 0.2 mL/min. The mobile-phase gradient was set as follows: the composition started with 3% B held constant for 5 min; increased to 90% B over 25 min, held for 10 min; increased to 95% B over 2 min, held for 6 min; and finally decreased to 3% B over 5 min and held for about 12 min until the pump pressure balanced. The ESI tandem MS was operated with nebulizer pressure at 0.8 bar and a dry gas flow of 10 L min−1. A solution of camphor sulfonic acid (Sigma-Aldrich, USA) was used to optimize ionization and fragmentation parameters to gain high intensity of [M−H]−, the resulting ionization voltage was 4 kV and the fragmentor was 120 V.

correlations observed between pNOSs and photochemically derived species.21 For isoprene OSs (iOSs), the major product, 2-methyltetrol sulfate ester, is formed through reactive uptake of low-NOx products, epoxydiols (IEPOX), onto acidic particles.27,28 High concentrations of IEPOX (up to 1.44 ppbv)29 and 2methyltetrol sulfate esters (up to 196.5 ng m−3)16 were reported in southeastern United States (U.S.). However, high NOx levels in polluted areas would depress IEPOX formation and accordingly result in low production of 2-methyltetrol sulfate ester. As the GEOS-Chem model predicted, IEPOX levels are much lower in South China than in the southeastern U.S., probably due to the depression of IEPOX formation under the influence of high anthropogenic NOx emissions.30 Thus, the levels of 2-methyltetrol sulfate esters might be also lower in South China, in spite of very high particle acidity.31 A previous study found that the correlation between aerosol acidity and 2-methyltetrols, which could be also formed by the uptake of IEPOX, was much weaker in the air of South China32 as compared with that in chamber studies.33,34 Thus, more field studies in highly polluted regions are needed to provide insight into the formation mechanisms of biogenic OSs under the impact of anthropogenic pollution. There is relatively little information about OSs in regions of Asia that are highly influenced by human activity. Stone et al. reported the characterization of OSs in Maldives, Korea, Singapore, and Pakistan.25 In China, there are only three studies available at present. OSs were detected but not quantified in humic-like substances (HULIS) in the Pearl River Delta (PRD) and Taiwan, and the results indicated that organosulfates and nitrooxy-organosulfates derived from monoterpenes were the dominant species.35 The pinenederived nitrooxy-organosulfates (pNOSs) with MW 295 were also observed in PM10 samples in Beijing.36 Ma et al. recently reported seasonal and diurnal trends of OSs in Shanghai.37 More studies in the air of Chinese city clusters are still necessary for further understanding in the formation mechanisms of OSs in heavily polluted areas. The PRD in South China is one of the most urbanized and industrialized regions in China. The economy has grown rapidly in the past decades accompanied by large increases in emissions of anthropogenic pollution (e.g., NOx and SO2).38 Located in the subtropics, the PRD region has an annual average temperature of 25 °C and high biogenic emissions throughout the year.39 Abundant sulfate and nitrate make the aerosols very acidic, e.g. the pH values of particles were reported in the range of −0.62 to 2.35 in Hong Kong.31 Moreover, increasing surface ozone and NOx concentrations indicates enhanced oxidation capacity of the air.40 All these factors would favor biogenic OSs formation, and thus, OSs should be important particulate components in the PRD. In this study, PM2.5 samples were collected at a regional background site in the PRD from September to November. The most abundant OSs from pinene and isoprene were measured, including pNOSs with MW 295, 2-methyltetrol sulfate ester with MW 216 from IEPOX, and 2-methylglyceric acid sulfate ester with MW 200 from methacrylic acid epoxide (MAE), to characterize these biogenic OSs and investigate their formation mechanisms in this heavily polluted region.

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EXPERIMENTAL SECTION Field Sampling. Field sampling was conducted at a regional background site, Wanqingsha (WQS, 22°42′N, 113°32′E). As 9237

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Table 1. Summary of Particulate and Gaseous Species in the Air of the PRD Region Summer mean PM2.5 (μg m−3) OC (μgC m−3) EC (μgC m−3) OM (μg m−3)a NO3− (μg m−3) SO42− (μg m−3) NH4+ (μg m−3) O3 (ppbv) NOx (ppbv) SO2 (ppbv) temperature (°C) RH (%) [H+] (nmol m−3)c

47.1 8.16 1.88 16.3 2.84 9.09 3.13 42.6 22.9 11.4 29.6 79.7 60.9

cis-pinonic acid pinic acid 3-methyl-1,2,3-butanetricarboxylic acid 3-hydroxyglutaric acid 3-hydroxy-4,4-dimethylglutaric acid sum of monoterpene tracers pNOSs iOS (MW 216) iOS (MW 200) pNOSs/OM (%) iOSs/OM (‰)

3.66 0.66 8.80 5.53 4.82 23.5 52.4 0.68 0.36 0.04

median

Fall min

max

39.9 17.6 91.8 8.36 3.72 14.1 1.83 1.08 2.91 16.7 7.4 28.2 2.32 0.71 6.21 8.35 2.91 17.6 3.21 0.70 5.60 41.5 13.1 66.9 22.6 13.9 35.4 9.43 5.11 30.2 29.8 28.3 31.1 78.5 75.6 85.3 56.6 32.4 143 SOA Tracers (ng m−3) 3.67 1.79 5.11 0.62 0.28 1.20 7.79 0.85 19.2 4.95 0.58 11.9 3.45 0.50 13.9 20.9 4.0 50.5 48.2 14.4 133 0.45 nd 2.09 nd nd nd 0.28 0.08 1.20 0.03 nd 0.10

mean

median

min

max

S/Fb

97.9 18.2 3.13 36.4 10.4 18.6 8.83 40.1 37.5 17.0 21.6 69.1 65.9

93.9 16.6 2.90 33.2 8.49 17.5 7.44 37.8 33.4 16.4 21.6 71.2 63.8

61.9 10.5 1.74 21.1 3.72 6.98 3.02 23.9 25.0 6.83 18.6 48.6 3.69

191 28.3 7.73 56.6 41.4 40.2 24.4 62.6 65.0 30.9 24.0 87.4 161

0.48 0.45 0.60 0.45 0.27 0.49 0.35 1.06 0.61 0.67 1.37 1.15 0.92

11.0 3.6 15.5 11.6 10.9 54.4 151 0.38 0.28 0.44 0.03

9.15 2.89 13.2 8.7 10.9 52.0 144 0.34 0.64 0.42 0.02

1.07 0.96 2.49 0.92 3.34 34.7 42.2 nd nd 0.08 nd

29.6 14.2 26.0 29.3 20.5 97.7 470 1.06 1.23 1.15 0.10

0.33 0.18 0.57 0.48 0.44 0.43 0.35 1.81 0.82 1.33

a c

Particulate organic matter (OM) was estimated by OC multiplying a factor of 2.0. bS/F means the ratios of components in summer and fall. Aerosol acidity was estimated by charge balance of SO42−, NO3−, and NH4+.

Data were acquired and processed using Mass Hunter B002.0 version. OSs were identified by comparison of mass spectra with literature data.9,46 MS2 spectra of product ions were obtained in the MS2-Scan mode to identify the structures of parent ions, [M−H]−. Figure S2 shows the MS/MS spectra of five OSs with parent ion m/z 199 (2-methylglyceric acid sulfate ester), 215 (2-methyltetrol sulfate ester), and 294 (pNOSs), respectively. All the MS/MS spectra displayed intense m/z 97 (HSO4−), m/z 96 (·SO4−) or m/z 80 (SO3−) product ions. The presence of 34S-containing isotopic peaks coincident with the target ions provided an additional confirmation of 32Scontaining formulas.35 For pNOSs, additional MS/MS product ions with m/z 247 (loss of HNO2) and m/z 231 (loss of HNO3) were detected.9 The structures of three pNOS isomers with MW 295 (pNOSa, pNOSb, pNOSc) were identified by comparing the detected spectra (Figure S2c−e) with those provided by Surratt et al.9 The isomer pNOSa was identified by extremely low intensity of m/z 231 ions and high intensity of m/z 220 (loss of CO and NO2) in MS2 spectra (Figure S2c); while the isomer pNOSc was identified by significant intensity of m/z 142 (−OSO3NO2) (Figure S2e). Due to the lack of commercial standard, OSs were quantified using camphor sulfonic acid.21,26 Seven-point linear calibration of peak area vs concentration (both normalized by the internal standard) were established with the correlation coefficient R2 > 0.99. Three pNOS isomers with MW 295, iOS with MW 216 and MW 200 were quantified since the intensity of their peaks were high enough for quantifications (signal/noise > 3). The use of surrogate standard thus adds uncertainty to the absolute concentrations since ESI response factors may be different for

different compounds. Nevertheless, these data can still provide important information on variation in levels and time-trends. A punch (1.5 cm × 1.0 cm) of each filter was taken for the measurements of OC and EC using the thermo-optical transmittance (TOT) method by an OC/EC Analyzer (Sunset Laboratory Inc.).47 An additional punch (Φ = 47 mm) was taken and extracted twice for 30 min in 10 mL of 18-MOhm milli-Q water under sonication for the analysis of sulfate (SO42−), nitrate (NO3−), and ammonium (NH4+) with an ion chromatograph (Metrohm 883, Switzerland). All the data of the major components in PM2.5 were corrected using field blanks. Table 1 lists all data obtained during our campaigns. Quality Assurance/Quality Control (QA/QC). Field and laboratory blank samples were analyzed in the same way as ambient samples. OSs were not detected in the field and laboratory blanks. Recoveries of camphor sulfonic acid in seven spiked samples (authentic standards spiked into solvent with prebaked quartz filter) were 94 ± 4%. The relative differences for target compounds in paired duplicate samples (n = 7) were all 0.27, p < 0.01), implying that BB events might enhance the pNOSs levels.53 All these explained the higher levels of pNOSs observed in fall. Five terpenoic acids from monoterpene-derived SOA (pinic acid, cis-pinonic acid, 3-hydroxyglutaric acid, 3-hydroxy-4,4-

0.577

77.3

Figure 1. Daily variations of pNOSs (MW 295) in summer and fall at a regional background site in the Perl River Delta. Three different isomers are shown with different colors, and the total contribution to organic mass (%) is shown to on the right-hand axis.

nd

0.4 3.6 140 0.63 27.1 0.2 1.51 81.5 0.2 6

instruments and standards were used for pNOSs quantification. The particulate OM was estimated as 2.0 × OC.43 The pNOSs accounted for 0.08−1.20% and 0.08−1.15% of OM in summer and fall, respectively (Figure 1), which was lower than those (1−30% of OM) over Europe and the U.S.9,19−23

Data were estimated based on the figure in ref 26. Summer.

0.277 nd one-year Southeastern US Shanghai, China

0.5 0.66

b

1.18

3.4

1.75

Article

a

ref instruments

HPLC/ESI-MS/MS HPLC/ESI-MS/MS UPLC/ESI-HR-TOFMS HPLC-ESI-QTOF-MS LC-LXQ linear ion trap MS LCT/ESI-TOF-MS

surrogate standards

camphor sulfonic acid camphor sulfonic acid camphor sulfonic acid camphor sulfonic acid octyl sulfate sinapic acid, suberic acid

max

133 470

min

14.4 42.2 48.2 144

median

52.4 151

mean max

75.9 127 4.3 4.17 49.2 0.3

min median

24.8 74.4 261 460 3.2

max min

12.4 80.4 0.1

median season

summer fall summer spring summer summer

location

WQS WQS California, USa Silkeborg, Denmark Antwerp, Belgium

79.5 214

overall night day

Table 2. Pinene Derived Nitrooxy-organosulfates (MW 295) in the PRD Region and Other Places (ng m−3)

this study this study 26 21 12 48


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Table 3. Correlations of pNOSs (MW 295) with Terpenoic Acidsa Summer (September 9−30) pNOSa pNOSb pNOSc

cis-pinonic acid

pinic acid

3-hydroxyglutaricacid

3-hydroxy-4,4-dimethylglutaric acid

3-methyl-1,2,3-butanetricarboxylic acid

0.7148 0.7178 0.7106

0.7036 0.7416 0.7386

0.5357 0.5662 0.5669

0.5326 0.5470 0.5636

0.7163 0.7640 0.7629

Fall (November 1−26) pNOSa pNOSb pNOSc a

cis-pinonic acid

pinic acid

3-hydroxyglutaricacid

3-hydroxy-4,4-dimethylglutaric acid

3-methyl-1,2,3-butanetricarboxylic acid

0.4262 0.4461 0.4148

0.1943 0.2749 0.1185

0.2247 0.2733 0.1714

0.2773 0.3451 0.1626

0.2647 0.2908 0.2519

Correlation coefficients in bold are indicative of p < 0.01.

dimethylglutaric acid, 3-methyl-1,2,3-butanetricarboxylic acid) were also measured and exhibited higher levels in fall (54.4 ± 16.3 ng m−3) than summer (23.5 ± 13.9 ng m−3), probably due to gas/particle partitioning which is favored by lower temperature in fall.41 These tracers were consistently 2−3 times less abundant than pNOSs in both seasons (Table 1). It is worth noting that the correlations among these tracers exhibited significant seasonal differences. In summer, the pNOSs and terpenoic acids exhibited strong correlations; while in fall, the correlations vanished (Table 3). These terpenoic acids are first- or second- generation oxidation products of monoterpene SOA mainly through homogeneous reactions.54−56 Aerosol acidity had negative influence on terpenoic acids’ levels in particle-phase, due to negative impact on gas/particle partitioning.32,33 And sulfate should have no significant impact on gas/particle partitioning of terpenoic acids. In contrast, pNOSs are formed by heterogeneous reactions on acidic particles.9,34,57 High sulfate levels would promote pNOSs’ formation.26 Since there was no significant difference in aerosol acidity between summer and fall (p > 0.05), aerosol acidity should not be the major factor leading to the seasonal differences in correlations between terpenoic acids and pNOSs. On the other hand, sulfate levels were about twice higher in fall than summer. During summer, when air masses mainly originated from oceans and coastal regions and sulfate levels were relatively lower, the formation of both terpenoic acids and pNOSs might be controlled by monoterpene emissions. Thus, good correlations between pNOSs and terpenoic acids were observed in summer. During fall, when air masses mainly passed through PRD city clusters, high sulfate levels would facilitate pNOSs formation but not so for terpenoic acids. Thus, the difference in formation mechanisms of the terpenoic acids and the pNOSs leads to poor correlations observed in fall. Day−Night Variation of pNOSs. Figure 2 presents the diurnal variation of pNOSs during the sampling period. Total pNOS levels were consistently a factor of ∼3 higher during the day than at night in both summer (92.0 vs 28.2 ng m−3 on average) and fall (234 vs 81.1 ng m−3 on average). Neither wind direction nor wind speed exhibited obvious diurnal variations. The cluster analysis revealed that the air masses at WQS exhibited no significant difference between daytime and nighttime in each season (Figure S3). Thus, the observed daynight difference of pNOSs was not due to transport. Monoterpene emissions positively depend on temperature and light.58,59 During the day, relatively high temperature and intense solar radiation enhance monoterpenes emissions. Moreover, ozone, NOx, aerosol acidity, and sulfate all exhibited

Figure 2. Diurnal variations of pNOSs (MW 295) in summer and fall. Daytime concentrations were consistently much larger than at night.

higher levels in daytime (Figure S4). In the presence of acidic sulfate seeds, all three pNOS isomers with m/z 294 could be produced through photo-oxidation. The reaction of α-pinene with OH radicals forms β-hydroxyalkyl radicals, which immediately react with O2 to form the respective hydroxyperoxy (RO2) radicals. Those RO2 further react with NO to produce hydroxynitrates (MW 215).9 Hydroxynitrates uptake onto acidic seeds generates the pNOSs. Thus, it is expected that daytime photochemistry played a dominant role in pNOSs formation over the PRD and accordingly high levels of pNOSs occurred in the daytime during our campaign. Unlike our observation in the PRD, high levels of pNOSs were observed in the nighttime and early morning in Antwerp, Belgium,12 K-puszta, Hungary,60 northeastern Bavaria, Germany,15 the Sierra Nevada Mountains, California,26 and Shanghai.37 Nighttime NO3 chemistry was proposed to account for the high levels of nighttime pNOSs. If nighttime NO3 oxidation played an important role in pNOSs formation, the specific pNOSc isomer that could be also formed via NO3 pathway should be relatively more abundant in nighttime samples and the relative abundance of the three isomers should show a significant diurnal variation. To check day-night differences in the relative abundance of the three pNOS isomers, we show a ternary plot of the daytime and nighttime samples in Figure 3. There was no significant difference between daytime and nighttime samples in either season, demonstrating that nighttime NO3 chemistry was not predominant for pNOSs formation in the PRD region. To further check the relative contributions of OH-initiated and NO3-initiated chemistry to the formation of the pNOSc 9240



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pinene of 6.21 × 10−12 cm3 molecules−1 s−1 with (k1).63 Thus, both of these steps will be rate-limited α-pinene emission. Because the daytime flux of α-pinene was about twice higher than the nighttime flux,39 the production rate of hydroperoxy radical 1 in daytime was also about twice higher than that of nitroperoxy radical 1 in nighttime. Both hydroperoxy radical 1 and nitroperoxy radical 1 are potential sources of hydroxynitrate, as shown in Scheme 1. During the day, NO would compete against RO2 and HO2 to react with hydroperoxy radical 1; and NO3 reaction could be ignored due to its high photolysis rate. Thus, the fractional yield of hydroxynitrate in the OH pathway is calculated by f OH = k7[NO] × 0.23/(k7[NO] + k8[RO2] + k10[HO2]). During the night, NO3 reaction should be the dominant process. Thus, the reactions of NO and HO2 with nitroperoxy radical 1 could be ignored. Thus, the fractional yield of hydroxynitrate in the NO3 pathway is calculated by f NO3 = k2[RO2] × 0.1/(k2[RO2] + k3[NO3]). During our campaign, [NO] were averagely 5.0 ppbv (1.35 × 1011 molecules cm−3) in daytime. [HO2] observed over the PRD were ∼1 × 109 molecules cm−3 in daytime and at least 1 order of magnitude lower in nighttime.61 Since [HO2] and [RO2] levels might be comparable in the ambient (e.g., 6.8 × 108 molecules cm−3 vs 4.5 × 108 molecules cm−3 in summer of Beijing),64 here we assumed that [RO2] had the same levels as [HO2] . Thus, the f OH and f NO3 were calculated as 0.23 and 1.2 × 10−4, respectively with the rate constants in Scheme 1 at T = 298.15 K. Although both hydroperoxy radical 1 and nitroperoxy radical 1 are potential sources of hydroxynitrate, the hydroperoxy pathway initiated by OH in the day is far more efficient. This is because the dominant sink of hydroperoxy radicals during the day in the PRD was reaction with NO which produces the hydroxynitrate with roughly 30% yield; while the dominant sink of nitroperoxy radicals at night was reaction with NO 3 , which is not a source of hydroxynitrate. The hydroxynitrate source at night is the asymmetric (molecular) pathway of the RO2 cross reaction, which is a minor pathway of a minor reaction.

Figure 3. Relative abundances of three pNOSs isomers (MW 295) in summer and fall, showing no systematic day−night difference and only modest seasonal variation.

isomer, we employed the Master Chemical Mechanism (MCM) to explore the formation of hydroxynitrates (MW 215) (http://mcm.leeds.ac.uk/MCM/browse.htt?species= APINENE) during daytime and nighttime, following the mechanisms proposed by Surratt et al.9 As shown in Scheme 1, the first step for hydroxynitrate formation is the reaction of α-pinene with OH to form a hydroperoxy radical or with NO3 radical to form a nitroperoxy radical. At a site near WQS, [OH] in daytime and [NO3] in nighttime were measured, respectively in July, 2006. During the day, [OH] was 1 × 107 molecules cm−3,61 resulting in an α-pinene lifetime of ∼30 min, given a rate constant for OH + a-pinene of 5.25 × 10−11 cm3 molecules−1 s−1 (k6). Likewise, at night, [NO3] was 21.8 ± 1.8 pptv (5.86 × 108 molecules cm−3),62 resulting in an αpinene lifetime of ∼4.5 min, given a rate constant for NO3 + α-

Scheme 1. Formation Mechanisms of pNOSc from α-Pinene Oxidized by NO3 and OH Radicals

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about twice as high in summer than in fall (0.68 vs 0.38 ng m−3, Table 1). Our observation were much lower than those reported in the southeastern U.S. (up to197 ng m−3)16,18,29 and in the Belgian forest (3.0−9.0 ng m−3).12 Worton et al. compared the quantified results of 2-methyltetrol sulfate ester using as surrogate standards either camphor sulfonic acid or OS formed from the dihydroxy epoxide of butadiene and pointed out that the former underestimated the 2-methyltetrol sulfate ester level by a factor of 4 compared with the latter.26 Even increasing our measured values by a factor of 4, the levels of 2methyltetrol sulfate ester in the PRD were still much lower than those reported in the southeastern U.S. and in the Belgian forest. Figure S5 shows a typical chromatogram of the sample at WQS. The highly polar 2-methyltetrol sulfate ester (m/z 215) showed much lower intensity as compared with the pNOSs (m/z 294). Conversely, 2-methyltetrol sulfate ester exhibited higher intensity than pNOSs in the southeastern U.S.9 So the question is why 2-methyltetrol sulfate ester exhibited trace amount in the PRD? 2-Methyltetrol sulfate ester is produced through the reactive uptake of the low-NOx intermediates, IEPOX onto acidic particles.27,28 Thus, aerosol acidity and IEPOX formation should have significant impact on 2-methyltetrol sulfate ester formation. Similar as the observation in the U.S.,16,29 2methyltetrol sulfate ester exhibited no correlation with aerosol acidity (p > 0.5) in both seasons, indicating that the variations of aerosol acidity had no strong impact on 2-methyltetrol sulfate ester levels in the PRD. On the other hand, IEPOX formation is limited by anthropogenic pollutants that the yield drops rapidly with increasing NOx levels.30 The PRD is one of the fastest developing regions in China. Anthropogenic activities here have led to immense emission of NOx. Previous studies demonstrated that ozone production over the PRD was VOCs-limited due to abundant amount of NOx.68 During our campaigns, daily and hourly NOx levels were up to 65.0 and 163 ppbv, respectively. Thus, the high NOx concentration could depress IEPOX formation and result in low levels of 2methyltetrol sulfate ester over the PRD. A recent model study predicted that, in urban (high-NOx) environments, 2methyltetrol sulfate ester would have no significant contribution to SOA at 80% RH and 20 μg m−3 sulfate.69 That is what we observed in the real atmosphere of PRD. 2-Methylglyceric acid sulfate ester (m/z 199) was only detected in 10 fall samples, with an average value of 0.72 (0.54−1.23) ng m−3. As chamber results showed, high-NOx levels shift the isoprene oxidation pathway from producing IEPOX to MAE; and the reactive uptake of MAE to acidic particles can produce 2-methylglyceric acid as well as its sulfate ester (m/z 199) and oligoesters.17,29,70 Additionally, temperature has important influence on the fate of peroxymethylacrylic acid epoxide (MPAN) which is the precursor of MAE. When temperature is over 20 °C, thermal decomposition of MPAN is very fast, leading to a decline in MAE formation.29 During our sampling, temperature was higher than 20 °C in both seasons, which would prevent MAE production from MPAN and further limit the formation of 2-methylglyceric acid sulfate ester. This explained why the signal of 2-methylglyceric acid sulfate ester was mostly below the detection limit.

We also employed the observation based model (OBM) to simulate the radical concentrations from November 18 to 26 based on our observation of VOCs (data not shown), NOx, O3, and meteorological parameters. The detailed information on OBM was described in a previous study in the PRD.65 Solar radiation (SR) was applied to distinguish between photochemistry and nighttime chemistry that when SR > 0 f OH was calculated; otherwise f NO3 was calculated. After all f OH and f NO3 were obtained, the radical-weighted averages of f OH and f NO3 ( fOH and fNO3 ) were calculated as fOH = (∑f OH[OH])/ ∑[OH] and fNO3 = (∑f NO3[NO3])/∑[NO3]. The fOH and

fNO3 were 0.23 and 4.7 × 10−4, respectively. Again, f NO3 was very minor, suggesting that the NO 3 route forming hydroxynitrates and pNOSsc was negligible during nighttime in the PRD, although the formation of nitroperoxy radical 1 with NO3 might be comparable with that of hydroperoxy radical 1 with OH. The f OH was about 30%, indicating that the daytime photochemistry was significant for the formation of hydroxynitrate and pNOSc. Here we can conclude that photochemistry played a dominant role in pNOSs formation and the contribution of nighttime NO3 chemistry was minor for pNOSs formation in the PRD. Minerath et al. demonstrated that alcohol sulfate esterification reactions are kinetically infeasible with particle pH values in the range of 0−4.0.66 However, aerosol acidity is high in the PRD. Xue et al. measured online ionic chemical compositions in Hong Kong using a particle-into-liquid system (PILS) coupled with two ion chromatographs.67 In-situ pH was calculated based on these online data and exhibited extreme low values (close to −2.0) at noon in some days of fall. Since Hong Kong is only about 60 km away from WQS, such low in situ pH should also exist at WQS during daytime. Assuming pH= −2.0 and all free H+ came from the dissociation of sulfuric acid, the equivalent H2SO4 content is roughly calculated as 50 mol kg−1 (83 wt %). Using the method proposed by Minerath et al.,66 the secondary order rate constant under this conditions is calculated to be 2.42 × 10−5 m−1 s−1, and the predicted lifetime for the alcohol sulfate esterification reactions is 0.23 h. Therefore, alcohol sulfate esterification should be a possible pathway to form pNOSs in the PRD, especially during daytime. It should be noted that although our calculations support the hydroxynitrate pathway, other pathways cannot be ruled out completely. The hydroxynitrate pathway originally proposed by Surratt et al.9 has not been conclusively demonstrated in prior lab studies. Applying chemical ionization mass spectrometry (CIMS) techniques has been critical for identifying the roles of epoxides in forming organosulfates from isoprene oxidation.17,28 However, CIMS techniques were not yet heavily used to identify gaseous precursors to pNOSs. Other gaseous oxidation products (such as epoxides) from α-pinene might explain these compounds that have not been identified yet with CIMS. Additionally, it is possible that NO3 reactions might occur in the residual layer overnight and then is mixed down to the surface as the boundary layer becomes mixed during the day. Isoprene OSs. The isoprene-derived OS, 2-methyltetrol sulfate ester (MW 216) was only detected in 40 of the 64 samples. The concentrations were in the range of below the detection limit to 2.09 ng m−3 with an average value of 0.45 ng m−3 and the fraction of 2-methyltetrol sulfate ester in total OM was very small (Table 1). We observed a significant seasonal difference (p < 0.01) for 2-methyltetrol sulfate ester with levels

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ASSOCIATED CONTENT

S Supporting Information *

Sampling site Wanqingsha (WQS) in the Pearl River Delta (Figure S1). MS/MS fragmentation spectra of five OSs (Figure 9242



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S2). Cluster analysis of air masses in September and November, respectively (Figure S3). Day−night difference of NO, O3, sulfate, and acidity in summer and fall at WQS (Figure S4). HPLC/(−)ESI-MS extracted chromatograms for a typical sample in the Perl River Delta (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org/

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-20-85290127. Fax: +86-20-85290706. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation of China (NSFC) (41025012/41273116), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA05100104/XDB05010200), and the State Key Laboratory of Organic Geochemistry (No. SKLOG2013A01)

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Organosulfates from Pinene and Isoprene over the Pearl River Delta

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