Analysis of Organic Sulfur Compounds in Atmospheric Aerosols at the

Feb 20, 2015 - (12) MSA is also suggested to play an important role in the marine stratocumulus cloud formation over the remote ocean.(13). Hong Kong ...
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Analysis of Organic Sulfur Compounds in Atmospheric Aerosols at the HKUST Supersite in Hong Kong Using HR-ToF-AMS Dan Dan Huang,† Yong Jie Li,‡,§ Berto P. Lee,‡ and Chak K. Chan*,†,‡ †

Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ‡ Division of Environment, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China S Supporting Information *

ABSTRACT: Organic sulfur compounds have been identified in ambient secondary organic aerosols, but their contribution to organic mass is not well quantified. In this study, using a high-resolution time-of-flight aerosol mass spectrometer (AMS), concentrations of organic sulfur compounds were estimated based on the high-resolution fragmentation patterns of methanesulfonic acid (MSA), and organosulfates (OS), including alkyl, phenyl, and cycloalkyl sulfates, obtained in laboratory experiments. Mass concentrations of MSA and minimum mass concentrations of OS were determined in a field campaign conducted at a coastal site of Hong Kong in September 2011. MSA and OS together accounted for at least 5% of AMS detected organics. MSA is of marine origin with its formation dominated by local photochemical activities and enhanced by aqueous phase processing. OS concentrations are better correlated with particle liquid water content (LWC) than with particle acidity. High-molecular-weight OS were detected in the continental influenced period probably because they had grown into larger molecules during long-range transport or they were formed from large anthropogenic precursors. This study highlights the importance of both aqueous-phase processing and regional influence, i.e., different air mass origins, on organic sulfur compound formation in coastal cities like Hong Kong.

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INTRODUCTION Secondary organic aerosols (SOA) can contribute substantially to atmospheric aerosol mass, but current models considerably underestimate ambient SOA formation. In recent years, organic sulfur compounds have been observed in ambient aerosols globally.1−3 These compounds are stable at high temperatures4 and have low volatility, making them possible contributors to organic matters that have not been included in the models. Organic sulfur compounds are surface active5 and may also play a role in cloud formation.6 Furthermore, organic sulfur compounds of high molecular weight were found in the humic-like substances7 and recently in samples from continental area with long-chain alkane as precursors.8 Highmolecular-weight organic sulfur compounds can be formed during aerosol processing9 or formed from high-molecularweight anthropogenic precursors.8 Most studies of organic sulfur compounds in ambient aerosols are qualitative because of their wide varieties and relatively low concentrations, not to mention the scarcity of appropriate laboratory standards.9,10 Organic sulfur compounds are likely to exist in forms of organosulfates (OS) and methanesulfonic acid (MSA) at this site. Previous field and chamber studies of organic aerosols suggest that OS (ROSO3−) are likely to be the most abundant species of organic sulfur compounds in continental environments,11 although the identities and precursors of organic sulfur compounds are not yet well understood. MSA is believed to be abundant in marine or coastal areas because it is © 2015 American Chemical Society

from the oxidation of dimethyl sulfide (DMS)a gaseous species emitted by marine organisms.12 MSA is also suggested to play an important role in the marine stratocumulus cloud formation over the remote ocean.13 Hong Kong (HK) is located southeast of the Pearl River Delta, bordered by Shenzhen to the north but facing the South China Sea to the south, east, and west. MSA is likely available in the atmosphere of HK because of the proximity to ocean. Rapid urbanization and industrial development in Pearl River Delta (PRD) have led to aggravating air pollution problems throughout the region. Air pollution in Hong Kong is often associated with coastal and continental air masses from the north or northwest.14 The relatively high concentrations of pollutants5 (e.g., VOCs and sulfate) and high relative humidity (RH) are conducive conditions for the formation of organic sulfur compounds in the PRD regions including HK. The objective of this study is to estimate the abundance of MSA and OS in ambient aerosols based on measurements made with a high-resolution time-of-flight aerosol mass spectrometer (AMS). The high-resolution real-time characterization of MSA and OS as well as major species such as sulfate, ammonium, and nitrate was useful to examine the sources of Received: Revised: Accepted: Published: 3672

November 19, 2014 February 16, 2015 February 20, 2015 February 20, 2015 DOI: 10.1021/es5056269 Environ. Sci. Technol. 2015, 49, 3672−3679

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Environmental Science & Technology

Figure 1. Fragmentation patterns of (a) CH4SO3, (b) CH3O4SNa, (c) C3H7O4SNa, (d) C8H17O4SNa, (e) C10H21O4SNa, (f) C6H5O5SK, (g) C7H7O4SK, (h) C9H11O7SK, and (i) C6H11O9SNa. The error bars are the standard deviations over multiple measurements. CxHyOzS ions on graph are magnified by 5, 20, 50, or 100, as indicated in the legends in each panel.

relationship between the abundance of OS and particle liquid water content and particle acidity will be discussed.

MSA and OS and to determine how their formation is associated with aerosol conditions, such as particle liquid water content and particle acidity. The V-mode AMS data have been commonly used to estimate species mass concentrations because of their higher sensitivity. However, neither MSA nor OS is available in the fragmentation table for standard V-mode data analysis and signals of MSA or OS have been assigned to either organics or sulfate. Hence, we first present laboratory experiments to obtain the fragmentation patterns of MSA and OS and the method developed to estimate MSA and minimum OS concentrations in AMS analysis. We then describe the time series and diurnal variations in MSA and OS and their relationships with meteorological parameters. Finally, the

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EXPERIMENTAL SECTION Laboratory Calibrations. The experiments were conducted with an aerodyne high-resolution time-of-flight aerosol mass spectrometer. Standard AMS data analysis toolkits (SQUIRREL v1.53C and PIKA 1.12C) based on Igor Pro (6.32A) were utilized to process the AMS data. We obtained the fragmentation pattern of MSA and OS by nebulizing 5 ppm MSA and OS aqueous solutions, respectively. The generated MSA/OS containing particles passed through a 1 m long diffusion dryer (BMI, San Francisco, CA) for water removal before they were sent to AMS. In AMS, particles are vaporized by impaction on a resistively heated surface (∼600 °C) and 3673

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0.003%, further confirmed the potential contribution of OS to CH3SO2+ is negligible compared with MSA. The ion CH3SO2+ can be separated very well in the W-mode field data (SI Figure S1) and thus MSA in field measurements can be quantified with high confidence. MSA signals will contribute to the entries of organics and sulfate in the fragmentation table of AMS.24 Phinney et al.22 modified the fragmentation table of Q-AMS to separate the MSA signal as an independent entry in the fragmentation table based on integer m/z 79 peak. However, using W-mode analysis in AMS, we identify four other ions, Br+ m/z 78.92, C5H3O+ m/z 78.02, C5H5N+ m/z 79.04, and C6H7+ m/z 79.05, would also contribute to integer m/z 79 in unit mass resolution in V-mode measurement (SI Figure S1). Therefore, modifying the fragmentation table in V-mode only is not adequate for quantifying MSA in our measurements. Ge et al.25 used the intensities of CH3SO2+ and two other adjacent ions in W-mode data to quantify MSA. However, this approach has not taken the advantage of the higher sensitivity of the V mode data for estimating concentrations. In this paper, we develop the following quantification method to combine both the V mode data for total concentrations and W mode data for high-resolution ions differentiation. The principle of converting the AMS signal intensities to mass concentrations of a specific species has been described by DeCarlo et al.15 We combined the V-mode and W-mode data and eq 1 is derived for estimating the MSA concentrations (in μg/m3), (see detailed information in SI-3):

ionized by electron impact ionization (70 eV). The generated positive ion fragments are detected by mass spectrometer. Eight commercially available OS compounds were characterized with AMS, including four straight chain alkyl sulfates, sodium methyl sulfate (CH3O4SNa, C1S), sodium propyl sulfate (C3H7O4SNa, C3S), sodium octyl sulfate (C8H17O4SNa, C8S) and sodium decyl sulfate (C10H21O4SNa, C10S); three benzyl phenyl sulfates: 4-hydroxyphenyl sulfate (C6H5O5SK, C6RS), 4methylphenyl sulfate potassium (C7H7O4SK, C7RS) and 4hydroxy-3-methoxyphenylglycol sulfate potassium salt (C9H11O7SK, C9RS); one cycloalkyl sulfate: D-galactose 6sulfate sodium salt (C6H11O9SNa, C6S). Relevant properties of these compounds are summarized in Supporting Information (SI) Table S1. Field Measurements. Ambient sampling period was from 1st September to 29th September 2011 at the HKUST Air Quality Research Supersite (22°20′N, 114°16′E) located on the roof of the seawater pump house on the campus of HKUST. The supersite sits on the east coast of Hong Kong, a suburban area facing Port Shelter without significant anthropogenic sources nearby. Meteorological data were measured at an automatic weather station next to the Supersite. The mean RH and temperature in September 2011 were 78% and 27.9 °C, respectively. The AMS was operated alternatively between the V+PToF combined mode and the W-mode for 5 min each. In V-mode, the ions travel a shorter path than W-mode and thus V-mode has better sensitivity but lower mass resolving power of 2000 than that of 4000 in W-mode.15 The operating procedure of the AMS has been detailed in Li et al.16 Collection efficiency (CE) of 0.5 was applied to the whole data set. Composition dependent CE, especially under the influences of different air mass origins, has been discussed and detailed in SI1. The effects of meteorology, local emissions, and long-range transport on the aerosol abundance, composition of NR-PM1 (nonrefractory components in particulate matter less than 1 μm) and hygroscopic properties at this site have been presented before.17−20 Since September is normally a transient periods from summer to autumn in HK,21 here we show field results of this month with active photochemistry but quite distinct air mass origins (marine/coastal in the first half and continental in the second), although we had measurements for a few months in other seasons.16

CMSA = fMSA × (Corg + CSO4)

(1)

∑k IMSA, k

fMSA =

RIEMSA ∑n Iorg, n RIEorg

+

∑t ISO4, t RIESO4

(2)

where k, n, and t represent fragments of MSA, organics and sulfate, respectively; ∑kIMSA,k, ∑nIorg,n and ∑tISO4,t represent total signal intensities of MSA, organics and sulfate respectively obtained from the W-mode data. Corg and CSO4 represent the organics and sulfate concentrations calculated from V-mode data, respectively. MSA as a single compound is ionized into fragments and the signals are included in the entries of organics and sulfate in the matrix that used to calculate species concentrations in AMS.24 The factor “f MSA” represents for the fractional contribution of MSA to the lumped species of organics and sulfate in W-mode data. The total signal intensity of MSA in W-mode data can be estimated based on the fragmentation pattern derived from the laboratory experiments (Figure 1a and tabulated in SI Table S2), where the fractional contribution of CH3SO2+ (m/z78.99) to the total signal intensity of pure MSA is 10 ± 2%, based on calibration measurements at different MSA concentrations. The variation (standard deviation) of CH3SO2+ fraction is included in the uncertainty analysis of MSA concentrations (see discussion in SI-4). The relative ionization efficiency for MSA (RIEMSA) was estimated as the average of RIEorg = 1.4 and RIESO4 = 1.2.26 Uncertainty in MSA concentration caused by RIEMSA is also included in the uncertainty analysis of MSA concentrations (SI4). Overall, the uncertainty in the MSA concentration was estimated to be about 44%, comparable to the typical uncertainty of 39% in organic concentrations measured with AMS.27 Filtered ambient air was collected daily for 30 min with a HEPA-filter placed in front of the inlet, defined as the filter

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QUANTIFICATION OF MSA AND OS MSA Concentrations. To make sure that we have a representative mass spectrum, we performed experiments with different concentrations (5 and 30 ppm) and found that the mass spectra were quite constant. The average high-resolution mass spectrum of pure MSA is shown in Figure 1a. The error bars represent the standard deviations of the fractional contribution of each m/z to total signal of pure MSA over 24 measurements. The ion CH3SO2+ (m/z 78.99) is one of the most dominant organic sulfur fragments from MSA and is used as a tracer ion of MSA22 because it cannot be generated by sulfate or sulfuric acid and it would not be affected by sulfate measured in AMS. The contribution of the CH3SO2+ fragment from OS is not significant either (associated discussion in SI-2). On the basis of the spectra of MSA and C5H12O7S, a possible contributor to CH3SO2+ fragment reported by Farmer et al.,23 the contribution from C5H12O7S to this ion (0.3%) is not important compared with MSA (9%), especially considering HKUST is a coastal site while C5H12O7S is from isoprene. Also, the fraction of CH3SO2+ in all the tested standards is less than 3674

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Figure 2. Time series of (a) MSA mass concentrations; (b) OS mass concentrations and RH; and (c) Particle liquid water content and pH.

Without further information on the OS varieties and mixing ratios in ambient samples, a conservative lower-bound estimate of OS concentration (OSmin) has been obtained in this study based on the calibration results of sodium methyl sulfate. Most of the CxHyOzS signals can be separated well. Shown in SI Figure S2 are examples of one major (CH3SO3+) and one minor (CSO+) CxHyOzS ions in the calibration of methyl sulfate. Similar to the quantification of MSA, the OSmin can be calculated from eq 3:

periods. The detection limits of species are defined as three times the standard deviations of the measured species concentrations in the filter periods.28 The detection limits of organics, sulfate, nitrate, ammonium, and MSA in this work were estimated to be 234, 17, 9.5, 180, and 1 ng/m3, respectively. Minimum OS Concentrations. As mentioned, several different commercialized OS standards were tested in the AMS to obtain their fragmentation patterns (shown in Figure 1b−i and tabulated in SI Table S2). These standards have been demonstrated to be useful for both qualitative and quantitative study of OS using various analytical techniques.2,29,30 For example, 4-methyl-phenyl sulfate (C7H7O4SK, C7RS) is atmospherically relevant as methyl-phenyl sulfate was detected by Stone et al. in field samples of Pakistan and Pasadena, California.31 Galactose sulfate has been shown similar to the ambient OS in molecular weight, elemental composition, and UPLC retention time and has been used as the OS standard in a previous study.2 Octyl sulfate and decyl sulfate were chosen to represent OS formed from long-chain alkanes.8 Besides, propyl sulfate is used to determine the characteristic fragments associated with sulfate esters in LC/MSMS32 and a standard to quantify the isoprene-derived OS.29 Therefore, we believe the eights standards used in this study with different characteristic structures can provide a sufficiently broad representation of different ambient OS from an analytical perspective. All of the tested OS standards produce S-bearing organic fragments (CxHyOzS) during the ionization owing to their characteristic functional group, ROSO3−. The total signal intensity of OS can thus be obtained based on the signal intensity of CxHyOzS and their fraction among the total signal intensity of OS. However, accurate determination of OS concentrations from the AMS data is difficult because of the complex composition and variety of OS. Nevertheless, as shown in Figure 1, a general trend from larger fractions of CxHyOzS toward smaller fractions of CxHyOzS is observed with increasing molecular weight of OS. Given that methyl sulfate is the simplest OS, the fraction of CxHyOzS of all the other OS is likely to be lower than that of methyl sulfate.

COSmin = fOS × (Corg + CSO4)

(3)

∑h IOS, h

fOS =

RIEOS ∑n Iorg, n RIEorg

+

∑t ISO4, t RIESO4

(4)

where h represents the fragments of OS and ∑hIOS,h represents the total signal intensity of OS obtained from the W-mode data. Using the same RIE as that of MSA, the OSmin can be estimated (see details in SI-5). As will be discussed later, the estimated OSmin are within the range of previous filter-based measurements reported in the literature. The detection limit and uncertainty of OSmin, 80 ng/m3, and 42%, are determined using the same approach used for MSA.

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RESULTS AND DISCUSSIONS Time series of MSA and OSmin concentrations are shown in Figure 2a,b. On the basis of the wind rose plot (SI Figure S3c) and supported by the backward trajectory analysis (SI-6), three periods under the influence of oceanic, coastal, and continental sources, denoted by OCE, CST, and CONT, respectively, were selected for further analysis (areas shaded in blue in Figure 2a,b). An overview of the meteorological conditions, fractions, and concentrations of major PM1 species during this campaign is presented in SI Figure S3. Overall, sulfate (SO4) was the dominant species (54%) in NR-PM1 with an average concentration of 8.6 μg/m3, followed by organics (org, 26%) of 4.2 μg/m3, whereas the corresponding contributions of LVOOA (SVOOA) to the total organics are 29.1 (52.7)% 3675

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Environmental Science & Technology reported in Li et al.16 The high sulfate and OOA contributions reflect the fact that this receptor site is affected mainly by regional pollution within the PRD and super-regional pollution from long-range transport outside the PRD area.33 This is supported by the lower PM1 concentrations observed in OCE (14.6 μg/m3) and CST (10.9 μg/m3) than those in CONT (24.7 μg/m3). OCE and CST were less influenced by longrange transported pollutants. We further calculate the particle LWC and pH (Figure 2c) by applying E-AIM II34 to explore the effects of aqueous processing and particle acidity on MSA and OS formation, and this will be discussed later in detail. Generally, OCE had the highest average LWC among these three periods, 19 μg/m3, followed by CONT (14 μg/m3) and CST (10 μg/m3). The overall pH of aerosol in Sept was 0.5, with CONT exhibiting an average pH of only 0.1. MSA. MSA is mainly from the oxidation of DMS, which is produced by marine organisms. In our study, MSA concentrations increased continuously to as high as 70 (±31) ng/m3 in OCE when the air mass origin in Hong Kong changed from a continental one to an oceanic one, suggesting its marine origin. Phinney et al.22 found correlations between MSA concentrations and oceanic biological activities over the northeast Pacific Ocean, and Gatson et al.35 showed high levels of MSA correlated well with blooms of DMS-producing organisms along California coast in an inland location of Riverside, CA. In CST, the MSA concentrations started to decrease and coincidently a decreasing trend in LWC was observed (Figure 2c). We further correlate MSA concentrations with particle LWC (Figure 3). The particle liquid water content is presented

particles.38 The high LWC of the particles is likely to promote the condensation of MSA produced in the gas-phase into the particle phase. Bardouki et al.39 reported the clear decrease of gas-phase MSA with increasing RH and most of the MSA are found in particulate form at high RH in a field study in the Eastern Mediterranean. However, Barnes et al.12 reported that the production of MSA can be enhanced by aqueous-phase processing of DMS or its intermediates due to their high reactivity with O3 or OH radicals in the aqueous medium. Ge et al.25 found good correlation between MSA and sulfate, which suggests that the aqueous-phase processing contributes to MSA formation because sulfate is mostly formed from aqueous-phase reactions in their campaign in the Central Valley of California. In CONT, organics and PM1 concentrations were high, but MSA concentrations were relatively low and stable because of its marine origin. Since there was no rainfall in CONT, the observed MSA concentration of 21 (±9) ng/m3 can be taken as an estimate of the background concentration of MSA in HK (without significant precipitation). Furthermore, a clear diurnal variation in MSA was observed over the whole month of September (Figure 4a). MSA started

Figure 4. Diurnal patterns of (a) MSA concentrations; (b) OS concentrations; and (c) Solar irradiance.

to increase noticeably at around noon as solar irradiance increased, and its concentration remained high during the afternoon. Solar irradiance decreased after 18:00, resulting in a decrease in MSA whose concentrations remained stable during the night. The near-noon peak of MSA suggests that the local photochemical activities play an import role in MSA formation. This is consistent with previous studies that oxidation of DMS is mainly initiated by the reaction with OH radicals during the daytime. OS. In September, OSmin concentrations in PM1 particles ranged from 80 to 2000 ng/m 3 . The average OS min concentration was 203 (±85) ng/m3, which is about 5% of AMS-detected organics and 2% of AMS-detected sulfate. Our results are comparable to those reported for continental aerosols measured over the southeast Pacific Ocean, where OSmin comprised 4% of organics.40 Unlike MSA, OSmin had lower concentrations in CST (125 ng/m3) than in OCE (215 ng/m3) and CONT (255 ng/m3) because of their association with different air mass origins. Surratt et al.30 demonstrated that OS could be formed in both photo-oxidation and night-time oxidation experiments. Tolocka et al.1 showed that OS concentrations appear to be higher in summer and fall when photochemical oxidation chemistry is most active. In our study, neither noon peak nor night peak is observed in OS (Figure 4c). OS detected in this campaign is likely brought in from elsewhere rather than formed near Hong Kong. Although formation mechanisms of OS are not completely understood yet, it has been suggested that particle-phase reactions play a significant role in OS formation.32 OS

Figure 3. MSA concentrations vs particle liquid water content.

using two fixed size bins. The width of the first 4 bins is 5 μg/ m3 and the last bin is 15 μg/m3, as few data points have LWCs above 25 μg/m3. MSA concentrations increased as the liquid water contents increased during the whole of September, including the CONT period (pink circles in Figure 3) when the precursor might be scarce. DMS is not very soluble in water with the Henry’s law coefficient of 0.48 M atm−1 at 298 K.36 MSA production is supposed to be dominated by gas-phase oxidation of DMS. Clegg et al.37 estimated that the lower limit of Henry’s law coefficient for MSA is 2 × 107 M atm−1, and MSA was primarily found in the accumulation mode of the 3676

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Figure 5. OS concentrations vs (a) particle liquid water content and (b) pH in September 2011.

observed, i.e., high sulfate mass concentration, high particle acidity. As mentioned, inorganic sulfate is of high concentration in this area, and will affect the LWC together with RH. No obvious correlations between OS and RH (Figure 2b) or inorganic sulfate (SI Figure S3) can be observed. High Molecular Weight OS Formation. High OS concentrations under highly acidic conditions (pH < 0.5, light purple circles in Figure 5b) were observed when Hong Kong was under the influence of continental sources. The observed OS in CONT were very likely to have transported from polluted continental areas of high sulfate content and particle acidity. High-resolution mass spectra of sulfur bearing organic fragments provide another evidence for the regional influence in OS formation in Hong Kong. Although most of the species detected by the HR-ToF-AMS including OS had undergone extensive fragmentation, examination of ion signals at higher m/z values (>100) was still useful in providing information regarding the molecular weight of the species.49 For example, DeCarlo et al.50 observed larger fractions of fragments above m/z 100 in biomass burning plumes than in urban SOA, proposing that larger molecules or molecules more resistant to fragmentation were formed in biomass burning plumes. The HR mass spectra of organic sulfur fragments indicate substantial contributions of signals at m/z values >100 (shown in SI Figure S5). The fraction was less than 20% in OCE and CST, but it increased to 38% in CONT. The longrange transport during CONT may have facilitated the formation of high molecular weight OS products via aerosol processing. The low pH of 0.1 in CONT may induce the formation of larger oligomers46 and result in the formation of high-molecular-weight OS. Alternatively, the precursors of OS in CONT may be larger than those in other periods. In most recent studies, high molecular weight OS from long chain alkanes and aromatic OS were detected in field samples of Pakistan, Guangdong,10 and Shanghai, China.51,8 They were supposed to be formed from combustion sources with possibly subsequent atmospheric processing. The characteristic of these locations, where long-chain alkyl sulfates or aromatic sulfates were found, is highly polluted with high particle acidity and sulfate content. These studies indicate the formations of high molecular weight OS from anthropogenic emissions are possible, especially in PRD region, where organic matter and sulfate are of high abundance. Atmospheric Implications. Our results highlight the regional influence and aqueous-phase processing on organic

formation through the direct reactions of sulfuric acid and alcohols are slow under tropospheric conditions with typical particle acidity of pH = 4−1.5.41 OS formation from epoxides have been proposed to be feasible under ambient conditions42,43 and was suggested to be important in OS formation in East Asia.10 In this reaction, reactive uptake of epoxides occurs by the acid-catalyzed ring opening of epoxydiol, followed by the addition of the inorganic sulfate.10,42,43 Besides, radical-initiated formation of OS was found to be possible in both bulk and chamber experiments.44 Hatch et al.45 also suggested that OS yield can be enhanced under high RH by promoting partitioning of semivolatile organic species into the particle phase. Hence, the inorganic sulfate concentration, particle acidity, and particle liquid water content likely affect OS formation. In our measurements, the sulfate level was sufficiently high at 8.6 ug/m3 and made up 55% of the aerosol mass on average. The aerosol sulfate mass concentration can hardly be the limitation for the OS formation in our campaign. OS was first found in acidic particles with inorganic sulfate but OS formation from nonacidified seed particles was also observed.32 Surratt et al.30 reported that the number of OS products increased as sulfuric acid concentrations increased in seed aerosols. In addition, Zhang et al.46 observed OS have higher yields under humid conditions due to the potential need for aerosol-phase water. However, high RH may also reduce acidity and yield.47 The combined effects of particle liquid water content and particle acidity on OS formation are still unknown. We correlate particle LWC and pH with OS concentrations. Interestingly, elevated OS were found under high LWC conditions, as shown in Figure 5a (bins are presented in the same way as MSA). In Figure 5b, no clear correlation between OS concentration and particle acidity can be observed. As the data are scattered, the particle acidity is presented using bins with two fixed sizes of 0.15 (the middle four bins) and 0.3 (first and last two bins), and mean OS concentrations are specified in each bin (marked as star). In total, 44% of the data points (blue circles in Figure 5b) fall into the three bins around 0.5, the average particle pH in Sept. Minerath et al.48 reported that formation of OS through epoxide reactions will likely be very efficient for a broad range of epoxides for the particle pH of 1.5 to 4 as hydrolysis of epoxides is kinetically feasible even under mildly acidic conditions. During our campaign, OS production in general had a stronger relationship with particle liquid water content than with particle acidity under the conditions 3677

DOI: 10.1021/es5056269 Environ. Sci. Technol. 2015, 49, 3672−3679

Article

Environmental Science & Technology

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sulfur compounds formation. MSA is of marine origin and the MSA-to-OS ratio doubled (18%) during the period under the influence of coastal air masses compared with the average ratio under continental influenced period (9%). The high concentrations detected during the continental influenced period indicate OS is likely to be an important contributor to the ambient SOA over the polluted continental area. Most of the current studies focus on the biogenic precursors, but the formation of OS from anthropogenic sources and the formation mechanisms deserves further study in the future. Furthermore, our results suggest aqueous-phase processing is important in both MSA and OS formation. MSA formation is enhanced under high liquid water content conditions and OS formation tends to be more sensitive to the particle liquid water content than to the particle acidity at this polluted humid coastal site during the campaign. But the conditions that favor OS formation may vary in different locations due to differences in meteorological conditions and precursors. The reported laboratory studies that concluded epoxide or radical initiated pathways in OS formation were conducted either under relatively low RH conditions (RH < 50%) or in the bulk solutions.3,42 The high molecular weight OS observed in CONT also raised the question whether the highly acidic conditions favor the high molecular weight OS formation. Further studies are necessary to examine the combined effects of RH and particle acidity on OS yields and formation mechanisms, especially under the high RH conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text: SI1−6, Figures S1−S5, and Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (852) 2358-7124; fax: (852) 2358-0054; e-mail: [email protected]. Present Address §

School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Hong Kong Research Grants Council General Research Fund (GRF 600413) and the Environment and Conservation Fund (project number ECWW09EG04).

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