Characterization of secondary organic aerosol generated from ozonolysis of α-pinene mixtures

Atmospheric Environment 67 (2013) 323e330

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

Characterization of secondary organic aerosol generated from ozonolysis of a-pinene mixtures Hardik S. Amin, Meagan L. Hatfield, Kara E. Huff Hartz* Department of Chemistry and Biochemistry, Southern Illinois University Carbondale, Carbondale, IL, USA

h i g h l i g h t s < Secondary organic aerosol from volatile organic compound mixtures was characterized. < Mixtures included a commercially available essential oil. < Product yields were measured as a function of mixture composition. < Changes in products yields were observed as the composition was changed.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 July 2012 Received in revised form 15 October 2012 Accepted 31 October 2012

In the atmosphere, multiple volatile organic compounds (VOCs) co-exist, and they can be oxidized concurrently and generate secondary organic aerosol (SOA). In this work, SOA is formed by the oxidation (in presence of excess ozone) of mixtures containing a-pinene and other VOCs. The VOC mixtures were made so their composition approached a commercially-available a-pinene-based essential oil, Siberian fir needle oil. The SOA products were sampled using filters, solvent extracted and analyzed by gas chromatography/mass spectrometry with trimethylsilyl derivatization. The individual product yields for SOA generated from a-pinene changed upon the addition of other VOCs. An increase in concentration of non-reactive VOCs (bornyl acetate, camphene, and borneol) lead to a decrease in individual product yields of characteristic a-pinene SOA products. Although these experiments were carried out under higher VOC and ozone concentrations in comparison to the atmosphere, this work suggests that the role of non-reactive VOCs should be explored in SOA products formation. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: VOC mixtures ozonolysis Secondary organic aerosol GC/MS characterization Siberian fir needle oil

1. Introduction The identities of many organic compounds in the atmosphere are unknown (Goldstein and Galbally, 2007). In laboratory experiments used to simulate atmospheric reactions, volatile organic compounds (VOCs) are used to generate secondary organic aerosol (SOA) by OH, NOx, and/or O3 oxidation. Although oxidation increases the number of organic compounds in aerosol, the majority of laboratory studies utilize a single VOC as the precursor to organic aerosol, which underestimates the complexity of atmospheric VOC composition. This could limit the characterization of organic aerosol if a mixture of VOCs gives a different set of aerosol products or different yields of aerosol products than individual VOCs. SOA contributes to atmospheric particulate matter (PM) concentrations, thus the study of the sources and chemical

* Corresponding author. E-mail address: [email protected] (K.E. Huff Hartz). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2012.10.063

formation mechanisms of SOA is important. Many laboratory studies of SOA use a-pinene as the SOA precursor for important reasons: among the nonmethane VOCs, a-pinene is the dominant monoterpene (Guenther et al., 1995; Seinfeld and Pankow, 2003), and the reaction of ozone with a-pinene is an important reaction pathway in troposphere, converting 40% of the emitted a-pinene (Griffin et al., 1999). Only a few reports of SOA generated from the ozonolysis reactions of a-pinene mixtures exist. Waring et al. (2011) investigated SOA formation from ozone reactions with binary and tertiary mixtures of limonene, a-pinene, and a-terpineol. The particle concentrations were affected by the composition of the SOA precursor mixture and depended on the ozone level. Forester and Wells (2011) quantified the OH radical formation during ozonolysis of a-pinene, limonene, and a-terpineol mixtures and pine oil cleaner. The OH yields were the highest for a binary mixture of limonene/a-pinene, followed by pine oil cleaner and the lowest for a-terpineol/a-pinene and a-terpineol/limonene mixtures. Hao et al. (2011) measured the SOA yields from pine and spruce emissions which contained a-pinene. Hatfield and Huff Hartz (2011)

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H.S. Amin et al. / Atmospheric Environment 67 (2013) 323e330

measured SOA yields from VOC mixtures containing a-pinene. When VOCs were added incrementally, the SOA yields were characteristic of the VOCs used to generate the SOA. Only a few reports of the product identification of SOA generated from biogenic VOC mixtures exist. Jaoui et al. (2008) measured the effect of isoprene on SOA production from the photooxidation of a-pinene/toluene/NOx mixture. The presence of isoprene decreased the SOA formed by 51%. A reduction in the concentration of toluene oxidation products (benzaldehyde and glyoxal) and an increase in the concentration of isoprene oxidation products (carbonyl and dicarbonyl products) were observed. Jaoui and Kamens (2003) generated SOA from a-pinene/b-pinene mixtures and identified twenty-nine products. The products formed from bpinene dominated the gas phase or were structurally similar to SOA products generated from a-pinene. In this work, the condensed phase products of SOA generated from the ozonolysis of a-pinene mixtures were characterized. The SOA that was generated by Hatfield and Huff Hartz (2011) was analyzed as a function of VOC mixture composition, and the product characterization is related to experiment yields. The effects of adding reactive monoterpenes and non-reactive VOCs on the PM characterization were determined. Finally, the PM from a complex, a-pinene based VOC mixture, Siberian fir needle oil (SFNO), was characterized. SFNO is a biogenically-derived essential oil; thus it provides a complex mixture with reproducible composition and is composed of VOCs which are not readily available as authentic standards. This work suggests that the ozonolysis of VOC mixtures gives different the individual product yields in comparison to the ozonolysis of a single VOC. 2. Material and methods 2.1. Collection of PM The experimental details for SOA generation and SOA yield measurement were described by Hatfield and Huff Hartz (2011). Briefly, the SOA was generated in a 5.5 m3 TeflonÒ chamber by reaction of ozone (450e600 ppb) with VOC mixtures. 2-butanol was used as a radical scavenger and the reactions were carried out at 23e29
C and relative humidity 1e4%. Particle concentration was monitored by a TSI scanning mobility particle sizer. The PM characterized in this work was collected during chamber experiments carried out by Hatfield and Huff Hartz (2011). PM samples were collected during SOA generation using Pallflex Tissuquartz filters (47 mm diameter) housed in a Gelman stainless steel filter holder. Prior to sampling, the filters were heated to 185
C for 4e8 h to reduce volatile contaminants. A blank filter was collected prior to injection of the VOCs in the chamber, and the PM sample was collected 300  30 min after the injection of VOC. No denuder was used. A vacuum pump (Gast) supplied a flow rate of 40  1 L min1, monitored using a flowmeter (Omega) for 30e 60 min, so approximately 100 mg of PM was collected on each filter. After sample collection, the filters wrapped in aluminum foil and stored in air tight containers at 18
C. 2.2. Solvent extraction and derivatization of SOA The PM collected on the filters was extracted with a solvent mixture and analyzed by GCMS (Huff Hartz et al., 2007 and Nolte et al., 2002). The filters were spiked with a deuterated standard mixture that contained d32-pentadecane (98%), d3-(methyl) myristic acid (98%), d31-palmitic acid (98%), d42-eicosane (98%), and d50-tetracosane (98%) (Cambridge Isotope). 10 mg of each deuterated standard was spiked on each filter. After spiking with deuterated standards, the filters were transferred to a 30 mL glass

centrifuge tube. 10 mL of the extraction solvent (2:3:5 dichloromethane: acetone: hexanes (v/v), Fisher) was added to each tube using a tilt pipette. After sonication at 30
C for 15 min, the supernatants were collected in a test tube. Each extraction was repeated twice, and the supernatant fractions were combined (30 mL). Fibers from the filters were removed by gravitational settling. The extracts were concentrated using a rotary evaporator (Yamato RE500) until the volumes were reduced to 1 mL. The extracts were reduced to 400 mL using a nitrogen gas (grade 5.0, Airgas), dried using a molecular sieve vapor trap (Supelco). 0.1 g of anhydrous sodium sulfate (Sigma Aldrich) was added to each sample to remove moisture from the extract. The samples were stored in the freezer at 18
C. Extracts were derivatized to improve separation by gas chromatography/mass spectroscopy (GC/MS). Immediately prior to GC/ MS analysis, 100 mL of extract was transferred into a pre-cleaned, silanized vial insert (National Scientific), and the insert was placed in a GC/MS vial. 125 mL of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) in 1% trimethyl chlorosilane (ThermoPierce) was added followed by the addition of 15 mL of pyridine catalyst (99%, ACROS). The vial was sealed using a polypropylene cap equipped with a PTFE/ silicon septum (National Scientific) and incubated at 65
C for 2 h (Nolte et al., 2002). 2.3. GC/MS analysis and quantification of SOA products SOA extracts were analyzed using a Varian Saturn 2100 GC/MS, which was equipped with a 3900 gas chromatograph and 2100T ion trap mass spectrometer. The 2.0 mL of each extract was injected in triplicate using an autosampler (Varian, 8400) into a split/splitless inlet maintained at 250
C. To reduce the loss of semivolatile species from the vials during analysis, the autosampler was cooled to 10e12
C using a water bath. Prefiltered (Varian CP 17973) helium carrier gas (Airgas, 99.999%) was maintained at a flow rate of 1.0 mL min1. The analytes were separated using a 5% diphenyl/ 95% dimethylpolysiloxane capillary column (Varian FactorFourÔ, 30 m  0.25 mm  0.25 mm), with the following temperature gradient: initial column temperature 50
C for 10 min, ramp to 80
C at a rate of 5
C min1, ramp to 320
C at a rate of 10
C min1, final hold for 20 min, giving a total runtime of 59 min. The mass spectrometric data were collected between 40 and 650 m/z and obtained in electron impact ionization (EI) mode (70 eV) and in chemical ionization mode (CI) using acetonitrile (Optima, Fisher) as reagent. The temperature parameters for MS were trap temperature 170
C, manifold temperature 40
C, and transferline temperature 320
C. The GCMS data were collected and processed using the Varian Workstation (ver. 6.9). Many of the SOA species generated by the ozonolysis of VOC mixtures containing a-pinene do not have authentic standards, and thus their identification was based on the interpretation of mass spectra of the derivatized compound in complimentary EI mode and CI mode. The CI mass spectrum was used for initial determination of molecular weight of the silylated derivative and number of functional groups (-COOH, -OH). BSTFA reacts with eCOOH and eOH groups to form trimethylsilyl esters and ethers, respectively. In the EI mass spectrum, the trimethylsilyl derivatives were identified using characteristic fragment ions with m/z values of 73 (Si(CH3)þ 3 ), 89 þ (OSi(CH3)þ 3 ), and 117 (C(O)OSi(CH3)3 ). Fragment ions of trimethylsilyl derivatives that are characteristic of molecular weight were also observed: Mþ (where M is the molecular weight of silylated compound), (M  15)þ, (M  73)þ, (M  89)þ, and (M  117)þ are observed. In CI mode, adduct ions with m/z values (M þ 1).þ and (M þ 73).þ were observed (Rontani and Aubert, 2004). The observed SOA products were quantified using calibration curves and surrogate standards, except pinonic acid, which has an

H.S. Amin et al. / Atmospheric Environment 67 (2013) 323e330

available standard. Surrogate standards were matched to SOA products on the basis of retention time and the number of derivatized polar hydrogens. For products with one polar hydrogen, cispinonic acid or dodecanoic acid (Aldrich) were used as the surrogate standard. For SOA products with two polar hydrogen atoms, sebacic acid or dodecanedioic acid (Aldrich) were used as the surrogate standard. The quantity of SOA product calculated was corrected for the percent of SOA recovered from the filter by the average percent recovery of d31-palmitic acid and myristic acid (methyl, d3). The percent recovery for standards was 96e125%. The fraction of individual SOA product in the PM was calculated by dividing the corrected mass of each SOA product by the mass of SOA collected on filters and expressed as a percent. The total PM identified was calculated by summing the fractions of individual SOA products and was expressed as a percent. The individual SOA yield for each product was calculated by multiplying the fraction of each individual SOA product in the PM by the overall SOA yield (which is mass-based as reported by Hatfield and Huff Hartz, 2011). 3. Results and discussion 3.1. SOA products from a-pinene As a base case, the SOA generated from the ozonolysis of a-pinene alone was characterized. Four filter samples were collected from SOA generated from 230 to 280 mg m3 a-pinene. On average, 61% of the collected PM mass was identified, and Table 1 summarizes these results. The identified products include monocarboxylic acids (pinalic-4-acid, norpinonic acid, pinonic acid, and terpenylic acid), dicarboxylic acids (pinic acid and norpinic acid), hydroxycarboxylic acids (10-OH norpinonic acid and 10-OH pinonic acid). The mass fragmentation patterns and the discussion for identification of these SOA products are given in the supporting information. The identified products are consistent with a-pinene ozonolysis products. Hoffmann et al. (1997) reported the presence of pinic acid, norpinic acid, and pinonic acid in SOA generated from apinene/ozone reaction. Other studies (Glasius et al., 2000; Yu et al., 1999; Jang and Kamens, 1999; Presto et al., 2005) also reported the identification of 10-OH pinonic acid, norpinonic acid, and pinalic-4acid. Jaoui and Kamens (2003) reported 10-OH norpinonic acid as one of the products of a-pinene ozonolysis. Terpenylic acid has been identified as SOA product generated by oxidation of a-pinene by Claeys et al. (2009) and Wells (2012). Previous studies have reported molar yields for the SOA products obtained by ozonolysis of a-pinene (Glasius et al., 2000 and Yu et al., 1999). Our work is in good agreement with the identities of the dominant products, pinic acid and pinonic acid. High molecular weight oligomers have also been reported as the products of the ozonolysis of a-pinene by Gao et al. (2004) and Hall and Johnston (2011). In this work all products have an unsilylated mass of less than 250 g mol1, thus no oligomer products were observed. Fragmentation during electron impact ionization and chemical ionization could explain the lack of observation (Nøjgaard et al., 2008). Also, the inlet temperature of the GC was 250
C, and oligomers may be thermally unstable at this temperature and cannot be observed (Hall and Johnston, 2012). 3.2. SOA products from a-pinene and reactive VOCs

b-pinene occurs in lower concentration than a-pinene in SFNO (Hatfield and Huff Hartz, 2011). In order to determine the effect of b-pinene on a-pinene SOA products, SOA was generated from a mixture containing 280 mg m3 a-pinene and 30 mg m3 b-pinene. In comparison to a-pinene alone (Table 1), a decrease in the total PM identified was observed (51.1%). This decrease is mostly due to a decrease in the individual SOA product yields for pinonic acid and

325

pinalic-4-acid. Student’s t-tests were used to identify statistically significant changes in the product yields. Probability values (p) lower than 0.05 were calculated for pinonic acid (p ¼ 0.02) and pinalic-4-acid (p ¼ 0.002), indicating that these products decreased when a-pinene SOA was generated in the presence of b-pinene. While a change in individual SOA product yields was observed upon addition of b-pinene, the addition of b-pinene to a-pinene did not change the SOA yield (Hatfield and Huff Hartz, 2011). Heaton et al. (2007) reported that SOA from b-pinene can react with a stabilized Criegee intermediate and generate dimers. If dimers from b-pinene SOA also occurred in our work, this might explain the decrease in yields for pinalic-4-acid and pinonic acid. Formation of dimers by reaction between b-pinene and Criegee intermediates for pinalic-4-acid and pinonic acid may explain the decrease in the amounts of pinalic-4-acid and pinonic acid. In this work, due to the methods used, dimers were not directly observed, but the dimers would be condensable and contribute to the overall SOA yield. The oxidation products from an a-pinene/b-pinene mixture have been reported by Jaoui and Kamens (2003). The authors reported SOA products pinic acid and pinonic acid as the major aerosol products, minor products (pinalic-4-acid, norpinonic acid, 10-OH pinonic acid, and norpinic acid, pinonaldehyde, nopinone), and products unique to a-pinene and b-pinene. Pinonaldehyde and nopinone were not observed in our work because they were masked by the solvent/BSTFA injection peak. While the remainder of these products were observed in this study as well, the characteristic products of b-pinene (for example, 3,7-dihydroxynopinone, myrtenol, 1-hydroxynopinone) were not observed. The characteristic products for b-pinene predominantly occur in the gas phase. The condensed phase products may occur in concentrations too low to detect, because the concentration of b-pinene in this mixture is small (30 mg m3). Another reactive VOC, 3-carene, was added to the VOC mixture. The condensed phase products from the ozonolysis of a mixture containing 280 mg m3 a-pinene, 30 mg m3 b-pinene, and 190 mg m3 3-carene were determined (Table 1). The products generated from this mixture were similar to the products generated from a mixture of a-pinene and b-pinene. The total PM identified is nearly the same (53.1% with 3-carene and 51.1% without 3-carene). The dominant products were pinonic acid (14.7% with 3-carene, 16.3% without 3-carene) and pinic acid (29.4% with 3-carene and 29.6% without 3-carene), and these differences are not significant. In addition to the compounds due to a-pinene, one additional compound was detected in the condensed phase of SOA. The mass spectrum (Fig. S10) for this SOA product is similar to nor-3-caralic acid, which is generated from the ozonolysis of 3-carene (Ma et al., 2009). Under these conditions, the addition of 3-carene to a mixture of a-pinene and b-pinene does not significantly change the condensed phase SOA product distribution in comparison to the a-pinene/b-pinene mixture. The SOA yield increased however, because the addition of 3-carene increased the reactive VOC concentration (Hatfield and Huff Hartz, 2011). SFNO contained a small amount of limonene, and the condensed phase products from the ozonolysis of 280 mg m3 a-pinene, 30 mg m3 b-pinene, 190 mg m3 3-carene, and 30 mg m3 limonene were determined (Table 1). Upon the addition of limonene, pinalic4-acid decreased (from 0.884% to 0.368%, p ¼ 0.02) and pinonic acid decreased (from 14.7% to 9.49%, p ¼ 0.02) while nor-3-caralic acid increased (from 3.05% to 4.43%, p ¼ 0.02), norpinic acid increased (from 0.613% to 1.10%, p ¼ 0.03), and 10-OH norpinonic acid increased (from 0.537% to 6.72%, p ¼ 0.0001). The increase in percentage of 10-OH norpinonic acid in the PM was more than tenfold. Limonene typically produces SOA with higher yields than apinene, b-pinene, or 3-carene, and Hatfield and Huff Hartz (2011)

326

Table 1 Distribution of SOA products and individual SOA product yields generated from the ozonolysis and a-pinene and a-pinene-based precursors mixtures.a Nor-pinonic acid

Nor-3-caralic acid

Camphene unknown 1

Pinonic acid

Norpinic acid

Camphene unknown 2

Terpenylic acid

10-OH norpinonic acid

Pinic acid

10-OH pinonic acid

Total PM identified

SOA Yieldb

0.521% 0.456%

n.d. n.d.

n.d. n.d.

24.5% 23.3%

0.524% 0.490%

n.d. n.d.

1.18% 1.22%

0.49% 0.65%

28.2% 27.3%

1.23% 1.96%

57.4% 56.3%

0.17 0.16

n.d. n.d. n.d. n.d.

32.0% 25.2% 26.2% 0.041

0.570% 0.780% 0.591% 0.00092

n.d. n.d. n.d. n.d.

1.22% 2.30% 1.48% 0.0020

0.41% 0.55% 0.526% 0.00083

31.6% 29.3% 29.1% 0.046

1.06% 2.24% 1.62 0.0025

68.3% 62.0% 61.0% 0.096

0.15 0.15 e 0.16

n.d. n.d.

16.3% 0.028

0.775% 0.0013

n.d. n.d.

1.52% 0.0026

0.619% 0.0011

29.6% 0.0503

1.19% 0.0020

51.1% 0.087

e 0.17

n.d. 14.3% n.d. 15.6% n.d. 14.2% n.d. 14.7% n.d. 0.036 mg m3 limonene n.d. 9.49% n.d. 0.026

0.490% 0.570% 0.780% 0.613% 0.0015

n.d. n.d. n.d. n.d. n.d.

1.22% 1.22% 2.30% 1.58% 0.0038

0.650% 0.411% 0.550% 0.537% 0.0013

27.3% 31.6% 29.3% 29.4% 0.071

1.96% 1.07% 2.24% 1.76% 0.0043

50.6% 54.7% 54.0% 53.1% 0.13

0.27 0.22 0.24 e 0.24

1.10% 0.0030

n.d. n.d.

2.33% 0.0063

6.72% 0.018

30.2% 0.082

1.48% 0.0040

57.4% 0.16

e 0.27

24.2%

0.756%

n.d.

1.93%

0.911%

25.2%

1.86%

56.3%

0.16

0.575% 0.786% n.d. n.d. 26.2% Average (n ¼ 2) 0.600% 0.838% n.d. n.d. 25.2% Yield 0.00087 0.0012 n.d. n.d. 0.036 a-pinene (310 mg m3) and 2860 mg m3 bornyl acetate 0.524% 0.346% n.d. n.d. 23.4% 1.00% 1.445% n.d. n.d. 22.5% 0.766% 0.352% n.d. n.d. 21.7% Average (n ¼ 3) 0.763% 0.714% n.d. n.d. 22.5% Yield 0.0011 0.0011 n.d. n.d. 0.033 a-pinene (350 mg m3) and 8900 mg m3 bornyl acetate 0.436% 0.165% n.d. n.d. 17.9% Yield 0.00070 0.000264 n.d. n.d. 0.029 3 3 3 a-pinene (290 mg m ) and 400 mg m bornyl acetate, 180 mg m camphene 0.510% 1.58% n.d. 1.41% 17.5% Yield 0.00097 0.0030 n.d. 0.0027 0.033 a-pinene (280 mg m3) and 400 mg m3 bornyl acetate, 177 mg m3camphene, 23 mg m3 borneol 0.291% 1.21% n.d. 1.08% 14.3% 0.378% 0.976% n.d. 0.857% 15.6% Average (n ¼ 2) 0.335% 1.09% n.d. 0.970% 14.9% Yield 0.00048 0.0021 n.d. 0.0018 0.028

0.986% 0.871% 0.0012

n.d. n.d. n.d.

2.36% 2.14% 0.0031

0.867% 0.890% 0.0013

27.3% 26.2% 0.038

2.17% 2.0% 0.0029

61.2% 58.7% 0.085

0.13 e 0.14

0.342% 0.388% 0.377% 0.369% 0.00054

n.d. n.d. n.d. n.d. n.d.

0.408% 1.36% 0.856% 0.369% 0.0013

0.411% 0.304% 0.459% 0.391% 0.00057

17.8% 12.8% 15.3% 15.3% 0.022

0.99% 1.325 1.17% 1.16% 0.0017

44.2% 41.2% 41.0% 42.1% 0.062

0.13 0.15 0.16 e 0.15

0.287% 0.00046

n.d. n.d.

0.465% 0.00074

0.336% 0.00054

11.6% 0.019

0.77% 0.0012

32.0% 0.051

e 0.16

0.829% 0.0016

2.94% 0.0056

1.62% 0.0028

1.03% 0.0020

24.5% 0.047

1.60% 0.0030

53.5% 0.10

e 0.19

0.829% 0.780% 0.805% 0.0015

3.03% 3.56% 3.30% 0.0063

1.30% 1.12% 1.21% 0.0023

1.42% 1.26% 1.34% 0.0025

27.9% 28.5% 28.2% 0.054

0.926% 0.756% 0.841% 0.0016

52.3% 53.7% 52.9% 0.10

0.19 0.11 e 0.15

Pinalic-4-acid

a-pinene (230 mg m3) a-pinene (280 mg Average (n ¼ 4) Yield a-pinene (280 mg Yield a-pinene (280 mg

Yield

0.3515 n.d. 0.884% n.d. 0.553% n.d. 0.00087 n.d. b-pinene 0.664% n.d. 0.0011 n.d. b-pinene, 190 mg m3 3-carene 0.456% 3.26% 0.351% 2.93% 0.884% 2.96% 0.564% 3.05% 0.0014 0.0075 b-pinene,150 mg m3 3-carene, 30 1.28% 4.43% 0.0035 0.012

a-pinene (280 mg m3) and 400 mg m3 bornyl acetate 0.625%

0.890%

n.d.

n.d.

a-pinene (300 mg m3) and 420 mg m3 bornyl acetate

SFNO Surrogate 1: a-pinene (290 mg m3) and 410 mg m3 bornyl acetate, 310 mg m3 camphene, 160 mg m3 3-carene, 35 mg m3 limonene, 24 mg m3 borneol, 31 mg m3b-pinene, 6.6 mg m3 b-caryophyllene, 8 mg m3 terpinolene, 6.2 mg m3a-caryophyllene, 3.7 mg m3 p-cymene 0.311% 1.23% 5.19% 0.558% 8.37% 1.43% 4.43% 1.43% 6.66% 29.5% 1.64% 60.7% 0.20 SFNO Surrogate 2: a-pinene (200 mg m3) and 450 mg m3 bornyl acetate, 290 mg m3camphene, 157 mg m3 3-carene, 70 mg m3 limonene, 27 mg m3 borneol, 28 mg m3b-pinene, 14 mg m3 b-caryophyllene, 8 mg m3 terpinolene, 7 mg m3a-caryophyllene, 4 mg m3 p-cymene, 2 mg m3 camphor, 29 mg m3a-terpinene 0.230% 0.895% 6.35% 0.782% 11.1% 1.49% 5.12% 1.34% 8.21% 33.2% 2.15% 70.8% 0.259 Average (n ¼ 2) 0.271% 1.06% 5.77% 0.607% 9.73% 1.46% 4.77% 1.38% 7.43% 31.3% 1.9% 65.8% e Yield 0.0061 0.0024 0.013 0.0016 0.023 0.0033 0.011 0.0032 0.017 0.072 0.0044 0.15 0.23

H.S. Amin et al. / Atmospheric Environment 67 (2013) 323e330

Average (n ¼ 3) Yield a-pinene (280 mg

0.810% 0.960% m ) 0.954% 0.739% 0.866% 0.0014 3 m ) and 30 mg m3 0.335% 0.00057 m3) and 30 mg m3 0.960% 0.954% 0.739% 0.884% 0.0022 m3) and 30 mg m3 0.368% 0.0010 3

a The fraction of individual SOA products in the PM was calculated by dividing the mass of SOA product determined in the extract by the amount of SOA collected on filters and as a percent. The total PM identified was calculated by summing the fractions of individual SOA products and expressed as a percent. The individual SOA yield for each product was calculated by multiplying the fraction of individual SOA product by the mass-based SOA yield. b Reported by Hatfield and Huff Hartz (2011). c The reactive VOCs are a-pinene, 3-carene, limonene, b-pinene, b-caryophyllene, terpinolene, a-caryophyllene, and a-terpinene.

e 0.245 24.5% 0.060 0.467% 0.0011 2.72% 0.0067 1.03% 0.0025

0.250% 0.00061

2.85% 0.0070

0.454% 0.0011

3.76% 0.0092

10.8% 0.026

26.9% 24.0% 25.4% 0.057 3.94% 3.06% 3.50% 0.0079

SFNO, reactive VOCsc ¼ 430 mg m3, non-reactive VOCs ¼ 774 mg m3 0.223% 0.655% 2.71% 0.124% 0.681% 2.44% Average (n ¼ 2) 0.173% 0.668% 2.57% Yield 0.00039 0.001502 0.0058 SFNO, reactive VOCsc ¼ 560 mg m3, non-reactive VOCs ¼ 1010 mg m3 0.104% 0.403% 2.51% Yield 0.00025 0.00099 0.0062

3.94% 3.35% 3.65% 0.0082

0.408% 0.365% 0.386% 0.00087

2.28% 2.26% 2.27% 0.0051

0.692% 0.747% 0.719% 0.0016

4.24% 3.11% 3.67% 0.0083

7.18% 7.47% 7.32% 0.016

0.641% 0.378% 0.509% 0.0012

0.23 0.22 e 0.22

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found that the SOA yields from this mixture increased slightly upon the addition of 30  1 mg m3 limonene. Despite this slight but measurable increase in yield, the filter analysis showed that no characteristic SOA products from limonene. Limonene was present in small concentration as compared to other reactive species like apinene (9 times the concentration of limonene) and 3-carene (5 times the concentration of limonene) which may explain the absence of any characteristic products of limonene. However, we cannot rule out that a SOA product that is characteristic of limonene, like 10-OH norlimononic acid, coeluted with 10-OH norpinonic acid and contributed toward the increase in amount of this product. 3.3. SOA products from a-pinene and non-reactive VOCs Bornyl acetate lacks a carbonecarbon double bond, does not react with ozone directly, and was a dominant VOC in SFNO. The condensed phase products from the ozonolysis of a 280e350 mg m3 a-pinene generated in the presence of 400e8900 mg m3 bornyl acetate were determined (Table 1). The types of SOA products generated from a-pinene in the presence of bornyl acetate are the same as the products generated from a-pinene alone. However, the individual SOA product yields decreased with increasing bornyl acetate. The dominant products for all levels of bornyl acetate were pinic acid and pinonic acid but the fractions of SOA products in the PM and the total amount of PM identified decreased with an increase in bornyl acetate concentration (Fig. 1). For example, when 8900 mg m3 bornyl acetate was present, the percent of pinonic acid decreased from 24% to 17.9% (p ¼ 0.02), and the percent of pinic acid decreased from 26.2% to 11.6% (p ¼ 0.0003). When the percent of individual products from the mixture containing 2860 mg m3 bornyl acetate were compared to the a-pinene alone, the decreases in percent of SOA products and individual product yields were statistically significant: for example, norpinic acid (p ¼ 0.04) and pinic acid (p ¼ 0.002). At the highest concentration of bornyl acetate (8900 mg m3), all products showed statistically significant decreases except 10-OH pinonic acid (p ¼ 0.06). The overall SOA yield did not change as the bornyl acetate concentration increased (Hatfield and Huff Hartz, 2011) because bornyl acetate does not react with ozone (Atkinson and Arey, 2003).

Fig. 1. a-Pinene SOA products distribution in the presence of bornyl acetate.

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However, the individual SOA product yields decreased as the concentration of bornyl acetate increased during SOA generation. The decrease in SOA product yields without a decrease in SOA yield would be possible if the bornyl acetate is present in concentrations surpassing its saturation concentration and the bornyl acetate condensed onto PM. However, due to the high saturation vapor pressure of bornyl acetate (17.7 Pa at 298 K, Niinemets and Reichstein, 2003) contribution of bornyl acetate in the condensed phase would be less than 0.1% at the highest concentration. A decrease of the individual SOA product yields without changes in overall SOA yield is possible if bornyl acetate adsorbed to the filter and reduced the sampling efficiency of other SOA products. Variability due to random errors introduced during SOA sampling process and extraction process can be as high as 50% (Yu et al., 1999; Subramanian et al., 2004). However, we observe a consistent decrease in SOA product yields with increase in bornyl acetate concentration is observed. Another explanation is that bornyl acetate reacts with intermediates to form adducts that are condensable, thus reducing the individual product yields without significantly changing the overall SOA yield. The kinetics for these reactions would have to be rapid relative to particle formation. In a recent study (Taatjes et al., 2012), the second-order rate constants between CH2OO Criegee biradicals with carbonyl compounds were reported: acetone (2.3  1013 cm3 molecule1 s1) and acetaldehyde (9.5  1013 cm3 molecule1 s1). If bornyl acetate reacts at similar rate, then the lifetime for a Criegee biradical in the presence of 400 mg m3 bornyl acetate could be 3.5 s. The carbonyl reacts with the Criegee biradical to form an adduct containing a trioxolane ring. If bornyl acetate, which contains a carbonyl, reacts with a-pinene Criegee intermediates, the products formed would be an adduct of bornyl acetate (Fig. 2). Oligomers of stabilized a-pinene Criegee intermediates have been observed with mass spectrometry using electrospray ionization (Heaton et al., 2009). However, no products characteristic to bornyl acetate are detected. The nominal molecular formula for a bornyl acetate/apinene Criegee adduct is C21H34O5 which may not be detectable by solvent extraction/GCMS using electron impact and chemical ionization methods. A kinetically competitive product formed from bornyl acetate and a-pinene Criegee intermediates would explain the observation that the SOA yield does not change while the fraction of a-pinene SOA products decreases. Camphene is a bicyclic monoterpene with an exo carbone carbon double. The camphene/ozone rate constant is smaller than other monoterpenes. Under our experimental conditions, 20%

camphene reacted with ozone when the filter was collected. Hatfield and Huff Hartz (2011) treated camphene as non-reactive because the presence of camphene did not affect the SOA yield. The characterization of the SOA generated from a 290 mg m3 apinene, 400 mg m3 bornyl acetate, and 180 mg m3 camphene mixture gave a product distribution that was similar to the SOA generated from 290 mg m3 a-pinene and 410 mg m3 bornyl acetate(Table 1). While the a-pinene characteristic products were the same, the presence of camphene changes the product distribution. Upon the addition of camphene, the percent of pinonic acid decreased (from 25.2% to 17.5%), the percent of norpinonic acid increased (from 0.838% to 1.58%), and the remaining a-pinene SOA products only showed insignificant changes. In addition to the apinene oxidation products, two other products were observed, and the molecular formulas have been tentatively identified as C9H16O2 and C8H12O4. (See Figs. S11 and S12 for mass spectra.) The percent SOA product were estimated to be low: C9H16O2 was 1.4% and for C8H12O4 was 2.9%. Despite the low reaction rate constant of camphene, peaks with similar retention times and mass spectra were observed only in SOA that was generated from VOC mixtures that contained camphene. These oxidation products were termed camphene unknown 1 and 2. Borneol is a bicyclic hydroxymonoterpene that doesn’t react with ozone within the timescale of these experiments. The effect on the SOA distribution of adding 23 mg m3 borneol to a mixture containing 280 mg m3 a-pinene, 400 mg m3 bornyl acetate, and 177 mg m3 camphene is small. Small decreases in the fraction of norpinic acid, the camphene unknown 1, pinonic acid, and 10-OH pinonic acid were observed, while small increases in the fraction of pinic acid and 10-OH norpinonic acid were observed, but these changes were not as significant as the changes observed upon addition of bornyl acetate. The effect on the a-pinene SOA product distribution of the addition of borneol, camphene, and bornyl acetate (total non-reactive VOCs ¼ 600 mg m3) was larger. For example, the pinonic acid (14.9%) from the mixture was lower than the pinonic acid obtained for a-pinene only experiments (26.2%). Given that adding borneol and camphene changed the SOA product distribution slightly, bornyl acetate was mostly responsible for the observed differences. 3.4. SOA products from SFNO surrogates and SFNO To determine the SOA product distribution from a complex VOC mixture, PM generated from SFNO and two SFNO surrogates was

Fig. 2. Proposed competition for a-pinene Criegee intermediate by bornyl acetate reaction.

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characterized (Table 1). SFNO surrogates were mixtures of VOCs created from standards, and they approached the composition of SFNO. The purpose of the surrogate mixtures was to determine if the SOA products from a complex mixture could be replicated if the mixture was made from the individual constituents of the mixture. Surrogate 1 contained eleven VOCs: a-pinene, bornyl acetate, camphene, 3-carene, limonene, borneol, b-pinene, b-caryophyllene, terpinolene, a-caryophyllene, and p-cymene. Surrogate 2 contained the VOCs in Surrogate 1 plus camphor and a-terpinene. In these surrogates, the following VOCs did not react completely under these experimental conditions: bornyl acetate, camphene, borneol, camphor, and p-cymene (in total, 745 mg m3 in Surrogate 1 and 775 mg m3 in Surrogate 2). The SOA product distribution for the SFNO surrogates was most similar to the SOA product distribution for a VOC mixture containing a-pinene, b-pinene, 3-carene, and limonene. The presence of nonreactive VOCs had little effect on the SOA products. However, when the SOA product distributions for the SFNO surrogates were compared to the SOA product distribution from a VOC mixture containing a-pinene, bornyl acetate, borneol, and camphene, a higher percent SOA products was observed for norpinic acid, camphene unknown 2, 10-OH norpinonic acid, and 10-OH pinonic acid. Decreases in percent pinonic acid and terpenylic acid were observed for the SFNO surrogates in comparison to the a-pinene, bornyl acetate, borneol, and camphene mixture, but the percent pinic acid was similar for both VOC mixtures. Although the SOA products from surrogates were similar to products generated from a-pinene alone, both reactive and non-reactive VOCs present in the mixture affect the quantitative product distribution. The SOA product distribution was determined at two SFNO concentrations, and the total reactive VOC concentrations (430 mg m3 and 560 mg m3, Table 1) are similar to the total reactive VOC concentrations in the surrogate mixtures. The identities of SOA products from SFNO were similar to surrogate mixtures. However, the total percent of PM identified decreased from 60 to 70% in the SFNO surrogates to 25% in SFNO. Although the SOA yield did not change (Hatfield and Huff Hartz, 2011), a significant decrease in the percent SOA products and individual product yields was observed. This occurred despite similar reactive VOC concentrations in the SFNO and SFNO surrogate experiments. The concentration of non-reactive VOCs in the SFNO experiments were 774 mg m3 and 1010 mg m3. The total of the nonreactive VOCs was similar or higher in SFNO in comparison to the surrogates and not different enough to explain the large decrease in the individual products. Another contributing factor that we cannot rule out is the presence of an unknown VOC in the SFNO, which reacted with an intermediate and formed a condensable product and caused a decrease in the individual SOA product yields. All but 5% of the VOCs were determined in SFNO (Hatfield and Huff Hartz, 2011). Furthermore, 5% of the ozone reactive VOCs and 3% of the nonreactive VOCs that were found in SFNO were not present in the SFNO surrogates. If these compounds form adducts, the SOA yields could remain constant for SFNO and a-pinene experiments in spite of a decrease in percent individual SOA product yields of individual products. Another factor that we cannot rule out is the collection of VOCs on the filters (Kirchstetter et al., 2001), if the VOCs in SFNO impact collection efficiency more than the VOCs in the surrogates. 4. Conclusions The composition of the fraction of SOA that can be sampled by filters, extracted by solvents, and analyzed by GCMS was determined. Eleven SOA products were found, and eight were attributed to a-pinene, which was the dominant ozone reactive monoterpene in the VOC mixtures. Two SOA products were attributed to

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camphene, and one was attributed to 3-carene. The fraction of identified SOA, the types of products, and the individual SOA product yields from a-pinene ozonolysis were similar to prior work (Yu et al., 1999; Glasius et al., 2000). A decrease in the individual SOA product yields correlated with an increase in bornyl acetate concentration, although the SOA yield did not change with bornyl acetate concentration. Adduct formation may explain this observation, and this suggests that this ozone nonreactive VOC may alter the SOA product distribution and enhance the formation of high molecular weight compounds. SOA precursor mixtures containing camphene generated two products that were unique to camphene-containing mixtures. These products have not been previously reported for ozonolysis of camphene, but it is not known if the presence of a precursor mixture is required to generate these products. When SOA was generated from SFNO and SFNO surrogates, the SOA product distributions were similar. SOA from SFNO surrogates yielded products which were similar to the products generated by ozonolysis VOC mixture containing a-pinene, b-pinene, 3-carene, and limonene, and no new products were observed. The SOA products from surrogates were characteristic of the dominant reactive species present in them (a-pinene and 3-carene). However, a comparison of two surrogates with the SFNO trial 2 experiment (where all three experiments used similar amounts of reactive VOCs) shows that a decrease in the individual product yields is observed when SFNO is the precursor mixture. This reduction in the amount of SOA products of SFNO may be contributed to alteration of reaction pathways to form higher molecular weight dimers, which explains the constant SOA yield and no detection of new products. This work suggests, via indirect evidence, that an alteration in reaction pathways or formation of new products occurs in presence of both reactive and non-reactive VOCs. While SOA generated from precursor mixtures takes on the characteristics of SOA generated from single precursors in terms of types of products and yield, the role of non-reactive VOCs and the composition of the reactive mixture may impact the concentrations of individual products. If the concentrations of the individual products impact the toxicity and/or the cloud properties of PM consisting of biogenic SOA, then SOA generated from precursor mixtures might be a way to distinguish differences between single-precursor SOA and SOA that is generated from a complex mixture of VOCs and more likely to occur in the atmosphere. Furthermore, this work suggests that under conditions where ozone is the dominant atmospheric oxidant, nonozone reactive VOCs may be incorporated into SOA via adduct formation. Although oxygenated monoterpenes like bornyl acetate are typically found at lower concentrations in emissions than monoterpenes (Geron et al., 2000), trees exposed to ozone can show increases in oxygenated VOC emissions (Pellegrini et al., 2012). The presence of seed aerosol and the seed aerosol acidity may further influence the product distribution, and the study of the effect of non-reactive VOCs on SOA product distributions under atmospherically relevant conditions is needed.

Acknowledgments This research was supported by SIUC Office of Sponsored Projects Administration Faculty Seed Grant.

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2012.10.063.

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