Organosulfates in Humic-like Substance Fraction Isolated from

Nov 16, 2012 - Organosulfates in Humic-like Substance Fraction Isolated from Aerosols at Seven Locations in East Asia: A Study by Ultra-High-Resolutio...
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Organosulfates in Humic-like Substance Fraction Isolated from Aerosols at Seven Locations in East Asia: A Study by Ultra-HighResolution Mass Spectrometry Peng Lin,†,‡ Jian Zhen Yu,*,†,‡ Guenter Engling,§ and Markus Kalberer*,∥ †

Department of Chemistry, and ‡Division of Environment, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China § Department of Biomedical Engineering & Environmental Sciences, National Tsing Hua University, Hsinchu, Taiwan ∥ Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom S Supporting Information *

ABSTRACT: Humic-like substances (HULIS) in ambient aerosols collected at seven locations in East Asia were analyzed using electrospray ionization (ESI) coupled with an ultra-high-resolution mass spectrometer (UHRMS). Locations included a 3 km high mountaintop site in Taiwan, rural, suburban, and urban locations in the Pearl River Delta (PRD), South China, and in Taiwan. Organosulfates (OS) in the HULIS fraction were tentatively identified through accurate mass measurements and MS/MS spectra interpretation. In the two mountaintop samples collected in regional background atmosphere, little OS were detected, while a few hundred OS formulas were identified in the six samples taken in Taiwan and PRD. Many of the OS ions were among the most intense peaks in the negative ESI−UHRMS spectra, and their elemental formulas were identical to OS derived from biogenic volatile organic compounds (BVOCs) (e.g., monoterpenes) that have been identified in chamber studies. With OS having less than 6 carbon atoms too hydrophilic to be effectively retained in the HULIS fraction, OS containing 10 carbon atoms were the most abundant, indicating monoterpenes as important precursors of OS in the HULIS fraction. Clear spatial variation in abundance of OS was found among different atmospheric environments, with enhanced coupling of BVOCs with anthropogenic acidic aerosols observed in the PRD samples over the Taiwan samples. The double bond equivalent (DBE) values indicate the majority of OS (>90%) in the HULIS fraction are aliphatic. The elemental compositions of OS compounds containing N atoms (defined as CHONS) indicate that they are probably nitrooxy OS. Some insights into OS formation mechanisms are also gained through examining the presence/absence of perceived reactant−product formula pairs in the mass spectra. The results suggest the dominant epoxide intermediate pathway for formation of OS compounds without N atoms (defined as CHOS) and confirm the more readily hydrolyzed characteristics of the −ONO2 group than the −OSO3 group. There is a lack of evidence for the epoxide pathway to account for the formation of OS in the CHONS subgroup.

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INTRODUCTION

The presence of OS in ambient aerosols has been confirmed through a number of analytical approaches, including liquid chromatography mass spectrometry,7,8 in situ measurement by single particle mass spectrometry,9,10 functional group analysis by Fourier transform infrared spectroscopy,11,12 electrospray ionization coupled with ultra-high-resolution mass spectrometry (ESI−UHRMS),13−15 and comparison of total S by X-ray fluorescence (XRF) spectroscopy versus inorganic sulfate by ion chromatography.16,17 Quantification of OS is challenging due to a lack of commercially available standards. As a result, studies on OS have largely remained at the qualitative level.7,14,15,18 Semiquantification methods applied in a few

East Asia is an important area where emissions of anthropogenic air pollutants (e.g., SO2, NOx, volatile organic compounds (VOCs), and particulate matter) are large and continue to increase due to expanding populations and industrial activities.1,2 A considerable amount of biogenic VOCs (BVOCs) is also emitted in the subtropical areas of this region.3 Gases and aerosol particles can interact with each other and form new compounds in the atmosphere. Recent laboratory smog chamber studies demonstrated that the interaction of BVOCs and acidic sulfate particles has substantial potential to form organosulfates (OS) through acid-catalyzed particle-phase reactions.4,5 Considering the high abundance of sulfate and organic matter in the fine aerosol in this region,6 OS may contribute a significant fraction of particulate organic matter (POM) in East Asia. © 2012 American Chemical Society

Received: Revised: Accepted: Published: 13118

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Table 1. Summary of the Eight Aerosol Samples Discussed in This Study sample ID

location and site characteristics

GZ

Guangzhou, PRD, urban

NS YC

Nansha, PRD, suburban Yangcun, PRD, rural

TP YLC

Taipei, Taiwan, urban Yunlin County, Taiwan, suburban Mt. Bei Tungyen, Taiwan, rural Mt. Lulin, Taiwan, regional background (2862 m a.s.l.) Mt. Lulin, Taiwan, regional background (2862 m a.s.l.)

BTY LABS-1 LABS-2

sampling period

HULISa (μg/m3)

OCb (μgC/ m3)

no. of formulas identified

no. of formulas containing Sc

no. of formulas with O/S ≥ 4d

no. of formulas CHOS/CHONS

16−20 Sep, 2004 12 Jan, 2009 18−19 Nov, 2008 6 Oct, 2006 22 Dec, 2006

11.5

10.9

803

302 (38%)

299 (99%)

228/71

17.6 16.8

21.7 29.3

848 796

155 (18%) 211 (27%)

155 (100%) 211 (100%)

108/47 152/59

2.2 6.4

3.2 11.3

776 807

198 (26%) 244 (30%)

196 (99%) 243 (99%)

128/68 162/81

8.7 2.4

10.4 2.7

429 612

143 (33%) 17 (2.7%)

143 (100%) 17 (100%)

119/24 16/1

19.2

18.9

802

0 (0%)

27 Mar, 2008 27 Feb−6 Mar, 2007 16 Mar, 2007

0 (0%)

0/0

a

HULIS concentration in ambient aerosol. bOrganic carbon (OC) air concentration in ambient aerosol. cValues in the parentheses are the percentages of S-containing ions among the total number of identified ions. dValues in the parentheses are the percentages of ions with O/S ≥ 4 among the total number of S-containing ions.

regional background aerosol samples were collected at the Lulin Atmospheric Background Station (LABS, http://lulin.tw/ index_en.htm), located on top of Mt. Lulin (23°28′07″ N, 120°52′25″ E, 2860 m a.s.l.) in central Taiwan. The samples were collected on prebaked quartz filters (8” × 10”) using highvolume aerosol samplers (Ecotech HiVol 3000, HI-Q HVP4300AFC, and MSP M310) equipped with PM2.5 inlets, except for the sample taken at Guangzhou, which was taken using a four-channel PM2.5 sampler (Tianhong Corp., China). Field blank samples were taken by placing the blank filters into the sampler for ∼10 min without airflow. The filter samples were stored at −18 °C in a freezer until analysis. Portions of the filters (9−40 cm2) were extracted with ultrapure water in an ultrasonic bath for 40 min and then filtered with a 0.45 μm Teflon syringe filter to remove insoluble suspensions. Inorganic constituents (e.g., ammonium, sulfate, and nitrate) were abundant in the resulting aerosol water extracts, and their concentrations greatly exceeded those of individual organics. Therefore, solid-phase extraction (SPE) was employed for desalting prior to the direct infusion UHRMS analysis of the organic aerosol components. The removal of inorganic sulfates and nitrates from the water extracts also eliminates the possibility that those ions form adducts with organic molecules during the ESI process. The extract was acidified to pH = 2 using HCl and then loaded on a SPE cartridge (Oasis HLB, Waters, U.S.). The majority of inorganic ions, low molecular weight organic molecules, and sugars were not retained by the cartridge.19 The organic compounds retained on the cartridge were eluted using 1.5 mL of methanol containing 2% aqueous ammonia (w/w). The eluate was immediately evaporated to dryness under a gentle N2 stream and redissolved in 4 mL of acetonitrile and water (1:1) for ESI−UHRMS analysis. The organic matter obtained by this procedure is usually termed as HULIS. HULIS consists of a large number of unresolved aerosol components and exerts a number of environmental effects in the atmosphere.20−22 ESI−UHRMS Analysis and Data Processing. The HULIS fractions were analyzed with a LTQ-Orbitrap Velos mass spectrometer (Thermo Scientific Inc., Bremen, Germany) with a heated electrospray ionization source. Samples were injected through a steel capillary by a Hamilton syringe at a flow rate of 5 μL/min. Liquid chromatography (LC) upfront of the UHRMS was not used in the current study because our initial

studies estimated that OS accounted for 1−30% of organic mass and 4−14% of total sulfate in fine aerosols collected in different locations.5,8,11,12,16,17 Most of the studies available in the literature concerning OS were conducted in Europe and North America, with only one study conducted in Asia.8 In this study, aerosols collected at seven locations in the subtropical region of East Asia were characterized by ESI− UHRMS. Characteristics of the OS compounds in the HULIS fractions isolated from these ambient aerosols are the focus of this study. The samples include typical urban, suburban, and rural aerosols from two regions, the Pearl River Delta (PRD), a fast-developing and heavily polluted region in South China, and Taiwan, an island with an advanced industrial economy. Two samples were collected at a 3 km high mountaintop site in Taiwan, representing atmospheric background aerosols. With the remarkable resolving power and mass accuracy of UHRMS, the molecular formulas of OS in these samples can be identified. The purpose of this study is to obtain snapshots of OS in atmospheric environments with different anthropogenic and biogenic emission mixtures and examine the spatial variation in the elemental compositions of OS. Major precursors of the OS are also discussed by comparing the formulas and atom distributions of OS identified in ambient aerosols with those determined in smog chamber experiments. The results in this study can be used to guide more quantitative assessment of OS in this region.

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EXPERIMENTAL SECTION Sample Collection and Preparation. Eight aerosol samples were collected at seven locations representing different atmospheric environments in the PRD region, South China and Taiwan. Table 1 provides details of the sample locations, concentrations of OC and HULIS in the samples, and the number of S-containing formulas identified in individual samples (to be discussed in a later section). Two urban aerosol samples were taken in the urban region of Guangzhou (23°06′40″N, 113°14′00″ E) and Taipei (25°02′31″N, 121°37′0.3″ E), which are the largest cities in PRD and Taiwan, respectively. The other sampling sites are Nansha (22°45′10″ N, 113°36′09″ E, suburban) and Yangcun (23°27′44″ N, 114°30′08″ E, rural) in the PRD, Yunlin County (23°42′54″ N, 120°34′18″ E, suburban), and Mt. Bei Tungyen (24°2′33″ N, 121°4′24″ E, rural) in Taiwan. Two 13119

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the parent ions.30,31 In the current study, the parent ions were fragmented by collision-induced dissociation (CID) in the linear ion trap (LTQ), and the fragment ions were measured for their accurate masses by the Orbitrap. Because of the limited amount of solutions, it was not practical to conduct MS/MS on all of the S-containing ions found in these six samples. As a result, only 25 of the intense peaks with Scontaining formula assignments were chosen for the MS/MS analysis. All of the MS/MS spectra display intense HSO4− or ·SO4− product ions, indicating that their parent ions contain a sulfate group connected to some organic moieties. Figure S1 shows six representative MS/MS spectra of OS. It should be noted that due to the limitations on the resolving power of LTQ, the minimum isolation width of the parent ions for MS/ MS study is about 0.4 m/z unit. As a result, in the MS/MS experiment, several isobaric ions with the same integer m/z could be isolated (the formulas of isobaric ions are listed in the Figure S1 caption). A recent study32 reported accelerated formation of OS via the evaporation of aqueous SOA extracts in the presence of (NH4)2SO4 at pH 4−9 or in the presence of H2SO4 at pH ≈ 2. However, in our experiments, the evaporation step was conducted after the SPE step during which almost all of the sulfate ions (as well as other inorganic ions) were removed. Hence, the OS found in the HULIS fraction were unlikely produced in the sample workup process. The formation of OS has been observed in several smog chamber experiments.4,5,33 Although the detailed formation pathways are still unclear, there is strong evidence to show that those reactions take place between the oxidation products of VOCs and acidified sulfate seed particles or sulfuric acid.33,34 Isoprene,35 monoterpenes,5 and sesquiterpenes4 are known biogenic VOCs of which oxidation products can serve as OS precursors. Whether oxidation products of other VOCs could also contribute to OS formation is still under investigation. Table S2 lists the assigned formulas of some intense Scontaining peaks in the six HULIS samples. It is worth noting that many of these compounds have elemental formulas identical to those OS that have been identified in smog chamber studies. The VOC precursors of these formulas are tentatively identified on the basis of comparison with those chamber studies.4,5 By examining the formula lists of OS in this study, we find most of the OS formulas are consistent with reaction products of monoterpenes (C10H16) and sesquiterpenes (C15H24). Although these six samples were taken at different locations and in different times, the common strong presence of these OS peaks indicates their ubiquity in environments of mixed anthropogenic and biogenic emissions. Only a small number (1−8) of OS having less than 6 carbons were detected in the individual samples and all were present at low relative intensity (Table S2). Two of them were identical to the OS derived from isoprene (C5H8). An analysis of sample pretreatment steps for HULIS isolation could explain the infrequent and low presence of isoprene-derived OS, despite the abundant emission of isoprene in the atmosphere.3 Isoprene-derived OS have small molecular weights and short carbon chain structures (e.g., 2-methyltetrol sulfates), which render them too hydrophilic to be effectively retained by the SPE sorbent.5,36,37 Hence, it should be emphasized that our current study focuses on the OS in the HULIS fraction, and no information is obtained on the presence and relative abundance of the isoprene-derived OS. The likely importance of isoprene-

goal was to acquire an overall picture about the elemental formula distribution of HULIS. A spray voltage of 2.5 kV was used in the analysis. The system was operated in negative ESI mode with a resolution of 100 000 at m/z 400. Signals in the range of m/z from 90 to 1000 were recorded and processed for formula calculations. Field blank filters were processed and analyzed following the same procedure. There were 17 Scontaining formulas identified in the blank, only 2 of which have O/S ≥ 4. Peaks present in the blank were not further considered in the data analysis, irrespective of their intensity. Other details of the experiment setup and discussion of possible analytical artifacts have been provided in our previous paper.23 The mass spectra were processed, and the mass lists were exported using the Xcalibur software (V2.1, Thermo Scientific). All of the ions with a relative intensity ≥0.5% were exported. All mathematically possible formulas for these ions were calculated using a mass tolerance of ±2 ppm, which was obtained by the root-mean-square of the measured mass errors of the standard mixture during calibration.23 In the Xcalibur software, up to 80 12C atoms, 200 1H atoms, 50 16O atoms, 5 14 N atoms, 2 32S atoms, 1 13C atom, and 1 34S atom were allowed in the molecular formula calculations. Several conservative rules were also applied to eliminate compounds not likely to be observed in nature; that is, the H/C, O/C, and N/C ratios were limited to 0.3−3.0, 0−3, and 0−0.5, respectively.24 The calculated formulas that disobey the nitrogen rule for even electron ions25 were excluded. The dominant single-charge status of the ions in the UHRMS spectra was confirmed by the m/z difference between a monoisotopic (e.g., all 12C) peak and its corresponding 13 12 C Cn−1 isotope peak.23,26 The presence of 34S isotope peaks for intensive ions supports the assignment of sulfurcontaining formulas (Table S1). The isotopic peaks are not included in the ensuing discussion because they have the same elemental compositions as their corresponding monoisotopic peaks.

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RESULTS AND DISCUSSION Identification of Organosulfates. Table 1 summarizes the number of ions with unambiguous formula assignments in each sample. In the six samples taken at urban, suburban, and rural regions of PRD and Taiwan, hundreds of ions (∼140− 300) were assigned with formulas containing sulfur atoms, accounting for 18−38% of all unambiguously assigned formulas. All of the sulfur-containing ions contain only one sulfur atom. More than 98% of them have O/S ratios greater than 4, which supports the assignment of a sulfate group in the molecules. Only very few sulfur-containing ions were observed in the mountain top samples (LABS-1 and LABS-2) despite relatively higher concentrations of HULIS in the spray solutions (365 and 566 μg/mL for LABS-1 and LABS-2, respectively) in the two samples. In comparison, the HULIS concentrations in the spray solutions of the other samples ranged from 84 to 222 μg/mL. These concentrations were lower than those of the fulvic acid (1200−2400 μg/mL), humic acid (∼1200 μg/mL), and fog waters (∼640 μgC/mL) measured by ESI−UHRMS in other studies.27−29 Considering the relatively high ionization efficiency of organosulfates, we believe ion suppression effects are unlikely a problem for OS in this study. Product ion spectra obtained by ESI−MS/MS analysis have been found to be helpful in deducing structural information of 13120

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Figure 1. Reconstructed mass spectra of S-containing species detected in eight aerosol samples. The red bars denote ions with the same formulas as those from BVOCs-derived organosulfates identified in chamber studies.5 The measured signal-to-noise ratio (s/n) of ions was normalized against sampled air volumes. These normalized mass spectra could indicate the relative abundance of OS in the HULIS fraction for the different locations. A, m/z = 294, C10H16O7NS−; B, m/z = 296, C9H14O8NS−; C, m/z = 373, C10H17O11N2S− are three formulas of monoterpene-derived OS.

derived OS38,39 in the whole OS family could not be assessed by data from this study. Spatial Variation. In ESI−MS spectra, peak intensity is the product of initial concentration and ionization efficiency of the neutral compound.40 The presence of a sulfate functional group on the organic molecules makes them readily ionized during the ESI process. It is reasonable to assume the different OS may have similar ionization efficiency40 as the ionization of OS takes place on the sulfate functional group. On the basis of this assumption and the fact that all of the spectra were acquired under the same ESI−MS conditions, the peak intensities of the OS ions could be compared to provide information on relative abundances among different locations by assuming that matrix effects were relatively constant in all samples. The signal-tonoise ratios (s/n) of all OS peaks are normalized against the sampled air volume and corrected for solvent dilution ratio of each sample. They are then plotted versus their corresponding m/z to get the reconstructed mass spectra (Figure 1). One distinct feature revealed is that several OS are detected in the regional background sample LABS-1, but the normalized

s/n of these OS (Figure 1g) are several orders of magnitude lower than those of OS observed in the urban, suburban, and rural locations of the PRD and Taiwan (Figure 1a−f). As it is shown in Table 1, comparable atmospheric HULIS concentrations were observed at the background site (LABS-1, 2.4 μg/ m3) and in the urban sample in Taiwan (2.2 μg/m3). Yet the normalized peak intensities (s/n) of OS in the urban Taiwan sample (Figure 1b) are ∼20 times higher than those in LABS-1 (Figure 1g), indicating significantly higher OS concentrations in urban Taiwan than in the background atmosphere. The HULIS concentration in the LABS-2 was the highest among the eight samples (Table 1). It was mainly caused by long-range transport of biomass burning aerosols from Indochina,41 evidenced by the high levoglucosan concentration (406 ng/ m3 in LABS-2 versus 25 ng/m3 in LABS-1). However, no OS was detected in the HULIS fraction of this sample. As we have noted earlier that isoprene-derived OS were not effectively retained in the HULIS fraction, whether the isoprene-derived OS could be present in LABS samples could not be assessed. 13121

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Figure 2. The number and intensity distributions of carbon atoms in organosulfates of the urban PRD sample. The color bar in (a) and (b) denotes the relative peak intensities (0−100%). All of the ions with relative intensity ≥7% were denoted by red color for enhanced visualization of the color plots. The regions marked in roman numerals (I−IV) in (e) and (f) represent composition domains of organosulfates originating from sesquiterpenes (I), monoterpenes (II), isoprene (III), and dimers of isoprene oxidation (IV), as reported in chamber studies.4,5,44

and sesquiterpenes) account for 20% of the total OS (Figure S2), lower than those in the other samples (26−49%), indicating the dominant presence of more other precursors in the heavily polluted urban atmosphere (GZ). The concentrations of total OS in the PRD HULIS samples were much higher than those in Taiwan. With the limited number of samples analyzed in this study, it is difficult to determine whether this is related to the more severe anthropogenic air pollution problems encountered in the PRD than in Taiwan. More efforts are needed to investigate OS in the atmosphere, especially in the subtropical region of East Asia where both anthropogenic air pollutants and BVOCs are abundant and coexist. It is worth noting that many OS derived from monoterpenes are usually the most intense peaks in the spectra. In Figure 1, peaks with the same formulas as those of OS originated from monoterpene and sesquiterpene precursors are marked in red, and other OS from unknown precursors are shown in black. The most intense peaks, denoted as A, B, and C in Figure 1, are three examples of monoterpene-derived OS. The terpenederived OS account for 20−49% of the overall OS in HULIS (Figure S2), suggesting that they are important precursors of OS in HULIS. Elemental Composition Domains. All of the S-containing compounds identified in this study contain only one sulfur atom in each formula. They can be classified into two

The reduced concentrations of OS in HULIS samples taken at LABS could be explained by the fact that the formation of OS requires the presence of both acidic sulfate seed particles and oxidation products of VOCs. All eight aerosol samples in this study were collected in the subtropical regions of East Asia, where biogenic emissions are expected to contribute a substantial fraction of VOCs in the atmosphere. Aerosols sampled at LABS are mainly characterized as regional background or long-range transport of Asian outflow (biomass burning or anthropogenic pollution).42 The concentrations of major inorganic ions in samples LABS-1 and LABS-2, both collected in March 2007, were not measured in this study. Using the monthly average ion composition data for the site available from Lee et al.,42 we found that the equivalent concentration ratio of cations versus anions was 0.98 for March 2007 and varied from 0.92 to 1.39 for the month of March in other years in the period of 2004−2009. Hence, the typical aerosols experienced at LABS in March may not be sufficiently acidic to catalyze the reactions for OS formation. Further work is needed to explore the relationships between OS formation and ambient particle acidity. Many OS formulas are common to the six urban, suburban, and rural samples of PRD and Taiwan (Table S2), but they have varied relative abundance, seen from normalized s/n, among these six samples (Figure 1a−f). In the urban sample of PRD (GZ), OS with known BVOCs precursors (monoterpenes 13122

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significantly lower, with the average values ≤1.7, which appear to possess more features of primary organic aerosols.45,46 As compared to measurements reported for OS in the literature, the OM/OC and MW of OS in this study are similar to those found in nonurban aerosols collected during the summer of 2010 at the Storm Peak Laboratory in Colorado, and in polluted urban fog waters.13,28 This similarity may reflect their comparable oxidation ages in the atmosphere. Unlike the aerosol samples, the OS in rainwater47,48 have much higher OM/OC ratios and lower MW, possibly indicating the longer oxidation time or more efficient oxidation in rainwater samples. The double bond equivalent (DBE) value of a molecule reflects the degree of its unsaturation (e.g., the minimum DBE value of an aromatic compound is 4). The DBE value (number of rings plus double bonds) is calculated as follows for the elemental composition CcHhOoNnSs:

subgroups, denoted as CHOS and CHONS compounds, according to whether there are nitrogen atoms in their formula. The numbers of formulas in these two groups in individual samples are summarized in Table 1. By number, 16−33% of OS contain one or two N atoms in their formula. All of the CHON1S formulas have seven or more O atoms, and all of the CHON2S formulas have 10 or more O atoms, implying that the nitrogen atoms may exist in the form of −NO3 groups and these CHONS compounds are probably nitrooxy-OS. Nitrooxy-OS have been demonstrated to form via photooxidation of biogenic VOCs in smog chamber experiments conducted under high NOx conditions.5 In this study, the CHONS ions that have been examined for MS/MS characteristics show neutral loss of HNO2 or HNO3 in their ESI−MS/MS spectra (Figure 1d−f). Such fragmentation features are also the general characteristics of nitrooxy-OS generated in smog chamber studies.5 A neutral loss of NO and NO2 fragments, which would indicate the presence of a nitro group in a molecule,43 was not observed in the CHONS molecules analyzed with MS/MS. Thus, while we could not completely exclude the possibility of CHONS compounds being nitro-OS, the evidence from limited MS/MS analysis does not support the presence of nitro-OS. The carbon number distributions of OS in the HULIS are shown in Figure 2a−d for the urban PRD sample and in Figures S3−S7 for the other samples. Figure 2e and f shows the distributions of carbon atoms in OS generated from oxidation of BVOCs in chamber simulations.4,5 OS derived from sesquiterpenes have a carbon number of 14−16 (region I), and those from monoterpenes are in the range of C6−C10 (region II). Most isoprene-derived OS are in the range of C2 to C5 (region III), but OS derived from isoprene oxidation dimer products, with carbon numbers exceeding 5 (region IV), are also reported in chamber studies.5,44 The carbon number of OS observed in the ambient samples covers the entire range of OS identified in chamber studies involving sesquiterpenes and monoterpenes. A small number ( 7 for CHONS compounds (nitrooxy-OS). In our samples, most of the CHOS compounds satisfy this criterion. Yet we also have several CHOS compounds with O = 4 and S = 1, which cannot be explained by the epoxide pathway. Similar cases were also found in the CHONS compounds. For instance, C10H16O7NS−, usually the most intensive ions in the mass spectra, has been identified as a nitrooxy-OS in which the O atoms are accounted for by one sulfate and one nitrooxy group, leaving no extra O atoms. Clearly, this nitroxy-OS could not have been formed from the epoxide pathway due to its lacking of an OH group. Evidence from field measurement50 and chamber simulation5 implied the importance of NO3-initiated nighttime reactions of monoterpenes in producing nitrooxy-OS. Further investigation of the link between nitrooxy-OS and NO 3-initiated oxidation chemistry is warranted. Water is an important component of atmospheric aerosols. It can act as a nuclephile to compete with sulfate ions to react with the epoxides, forming diol compounds, which is usually the branch reaction with the higher yield.34,56 Hence, for each OS (denoted as R−OSO3H) formed through the epoxide pathway, its corresponding alcohol compound (R−OH) is expected to form as well. Moreover, some OS (R−OSO3H) could also convert to their corresponding alcohol compounds (R−OH) through acid-dependent hydrolysis reaction.58 Here, we remove −SO3 from the S-bearing compounds to get corresponding formulas of alcohols and examine their presence

Table 2. Numbers and Percentages of Organosulfates, For Which Corresponding Formulas Were Present in the Samples with Mass Shifts Reflecting the Formation and Hydrolysis Reactions number and percentage occurrences of the plausible reactant− product pairs sample type

CHOS − SO3 → CHO (1)

CHONS − SO3 → CHON (2)

CHONS + OH − NO3 → CHOS (3)

PRD urban PRD suburban PRD rural Taiwan urban Taiwan suburban Taiwan rural

148 (65%) 74 (69%)

2 (2.8%) 7 (15%)

52 (75%) 22 (47%)

113 (75%) 112 (86%)

3 (5.1%) 2 (2.9%)

38 (69%) 36 (55%)

98 (60%)

5 (6.2%)

52 (70%)

76 (64%)

0 (0.0%)

20 (91%)

corresponding CHO formula was present in the samples after SO3 was subtracted from the OS formula. On average, for over 60% of the CHOS, the corresponding CHO alcohol formulas were found, consistent with the epoxide intermediate pathway for OS formation. The remaining ∼40% of CHOS, which have no corresponding alcohols, may be formed from other pathways. In contrast, only for a small fraction of CHONS OS were the corresponding CHON alcohol formulas observed, implying that the epoxide pathway is insignificant for the formation of OS in the CHONS subgroup. Recent laboratory work showed that organonitrates usually hydrolyze more rapidly than OS.59 This hydrolysis process substitutes the nitrooxy group with a hydroxyl group (i.e., −HNO3 + H2O, Scheme S1). Such reactions may take place in the atmosphere and/or during the experimental workup procedure. In our samples, we found six OS containing 2 N atoms and 10 or 11 O atoms (Table S4). Assuming that both N atoms are present as nitrooxy group in these compounds, we deduce the formulas of their corresponding hydrolysis products with 1 and 2 nitrooxy groups substituted by hydroxyl groups and denote as N = 1 and N = 0 series, respectively. All of the N = 0 and N = 1 OS were also found in our samples, consistent with the proposition that these CHONS compounds are most likely nitrooxy-OS. On average for 50−90% the CHONS OS, a corresponding CHOS OS is present in the samples (Table 2), which could be explained by the hydrolysis of nitrooxy groups of OS.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of HULIS and OC determination (Appendix S1), four supporting tables (Tables S1−S4), and 11 supporting figures (Figures S1−S11). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Z.Y.); [email protected]. ac.uk (M.K.). Notes

The authors declare no competing financial interest. 13124

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ACKNOWLEDGMENTS This study was partly supported by the Research Grants Council of Hong Kong (621510) and the HKUST Oversea Research Awards for Ph.D. students. We thank Dr. Ling-yan He for sharing with us the Guangzhou Urban sample and Dr. Angela G. Rincon for checking MS data of a few biomass burning samples.

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