Secondary organic aerosol formation from photooxidation of a mixture of dimethyl sulfide and isoprene

Atmospheric Environment 46 (2012) 271e278

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Secondary organic aerosol formation from photooxidation of a mixture of dimethyl sulfide and isoprene Tianyi Chen, Myoseon Jang* Department of Environmental Engineering Sciences, P.O. Box 116450, University of Florida, Gainesville, FL 32611, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 August 2010 Received in revised form 5 September 2011 Accepted 27 September 2011

Secondary organic aerosol (SOA) created from the photooxidation of a mixture of isoprene and dimethyl sulfide (DMS) was studied at different NOx concentrations (40e220 ppb) and humidities (12%, 42% and 80%) using a Teflon film indoor chamber. To study the effect of isoprene on DMS products, the major DMS photooxidation products, such as sulfuric acid, methanesulfonic acid (MSA) and methanesulfinic acid (MSIA), were quantified in both the presence and the absence of isoprene using a Particle-Into-LiquidSampler coupled with Ion Chromatography (PILS-IC). The resulting PILS-IC data showed that the DMS aerosol yield significantly decreased due to the photooxidation of isoprene. A 35.2% DMS aerosol yield reduction was observed due to 800 ppb isoprene in 185 ppb NOx and 140 ppb DMS. Among the aerosolphase DMS oxidation products, MSA was the most sensitive to the presence of isoprene (e.g., 46% reduction). The DMS aerosol product analysis indicates that isoprene oxidation affects the pathways of MSA formation on the aerosol surface. Using a new approach that implements an Organic Carbon (OC) analyzer, the isoprene SOA yield (Yiso) in the DMS/isoprene/NOx system was also estimated. The OC data showed that Yiso increased significantly with DMS compared to the Yiso without DMS. For example, Yiso with 80 ppb NOx and 840 ppb isoprene was increased by 124.6% due to 100 ppb DMS at RH ¼ 42%. Our study suggests that the heterogeneous reactions of isoprene oxidation products with the highly acidic products (e.g., MSA and sulfuric acid) from DMS photooxidation can considerably contribute to the Yiso increase. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Secondary organic aerosol Heterogeneous reaction Isoprene Dimethyl sulfide Methanesulfonic acid

1. Introduction Dimethyl sulfide (DMS) is an important marine-borne reduced sulfur compound with an estimated flux of 15.4e28.0 Tg S yr1 (Aumont et al., 2002; Bopp et al., 2003; Kloster et al., 2006). A large amount of ambient monitoring data suggests that DMS photooxidation produces fine particles which consist primarily of sulfuric acid and methanesulfonic acid (MSA) (Bardouki et al., 2003; Barone et al., 1995; Gaston et al., 2010; Lukacs et al., 2009). These fine particles have the potential effect on the Earth’s radiation budget through the production of cloud condensation nuclei (CCN) over the oceans, leading to increases in planetary albedo (Charlson et al., 1987). Thus, studies of DMS oxidation have been investigated by many researchers to understand DMS products and their impact on cloud chemistry. Despite many studies through controlled chamber experiments and field observations, many details about DMS photooxidation

* Corresponding author. Tel.: þ1 352 846 1744; fax: þ1 352 392 3076. E-mail address: mjang@ufl.edu (M. Jang). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.09.082

mechanisms, the aerosol formation, and the kinetic model remain controversial. For example, most chamber studies of DMS oxidation products have been operated solely for the reactions of DMS with OH radicals under both NOx free and NOx containing conditions (Arsene et al., 1999; Barnes et al., 1996; Librando et al., 2004; Yin et al., 1990a, 1990b), but no impact of coexisting volatile organic compounds (VOCs) has been considered. The atmospheric VOCs coexisting with DMS can modify gas phase chemistry of DMS, such as RO2 radical chemistry, the production of OH radicals, and the ozone formation associated with the production of O(1D). These modified gas phase reactions due to VOCs can also influence chemical distributions of DMS oxidation products that are directly related to DMS aerosol formation. In addition to the complexity of DMS oxidation due to the coexisting VOCs, the aerosol phase heterogeneous reactions of DMS products with atmospheric organic compounds have not been elucidated yet. For example, Jang et al. (2002) have proposed that atmospheric organics, which are partitioned to aerosols, can be further transformed via heterogeneous reactions, particularly in the presence of submicron sulfuric acid aerosols. The direct outcome of heterogeneous acid catalyzed reactions is the formation of

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oligomeric matter in aerosol and an increase in SOA mass. Besides sulfuric acid, DMS produces MSA which, being a strong acid (pKa ¼ 2), may also enhance the heterogeneous reactions of atmospheric organic compounds increasing SOA production. Up to date, the simulation of DMS photooxidation against field data has been conducted using an explicit gas-phase mechanism coupled with a few simple aerosol phase reactions (Karl et al., 2007; Lucas and Prinn, 2002, 2005) or the semiempirical gas and particle kinetic models based on the fitting parameters to field data (Chen et al., 2000; Davis et al., 1999; Mari et al., 1999). However, these simplified DMS oxidation mechanisms are not satisfactory in prediction of DMS oxidation products such as dimethyl sulfoxide (DMSO) (Chen et al., 2000), H2SO4 and MSA (Lucas and Prinn, 2002). This deviation is caused by the uncertainty of the gas phase reaction rate constants for both DMS and DMS products and the lack of MSA formation mechanisms through heterogeneous reactions of DMS oxidation products on the surface of aerosol. In addition, none of previous models included the aerosol phase heterogeneous reactions of DMS products with atmospheric semivolatile oxygenated compounds, which are mainly originated from secondary organic aerosol (SOA) due to the photooxidation of VOCs. In this study, in an attempt to understand the interaction between the oxidations of DMS and SOA precursors, the photooxidation of a mixture of isoprene and DMS was explored using an indoor Teflon film chamber under various NOx concentrations and relative humidities (RH). Isoprene was chosen as a representative of biogenic SOA precursors not only because it has a large emission (440e660 TgC yr1, about same as the global methane flux) (Guenther et al., 2006), but also because its SOA yields are highly sensitive to aerosol acidity. For example, Czoschke et al. (2003) reported that the SOA yield produced from the ozonolysis of isoprene increases by almost 200% when there is preexisting sulfuric acid aerosol. Edney et al. (2005) showed a 1000% increase in the isoprene SOA yield in the presence of SO2. To investigate the impact of isoprene on the DMS aerosol, the DMS aerosol products (with and without isoprene) were quantified using a Particle-Into-Liquid-Sampler coupled with Ion Chromatography (PILS-IC). To evaluate the DMS impact on the isoprene SOA formation, the isoprene SOA yields in the DMS/isoprene/NOx system were also calculated using a new approach implementing an Organic Carbon (OC) analyzer, and compared with those in the absence of DMS.

2. Experimental section 2.1. Teflon film indoor chamber experiments All SOA experiments were conducted in a 2 m3 indoor Teflon film chamber equipped with UVeVisible lamps (Solarc Systems Inc., FS40T12/UVB) covering all wavelengths ranging between 280 and 900 nm. Prior to each experiment, the chamber was flushed using air from clean air generators (Aadco Model 737, Rockville, MD; Whatman Model 75-52, Haverhill, MA). Precursor organics were added to the chamber by passing clean air through a T union where the chemicals were injected using a syringe, while NOx gas was introduced using a syringe through a certificate NO tank (99.5% nitric oxide, Linde Gas). SOA experimental conditions and resulting SOA data at different relative humidity (RH ¼ 12%, 42% and 80%) are summarized in Tables 1e3. 2.2. Chemicals and instruments All the organic chemicals (purity levels > 98%) were purchased from Aldrich (Milwaukee, WI). The ozone concentration was measured by a photometric ozone analyzer (Teledyne model 400E), while the NOx concentration was measured using a chemiluminescence NO/NOx analyzer (Teledyne Model 200E). A fluorescence analyzer (Teledyne Model 102E) was used to measure SO2 and Total Reduced Sulfur (TRS, gas phase) concentration. The particle size distribution was monitored with a Scanning Mobility Particle Sizer (SMPS, TSI, Model 3080, MN) combined with a condensation nuclei counter (CNC, TSI, Model 3025A). The SMPS data was corrected for the aerosol loss due to the chamber wall with a first-order decay (Mcmurry and Grosjean, 1985). The gasphase concentration of the precursors was measured using an HP 5890 Gas Chromatography-Flame Ionization Detector (GC-FID). A PILS-IC (Metrohm, 761 Compact) was used to measure the major aerosol products produced from DMS photooxidation. The PILS-IC sample was collected at the end of each chamber experiment (Exp. M-1, M-2 and D-1). The detection limit of PILS-IC is 0.2 mg m3 and the associated error is 6%. The aerosol sampling flow rate was 13 L min1 and the liquid flow rate in the anion column was 0.7 mL min1. A basic denuder, coated with 1% glycerol and 2% K2CO3 in ethanol-water (1:1), was placed upstream of the PILS to remove gaseous acids. For measuring OC data, the aerosol was

Table 1 DMS aerosol phase products measured using a PILS-IC at two different RH levels (10% and 45%). Yield  100%c Exp.a

NOxb ppb

RH ¼ 10% M-1 185 D-1 200 RH ¼ 45% M-2 181 a

YDMS  100% massd

Total S yield  100% molarf

xDMS  100% (IC)g

OCDMS/ OMDMS (IC)h

19.2 12.3

39.6 61.1

76.3 88.9

68 100

0.10 0.11

14.0

50.0

79.8

56

0.10

Isopreneb ppb

DMSb ppb (mg m3)

DDMS ppb (mg m3)

OM mg m3

MSIAd mass (molar)

MSAd mass (molar)

H2SO4d mass (molar)

SO2 molar

TRS molare

800 0

140 (354.0) 170 (431.0)

70 (177.0) 71 (177.0)

103.8 108.3

4.4 (3.4) 4.8 (3.7)

25.8 (16.7) 48.0 (31.0)

9.4 (7.3) 8.4 (4.6)

29.7 37.3

847

139 (352.0)

56 (142.0)

126.7

6.8 (5.3)

33.4 (21.6)

9.8 (6.2)

32.8

M: the mixture of isoprene and DMS experiment; D: the photooxidation of DMS only. b Initial concentration. c All the aerosol yield data were corrected for the wall loss using the first-order decay rate. The estimated uncertainty (10%) is calculated from errors associated with SMPS (2%), PILS-IC (6%), SOA density (5%) and DROGiso (6%). d Aerosol phase products only. e TRS: Total reduced sulfur including all the gas phase sulfur other than DMS. The TRS yield was not corrected for the wall loss. f Total sulfur yield ¼ MSIA aerosol yield þ MSA aerosol yield þ H2SO4 aerosol yield þ SO2 yield þ TRS yield. MSIA, MSA and H2SO4 aerosol yield have been corrected for wall loss. g xDMS is defined by Eq. (3). xDMS (IC) is determined using PILS-IC assuming that MSA, MSIA and H2SO4 are the only aerosol phase products of DMS. xDMS (IC) ¼ (MSIA mass þ MSA mass þ H2SO4 mass)/total aerosol mass (OM). The assumption here is valid because the calculated xDMS of D-1 (no isoprene) reaches the theoretical value, 100%. h OCDMS/OMDMS (IC) ¼ (0.15  MSIA mass þ 0.125  MSA mass)/(MSA mass þ MSIA mass þ H2SO4 mass). The MSIA and MSA carbon fractions of the total molecular weight are 0.15 and 0.125, respectively.

T. Chen, M. Jang / Atmospheric Environment 46 (2012) 271e278

273

Table 2 Isoprene SOA experiments with and without DMS in the presence of NOx at RH ¼ 12%, 42% and 80%.a Exp.b

NOx ppb

RH ¼ (12  2)% M-3 48 I-3 40 M-4 86 I-4 87 M-5 220 I-5 210 M-6 42 I-6 55 M-7 44 I-7 57 RH ¼ (42  2)% M-8 40 I-8 40 M-9 81 I-9 95 RH ¼ (80  2)% M-10 30 I-10 40 M-11 85 I-11 81

DMS ppb

Isoprene ppb

DROG mg m3 DDMS mg m3 DOM mg m3 OC/V c xiso  100%d,h Yiso  100%e,h YDMS  100%f,h % DYisog,h

119 0 103 0 129 0 30 0 15 0

850 870 872 860 880 920 210 210 100 100

241.0 478.6 784.0 651.0 2355.1 2437.7 475.1 541.2 257.5 257.0

26.6 0 33.9 0 203.5 0 27.3 0 n.a. 0

100 0 100 0

820 810 840 880

446.4 445.0 681.8 1051.6

n.a. 0 n.a. 0

120 0 130 0

851 1030 1050 1040

865.1 1635.6 1335.6 1380.9

n.a. 0 n.a. 0

6.8 5.1 19.0 6.3 116.3 22.9 10.5 5.2 6.3 2.3

0.38 0.56 0.37 0.56 0.31 0.59 0.40 0.59 0.42 0.59

56.7 100 54.3 100 37.8 100 57.4 100 61.3 100

9.2 2.9 18.6 5.96

0.36 0.53 0.31 0.50

23.4 9.4 30.6 6.3

0.31 0.54 0.33 0.54

(53.2)

1.60 (1.50) 1.06 1.31 (1.23) 0.97 1.87 (1.64) 0.94 1.26 (1.20) 0.95 1.5 (1.43) 0.89

11.1 n.a. 25.6 n.a. 35.5 n.a. 16.4 n.a. n.a. n.a.

55.9 (52.1) 100 46.1 (41.2) 100

1.16 (1.07) 0.65 1.28 (1.12) 0.57

n.a. n.a. n.a. n.a.

41.8 (37.0) 100 47.2 (42.7) 100

1.13 (1.00) 0.57 1.08 (0.98) 0.45

n.a. n.a. n.a. n.a.

(50.7) (33.2) (54.2) (58.5)

(11.90)

50.9% (41.6%)

(27.50)

35.1% (26.7%)

(37.90)

98.9% (74.4%)

(17.50)

32.6% (26.1%) 68.5% (60.8%)

78.5% (65.2%) 124.6% (97.2%)

98.2% (75.4%) 146.7% (124.4%)

Temperature was 21e24  C and the aerosol concentrations in the background air were 0.1e0.2 mg m3. Refer to the superscripts in Table 1. I: isoprene experiment (no DMS). c The ratio of the OC mass concentration to total volume concentration of aerosols. OC/V values (g cm3) were determined using an OC/EC analyzer and SMPS. d xiso is defined by Eq. (2). Its uncertainty (11.4%) was estimated from errors associated with SMPS, OC/EC (10%) and SOA density. e The maximal isoprene SOA yield with and without DMS (see Eq. (4)). The uncertainty was estimated from errors associated with SMPS, OC/EC, SOA density and DROG. The uncertainty for the single precursor SOA yield was 8% and that for SOA yields in the mixture was 13.8%. f The maximum DMS aerosol yield (YDMS) in the presence of isoprene. YDMS ¼ OM  (1  xiso)/DDMS. The uncertainty associated with YDMS is 13.8%. g 0 e Y )/Y , where Y 0 is the isoprene SOA yield in the presence of DMS and Y Percentage of Yiso increase due to DMS. %DYiso ¼ (Yiso iso iso iso is the yield in the absence of DMS. iso h Data in brackets are corrected based on water loss due to the maximum esterification reactions. n.a.: not applicable. a

b

analyzed at the end of each experiment with a semi-continuous OC/EC carbon aerosol analyzer (Sunset Laboratory Model 4) using the NIOSH 5040 method. The detection limit of OC analysis is 0.3 mg m3 (for 120 min sampling) and the associated error is 10%. The sampling flow rate for OC analyses was 8 L min1. The sampling time varied between 40 and 120 min, depending on the mass concentration of aerosols in the chamber. 3. Results and discussion 3.1. PILS-IC data to study the impact of isoprene on DMS aerosol formation

a mixture of 1% glycerol and 2% K2CO3 in ethanol-water upstream the PILS removes these carboxylic acids, which are produced from photooxidation of isoprene. Table 1 summarizes aerosol-phase DMS product distribution in both the presence (M-1 and M-2) and the absence (D-1) of isoprene. MSA accounts for the highest aerosol mass fraction (70% on average, see Fig. 2) among the three major DMS aerosol products. As shown in Table 1, the total aerosol mass yield of DMS (YDMS), estimated from the sum of MSA, MSIA and H2SO4 mass yield, was 39.6% with isoprene (M-1) and 61.1% without isoprene (D-1), resulting in a 35.2% decrease in YDMS due to isoprene. The aerosol

The products of DMS/NOx photooxidation with isoprene were characterized using a PILS-IC and compared to those without isoprene. Fig. 1 shows a typical ion chromatogram for DMS photooxidation products, which mainly includes methanesulfinic acid (MSIA), MSA and sulfuric acid. For the analysis of MSIA and MSA, no interference due to gaseous carboxylic acids (e.g., formic acid and acetic acid) appears when a denuder coated with

Table 3 SOA formation from DMS photooxidation using an indoor chamber at RH ¼ 12%.a Exp.

NOx ppb

DMS ppb

DDMS mg m3

OM mg m3

OCDMS/ VDMS

OCDMS/ OMDMSb

YDMS  100%c

D-2 D-3 D-4 D-5

280 310 210 43

136 127 170 116

255.1 209.6 113.9 121.5

192 156.8 73 65.1

0.13 n.a. 0.14 0.14

0.11 n.a. 0.12 0.12

75.3 74.8 64.1 53.6

a

Refer to the superscripts in Table 2. OCDMS/OMDMS ¼ OCDMS/VDMS/1.2. The density of the DMS aerosol is 1.2 g cm3. OCDMS/VDMS values (g cm3) were determined using an OC/EC analyzer and SMPS. c Aerosol yield for DMS. YDMS ¼ OM/DDMS. The uncertainty for the single precursor SOA yield was 8%. b

Fig. 1. Anion chromatogram of the aerosol sample of D-1 showing MSA, MSIA and sulfate.

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To estimate the SOA yield for each precursor in the mixture of DMS and isoprene, the total aerosol mass, OMDMSiso , is separated into the OM associated with DMS ðOMDMS Þ and the OM associated with isoprene ðOMiso Þ. We define the mass fraction of isoprene SOA in OMDMSiso as xiso and the mass fraction of DMS aerosol in OMDMSiso as xDMS.

xiso ¼

OMiso OMDMSiso

xDMS ¼ 1  xiso ¼ Fig. 2. Major acid (MSIA, MSA and sulfuric acid) mass fraction of the total acidic aerosol mass in M-1, M-2 and D-1 (assuming MSIA, MSA and sulfuric acid are the only acidic aerosol products).

phase MSA yield with isoprene (M-1) was 46% lower than that without isoprene (D-1), indicating that isoprene significantly decreases the DMS aerosol yield mainly through the reduction of the aerosol phase MSA. It is currently unclear why the DMS aerosol yield becomes lower in the presence of isoprene. There are two possible reasons. First, isoprene and its gas-phase products compete for atmospheric oxidants with the DMS and DMS gas-phase products. Due to the presence of isoprene, the reaction system produces different amounts of atmospheric oxidants and radicals, such as RO2, NO3, O3P and OH radicals. This modification changes DMS oxidation pathways that lead to different yields of MSA. Second, the aqueous facilitated heterogeneous reaction of MSIA that produces MSA (Bardouki et al., 2002; Hopkins et al., 2008) is unfavorable on the surface of the relatively hydrophobic aerosol containing isoprene oxidation products. Unlike the DMS/NOx system mainly comprising MSA, MSIA and sulfuric acid, the aerosol produced from the DMS/isoprene/NOx system may also contain irreversible organosulfates (nonelectrolytes) that are formed through the reaction of isoprene products and the acids originated from DMS photooxidation. The PILS-IC method may underestimate the sulfur compounds in the DMS aerosol in the presence of isoprene. However, the DMS yields using the PILS-IC data agree well with those obtained from the OC data (see “Section 3.3. Comparison of PILS-IC and OC data”). Such a tendency suggests that the organosulfates produced, if any, are reversible during the PILS-IC analysis where hot water steam is used for the aerosol extraction. The total sulfur molar yields in M-1 and M-2 were calculated (Table 1) without the wall loss correction for the total reduced sulfur (TRS). The total identified sulfur molar yield without isoprene (D-1) reaches almost 89% and that with isoprene (M-1 and M-2), on average 78% (Table 1). The wall loss of polar products (e.g., MSA and MSIA) and the heterogeneous reaction of DMS products with isoprene products on the chamber wall would contribute to the unidentified sulfur molar yields (11% for D-1 and 22% for M-1 and M-2). 3.2. OC analysis to study the impact of DMS on isoprene SOA yields The typical SOA yield for a single precursor is described as (Odum et al., 1996)

Y ¼

OM

DROG

(1)

where OM is the mass concentration (mg m3) of organic matter formed and DROG is the mass concentration (mg m3) of the consumed reactive organic gas.

(2) OMDMS OMDMSiso

(3)

Both xiso and xDMS range between 0 and 1. Then, the isoprene SOA yield (Yiso) in the presence of DMS can be expressed as:

Yiso ¼

OMiso OMDMSiso ¼ xiso DROGiso DROGiso

(4)

where DROGiso is the consumed isoprene mass concentration. In this study, an OC analysis was used to estimate xiso and xDMS. The organic carbon (OCDMSiso ) in the aerosol produced from photooxidation of the isoprene/DMS/NOx system is the sum of OC contributed by isoprene ðOCiso Þ and DMS ðOCDMS Þ.

OCDMSiso ¼ OCDMS þ OCiso

(5)

Each side in Eq. (5) is divided by OMDMSiso.

OCDMSiso OCiso OCDMS ¼ þ OMDMSiso OMDMSiso OMDMSiso

(6)

By adding Eqs. (2) and (3) into Eq. (6), the equation can be transformed into

OCDMSiso OCiso OCDMS ¼ ð1  xiso Þ  xiso þ OMDMSiso OMiso OMDMS

(7)

We assume that OCiso =OMiso for the DMS/isoprene/NOx system is not significantly different from OCiso =OMiso for the isoprene/NOx system, where OCiso is the organic carbon mass formed from the isoprene/NOx system (no DMS) at a given experimental condition and OMiso is the total aerosol mass in the same system. Similarly, we assume OCDMS =OMDMS zOCDMS =OMDMS , where OCDMS and OMDMS are for the aerosol from the DMS/NOx system (no isoprene), respectively. Table 1 shows OCDMS/OMDMS values for M-1, M-2 and D-1, which are estimated using the chemical composition of DMS aerosol products (MSA, MSIA and sulfuric acid) (Fig. 2) and the carbon fractions of each acid (0.125 for MSA, 0 for sulfuric acid and 0.15 for MSIA). The resulting OCDMS/OMDMS values of the aerosol from the DMS/NOx system were 0.10 and close to OCDMS =OMDMS (0.11) of the DMS/isoprene/NOx aerosol, supporting the assumption OCDMS =OMDMS zOCDMS =OMDMS (within the 8.5% statistical error, see superscript h in Table 1). Then, Eq. (7) can be rewritten as

OCDMSiso OCiso OCDMS ¼ x þ ð1  xiso Þ OMDMSiso OMiso iso OMDMS

(8)

The organic mass (OMiso) in the isoprene/NOx system can be estimated from the isoprene SOA density (riso) and isoprene SOA volume (Viso) using SMPS data.

OMiso ¼ Viso riso

(9)

The organic mass (OMDMS) in the DMS/NOx system and the organic mass (OMDMSiso ) in the isoprene/DMS/NOx system can be described in a similar way,

T. Chen, M. Jang / Atmospheric Environment 46 (2012) 271e278

OMDMS ¼ VDMS rDMS

(10)

OMDMSiso ¼ VDMSiso rDMSiso

(11)

where VDMS is the DMS aerosol volume and rDMS is the DMS aerosol density. VDMSiso and rDMSiso correspond to the volume and the density of the aerosol of the isoprene and DMS mixture system, respectively. rDMSiso is estimated approximately by the following equation.

rDMSiso ¼ riso xiso þ rDMS ð1  xiso Þ

(12)

Using Eqs. (9)e(12), Eq. (8) can be rewritten as

OCDMSiso OCiso x ¼ VDMSiso ½riso xiso þ rDMS ð1  xiso Þ Viso riso iso OCDMS þ ð1  xiso Þ ð13Þ VDMS rDMS Eq. 13 is rearranged to solve for xiso.

xiso ¼

  1 OCiso    r OCiso OCDMS Viso riso DMS 2ðriso  rDMS Þ  Viso riso VDMS rDMS   OCDMS OCiso r þ ðriso 2rDMS Þ þ VDMS rDMS Viso riso DMS 2  OCDMS OCiso þ ðriso 2rDMS Þ 4ðriso  rDMS Þ VDMS rDMS Viso riso  0:5  OCDMS OCDMS OCDMSiso ð14Þ   VDMS rDMS VDMS VDMSiso

where riso (1.1 g cm3) and rDMS (1.2 g cm3) are experimentally determined using SMPS data and the aerosol mass collected on the filter. OCiso, OCDMS and OCDMSiso are determined by the OC/EC analyzer. Viso, VDMS and VDMSiso are analyzed by the SMPS. The OCiso/Viso values of the isoprene/NOx system range from 0.56 to 0.59 at RH ¼ 12% and those of the isoprene/DMS/NOx system, 0.31e0.42 (Table 2). The OCDMS/VDMS value in this study is fixed at 0.13 for NOx reactions (OCDMS/VDMS in Table 3). The resulting xiso values are listed in Table 2. The Yiso values reported in Table 2 are estimated using Eq. (2) with OMiso, OMisoDMS , xiso and DROGiso. When we take into consideration the esterification of DMS major products (MSA and sulfuric acid) with isoprene products, OCDMS =OMDMS can be slightly larger than OCDMS =OMDMS because OMDMS decreases with water evaporation e a concurrent process associated with esterification. Considering esterification is therefore needed to correct the OCDMS =OMDMS calculation. When we assume that all the produced MSA and sulfuric acid are converted to esters (maximum esterification), the mass loss fraction of MSA due to esterification is 18/96 where 18 and 96 are the molecular weight of water and MSA, respectively; the loss fraction of sulfuric acid is 36/98 where 36 is the molecular weight of 2 mol of water and 98 is that of 1 mol of sulfuric acid. Using the chemical composition information (Fig. 2), the estimated mass loss fraction of OMDMS due to esterification is 0.20 suggesting that OCDMS =OMDMS ¼ 1:25  OCDMS =OMDMS (no reaction). xiso (maximum esterification) is only 7.2% smaller than xiso (no reaction), which is within the 11.4% error range (see “xiso” in Table 2). Thus, we will only report the data without water loss correction in the following discussion. 3.3. Comparison of PILS-IC and OC data The OC/EC data are compared to the PILS-IC data to evaluate the validity of the two methods. For example, the PILS-IC measurement

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in M-1 (Table 1) and the OC/EC analysis in M-5 (Table 2) were conducted in similar experimental conditions. Fig. 3 summarizes xiso and YDMS from M-1 and M-5. Both methods show similar results in either xiso or YDMS: e.g., xiso ¼ 32.0% based on IC data and xiso ¼ 37.8% from OC data; YDMS ¼ 39.6% based on IC data and YDMS ¼ 35.5% from OC data. Consistency between the two methods is also observed for the OCDMS/OMDMS data: 0.11 for D-1 using IC (Table 1) and 0.12 for D-4 using OC/EC (Table 3). Thus, we conclude that the OC/EC approach is an appropriate method to estimate the decoupled aerosol yields (Yiso and YDMS) in the binary mixture and that the PILS-IC analysis reasonably quantifies the major DMS aerosol-phase products in the presence of isoprene. 3.4. The impact of DMS on the isoprene SOA yield: NOx effects The impact of DMS on Yiso was analyzed by comparing Yiso with DMS to that without DMS at given NOx and isoprene concentrations. Fig. 4A presents Yiso at a given isoprene concentration (850 ppb) and humidity (RH ¼ 12%) with varying NOx concentrations (40e220 ppb) with and without DMS. Fig. 4B shows Yiso at a given NOx concentration (40 ppb) and humidity (RH ¼ 12%) with varying isoprene concentrations (100e850 ppb) with and without DMS. The percentage increase of Yiso due to DMS (%DYiso) is summarized in Table 2. The resulting %DYiso values (between 32.6% and 98.9%) suggest that Yiso is significantly elevated by DMS in a wide range of NOx concentrations. 3.5. The impact of DMS on isoprene SOA formation: RH effects In Fig. 5, the Yiso both with DMS and without DMS at RH ¼ 12% were compared with those at higher RHs (42% and 80%), with two initial NOx concentrations (40 ppb and 80 ppb) at a given isoprene concentration (850 ppb). Overall, %DYiso (Fig. 5) was much higher at high RH compared to that at low RH. Humidity can influence both gas-phase chemistry and aerosolphase reactions inside the chamber. In the presence of NOx, higher OH radicals can be formed though the reaction of O(1D) with a water molecule. Under our experimental condition, isoprene is present over the entire chamber experiment. Thus, the additional OH radicals due to higher humidity are more likely consumed by isoprene instead of producing highly oxidized products. On the other hand, the higher humidity can reduce HO2 radicals in the gas phase (Kanno et al., 2005), influencing the formation of ROOH. It is

Fig. 3. xiso and YDMS values associated with M-1 (IC data) compared to those with M-5 (OC/EC data). Experimental conditions of M-1 and M-5 are similar. xiso and YDMS associated with OC/EC data are found in Table 2 and YDMS associated with IC data is in Table 1. xiso associated with IC data ¼ 1  xDMS (Table 1).

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Fig. 4. (A) Isoprene SOA yields at different NOx concentration with and without DMS (isoprene concentration ¼ 850 ppb and RH ¼ 12%). (B) Isoprene SOA yields at different isoprene concentration with and without DMS (NOx concentration ¼ 40 ppb and RH ¼ 12%). The isoprene SOA yield was calculated using Eq. (4).

known that ROOH can further react with an aldehyde and produce high molecular weight hemiacetal in the aerosol (Johnson et al., 2004). Hence, isoprene SOA yields decrease with increasing humidity. The greater %DYiso at the high RH condition (Fig. 5) is mainly due to the higher amount of acid formed from DMS photooxidation. The formation of MSA on the aerosol surface is enhanced by higher RH due to the aqueous processing from MSIA. The higher MSA accelerates aerosol phase heterogeneous reactions of organic compounds increasing SOA formation. This explanation is supported by the MSA mass yield and DMS aerosol mass fraction (xDMS) of the total aerosol mass listed in Table 1. For example, the MSA mass yield is 25.8% at RH ¼ 10% (M-1 in Table 1) compared with 33.4% at RH ¼ 45% (M-2 in Table 1). In addition, the DMS aerosol fraction in the total aerosol (xDMS) is 68.0% (M-1) at RH ¼ 10% and 56.0% (M-2) at RH ¼ 45%, indicating stronger heterogeneous reactions at higher RH that drive more volatile isoprene products onto the aerosol. 3.6. The impact of DMS on isoprene SOA formation: aerosol growth pattern The impact of DMS on Yiso was also investigated for the aerosol growth pattern between the isoprene-only system and the mixture system. The time profiles of the SOA mass and isoprene concentrations with DMS (M-4) and without DMS (I-4) are shown in Fig. 6A. The time profile for DMS aerosol mass and DMS concentration (D-5) is shown in Fig. 6B. In Fig. 6A, the photoirradiation of the isoprene-only system shows a 40-min induction period (no significant SOA growth), the time to reach the saturation of the

gas-phase isoprene oxidation products. In contrast, rapid aerosol growth occurred from the beginning of SOA experiment in the DMS photoirradiation system owing to the self-nucleation of H2SO4. The dynamics of aerosol growth patterns shown in Fig. 6 indicate that the DMS aerosol forms before the condensation of isoprene oxidation products when the mixture of isoprene and DMS are photoirradiated together. The pre-formed acidic products from DMS photooxidation can serve not only as catalysts that facilitate the formation of methyl-tetrols and nonvolatile oligomers from reactive organic compounds (e.g., glyoxal and methyl glyoxal), but also as reactants that produce organosulfates (Iinuma et al., 2007; Liggio and Li, 2008; Minerath et al., 2008, 2009; Paulot et al., 2009; Surratt et al., 2010). 4. Uncertainties The uncertainties associated with the use of the indoor chamber include the temperature increase (w6 K) due to UV photoirradiation lamps. The temperature change can affect the gasparticle partitioning of semivolatile products onto aerosols (Carlton et al., 2009). The artificial light sources producing wavelengths in the range of 280 and 900 nm, different from the actual natural sunlight, can lead to different product compositions (Warren et al., 2008). In addition, the progression of heterogeneous chemistry on the chamber wall between organic and inorganic species could potentially reduce the effects of heterogeneous reactions on the surface of the SOA. To minimize the uncertainties, a large outdoor chamber facility at the University of Florida will be used for future studies. In addition, the detailed clarification of the

Fig. 5. (A) Isoprene SOA yields at different RH with and without DMS (initial isoprene Concentration ¼ 850 ppb and NOx ¼ 40 ppb). (B) Isoprene SOA yields at different RH with and without DMS (initial isoprene concentration ¼ 850 ppb and NOx ¼ 80 ppb). The isoprene SOA yield (Yiso) was calculated using Eq. (4) and the percentage increase of isoprene SOA yield (%DYiso) due to DMS was marked in the figure.

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Their observation suggests that heterogeneous reactions of isoprene SOA products on the surface of the acidic aerosol, which is produced from DMS oxidation, can increase isoprene SOA yields in the ambient air. Further field studies are required in the future to find more evidence for the effect of DMS on isoprene SOA formation in the ambient atmosphere. Acknowledgments This work was supported by grants from the National Science Foundation (ATM-0852747). We thank David Smith in the Sunset Laboratory for correcting OC/EC data. References

Fig. 6. (A) Time profiles of aerosol growth and isoprene decay for the isoprene/NOx photooxidation with and without DMS (Exp. I-4 and M-4). (B) Time profiles of aerosol growth and the DMS decay for the DMS/NOx photooxidation (Exp. D-5).

mechanisms of the impact of DMS on isoprene SOA formation is needed based on the analytical studies of both the gas and aerosol phase products. 5. Atmospheric implications Our study is beneficial for evaluating the potential impact of DMS as a reduced sulfur compound on SOA formation in the ambient air especially near seaside areas. We have observed significant Yiso increases due to DMS at an atmospheric relevant RH (42%) conditions using the given mixing ratio (Table 2). The isoprene concentration in the coastal area is 200e800 ppt, varying with season and location (Bottenheim and Shepherd, 1995; Holzinger et al., 2002; Yokouchi, 1994), and the coastal concentration of DMS is usually 50e200 ppt (Ramanathan et al., 2001). Thus, possible mixing ratios of isoprene to DMS can range from 1 to 16 in coastal areas. Therefore, the mixing ratio of 7.5 used in our study falls into the actual ambient mixing ratio range. Based on the tendency in Yiso as a function of NOx (Fig. 4A), Yiso value of this study was not strongly dependent of NOx concentrations but was significantly affected by the presence of DMS. Although concentrations of isoprene, DMS, and NOx are higher in our study than those in ambient, the similar impact of DMS on Yiso would occur in the ambient air. For example, Froyd et al. (2010) have recently discovered a significant loss of gaseous epoxide (characteristic product of isoprene photooxidation in ambient) to the acidic aerosol during the maritime convective lofting of DMS.

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