Photooxidation of α-pinene at high relative humidity in the presence of increasing concentrations of NOx

ARTICLE IN PRESS

Atmospheric Environment 42 (2008) 5044–5060 www.elsevier.com/locate/atmosenv

Photooxidation of a-pinene at high relative humidity in the presence of increasing concentrations of NOx Yong Yua, Michael J. Ezella, Alla Zelenyukb, Dan Imrec, Liz Alexanderb, John Ortegab, Barbara D’Annad, Chris W. Harmona, Stanley N. Johnsona, Barbara J. Finlayson-Pittsa, b

a Department of Chemistry, University of California Irvine, Irvine, CA 92697-2025, USA Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, USA c Imre Consulting, Richland, WA 99352, USA d IRCELYON CNRS, Universite de Lyon, F-69629, Villevrbanne, France

Received 21 November 2007; received in revised form 11 January 2008; accepted 9 February 2008

Abstract The photooxidation of 1 ppm a-pinene in the presence of increasing concentrations of NO2 was studied in a Teflons chamber at 72–88% relative humidity and 296–304 K. The loss of a-pinene and formation of gas-phase products were followed using proton-transfer reaction mass spectrometry (PTR-MS). Gas-phase reaction products (and their yields) include formaldehyde (571%), formic acid (2.571.4%), methanol (0.670.3%), acetaldehyde (3.971.7%), acetic acid (8.671.9%), acetone (1273%), pinonaldehyde (2276%), and pinene oxide (0.970.1%). There was evidence of organic nitrates; and small peaks were tentatively assigned to norpinonaldehyde, 4-oxopinonaldehyde, propanedial, 2,3dioxobutanal and 3,5,6-trioxoheptanal or 3-hydroxymethyl-2,2-dimethylcyclobutylethanone. The formation and growth of new particles were followed using a scanning mobility particle sizer (SMPS), and their chemical composition and density probed using single particle mass spectrometry (SPLAT II). SPLAT II showed that the suspended SOA consisted of a complex mixture of organic nitrates and oxygenates having a density of 1.2170.02 g cm3, 20% larger than often assumed in calculating SOA yields. Three-wavelength light scattering measurements were consistent with particles having a refractive index characteristic of organic compounds, but the data could not be well matched at all three wavelengths with a single refractive index. The effect of addition of cyclohexane or NO on particle formation showed that ozonolysis was the major mechanism of SOA formation in this system. However, unlike simple ozonolysis, organic nitrates are formed in both the gas and particle phases. Identifying and measuring specific organic nitrates in both the gas and particle phases in air may help to elucidate why SOA formation has been reported in field studies to be associated with polluted urban areas, yet the carbon in these particles is largely contemporary, i.e., non-fossil fuel carbon. r 2008 Elsevier Ltd. All rights reserved. Keywords: a-Pinene; NOx; Photooxidation; Nitrates; SOA

1. Introduction Corresponding author. Tel.: +1 949 824 7670;

fax: +1 949 824 2420. E-mail address: bjfi[email protected] (B.J. Finlayson-Pitts). 1352-2310/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.02.026

Airborne particles are important for visibility, health effects, heterogeneous chemistry and impacts

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on radiation from local to global scales (FinlaysonPitts and Pitts, 2000; NARSTO, 2003; Po¨schl, 2005; Ghan and Schwartz, 2007). The effects depend on the size and chemical composition of the particles, which in turn are determined by their sources and transformations in the atmosphere. An area of major uncertainty involves the origin of secondary organic aerosols (SOA) from both anthropogenic and biogenic emissions (Po¨schl, 2005; Ghan and Schwartz, 2007; Rudich et al., 2007). For example, predicted SOA is typically about an order of magnitude smaller than actually measured (Volkamer et al., 2006). While recent studies (Robinson et al., 2007) suggest that oxidation of semi-volatile emissions from diesel exhaust may contribute significantly to resolving such discrepancies, this would not explain the large discrepancies between free tropospheric measurements and models of SOA in the ACE-Asia studies (Heald et al., 2005), nor would it explain increased organics in particles above clouds measured during the MASE campaign off the coast of northern California (Ghan and Schwartz, 2007; Velasco et al., 2007). In addition, this hypothesis is not consistent with the large fraction of contemporary carbon typically found in carbonaceous aerosol particles (Hildemann et al., 1994; Weber et al., 2007). Clearly, there are a number of other processes that are also contributing to formation and growth of SOA that are not properly represented in current models. a-Pinene is a significant biogenic hydrocarbon constituent in many regions (Atkinson and Arey, 1998, 2003; Calogirou et al., 1999; Fuentes et al., 2000) and a well-known SOA precursor through its reaction with O3, and to a lesser extent, with OH and NO3 (Yokouchi and Ambe, 1985; Hatakeyama et al., 1989; Zhang et al., 1992; Hoffmann et al., 1997; Christofferson et al., 1998; Griffin et al., 1999; Jang and Kamens, 1999; Yu et al., 1999b; Andersson-Sko¨ld and Simpson, 2001; Bonn and Moortgat, 2002; Bonn et al., 2002, 2007; Berndt et al., 2003; Winterhalter et al., 2003; Gao et al., 2004a; Iinuma et al., 2004; Jenkin, 2004; Tolocka et al., 2004; Bahreini et al., 2005; Docherty et al., 2005; Alfarra et al., 2006;Czoschke and Jang, 2006; Jonsson et al., 2006; Kleindienst et al., 2006; Lee et al., 2006; Presto and Donahue, 2006; Northcross and Jang, 2007; Stanier et al., 2007). Photooxidation of a-pinene by NOx also generates SOA, which is not surprising given that O3, OH, and NO3 are formed. While there have been laboratory studies of NOx photooxidation of a-pinene, they have generally been carried out with an initial concentration of

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NO2 that decreases during the experiment, and at low relative humidities. We report here studies of the photooxidation of a-pinene in the presence of increasing NO2 concentrations and at high relative humidities, which are more typical of morning conditions in many areas. The loss of a-pinene and the formation of gas-phase products as well as the formation, composition, and density of particles were followed in real time using proton-transfer reaction mass spectrometry (PTR-MS) and single particle laser mass spectrometry (SPLAT II). The size distribution of particle formation and growth was followed using an SMPS consisting of a differential mobility analyzer (DMA) with a condensation particle counter (CPC). While particle formation is shown to be due almost exclusively to reaction of the a-pinene with O3, there are important differences compared to results from the simple ozonolysis of a-pinene that must be taken into account in extrapolating laboratory studies to urban atmospheres. 2. Experimental Fig. 1 is a schematic diagram of the reaction system, the central part of which is a collapsible Teflon reaction chamber of maximum total volume 100 L. Irradiation of the chamber was carried out using two banks of blacklamps (Sylvania 350, 20 W, F20T12/350 BL, 300–400 nm) located on opposite sides of the chambers. Two different sets of lights were used during the experiments, and they had different intensities. The light intensity was measured by following the loss of NO2 in nitrogen during photolysis (Holmes et al., 1973), giving J-values of (1.870.2)  103 s1 and (1.170.1)  103 s1, respectively. The size distributions and number concentrations of the particles formed by the photochemistry were measured using a SMPS consisting of either a nano or long DMA (TSI Model 3080 or 3085) and a CPC (TSI Model 3022A and 3786). The increases in NO2 and NO were measured using a Thermo 42C chemiluminescence Trace Gas NOx analyzer. At shorter reaction times, the concentrations of gases other than NO and NO2 are expected to be sufficiently small that the NO2 instrument reading can be taken as the NO2 concentration. At longer reaction times, there may be some contribution from organic nitrates. Total light scattering (7–1701) was measured using an integrating nephelometer (TSI Model 3563) at three different wavelengths: 450, 550, and 700 nm. During the measurements, the

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Particle velocity measurement

MS

T PM

PM

T

analyzer

Nebulizer

SPLAT-MS UVIRionization laser ablation

Teflon Chamber

Particle lens

Air

APS or

α-Pinene

α -pinene

Syringe

Black Lamps

SMPS aerosol size & number

RH / Temp. Probe

Nephelometer (light scattering)

NOx box Quadrupole

H2O vapor

Drift Tube Ion source

Detector pump

Fig. 1. Schematic diagram of the experimental apparatus.

maximum temperature rise in the nephelometer was 3 1C. a-Pinene and its gaseous products were measured using PTR-MS (Ionicon Analytik) (Lindinger et al., 1998; Wisthaler et al., 2001; Hayward et al., 2002; Prazeller et al., 2003; Tani et al., 2004; Jobson et al., 2005; Lee et al., 2006; DeGouw and Warneke, 2007). This method is based on proton transfer from H3O+ to compounds with larger proton affinities than the ionizing agent, giving an [M+1] peak. While fragmentation of the parent molecule occurs, it is less severe than for electron impact ionization. Low molecular weight compounds show less fragmentation, and have the largest ion peak at [M+1], while larger species have slightly more fragmentation and may have a fragment ion as the largest peak. One common mechanism for fragmentation of oxygenated species is the loss of a neutral water molecule following protonation. The largest peak from pinonaldehyde (MW ¼ 168) which is found at m/z ¼ 151 results from this mechanism. The lack of extensive fragmentation also contributes to the sensitivity of the PTR-MS, providing highly sensitive detection (100 ppt) of the parent molecules.

Daily calibrations for a-pinene, methanol, acetaldehyde, and acetone were performed using a certified multi-component gas-phase standard containing 179 ppb a-pinene, 641 ppb CH3OH, 437 ppb CH3CHO, and 390 ppb CH3COCH3. For products for which standards were not available, the concentration was determined based on the following relationship (Lindinger et al., 1998): ½X ¼

I XHþ , ðI H3 Oþ Þkt

(I)

where [X] is the concentration of the compound of interest, I XHþ is the intensity of the M+1 peak, I H3 Oþ is the intensity of the protonated water cluster, k is the rate constant for proton transfer between H3O+, and t is the time in the drift tube. The ratio of concentrations of two compounds X1 and X2 is then given by ½X2  I X2 Hþ k1 ¼  . ½X1  I X1 Hþ k2

(II)

The proton-transfer rate constants for acetaldehyde, acetone, formaldehyde, formic acid, acetic acid,

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pinonaldehyde, pinene oxide, and a-pinene were 3.55, 3.8, 3.0, 2.5, 3.0, 3.8, 3.4, and 2.4  1010 cm1 molecule1 s1 (Anicich, 1993; Schoon et al., 2004), respectively. For HCHO and HCOOH, the intensities of the M+1 peaks at m/z ¼ 31 and 47, respectively, were used in this calculation and X1 was methanol for the HCHO determination and X1 was acetaldehyde for calculating the HCOOH concentrations. However, the protonated form of HCHO undergoes some back reaction, which leads to an underestimate of its concentrations. In a separate intercomparison field study in which HCHO was measured simultaneously by an absolute method, PTR-MS was found to underestimate HCHO by a factor of 4.5. As a result, the values reported here have been multiplied by 4.5. A further uncertainty in the HCHO measurements is the dependence on relative humidity for which corrections were not made. For CH3COOH (X1 ¼ acetone), to take into account that there is some fragmentation to m/z ¼ 43, the signal at m/z ¼ 61 was increased by a factor of 2.4 using the relative peak intensities at 43 and 61 measured from an authentic sample. This procedure avoids including contributions from other compounds to m/z ¼ 43, since it is a common fragment of many organic compounds. For pinene oxide, the major peak at 135 was used and corrected for the parent at m/z ¼ 153 and the fragment at m/z ¼ 109 using the fragmentation pattern measured for an authentic sample. For pinonaldehyde, peaks at m/z ¼ 107, 123, and 169 were in the same ratio to that at 151 as reported by Wisthaler et al. (2001), but those at m/z ¼ 71, 99, and 109 were larger, suggesting contributions from other products. The mass dependence for the transmission efficiency of the quadrupole was measured by adding individual compounds which do not fragment in sufficient quantity to completely react the H3O+. The predominant [M+1] signals were measured for each compound, covering the range from m/z ¼ 33 (methanol) to 177 (bromofluorobenzene). The sum of (107+123+ 151+169), corrected for the mass dependence of the quadrupole transmission, was multiplied by a factor of 1.47 to obtain the total ion intensity needed to calculate the pinonaldehyde concentrations. Particle composition and density were measured using SPLAT II, a second generation of single particle mass spectrometer, which is described in detail elsewhere (Zelenyuk et al., 2005, 2006;

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Zelenyuk and Imre, 2005). Briefly, density is determined by selecting particles with a specific electrical mobility diameter using the DMA and then sending them through an aerodynamic lens system into the SPLAT II, where their vacuum aerodynamic diameter is measured. The latter is accomplished using the particle time-of-flight between two frequency doubled Nd:YAG (532 nm) lasers whose light beams are scattered and detected as the particles pass through the laser beams. The ratio of the vacuum aerodynamic and the mobility diameters yields the particle effective density which is a function of the particle shape and density. For spherical particles, this ratio is equal to the true particle density. The high resolution offered by SPLAT II can be used to distinguish between spherical and aspherical particles on the basis of the width of the vacuum aerodynamic size distribution of the mobility classified particles (Zelenyuk et al., 2005), with spherical particles exhibiting narrow line widths. In the case of the newly formed SOA particles in this set of the experiments, we found particles to be spherical. The particles then travel to the evaporation/ionization region of the mass spectrometer, where they are evaporated using a pulsed CO2 laser (80 mJ pulse1, 50 ns pulse width) and after a delay of 3 ms, the expanding plume of gases from the original particle is photoionized using 193 nm light from a pulsed excimer laser (1.0 mJ pulse1, 15 ns pulse width). Reference SPLAT II mass spectra of a-pinene oxide (SigmaAldrich, 97%), trans-sobrerol (trans-p-menth-6-ene2,8-diol, Sigma-Aldrich, 99%), cis-pinonic acid (Sigma-Aldrich, 98%), and pinic acid (SigmaAldrich, no specified purity) were generated by atomizing aqueous solutions of each of the individual compounds. The a-pinene oxide spectrum is very similar to that of trans-sobrerol, which may reflect some hydrolysis of the oxide to the alcohol in the solution. Increasing concentrations of NO2 were provided during an experiment by the continuous addition of NO2 to the chamber. The chamber was initially filled with 80 L of Ultra Zero grade air (Oxygen Services Company, synthetic blend of O2 and N2, THCo0.01 ppm, H2Oo2.0 ppm, COo0.5 ppm, CO2o0.5 ppm) which had been humidified by passing through a bubbler containing nanopure water (Barnstead 18 MO cm). The chamber was not filled to capacity to allow for the subsequent addition of NO2. The relative humidity and temperature were measured using a Vaisala probe

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6000 59

Signal (cps)

5000 4000 3000

151

71

43

107

2000 1000

31

61 45 46 73 83 47

99

109

101

125 157

0 20

40

60

80

169

183

100 120 140 160 180 200 m/z

α-pinene

1000

Acetaldehyde (ppbv)

α-pinene, NO2 (ppbv)

Fig. 2. PTR-MS spectra of gas-phase products.

3. Results and discussion 3.1. Gas-phase products Fig. 2 shows a typical PTR-MS of the products during a run in which the gas phase was continuously monitored by PTR-MS and the particles followed using a SMPS for size distributions and SPLAT II for their composition. Fig. 3 shows the concentration-time profiles for a-pinene, NO2 and the identified gaseous products. After a short delay, a-pinene decreases while NO2 (which is being added continuously) and gaseous products measured by PTR-MS increase. The concentration of NO remains at 2–3 ppb throughout the experiments. Table 1 summarizes the assignment of the major peaks in the spectrum and the product yields. In addition to these products, there is a peak at m/z ¼ 46 characteristic of organic nitrates (DeGouw et al., 2003). Nozie`re et al. (1999) reported the formation of organic nitrates in the gas phase in the OH oxidation of a-pinene in the presence of NOx. As discussed below, there is also evidence for organic nitrates in the particles from the SPLAT II-MS measurements. The major products are clearly pinonaldehyde, acetone, and acetic acid, with smaller yields of

Pinonaldehyde

200

NO2

100

500

0 40

Acetaldehyde

Acetic acid

0 100

Acetone

50

20

Formaldehyde Formic acid (ppbv)

0

0 Formaldehyde

40

Pinonaldehyde (ppbv)

(Model HMP-238). a-Pinene was added to the chamber by injecting the appropriate volume of liquid [(1R)(+)a-pinene] (Aldrich 99+%), which vaporized and mixed within a few minutes. The blacklamps were then turned on and NO2 was added continuously to the chamber by metering in a small flow (o0.1 L min1) of a 4.64 ppm NO2/N2 mixture (Scott–Marrin). Experiments were carried out at 30074 K and at atmospheric pressure. The relative humidity was in the range from 72% to 88%.

Acetic acid, Acetone (ppbv)

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Formic acid

20

0 0

1000

2000 Time (s)

3000

4000

Fig. 3. NO2 and a-pinene concentrations, and identified gas-phase products measured by PTR-MS for a typical photolysis of an NO2–apinene (963 ppb) mixture in air at 303 K, RH varied from 86% to 72%.

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Table 1 Tentative peak assignments and percent molar yields of gas phase products observed by PTR-MS m/z

Peak assignment

31

HCHO

33 45 47

CH3OH CH3CHO HCOOH

0.670.3 3.971.7 2.571.4

59a

CH3COCH3

1273

61, 43 CH3COOH 169, 151, 123, Pinonaldehyde, C10H16O2 107

73 101 153,135, 109

Propanedial, OCHCH2CHO 2,3-Dioxobutanal, C4H4O3 Pinene oxide, C10H16O

155,137 157

Norpinaldehyde 3,5,6-Trioxoheptanal, C7H8O4 3-Hydroxymethyl-2,2dimethylcyclobutylethanone, C9H16O2 4-Oxopinonaldehyde, C10H14O3

183,165

Yield (%), % Molar yield in O3 reaction this work 571

8.671.9 2276

0.970.1

2873 (Lee et al., 2006) 2271 (Hatakeyama et al., 1989)

% Molar yield in OH reaction

9–148 (Librando and Tringali, 2005) 871 (Larsen et al., 2001) 1975 (Orlando et al., 2000) 2379 with NO (Nozie`re et al., 1999) 871 without NO (Nozie`re et al., 1999)

2.970. 3 (Lee et al., 2006) 7.570.7 (Lee et al., 2006) 2–10 (Hatakeyama et al., 1989)

5–18 (Librando and Tringali, 2005) 2873 (Larsen et al., 2001) 772 (Orlando et al., 2000) 5.970.5 (Lee et al., 2006) 12 (Librando and Tringali, 2005) 371 (Orlando et al., 2000) 1173 (Larsen et al., 2001) 872 (Reissell et al., 1999) 1172 (Wisthaler et al., 2001) 572 (Orlando et al., 2000) 976 with NO (Nozie`re et al., 1999) 772 without NO (Nozie`re et al., 1999) 1173 (Aschmann et al., 1998) 15710 (Gu et al., 1984) 870.9 (Lee et al., 2006) 0 (Librando and Tringali, 2005) 19–34 (Lee et al., 2006) 2875 (Aschmann et al., 2002) 10 (Ng et al., 2006) 672 (Larsen et al., 2001) 32–42 (Berndt et al., 2003) 3479 (Wisthaler et al., 2001) 6–18 (Yu et al., 1999a) 87720 with NO (Nozie`re et al., 1999) 6–8 (Reissell et al., 1999) 3777 without NO (Nozie`re et al., 1999) 5375b (Warscheid and Hoffmann, 2875 (Hakola et al., 1994) 2001) 14.372.4 (Alvarado et al., 1998a) 5674 (Hatakeyama et al., 1991) 16.472.9 (Baker et al., 2002) 29 (Arey et al., 1990) 1974 (Hakola et al., 1994) 5176c (Hatakeyama et al., 1989) 0.3 (Fick et al., 2003)

3.071.5 (Berndt et al., 2003) 2.170.7 (Alvarado et al., 1998b)

a

Propanal and glyoxal (CHO)2, which has been reported as a product of the OH oxidation (Grosjean et al., 1992; Fick et al., 2003), potentially also contribute to the peak at m/z ¼ 59. However, there are apparently no prior reports of propanal generated in a-pinene oxidation, and acetone is a product of both the O3 and OH reactions. Since glyoxal is not a dominant product, we assign the m/z ¼ 59 peak to acetone. b At 60% RH; yield decreased with decreasing RH. c Assuming all formyl groups measured by FTIR are pinonaldehyde.

HCHO, HCOOH, CH3CHO, pinene oxide, and higher molecular weight species. In NOx photooxidation experiments of a-pinene, Jaoui and Kamens

(2001) reported a molar yield of pinonaldehyde in the gas phase of 22–24% for experiments carried out over RH of 18–40%, very similar to

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our experiments. Ng et al. (2006) reported a much larger pinonaldehyde yield of 60% for experiments using ammonium sulfate seed particles at 40–50% RH. Table 1 also summarizes measurements of gasphase products from ozonolysis and OH reactions which are relevant because both O3 and OH are formed by NO2 photochemistry (Finlayson-Pitts and Pitts, 2000). Pinonaldehyde is a major product in most cases, with reported yields from 0.3% to 87%. Acetone yields in the range of 3–8% have been reported from the ozonolysis and from 5% to 15% in the OH oxidation, similar to that measured in the present studies and there are reasonable mechanisms for its production in both systems (Jaoui and Kamens, 2001; Atkinson and Arey, 2003; Capouet et al., 2004; Jenkin, 2004). Acetic acid was reported as a product of the ozonolysis of a-pinene by Lee et al. (2006) but no mechanism for its

formation was proposed. Scheme 1 proposes one possible route to its formation via alkoxy radical decomposition and reaction of an RO2 fragment with HO2 (Sander et al., 2006). Capouet et al. (2004) predict the formation of CH3COOH from the addition of OH to the double bond, followed by conversion to an alkoxy radical and its decomposition as shown in Scheme 1. However, radical A in Scheme 1 is an a-hydroxy radical which would be expected to react with O2 to form the ketone and HO2, rather than forming RO2 and then RO (Finlayson-Pitts and Pitts, 2000). In addition, Librando and Tringali (2005) did not observe CH3COOH in the OH-oxidation of a-pinene. In short, the mechanism of formation of CH3COOH in this system remains unclear. Photolysis of NO2 generates O(3P) and then O3: NO2 þ hn ! Oð3 PÞ þ NO

Scheme 1. Partial reaction scheme for formation of products from reaction of a-pinene with O3 or OH.

(1)

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3.2. New particle formation and growth Fig. 4 shows that the changes in the gas phase are accompanied by the formation of new particles. New particle formation ceases after approximately l000 s, and is followed by growth of the existing particles. The O3 reaction with a-pinene not only generates SOA but also OH radicals (Atkinson et al., 1992; Paulson and Seinfeld, 1992; Chew and Atkinson,

2

N

4x104 N (# cm-3)

Pinene oxide is formed in high yield (77%) from the direct reaction of O(3P) with a-pinene (Alvarado et al., 1998b). It is also generated in small yields (2–3%) in the ozone oxidation (Alvarado et al., 1998b; Berndt et al., 2003; Czoschke and Jang, 2006). Given effective bimolecular room temperature rate constants for the O(3P)+a-pinene reaction (Paulson et al., 1995; Atkinson et al., 2007) of (3.770.6)  1011 cm3 molecule1 s1 and 1.5  1014 cm3 molecule1 s1 for O(3P)+O2 at 1 atm in air, about 1% of the O(3P) will be trapped initially by 1 ppm a-pinene. However, modeling of this system assuming a 3% yield of pinene oxide from the ozone reaction shows that about 70% of the small amount of pinene oxide formed in this system is from the O3 reaction. Scheme 1 also summarizes one typical potential route of formation of pinonaldehyde, acetone, and organic nitrates as well as some of the minor multifunctional products tentatively identified in these studies. An 86-step simplified model was developed for this system. In addition, the model of Andersson-Sko¨ld and Simpson (2001) modified by the work of Kamens and Jaoui (2001) was also used. The models were consistent in showing that the O3 reaction accounts for slightly less than half the pinene loss, OH for 50% early in the reaction dropping to 35% at later times, and NO3 increasing from a few percent to 15% as the OH contribution dropped. The models also showed that the direct NO3-pinene reaction plays only a minor (p10%) role in the formation of organic nitrates, with RO2+NO being the dominant source. Detailed reaction mechanisms for the ozonolysis of a-pinene in the absence of NOx and the formation of some of the products measured here are discussed elsewhere (e.g., see Jaoui and Kamens, 2001; Baker et al., 2002; Atkinson and Arey, 2003; Capouet et al., 2004; Jenkin, 2004; Docherty et al., 2005).

5x104

3x104 1

2x104

V

1x104 0 0

1000

2000 3000 Time (s)

4000

0 5000

Volume per particle (107 nm3)

(2)

Fig. 4. New particle formation and growth for typical NO2–apinene photoxidation. Experiment same as that in Fig. 3.

1.2x1012 Aerosol volume (nm3 cm-3)

Oð3 PÞ þ O2 þ M ! O3 þ M

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Without scavenger added C6H12 added NO

8.0x1011

4.0x1011

0.0 0

2000

4000 Time (s)

6000

8000

Fig. 5. Comparison of increase in particle volume in a typical experiment without and then with 400 ppm C6H12 or 1 ppm NO added.

1996; Gutbrod et al., 1996; Paulson et al., 1998, 1999; Rickard et al., 1999; Seise et al., 2001; Aschmann et al., 2002; Berndt et al., 2003), and OH can also react with a-pinene to generate SOA, although the aerosol yields are significantly smaller (Griffin et al., 1999; Bonn and Moortgat, 2002; Bonn et al., 2002). Thus, a series of experiments was carried out to test for the contribution of both gas phase O3 and OH oxidation to the formation of new particles suspended in the chamber by adding traps for OH or O3. The OH free radical was trapped using cyclohexane (Docherty and Ziemann, 2003; Keywood et al., 2004), and in separate experiments, concentrations of O3 were kept small by adding NO: NO þ O3 ! NO2 þ O2

(3)

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Fig. 5 shows the formation and growth of new particles in typical experiments to which either 400 ppm cyclohexane or 1 ppm of NO had been added. The rate constants for the gas-phase reactions of OH free radical with cyclohexane and a-pinene are 7.2  1012 cm3 molecule1 s1 and 5.3  1011 cm3 molecule1 s1, respectively (Aschmann and Atkinson, 1997; Atkinson et al., 2007). Thus, at 400 ppm cyclohexane and 1 ppm a-pinene, 98% of the OH is scavenged by the cyclohexane. The addition of cyclohexane decreases the aerosol volume to some extent. However, the addition of 1 ppm NO completely suppresses new particle formation, at least up to 2 h reaction time (Fig. 5). The loss of pinene is also suppressed (o10% loss) by the addition of NO since the O3 concentrations remain too small to remove significant amounts of the pinene. Because the O3-pinene reaction is also the major source of OH in this system, OH formation and its contribution to pinene loss is also minimized. In the case of the cyclohexane addition, OH concentrations are suppressed but the precursor O3 is not. As a result, loss of pinene does occur but is about half of that in the absence of cyclohexane, in agreement with the model predictions discussed above. In short, these experiments confirm that the O3 reaction with a-pinene is the major source of the suspended particles in this system. The results are consistent with the studies of Griffin et al. (1999) and Bonn and co-workers (Bonn and Moortgat, 2002; Bonn et al., 2002) who showed that oxidation by OH and NO3 radicals is much less effective in forming particles than the ozone reaction. In our experiments, as NO2 increases in the chamber, it is photolyzed to generate O3 and NO. O3 reacts with the a-pinene to form gaseous products, some of which are of sufficiently low volatility to form new particles. Fig. 6 shows five averaged individual particle mass spectra obtained by SPLAT II. The top panel (Fig. 6a) shows the mass spectrum that was obtained when a-pinene was oxidized in the presence of NOx. Fig. 6b–e shows the average mass spectra of individual particles composed of pinene oxide, its hydrolysis product trans-sobrerol, pinic acid and pinonic acids. Pinic and pinonic acids are common components of particulate matter from a-pinene oxidation (Hatakeyama et al., 1989; Hoffmann et al., 1997; Christofferson et al., 1998; Yu et al., 1998, 1999a; Jang and Kamens, 1999; Glasius et al., 2000; Koch et al., 2000; Ku¨ckelmann et al., 2000; Jaoui and Kamens, 2001; Fick et al.,

2003; Winterhalter et al., 2003; Capouet et al., 2004; Iinuma et al., 2004; Lee et al., 2004; Jaoui et al., 2005; Lee and Kamens, 2005; Librando and Tringali, 2005; Presto et al., 2005; Czoschke and Jang, 2006; Kleindienst et al., 2006; Claeys et al., 2007; Ma et al., 2007). A comparison between these mass spectra and the mass spectrum in the top panel shows that some of the fragments are common, but that the overall pattern indicates that they do not account for the majority of the particle composition. Comparison of the mass spectra in Fig. 6 reveals that the most striking feature of the mass spectrum of the SOA particles that were formed in the present experiment (Fig. 6a) is that the most intense peak in that spectrum is at m/z ¼ 30, which is a clear indication of the presence of a large amount of nitrates in these particles. Furthermore, the m/z ¼ 46 peak in the PTR-MS (Fig. 2) supports that organic nitrates are formed in this system. Although the SPLAT mass spectra do not distinguish between organic and inorganic nitrate, given the chemistry in the present experiments it is reasonable to assume that the nitrates here are organic. There were no significant peaks at higher molecular weights such as those attributed to oligomer formation (Gao et al., 2004a, b; Tolocka et al., 2004, 2006; Docherty et al., 2005), which indicates either their absence or that they fragment during the evaporation or ionization steps. The partial mechanism in Scheme 1 shows that the formation of higher molecular weight nitrates is expected in this system from the reactions of RO2 radicals with NO. Modeling of this system assuming that the RO2+NO reaction gives 25% RONO2 predicts that the total yield of organic nitrates will be approximately 20%. Even at 1 ppb NO, the RO2+NO reaction dominates over the RO2+HO2 reaction, although the latter does contribute. For example, with peak HO2 concentrations of 6  109 cm3 and NO of 2 ppb, the ratio of the NO reaction to the HO2 reaction is about 5. Hydroxyl radical oxidation of a-pinene in the presence of NOx also generates organic nitrates such as PAN and/or hydroxy- and dihydroxynitrates (Wa¨ngberg et al., 1997; Aschmann et al., 1998; Nozie`re et al., 1999). Furthermore, a-pinonyl peroxynitrate (B in Scheme 1) has been reported to be a major aerosol product of pinonaldehyde oxidation by OH or NO3 under NOx-rich conditions (Nozie`re and Barnes, 1998). Total alkylnitrate yields in the range of 14–19% have been reported

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Fig. 6. Average SPLAT II spectra of particles from (a) a-pinene photoxidation, and authentic samples of (b) pinene oxide; (c) transsobrerol; (d) pinic acid; and (e) cis-pinonic acid.

for the reaction of NO3 with pinene (Wa¨ngberg et al., 1997; Hallquist et al., 1999). However, modeling of this system shows that the nitrate

radical concentrations are sufficiently small that 90% or more of the nitrates formed will be from the RO2+NO chemistry.

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Presto and Donahue (2005) identified organic nitrates in particles from the ozonolysis of a-pinene in the presence of NOx using FTIR of particles collected on filters. Their experimental conditions were quite different than the ones reported here in that initial ozone concentrations of 150–570 ppb and initial NOx concentrations of 7–2000 ppb were used, their reactant concentrations decreased with time, the relative humidity was low, and the reaction was carried out in the dark. In the present case, the relative humidity was high, the NOx concentration increased with time as the mixture was irradiated, and the peak O3 generated from the NOx photochemistry in these systems is o200 ppb. Alfarra et al. (2006) also reported the formation of nitrogen-containing organics in the NOx photooxidation of a-pinene in the presence of propene at 50% RH using an aerosol mass spectrometer. It is interesting that Winterhalter et al. (2003) did not detect organic nitrates in the NOx photooxidation of a-pinene using LC-MS, in contrast to their expectations. They suggest that the lack of observation may be due to instability of the nitrates during sampling and/or analysis. Density is an important physical property of aerosol particles. For example, it is needed to convert measured size distributions to mass loading and to derive aerosol yields in laboratory experiments. In many studies of aerosol formation, the aerosol density was assumed to be 1.0 g cm3 for aerosol mass and yield calculations. SPLAT II was used to measure the aerodynamic diameter of particles that had been size selected for their electrical mobility diameter using the DMA. As described in detail elsewhere (Zelenyuk et al., 2005; Zelenyuk and Imre, 2005), this measurement can be used to obtain the density of the particles or the effective density: rp ¼ r0

d va . dm

(III)

In Eq. (III), rp is the particle effective density which is a function of particle shape and density, r0 is 1.0 g cm3, dva is the vacuum aerodynamic diameter, and dm is the mobility diameter. For spherical particles, the effective density is equal to the true particle density. Zelenyuk et al. (2005, 2008) have shown that it is possible to distinguish between spherical and aspherical particles on the basis of the width of the vacuum aerodynamic diameter size distribution that is measured by SPLAT II. The measurements of the vacuum aerodynamic size

distributions of the particles that were formed here by the oxidation of a-pinene in the presence of NOx show that these particles are spherical. On that basis, the measured mobility and vacuum aerodynamic diameters can be used to obtain the true particle density, which in this case was found to be 1.2170.02 g cm3 and used to calculate the aerosol mass in these experiments. This value is similar to an estimated density of 1.19 g cm3 for particles formed in the ozonolysis of a-pinene reported by Bahreini et al. (2005) and densities of 1.2137 0.003 g cm3 and 1.19870.004 g cm3 measured for particles formed in the ozonolysis of a-pinene with and without OH scavenger by Zelenyuk et al. (2008). Alfarra et al. (2006) reported particle densities in the range 1.29–1.32 g cm3 in the NOx photooxidation of a-pinene. Offenberg et al. (2007) reported a particle density of 1.0670.07 g cm3 from the NOx photooxidation of a-pinene. This is based on the mass of particles collected on a filter and their volume measured using an SMPS system, which could lead to an underestimate of the density if there is some loss of products from the filters. It has been assumed in most laboratory studies that the particle density is 1.0 g cm3, and the volume is then used to calculate the mass of SOA formed and the SOA yield. Our data suggest that these yields have been underestimated by about 20%. The increase in mass per particle, the number concentration (Fig. 4) and the measured particle density can be combined with the loss of a-pinene (Fig. 3) to estimate an aerosol yield, that is, the fraction of mass of reacted pinene that is incorporated into particles. This will be a lower limit to the aerosol yield because it does not take into account the loss of low volatility products to the walls of the chamber. However, with this caveat in mind, a lower limit to the aerosol yield of approximately 25% is calculated. Combined with the yields of those gas-phase products which could be identified and measured, this represents a carbon mass balance of about 50%. Our SOA yield is similar to a value of 33% reported for the NOx photooxidation of a-pinene at 50% RH and at similar aerosol mass loadings of 100 mg m3 (Ng et al., 2006). The mass loadings in the Jaoui and Kamens (2001) studies were much larger, 1000 mg m3, but they also reported SOA yields of 19–27% for studies at an RH of 18–40%. SOA yields are expected to increase with mass loading because the existing particles provide an organic matrix for partitioning

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6 ηd=1.56 ± 0.04 Total Scattering (10-4 m-1)

of semi-volatile organics into the condensed phase (Pankow, 1994a, b; Presto and Donahue, 2006). For example, Verheggen et al. (2007) reported an SOA yield of only 771% using an initial pinene concentration of 14 ppb which gave a final aerosol mass concentration of 5 mg m3 (corrected to a density of 1.21 g cm3). SOA yields in the range of 14–67% have been reported in the ozonolysis of a-pinene (Hoffmann et al., 1997; Griffin et al., 1999; Yu et al., 1999a; Winterhalter et al., 2003; Docherty et al., 2005; Czoschke and Jang, 2006; Lee et al., 2006; Ng et al., 2006). Presto and Donahue (2005) studied SOA formation during the ozonolysis of a-pinene in the presence of NOx and reported SOA yields in the range from 8% to 30% depending on the aerosol mass, with lower yields under higher NOx (smaller VOC/NOx) conditions. Similarly, Nøjgaard et al. (2006) reported a large reduction in both the number concentration and the volume of SOA when NO2 was present during the ozonolysis of a-pinene. Docherty et al. (2005) measured SOA yields of 45712% in the ozonolysis of a-pinene at 50% RH in the presence of cyclohexane. Jenkin (2004) modeled this system and showed that SOA yields should be larger in the presence of an OH scavenger by about a factor of two, consistent with our lower limit value of 25%. Aerosol particles play a key role in the radiative forcing of the earth’s climate mostly through scattering and absorption of radiation. We also carried out light scattering measurements of the particles formed in this system and for comparison, Mie theory calculations. As expected, light scattering increases as particles are formed. Fig. 7 shows the light scattering at three different wavelengths in a typical experiment. If the particle size distribution, number concentration and refractive index are known, the light scattering as a function of wavelength can be calculated using Mie theory (Hinds, 1982; Bohren and Huffman, 1983). The colored horizontal bars show the expected Mie scattering for particles at these three wavelengths calculated from the measured particle size distribution and concentrations but using different indices of refraction (Zd). Scattering at all three wavelengths could not be matched simultaneously by a single refractive index. The best-fits are for an index of refraction of 1.46 at 700 nm, 1.51 at 550 nm, and 1.56 at 450 nm. These are consistent with indices of refraction of many organics (Lide, 1994), which are typically in the range of 1.4–1.6. It is not clear why

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450 nm 4

ηd=1.51 ± 0.03 550 nm

2

ηd=1.46 ± 0.03 700 nm

0 Time (min) Fig. 7. Mie scattering coefficient measured at 450, 550, and 700 nm. Horizontal bars show calculated values using Mie theory for particles using a best-fit value for the refractive index at each wavelength. The error bars are from the uncertainty in particle size and concentration measurements.

these differences are greater than expected from the typical wavelength dependence of Zd for single organics (0.01–0.02) (Moreels et al., 1984). However, this is potentially very important when extrapolating the results of laboratory studies to the atmosphere, since it indicates that the use of a single refractive index for SOA even from the oxidation of one organic precursor may not be appropriate. 4. Conclusions These experiments provide the first real-time studies of gases and particles generated in the photooxidation of a-pinene at high relative humidity under conditions of increasing NOx. Such circumstances are found in urban areas in the morning. In addition to a complex mixture of gaseous and particulate oxygenated organics, organic nitrates were components of both the gas and particle phases. These are likely to be high molecular weight compounds (e.g., see Scheme 1) which would not be easily identified in the field by existing techniques. This is consistent with the observation that measurement of alkyl nitrates as a group generally gives higher concentrations than the sum of those measured individually (e.g., see Rosen et al., 2004). If higher molecular weight organic nitrates from biogenic precursors are significant components of SOA, it could explain at least in part why SOA formation is linked with polluted urban air, yet the

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particulate carbon is largely non-fossil fuel derived (e.g., see Weber et al., 2007). In addition, if specific organic nitrates can be identified in air, they may provide a much more robust test of airshed models than do major air pollutants. For example, trace compounds such as HCHO formed during photooxidation in air have been shown to be more sensitive measures of model performance than is ozone (Kuhn et al., 1998), and organic nitrates may fulfill a similar role. It is therefore important that techniques be developed to separate and analyze individual organic nitrates in both the gas and particle phases in order to better understand the atmospheric oxidation mechanisms for organics, and the implications for health, visibility, and radiation. Finally, the finding that the density of the SOA formed is 1.21 g cm3 suggests that SOA formation may have been underestimated significantly in other studies where a density of 1.0 g cm3 was assumed. Acknowledgments We are grateful to the US Department of Energy (Grant no. DE-FG02-05ER64000) for support of this work. This research was in part performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the US Department of Energy’s Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL) and supported by the US Department of Energy Office of Basic Energy Sciences, Chemical Sciences Division. PNNL is operated by the US Department of Energy by Battelle Memorial Institute under Contract no. DEAC06-76RL0 1830. Additional support was provided by the Collaborative Research in Chemistry program (Grant no. CHE-0209719) and the AirUCI Environmental Molecular Science Institute (Grant no. CHE-0431512) funded by the National Science Foundation. We also thank A. Campbell and R. Gephart for facilitating the SPLAT II and PTR-MS collaborative studies; C.M. Berkowitz, J. Hubbe, and W. Wang for technical assistance; and T.E. Kleindienst for helpful discussions. References Alfarra, M.R., Paulsen, D., Gysel, M., Garforth, A.A., Dommen, J., Pre´voˆt, A.S.H., Worsnop, D.R., Baltensperger, U., Coe, H., 2006. A mass spectrometric studie of secondary organic aerosols formed from the photooxidation of anthropogenic

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