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Atmospheric Environment 42 (2008) 6851–6861

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Atmospheric Environment

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

Detection of nitrooxypolyols in secondary organic aerosol formed fromthe photooxidation of conjugated dienes under high-NOx conditions

Kei Sato*

Asian Environment Research Group, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan

a r t i c l e i n f o

Article history:Received 19 March 2008Received in revised form 24 April 2008Accepted 1 May 2008

Keywords:Urban atmosphereLaboratory smog chamberPolyolIsoprene tetrolIsoprene nitrateCloud processing

* Tel.: þ81 29 850 2414; fax: þ81 29 850 2579.E-mail address: kei@nies.go.jp

1352-2310/$ – see front matter � 2008 Elsevier Ltddoi:10.1016/j.atmosenv.2008.05.010

a b s t r a c t

2-Methyltetrols produced by the oxidation of isoprene have been recently found tocontribute toward the formation of atmospheric secondary organic aerosol (SOA).However, the oxidation mechanism relevant to the formation of these polyols has notbeen completely understood. In this study, the photooxidation of four conjugated dienes(isoprene, 1,3-butadiene, 2,3-dimethyl-1,3-butadiene and 2,4-hexadiene) in the presenceof 0.2–1 ppmv NO was examined by a series of laboratory experiments, and the polyols,organic acids and nitric acid in an aqueous solution of the resulting SOA were analysedby using ion-exclusion liquid chromatography/mass spectrometry (LC–MS). In the experi-ments performed using isoprene, 2-methyltetrols (comprising 0.5–2% of aerosol mass),methylnitrooxybutanetriols (comprising 1–7% of aerosol mass), methyldinitrooxybutane-diols (comprising 0.3–8% of aerosol mass) and nitric acid (comprising 4–9% of aerosolmass) were found in the aqueous solution of the SOA samples. Three days after the extrac-tion, the concentrations of nitrooxypolyols (i.e. methylnitrooxybutanetriols and methyldi-nitrooxybutanediols) decreased, whereas the concentrations of polyols and nitric acidincreased. Similar results were obtained for all the four dienes. Nitrooxypolyols, whichare produced by the gas-phase oxidation of dienes in the presence of NOx, contributetoward the SOA formation, and these compounds can decompose to polyols and nitricacid in an aqueous solution. The polyols and the nitric acid present in the aqueous solutionare hydrolysis products, and not real constituents of aerosol. The direct gas-phase forma-tion of polyols from the diene oxidation is suppressed in the presence of NOx.

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1. Introduction

The atmospheric photooxidation of conjugated dienes(e.g. 1,3-butadiene from anthropogenic emission sourcesand isoprene from both anthropogenic and biogenic sour-ces) is a potential source of photochemical ozone in urbanair (Singh, 1995; Angove et al., 2006; Jenkin et al., 2003;Jemma et al., 1995). Secondary organic aerosol (SOA) wasknown to be produced during the oxidation of dienes withsix or more carbons; however, the contributions from lowerdienes were believed to be negligible by early workers

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(Grosjean and Seinfeld, 1989). However, 2-methyltetrols(i.e. 2-methylerythritol and 2-methylthreitol), which havean isoprene molecular skeleton, have recently been foundin aerosols collected around tropical rainforests (Claeyset al., 2004a; Schkolnik et al., 2005), forests in middle lati-tudes (Claeys et al., 2004b; Ion et al., 2005; Kourtchevet al., 2005; Xia and Hopke, 2006) and urban and suburbanareas (Clements and Seinfeld, 2007; Edney et al., 2005).Furthermore, the SOA formation from the isoprene oxida-tion has also been observed in number of laboratorychamber studies (Pandis et al.,1991; Edney et al., 2005; Krollet al., 2005; Boge et al., 2006; Dommen et al., 2006; Kalbereret al., 2006; Surratt et al., 2006; Kleindienst et al., 2007;Szmigielski et al., 2007). These recent field and laboratorystudies have shown that the oxidation of isoprene does in

K. Sato / Atmospheric Environment 42 (2008) 6851–68616852

fact contribute toward the atmospheric formation of SOA inurban areas as well as forest areas and should not beneglected. Claeys et al. (2004a) suggested that 2-methyltetrols are produced by the gas-phase oxidation ofisoprene under low NOx conditions (Scheme 1 in Fig. 1).Under such conditions, peroxy radicals (RO2) produced bythe addition of O2 to isoprene–OH adducts are assumed toproceed according to the reaction RO2þ R0CH2O2 /

ROHþO2þ R0CHO, which results in the formation of unsat-urated diols (R and R0 represent organic groups). The subse-quent oxidation reactions of unsaturated diols lead to theformation of 2-methyltetrols. In addition, the acid-catalysedheterogeneous reactions of hydrogen peroxide withgaseous isoprene oxidation products [Fig. 1, Scheme 2(Surratt et al., 2006) and Scheme 3 (Boge et al., 2006)]should also contribute toward the formation of 2-methyltetrols in the presence of acidic particles.

Recently, Surratt et al. (2006) investigated the impact ofthe initial NOx levels on the chemical composition of SOAproduced from the oxidation of isoprene; by means of gaschromatography/mass spectrometry (GC–MS), they identi-fied 2-methyltetrols in the SOA produced in the absence ofNOx. However, they did not detect 2-methyltetrols for aninitial NOx concentration of w1 ppmv. The suppression ofthe 2-methyltetrol formation in the presence of NOx canbe explained in terms of the removal of RO2 radicals bythe gas-phase reaction RO2þNO / ROþNO2. On the otherhand, Angove et al. (2006) studied the photooxidation of1,3-butadiene for an initial NOx concentration of1–2 ppmv, and they analysed the aqueous solutions of theresultant SOA by using nuclear magnetic resonance(NMR) spectrometry. Although broad NMR spectra attrib-uted to polymeric products were observed in the samplesimmediately after the extraction, low-molecular-weight

Fig. 1. Believed mechanism of 2-methyltetrol formation from pho

products, including tetrols (erythritol and threitol), werefound in aqueous solution samples stored at 323 K for threedays. It was concluded that tetrols are produced by the gas-phase oxidation of 1,3-butadiene, even in the presence ofNOx; these tetrols undergo heterogeneous reactions toform reversely decomposable polymers. As describedabove, the pathways of the polyol formation from theoxidation of conjugated dienes have not been completelyunderstood.

For better understanding of the polyol formation, weattempted to study the compositions of the SOA producedfrom the diene/NOx photooxidation and their decomposi-tion products in aqueous solutions by using liquid chroma-tography–mass spectrometry (LC–MS). Among the SOAproducts obtained from the oxidation of isoprene, notonly 2-methyltetrols but also organic acids (such as2-methylglyceric acid) and oligomers have been identified(Surratt et al., 2006; Szmigielski et al., 2007; Dommenet al., 2006; Kalberer et al., 2006). Organic acids and oligo-mers can be analysed by using LC–MS. Polyols can also bedetected by using electrospray ionization mass spectrom-etry (ESI-MS); however, the analytes were not successfullyseparated by using a widely used octadecyl silica gelcolumn (Surratt et al., 2006). On the other hand, Schkolniket al. (2005) succeeded in separating 2-methyltetrols byemploying a method based on ion-exclusion liquid chro-matography (IEC-LC) with UV detection.

In this study, we employed an IEC-LC method combinedwith an ESI-MS method for the analysis of polyols andorganic acids present in an aqueous solution of SOA obtainedfrom diene oxidation. Isoprene,1,3-butadiene, 2,3-dimethyl-1,3-butadiene and 2,4-hexadiene were selected as theprecursor dienes. Comparisons among the results obtainedfor the four dienes should confirm the identifications of

tooxidation of isoprene in the presence of acidic particles.

K. Sato / Atmospheric Environment 42 (2008) 6851–6861 6853

the resultant products. The aims of this study are asfollows: (1) to establish a method for the simultaneousanalysis of polyols and organic acids by employingan LC–MS method, (2) to study the formation of polyolsin an aqueous solution of SOA obtained from diene photo-oxidation and (3) to discuss the gas-phase reactionmechanisms involved in the NOx/diene photooxidation.

2. Experimental

2.1. Chamber experiments

Six experiments were conducted on the diene/NO/CH3ONO/air photoirradiation systems by using a 6-m3,Teflon-coated, stainless steel laboratory smog chamber(Akimoto et al., 1979; Sato et al., 2004a) in the absence ofseed particles (Table 1). Prior to each experiment, thechamber was filled with 758–765 Torr of purified air. Therelative humidity of the purified air was less than 1 ppmv.The diene, NO and methyl nitrite reactants at the desiredpartial pressures were introduced into calibrated bulbsand then flushed into the chamber by using pure nitrogencarrier gas. In all the experiments, the initial concentrationof the diene was set to 2 ppmv. In the experiments per-formed on isoprene, the initial NO concentrations wereset to 0.2, 0.5 and 1 ppmv for experimental runs 1, 2 and3, respectively. For the other dienes, all the experimentswere conducted at an initial NO concentration of0.5 ppmv (runs 4–6). Approximately 10-ppbv methyl nitritewas added to the reaction mixture as an OH radical source.Methyl nitrite was prepared by the reaction of sodiumnitrite with methanol in the presence of sulfuric acid, andit was purified by vacuum distillation to remove any meth-anol and water impurities. The synthesized methyl nitritesample was stored in a glass sample tube at 77 K until itwas used for the chamber experiments. The temperaturesof the chamber wall were controlled to within the range298–299 K. The gas mixtures were irradiated by Pyrex-filtered light from xenon arc lamps (19 kW). The rate coef-ficient for NO2 photolysis was (4.7� 0.2)� 10�3 s�1.The concentrations of the diene, NO, NO2 and O3

were monitored by using a Fourier-transform infrared(FT-IR) spectroscope (Nicolet, Nexus 670) combined witha multi-reflection mirror system with an optical pathlength of 221.5 m (Miyoshi et al., 1994). The volumeconcentration of the SOA (referred to as V in this paper)was measured by using a scanning mobility particle sizer(SMPS; TSI, model 3934) spectrometer. SOA particles

Table 1Initial diene and NO concentrations, total concentration of hydrocarbon reacted (experiments

Code Precursor [HC]0 (ppbv) [NO]

Run 1 Isoprene 2001 196Run 2 Isoprene 2017 496Run 3 Isoprene 2012 988Run 4 1,3-Butadiene 2024 503Run 5 2,3-Dimethyl-1,3-butadiene 2028 506Run 6 2,4-Hexadiene 2013 501

a SOA mass concentration determined from the SMPS volume concentration d

were collected through a stainless steel tube (outer diam-eter: 0.64 cm; length: 60 cm) at a flow rate of 0.3 L min�1.The irradiation was continued until the volume concentra-tion of the SOA leveled off. The durations of irradiationwere 180 min. After the xenon lamps had been turned off,the SOA particles were collected on a filter for subsequentanalysis of the SOA products (described below). Then, therate coefficient for wall deposition loss of the SOA remain-ing in the chamber was measured (Sato et al., 2004b). Inorder to suppress reactions with O3 and NO3 radicalsduring the wall-loss measurement, an excess of NO wasadded to the gas mixture. The rate coefficients for wallloss were independent of the particle diameter; the coeffi-cients were determined to be (3–5)� 10�5 s�1. The volumeconcentrations measured by using the SMPS spectrometerwere corrected by using the determined rate coefficient;the corrected volume concentration is referred to as Vc

elsewhere in this paper.

2.2. Sample collections and pretreatments

For the analysis of the SOA products, the gas mixturefrom the chamber was collected through a Teflonmembrane filter (Sumitomo Electric, 47-mm diameter,1-mm pore size) by using a low-volume air sampler (Shi-bata, LV40B). Another sampling tube was used for this filtersampling. The length and outer diameter of the samplingtube were 60 and 1.27 cm, respectively. The rate of collec-tion was 16.7 L min�1 and the duration was 1–2 h. Duringfilter sampling, purified air was introduced into thechamber at 17.0 L min�1 to maintain the chamber atambient pressure. The filter mass before sampling andthat after sampling were measured by using an electricsemi-micro balance (Mettler Toledo, AG285), and the totalmass of the collected SOA was calculated from the differ-ence between the masses before and after sampling.Pretreatments of the filter sample were performed imme-diately after the gravimetric measurements. The filter wascut into small pieces, which were sonicated in 3 mL ofmethanol for 40 min. The extract (2.5 mL) was placed ina 20-mL vial and concentrated to near dryness undera gentle stream of nitrogen gas. The concentrated samplewas mixed with 1 mL of distilled water and sonicated for20 min. A 0.5-mL aliquot of the filtered aqueous solutionwas placed in a 1.5-mL vial and analysed immediately afterthe preparation of the sample. The remaining 0.5-mLaliquot was placed in a 1.5-mL umber brown vial witha Teflon septum cap. This sample was stored at 323 K and

D[HC]), total concentration of SOA produced (Mc) and SOA yield (Y) for all

0 (ppbv) D[HC] (mg m�3) Mca (mg m�3) Y (w/w)

4058 33 0.0085330 69 0.0135591 195 0.0354243 105 0.0256267 28 0.0046644 297 0.045

ata by assuming the SOA density to be 1.3 (see text).

K. Sato / Atmospheric Environment 42 (2008) 6851–68616854

analysed three days after the extraction. Althoughthis storage temperature was considerably high, it waschosen in order to verify the results of the previous exper-iments conducted at the same temperature by Angove et al.(2006).

2.3. IEC-LC/MS analysis

Loop-injected analytical samples (20 mL) were analysedby using a Shimadzu LC–MS QP-8000a system (Sato et al.,2007). A sulfonated poly(styrene-divinylbenzene) Hþ-form cation-exchange resin column (Supelco, SupelcogelH, 9-mm particle size, 25-cm length� 4.6-mm inner diam-eter) was used to separate the organic acids and polyols.The eluent was a 5-mM aqueous solution of either ammo-nium acetate or ammonium formate. The flow rate of theeluent was 0.4 mL min�1, and the column temperaturewas maintained at 298 K. Cation-exchange resin columnsare normally used with a strong acid buffer; however,acid buffers were not used in this study because the ioniza-tion efficiency in negative-ion mass spectrometrydecreases in the presence of strong acids. For the analysesof polyols and sugar compounds, salt is often added tothe eluent for increasing the ionization efficiencies. Thebuffer salts used in this study were selected because theirhigh volatilities are compatible with mass spectrometry.Because the degradation lifetimes of cation-exchange resincolumns are shorter than those of commonly used silica gelcolumns, one cation-exchange resin column was usedduring the preliminary experiments and another wasused during the main series of studies; thus, the effect ofthe degradation of the column during the experimentswas minimized.

The eluent from the column was introduced into a nega-tive-ion mode ESI probe at �3.5 kV. The eluent was nebu-lized from the probe by a nitrogen gas flow at 4.5 L min�1.Negatively charged droplets from the ESI probe were intro-duced into a vacuum chamber through a curved desolva-tion line at 523 K and 30 V. Negative ions from thedesolvation line were focused by using four deflector plates(�20 V for each) and injected into a quadruple mass spec-trometer. The region with mass-to-charge ratios (m/z)50–1000 was scanned at 500 Da s�1 (scan mode), or theselected ions were monitored at 12.5 Da s�1 in order toobtain higher sensitivities (SIM mode). The mass-selectedions were detected by a secondary electron multiplierat �1.8 kV. The outputs from the secondary electron multi-plier were integrated for 2 s to obtain a mass spectrum. Inorder to estimate the molecular structures of the unidenti-fied compounds, the secondary fragmentation patterns ofthe ions produced by the ESI method were studied byemploying a collision-induced dissociation (CID) method.The voltage on the deflector was set to �40 V in order todissociate the ions by collisions with neutral molecules inthe deflector region. In order to estimate the recovery ofpolyols in the present analytical method, 100 mL of a meth-anolic solution of meso-erythritol (500 ng mL�1) was spikedon a Teflon filter, and the filter was then analysed. Therecovery of polyols was confirmed to be >90%. Chromato-graphic peaks were not observed during the analysis ofa blank filter.

2.4. Analysis of standard samples

Twelve standard samples of compounds expected to beproduced in SOA (Claeys et al., 2004b; Edney et al., 2005;Surratt et al., 2006; Boge et al., 2006) and aqueous solutionsof SOA (Angove et al., 2006; Carlton et al., 2006) were ana-lysed in order to measure their retention times and toproduce calibration curves (Table 2). Among the 12 stan-dard samples, 7 samples (compounds 1–7) were authenticstandard compounds, including nitric acid, organic acidsand tetrols. Equimolar mixed aqueous solution samples ofthese 7 compounds (0, 1, 2 and 5 ng mL�1) were analysed.The other standard samples were dihydroxyalkanoic acidsand polyols for which authentic standards are not available.These compounds were prepared by using a methodproposed by Claeys et al. (2004b). 2-Methylglyceric acidand 2,3-dihydroxybutanoic acid were synthesized by thesulfuric acid-catalysed reactions of hydrogen peroxidewith methacrylic acid and 3-butenoic acid, respectively.2-Methyltetrols, 2,3-dimethyltetrols and 2,3,4,5-hexane-tetrols were synthesized by similar reactions involvingisoprene, 2,3-dimethyl-1,3-butadiene and 2,4-hexadiene,respectively. The synthesized standards for dihydroxyalka-noic acids and polyols were used only for the measure-ments of the retention times. In the mass spectrometricanalysis of acids (compounds 1–5, 8 and 9), the quasi-molecular ions at m/z¼MW�H produced by the deproto-nation of the parent molecule were determined to be themost abundant fragment (Table 2). On the other hand,the most abundant ions in the polyols (compounds 6, 7,10–12) were the adduct ions at m/z¼MWþ 59 (CH3COO�)or MWþ 45 (HCOO�) when ammonium acetate or ammo-nium formate were used as buffer salts, respectively. Theretention times of all the standard compounds were inde-pendent of the buffer salts. Although the retention timesof nitric acid and oxalic acid were considerably short(3.6 min), the other compounds were successfully sepa-rated by the present method. Since the correlation coeffi-cients of the calibration curves of the seven standards(compounds 1–7) were >0.99, these curves were approxi-mated by straight lines so that they could be used for thepresent quantifications. In the SIM mode, the lower detec-tion limits of the seven standards were 0.64–5.8 ng.

3. Results

3.1. SOA density and SOA yield

The average SOA mass concentration during the SOAfilter sampling was determined from the ratio of themass of SOA collected on the filter to the volume of thecollected air. Since the corrected SOA masses during exper-imental runs 1 and 5 were close to the lower limit of thebalance (10 mg), the average SOA mass concentrationsduring these experimental runs could not be determined.However, the average SOA mass concentrations (<M>)were successfully determined to be 50–125 mg m�3 in theother runs. From the SMPS results obtained from experi-ments other than runs 1 and 5, the average SOA volumeconcentrations during the sampling (<V>) were deter-mined to be 30–120 mm3 cm�3. The density of the SOA

Table 2Molecular weight (MW), abundant ion detected in the negative-ion mode (m/z) and retention time (RT) of the standard compounds

Compound no. Compound MW (g mol�1) m/z (amu) RT (min) Note

1 Nitric acid 63 62 (MW�H) 3.62 Glyoxylic acid 74 73 (MW�H) 4.1 b,c

3 Pyruvic acid 88 87 (MW�H) 3.8 c

4 Oxalic acid 90 89 (MW�H) 3.6 c

5 DL-Glyceric acid 106 105 (MW�H) 4.8 b

6 meso-Erythritol 136 181 (MWþ 59)a 6.7 b

7 DL-Threitol 136 181 (MWþ 59)a 6.9 b

8 2-Methylglyceric acid 120 119 (MW�H) 4.9 d–g

9 2,3-Dihydroxybutanoic acid (2 diastereomers) 120 119 (MW�H) 4.5, 4.9 h

10 2-Methyltetrol (2 diastereomers) 136 195 (MWþ 59)a 6.7, 7.0 d–g

11 2,3-Dimethyltetrol (2 diastereomers) 150 209 (MWþ 59)a 7.4, 7.7 i

12 2,3,4,5-Hexanetetrol (6 diastereomers) 150 209 (MWþ 59)a 5.9–8.7 h

a Abundant ion detected by using ammonium acetate buffer.b Product found by Angove et al. (2006).c Product found by Carlton et al. (2006).d Product found by Claeys et al. (2004b).e Product found by Surratt et al. (2006).f Product found by Edney et al. (2005).g Product found by Boge et al. (2006).h Product expected from the oxidation of 2,4-hexadiene.i Product expected from the oxidation of 2,3-dimethyl-1,3-butadiene.

Fig. 2. Concentrations of isoprene, NO, NO2, O3, SOA (V) and SOA (Vc),measured as a function of time (run 2).

K. Sato / Atmospheric Environment 42 (2008) 6851–6861 6855

(d¼<M>/<V>) was calculated for each experiment. Theaverage density was determined to be 1.3� 0.3 g cm�3

(where the error is represented by the standard deviation).Since the volume concentration could be measured in allthe experiments, the mass concentrations were convertedfrom the volume concentrations by assuming the densityof the SOA to be 1.3 g cm�3. In this study, these massconcentrations obtained from the SMPS results wereused to calculate the SOA yields and the distributions ofthe SOA constituents.

The concentrations of isoprene, NO, NO2, O3 and SOAduring the chamber experiment (run 2) were plotted asa function of time (Fig. 2). The isoprene concentrationdecreased, and the SOA concentration increased duringthe experiments. The total consumption of dienes(D[HC]), total mass concentrations of the produced SOA(Mc¼ d$Vc) and the SOA yields (Y¼Mc/D[HC]) are summa-rized in Table 1. The range of the isoprene SOA yields deter-mined in the present study (0.008–0.035) is roughlycomparable with those determined by Kroll et al. (2005)(0.009–0.030) and Dommen et al. (2006) (0.002–0.053),although the initial reactant concentrations and tempera-tures were different. Since the SOA yield of a specific dienewas only measured under a single condition or few condi-tions, the results of the SOA yields are not discussed furtherin this study.

3.2. Qualitative analysis of SOA samples

The SOA sample obtained from the experiment onisoprene was analysed by using the ammonium acetatebuffer, and the mass spectra recorded in the scan modewere integrated over the retention time in the interval3–20 min in order to obtain the mass spectrum of the totaldetectable ions. In the mass spectrum measured immedi-ately after the extraction, intense mass signals appearedat m/z¼ 62, 73, 87, 119, 195, 240 and 285. Signals from olig-omers were not detected in the present analytical

conditions. When acetonitrile was added to the eluent,the signals from the oligomers were detected in the regionm/z< 700. The average molecular weight calculated fromthis mass spectrum was 516�183 Da (where the error isrepresented by the standard deviation). However, the olig-omer structures could not be estimated because the signalintensities from the oligomers were not sufficiently high.

By selecting abundant ions, the sample in experimentalrun 2 was analysed in the SIM mode (Fig. 3). Chromato-graphic peaks were observed for the retention time (RT)in the interval 3–20 min of the total-ion chromatogram(TIC) (Fig. 3a). In the extracted-ion chromatogram (EIC) ofm/z¼ 62, a single chromatographic peak is detected atRT¼ 3.6 min (Fig. 3b). The mass-to-charge ratio and theretention time for this peak correspond to those for nitricacid. In the EIC of m/z¼ 89, a chromatographic peak isalso observed at RT¼ 3.6 min. The mass-to-charge ratioand the retention time for this peak correspond to thosefor oxalic acid. However, since the peak retention times ofm/z¼ 62 and 89 are considerably short, ions formed from

Fig. 3. Chromatograms measured in (�)ESI/LC–MS analysis of isoprene SOAsample (run 2): (a) total-ion chromatogram and extracted-ion chromato-grams of m/z¼ (b) 62, (c) 119, (d) 195, (e) 240 and (f) 285.

Fig. 4. Mass spectra measured for compound eluting at retention time of8.2 min (run 2) by using (a) soft ionization method with ammonium acetatebuffer, (b) soft ionization method with ammonium formate buffer and (c)collision-induced dissociation method with ammonium acetate buffer.

K. Sato / Atmospheric Environment 42 (2008) 6851–68616856

other compounds may contribute toward these peaks. Inthe EIC of m/z¼ 119, a chromatographic peak appears atRT¼ 4.9 min (Fig. 3c). By comparing the above values ofm/z and RT with those for a standard sample, the above-mentioned chromatographic peak is identified as thatcorresponding to 2-methylglyceric acid. In the EIC ofm/z¼ 195, two intense peaks are observed at RT¼ 6.7 and7.0 min (Fig. 3d). These peaks are identified as those arisingfrom two diastereomeric 2-methyltetrols. Although notshown in Fig. 3, the peaks assigned to glyoxylic acid andpyruvic acid are also observed in the extracted-ion chro-matograms at m/z¼ 73 and 87, respectively.

The peaks at 7.2, 8.2 and 8.9 min in the EIC of m/z¼ 240(Fig. 3e) and those at 11.5 and 13.4 min in the EIC ofm/z¼ 285 (Fig. 3f) are observed as unidentified products.It must be noted that the mass numbers of 195, 240 and285 have a regular mass difference of 45. The fact thatmultiple peaks are observed in each of the extracted-ionchromatograms shows that these unidentified productshave isomers. We determined whether or not the productsare organic acids or polyols by varying the buffer salts(Fig. 4). When the product is a polyol, the mass number

of the adduct ion produced should depend on the buffersalt. The mass spectrum of the chromatographic peak atRT¼ 8.2 min was measured by using the ammoniumacetate buffer for the SOA sample in experimental run 2.Under analytical conditions, the mass peak at m/z¼ 240 isthe most abundant (Fig. 4a). On varying the buffer salt toammonium formate, the mass peak at m/z¼ 240 disap-pears and a peak at m/z¼ 226 appears (Fig. 4b). Theseresults imply that the product eluting at RT¼ 8.2 min isa polyol compound with a molecular weight of 181(¼ 240–59). The even mass number of the parent quasi-molecular ion shows that an odd number of nitrogen atomsare present in the product molecule. In order to study themolecular structure of this polyol, the secondary fragmen-tation pattern of the ion at m/z¼ 240 was measured byusing the CID method. In the mass spectrum measured byusing the CID method, two intense mass peaks appear atm/z¼ 240 and 62 (Fig. 4c). The new ion at m/z¼ 62 is a frag-ment ion and is identified as the nitrate ion (NO3

�). Thisresult implies that the product eluting at RT¼ 8.2 mincontains nitrooxy groups in the product molecule. Fromthe molecular structure of isoprene as well as the resultsdescribed above, the peaks detected at m/z¼ 240 areassigned to methylnitrooxybutanetriol isomers (e.g. thecompound shown in Fig. 3e). By means of a similar analysis,the products detected at m/z¼ 285 with the ammoniumacetate buffer are shown to be nitrooxy group-containingpolyols with a molecular weight of 226. These productscontain an even number of nitrogen atoms in each mole-cule. The peaks detected at m/z¼ 285 are assigned to meth-yldinitrooxybutanediol isomers (e.g. the compound shownin Fig. 3f).

K. Sato / Atmospheric Environment 42 (2008) 6851–6861 6857

In a similar manner, nitric acid, glyoxylic acid, pyruvicacid, oxalic acid, dihydroxyalkanoic acid and polyol wereidentified in each SOA sample used for the oxidation ofthe four dienes. Ions that showed behaviours similar tothose at m/z¼ 240 from the isoprene sample were detectedin the samples of 1,3-butadiene, 2,3-dimethyl-1,3-buta-diene and 2,4-hexadiene at m/z¼ 226, 254 and 254, respec-tively. Each peak was assigned to a nitrooxytriol isomerwith the precursor molecular skeleton. Furthermore, ineach diene sample, an ion assigned to the dinitrooxydiolisomers with the precursor molecular skeleton was alsodetected.

3.3. Quantitative analysis of SOA samples

Qualitative analysis was performed in the SIM mode forall the samples, immediately after the extraction, anda similar analysis was also performed three days afterthe extraction. The mass concentrations of the productsin the SOA samples were determined by employing anexternal standard method. Glyceric acid was used assurrogate for all the dihydroxyalkanoic acid products,and meso-erythritol was used as surrogate for all the pol-yol and nitrooxypolyol (i.e. nitrooxytriol and dinitrooxy-diol) products. The ratios of the products to the totalSOA mass were determined from the concentrations ofthe quantified products and that of the total SOA in eachsample solution. The error in each determined ratio isa result of mainly the errors in product recovery (�10%),chromatographic peak area (�10%), concentration of thereference standard (�5%), SOA density (w20%) andvolume concentration measured by SMPS spectrometry(�10%). On the basis of the error-propagation law, the totalerrors are determined to be �27%. In addition, the errorsobtained for products with short retention times (nitricacid and oxalic acid) and the products quantified by surro-gates (dihydroxyalkanoic acids other than glyceric acid,polyols other than tetrols and all the nitrooxypolyols)may be greater than the estimated errors. This impliesthat the present quantifications are merely order estima-tions. This is taken into account in the later discussionson the present quantitative results.

The present quantitative results of nitric acid, organicacids, polyols and nitrooxypolyols are summarized inTable 3. In the experiments with isoprene, nitric acid(4–10%), 2-methyltetrols (0.5–2%), methylnitrooxybutane-triols (1–7%) and methyldinitrooxybutanediols (0.3–8%)were found in the aqueous solution of the SOA samples.Three days after the extraction, the concentrations of nitro-oxypolyols decreased, whereas the concentrations of poly-ols and nitric acid increased. Similar results were obtainedfrom the experiments on all the four dienes. The total ratiosof all the quantified products were 12–27 wt% immediatelyafter the extraction; this indicates that these productscomprise a substantial portion of the SOA mass. The SOAsample analysed immediately after the extraction (run 2)was stored at 258 K, and it was then re-analysed threedays after the extraction. However, the product distributionobtained by this analysis was basically the same as thatobtained immediately after the extraction.

4. Discussion

4.1. Gas-phase nitrooxypolyol formation and itsdecomposition to polyols

First of all, the reaction mechanism for the formation ofnitrooxypolyols found in the SOA in this study is discussedon the basis of previous knowledge of the explicit reactionmechanism for isoprene oxidation in the gas phase. Thephotooxidation of isoprene in the presence of NOx atroom temperature is initiated by reactions with OH radi-cals [k¼ 1.0�10�10 cm3 molecule�1 s�1 (Atkinson, 1986)]and with O3 [k¼ 1.2�10�17 cm3 molecule�1 s�1 (Atkinsonet al., 1982)]. If we assume that the concentrations of OHradicals and O3 are 2�106 and 1012 molecule cm�3,respectively, the rate of the OH radical reaction is 17 timesthat of the O3 reaction. Furthermore, the SOA yield duringthe ozonolysis of isoprene is lower than that during theOH radical reaction of isoprene (Kleindienst et al., 2007).Thus, the contribution of the O3 reaction to the SOA forma-tion is negligible. Nitrate (NO3) radicals are also possibleoxidizers of isoprene in the dark; however, the NO3 reac-tion is slower than the OH reaction under irradiated condi-tions. Actually, the rate of the OH radical reaction is �15times that of the NO3 reaction if we assume that theconcentration of NO3 radicals is �2�107 molecule cm�3.Very recently, Ng et al. (2008) reported in the discussionpaper that the SOA yields from the NO3þ isoprene reac-tion are 0.04–0.24 and are higher than those from theOHþ isoprene reaction. If we employ these very recentdata, the contribution of the NO3 reaction to the SOAformation may not be ruled out. However, even if weemploy them, the reaction of isoprene with OH radicalsis the largest factor responsible for SOA formation duringthe photooxidation of isoprene.

The reaction of isoprene with OH radicals proceedsthrough addition to one of the double bonds of isoprene(Fig. 5). The adduct radical thus produced has two resonancestructures [i.e. CH2OHC�(CH3)CH]CH2 and CH2OHC(CH3)]CHC�H2] as a result of the addition of an OH radical toa terminal carbon (Tuazon and Atkinson, 1990); however,in the figure, for simplicity, only the subsequent reactionsof the former radical are shown. The adduct radicals reactwith O2 under atmospheric conditions to form peroxy radi-cals. Generally, peroxy radicals (RO2) react with NO to formeither alkoxy radicals or organic nitrates in the presenceof NO (Chen et al., 1998).

RO2 þ NO / RO þ NO2 (R1a)

RO2 þ NO / RONO2 (R1b)

The organic nitrates produced through Reaction (R1b)during the isoprene oxidation are methylnitrooxybutenolisomers. Subsequent oxidation of methylnitrooxybutenolscan form methyldinitrooxybutanediol isomers (Fig. 5,product A). On the other hand, for low concentrations ofNO, peroxy radicals (RO2) react with HO2 radicals to formhydroperoxides or they react with other peroxy radicals(R0CH2O2) to form alcohols.

Table 3Ratios of the total SOA mass measured immediately after extraction (values without parenthesis) to that measured three days after the extraction (values inparenthesis) expressed in units of wt%

Product MW (g mol�1) Isoprene 1,3-Butadiene 2,3-Dimethyl-1,3-butadiene 2,4-Hexadiene

Run 1 Run 2 Run 3 Run 4 Run 5 Run 6

Nitric acida 63 6.4 (12.4) 9.7 (16.2) 4.1 (13.1) 6.2 (9.3) 4.0 (14.7) 1.9 (5.4)Glyoxylic acid 74 0.9 (–) 0.8 (–) 0.8 (0.5) 1.2 (0.1) 0.4 (–)Pyruvic acid 88 1.2 (0.1) 0.9 (0.3) 1.0 (0.2) 0.1 (0.3) 1.0 (0.3) 0.7 (2.7)Oxalic acida 90 1.9 (5.8) 2.6 (4.3) 0.6 (2.8) 2.6 (6.8) 1.9 (4.1) 2.3 (4.5)Glyceric acid 106 – – – 0.2 (5.3) – –2-Methylglyceric acidb 120 0.8 (7.5) 2.4 (6.2) 1.4 (29.1) – 2.7 (18.9) –2,3-Dihydroxybutanoic acidb 120 – – – – – 3.8 (5.4)Tetrol 122 – – – 1.0 (6.5) – –2-Methyltetrolc 136 0.9 (6.3) 1.6 (8.3) 0.5 (6.0) – – –2,3-Dimethyltetrolc 150 – – – – 0.4 (2.8) –2,3,4,5-Hexanetetrolc 150 – – – – – 1.0 (1.9)Nitrooxybutanetriolsc 167 – – – 9.7 (13.5) – –Methylnitrooxybutanetriolsc 181 1.4 (–) 6.5 (–) 3.1 (2.2) – – –Dimethylnitrooxybutanetriolsc 195 – – – – 0.6 (–) –Nitrooxyhexanetriolsc 195 – – – – – 6.1 (3.1)Dinitrooxybutanediolsc 212 – – – 4.3 (7.8) – –Methyldinitrooxybutanediolsc 226 0.3 (–) 3.0 (–) 7.7 (1.0) – – –Dimethyldinitrooxybutanediolsc 240 – – – – 0.6 (–) –Dinitooxyhexanediolsc 240 – – – – – 2.3 (–)Total 13.8 (32.2) 27.4 (35.3) 19.2 (54.8) 25.3 (49.6) 11.5 (40.8) 19.7 (24.8)

a Result is upper limit.b Glyceric acid was used as the standard surrogate.c meso-Erythritol was used as the standard surrogate.

K. Sato / Atmospheric Environment 42 (2008) 6851–68616858

RO2 þ HO2 / ROOH þ O2 (R2)

RO2 þ R0CH2O2 / ROH þ O2 þ R0CHO (R3)

Claeys et al. (2004a) proposed that the alcoholsproduced by Reaction (R3) during the isoprene oxidation

Fig. 5. Proposed mechanism of methyldinitroxybutenediols, methylnitrooxybutentpresence of NOx.

(i.e. methylbutenediols) undergo subsequent oxidation toform 2-methyltetrols (Fig. 5, product C). If Reaction (R3) oc-curs, methylnitrooxybutanetriol isomers (Fig. 5, product B)are also formed. These RO2þ R0CH2O2 type reaction mayoccur at very low NO levels observed at the time of theSOA formation in the present study (see Fig. 2). However,the concentrations of the 2-methyltetrols and the methyl-nitrooxybutanetriols produced should be lower than thatof the nitrooxypolyols in the presence of NOx.

oriols and 2-methyltetrol formation from photooxidation of isoprene in the

K. Sato / Atmospheric Environment 42 (2008) 6851–6861 6859

Nitrooxypolyols (Fig. 5, products A and B) produced bythe gas-phase reaction are absorbed on the existing parti-cles following the establishment of a gas–particle equilib-rium. Since nitrooxypolyols include hydroxyl groups,these compounds may proceed to heterogeneous reac-tions with carbonyl compounds to form oligomericcompounds. However, these secondary processes thatoccur in the particle phase remain unclear because exper-imental evidence for the structures of oligomers was notobtained. In this study, the reactions of SOA products inthe aqueous phase were also monitored. Organic nitrates(RONO2), including nitrooxypolyols, hydrolyze to formalcohols and nitric acid in the aqueous phase (Boschanet al., 1955).

RONO2 þ H2O / ROH þ HNO3 (R4)

Methyldinitrooxybutanediols (Fig. 5, product A) hydro-lyze to form methylnitrooxybutanetriols (Fig. 5, product B)and nitric acid, and methylnitrooxybutanetriols proceedtoward further hydrolysis to form 2-methyltetrols (Fig. 5,product C) and nitric acid. Since the chamber experimentsin this study were conducted under considerably dry condi-tions, the abovementioned hydrolysis is unlikely to occur inthe aerosols formed in the chamber. The nitrooxypolyolcompounds should decompose in the aqueous solutionafter the extraction. As discussed above, nitrooxypolyols,which are produced by the gas-phase oxidation of dienesin the presence of NOx, contribute toward the SOA forma-tion, and these compounds can decompose to polyols andnitric acid in an aqueous solution. To our knowledge, thismechanism shown in Fig. 5 is fundamentally new.

4.2. Interpretation of previous chamber experiment results

Surratt et al. (2006) examined the photooxidation ofisoprene in the presence of w1 ppmv NOx. They analysedthe resultant SOA by using the derivatization–GC-MS tech-nique and reported that 2-methyltetrols were not detectedunder the above NOx conditions. This is probably due tohigh NO levels in their experiments. In this previous study,the formation of tetrols may be completely suppressed interms of the removal of RO2 radicals by the RO2þNO reac-tion. Small amounts of 2-methyltetrols (comprising 0.5–2%of aerosol mass) were detected in the present SOA samplesimmediately after the extraction. However, these 2-methyltetrols were detected together with nitric acid,which is the counterpart product of the hydrolysis of nitro-oxypolyols. Since the saturated vapor pressure of nitric acidis considerably high (60 Torr) at 298 K (Lide, 2001), onlya negligible amount of this compound can be absorbed tothe SOA. Nitric acid present in the aqueous solution of theSOA should be produced by the decomposition of low-vola-tility products present in the SOA. These results suggestthat the small amounts of 2-methyltetrols found in theaqueous solution samples in this study were mainlyproduced by the hydrolysis of nitrooxypolyols at roomtemperature during pretreatments. The previous resultsobtained by Surratt et al. (2006) for an initial NOx concen-tration of w1 ppmv are consistent with the present resultsobtained immediately after the extraction. The present

experimental results and the proposed gas-phase reactionmechanism suggest that the direct formation of 2-methyl-tetrol by gas-phase oxidation is a minor reaction pathwayin the presence of NOx.

Angove et al. (2006) examined the photooxidation of1,3-butadiene in the presence of NOx, and they analysedthe aqueous extract of the filter samples of the SOA by usingan NMR method. They showed the presence of oligomericcompounds in the samples immediately after extraction,whereas tetrols were found in the samples three days afterthe extraction. Their experimental results are basicallyconsistent with the present results for 1,3-butadiene.Angove et al. (2006) concluded that the reversely decom-posable oligomeric compounds are produced by theheterogeneous reactions of tetrol monomers that occurdue to the gas-phase oxidation of isoprene. However, thepresent results suggest that nitrooxypolyols are the majorproducts obtained through the 1,3-butadiene oxidationunder high-NOx conditions, and the direct formation ofpolyols is suppressed. Angove et al. (2006) studied thefunctional groups contained in the polymers by using FT-IR analysis of the SOA samples; however, they did notprovide any direct evidence for the formation of the poly-mers from tetrol monomers. Although their mechanismcannot be completely excluded, we concluded that the pol-yols found in their study are mainly produced by thedecomposition of nitrooxypolyols in the aqueous solutionof SOA. As demonstrated in this study, polyols can beproduced through the hydrolysis of nitrooxypolyols duringthe off-line analysis of SOA produced by laboratory dieneoxidation experiments. In order to minimize the effect ofthis hydrolysis on the analytical results, the SOA sampleshould be analysed immediately after the extraction or itshould be stored under sufficiently low temperatures untilthe analysis.

2-Methyltetrols have been found in ambient aerosolsamples collected in urban and suburban areas (Clementsand Seinfeld, 2007; Edney et al., 2005). On the basis of thisstudy, the direct gas-phase formation of 2-methyltetrols issuppressed in urban air. Edney et al. (2005) reported thatthe yields for 2-methyltetrols from the isoprene oxidationin the presence of NOx increased with the addition of SO2

to the reaction system. In the urban atmosphere, the acid-catalysed heterogeneous reactions of isoprene oxidationproducts (Fig. 1, Schemes 2 and 3) may play major rolesin the formation of 2-methyltetrols. If the nitrooxypolyolsproduced by the isoprene oxidation contribute toward theambient SOA formation, these nitrooxypolyols maydecompose through cloud processing in a humid atmo-sphere and/or hydrolysis proceeding on the surface offilters during aerosol filter sampling. However, it remainsunclear whether or not the nitrooxypolyols produced bythe isoprene photooxidation contribute toward the atmo-spheric SOA formation, because the initial reactantconcentrations in this laboratory study are considerablyhigher than the ambient levels. In order to discusswhether or not nitrooxypolyols contribute toward theambient SOA formation, laboratory experiments that aremore realistic and achieved by the developments of bothexperimental and analytical techniques would benecessary.

K. Sato / Atmospheric Environment 42 (2008) 6851–68616860

5. Conclusions

In this study, the photooxidation of four dienes in thepresence of 0.2–1 ppmv of NO was examined through labo-ratory experiments. Polyols, nitrooxypolyols, organic acids,and nitric acid present in the aqueous solutions of the SOAproduced by the photooxidation reactions were success-fully analysed by using the IEC-LC/MS method. In the caseof isoprene, nitric acid, 2-methylglyceric acid, 2-methyl-tetrols and nitrooxypolyols were identified as the majorproducts in the SOA samples. The concentrations of thenitrooxypolyol products decreased three days after theextraction, whereas the concentrations of nitric acid and2-methyltetrols increased. Similar results were obtainedfrom the experiments on all the four dienes. These resultssuggest that the nitrooxypolyols produced by the dieneoxidation in the presence of NOx contribute toward theSOA formation and that they can be decomposed into poly-ols and nitric acid in the aqueous solution. It was concludedthat the direct formation of 2-methyltetrol by gas-phasediene oxidation is suppressed in the presence of NOx.

Acknowledgement

This study was supported by a Grant-in-Aid for YoungScientists from the Ministry of Education, Culture, Sports,Science and Technology of Japan (No. 187180016, FY:2006–2007). The Author thanks Dr. Takashi Imamura ofthis institute for his encouragements.

References

Akimoto, H., Hoshino, M., Inoue, G., Sakamaki, F., Washida, N., Okuda, M.,1979. Design and characterization of the evacuable and bankablephotochemical smog chamber. Environmental Science & Technology13, 471–475.

Angove, D.E., Fookes, C.J.R., Hynes, R.G., Walters, C.K., Azzi, M., 2006. Thecharacterization of secondary organic aerosol formed during thephotodecomposition of 1,3-butadiene in air containing nitric oxide.Atmospheric Environment 40, 4597–4607.

Atkinson, R., Winer, A.M., Pitts Jr., J.N., 1982. Rate constants for the gas-phase reactions of O3 with the natural hydrocarbons isoprene anda-pinene and b-pinene. Atmospheric Environment 16, 1017–1020.

Atkinson, R., 1986. Kinetics and mechanisms of the gas-phase reactions ofthe hydroxyl radical with organic-compounds under atmosphericconditions. Chemical Reviews 86, 69–201.

Boge, O., Miao, Y., Plewka, A., Herrmann, H., 2006. Formation of secondaryorganic particle phase compounds from isoprene gas-phase oxidationproducts: an aerosol chamber and field study. Atmospheric Environ-ment 40, 2501–2509.

Boschan, R., Merrow, R.T., Van Dolah, R.W., 1955. The chemistry of nitrateesters. Chemical Reviews 55, 485–510.

Carlton, A.G., Turpin, B.J., Lim, H.J., Altieri, K.E., Seitzinger, S., 2006. Linkbetween isoprene and secondary organic aerosol (SOA): pyruvicacid oxidation yields low volatility organic acids in clouds. Geophys-ical Research Letters 33 (6), L06822, doi:10.1029/2005GL025374.

Chen, X.H., Hulbert, D., Shepson, P.B., 1998. Measurement of the organicnitrate yield from OH reaction with isoprene. Journal of GeophysicalResearch – Atmospheres 103, 25563–25568.

Claeys, M., Graham, B., Vas, G., Wang, W., Vermeylen, R., Pashynska, V.,Cafmeyer, J., Guyon, P., Andreae, M.O., Artaxo, P., Maenhaut, W.,2004a. Formation of secondary organic aerosols through photooxida-tion of isoprene. Science 303, 1173–1176.

Claeys, M., Wang, W., Ion, A.C., Kourtchev, I., Gelencser, A., Maenhaut, W.,2004b. Formation of secondary organic aerosols from isoprene and itsgas-phase oxidation products through reaction with hydrogenperoxide. Atmospheric Environment 38, 4093–4098.

Clements, A.L., Seinfeld, J.H., 2007. Detection and quantification of2-methyltetrols in ambient aerosol in the southeastern United States.Atmospheric Environment 41, 1825–1830.

Dommen, J., Metzger, A., Duplissy, J., Kalberer, M., Alfarra, M.R., Gascho, A.,Weingartner, E., Prevot, A.S.H., Verheggen, B., Baltensperger, U., 2006.Laboratory observation of oligomers in the aerosol from isoprene/NOxphotooxidation. Geophysical Research Letters 33 (13), L13805, doi:10.1029/2006GL026523.

Edney, E.O., Kleindienst, T.E., Jaoui, M., Lewandowski, M., Offenberg, J.H.,Wang, W., Claeys, M., 2005. Formation of 2-methyl tetrols and2-methylglyceric acid in secondary organic aerosol from laboratoryirradiated isoprene/NOx/SO2/air mixtures and their detection inambient PM2.5 samples collected in the eastern United States. Atmo-spheric Environment 39, 5281–5289.

Grosjean, D., Seinfeld, J.H., 1989. Parameterization of the formation poten-tial of secondary organic aerosols. Atmospheric Environment 23,1733–1747.

Ion, A.C., Vermeylen, R., Kourtchev, I., Cafmeyer, J., Chi, X., Gelencser, A.,Maenhaut, W., Claeys, M., 2005. Polar organic compounds in ruralPM2.5 aerosols from K-puszta, Hungary, during a 2003 summer fieldcampaign: sources and diel variations. Atmospheric Chemistry andPhysics 5, 1805–1814.

Jemma, C.A., Shore, P.R., Widdicombe, K.A., 1995. Analysis of C1–C16hydrocarbons using dual-column capillary GC-application to exhaustemissions from passenger car and motorcycle engines. Journal ofChromatographic Science 33, 34–48.

Jenkin, M.E., Sorensen, M., Hurley, M.D., Wallington, T.J., 2003. Kineticsand product study of the OH- and NO2-initiated oxidation of2,4-hexadiene in the gas phase. Journal of Physical Chemistry A 107,5743–5754.

Kalberer, M., Sax, M., Samburova, V., 2006. Molecular size evolution ofoligomers in organic aerosols collected in urban atmospheres andgenerated in a smog chamber. Environmental Science & Technology40, 5917–5922.

Kleindienst, T.E., Lewandowski, M., Offenberg, J.H., Jaoui, M., Edney, E.O.,2007. Ozone–isoprene reaction: re-examination of the formation ofsecondary organic aerosol. Geophysical Research Letters 34 (1),L0185, doi:10.1029/2006GL027485.

Kourtchev, I., Ruuskanen, T., Maenhaut, W., Kulmala, M., Claeys, M., 2005.Observation of 2-methyltetrols and related photooxidation productsof isoprene in boreal forest aerosols from Hyytiala, Finland.Atmospheric Chemistry and Physics 5, 2761–2770.

Kroll, J.H., Ng, N.L., Murphy, S.M., Flagan, R.C., Seinfeld, J.H., 2005.Secondary organic aerosol formation from isoprene photooxidationunder high-NOx conditions. Geophysical Research Letters 32 (18),L18808, doi:10.1029/2005GL023637.

Lide, D.R., 2001. CRC Handbook of Chemistry and Physics, 82nd ed. CRCPress, Boca Raton.

Miyoshi, A., Hatakeyama, S., Washida, N., 1994. OH radical-initiatedphotooxidation of isoprene – an estimate of global CO produc-tion. Journal of Geophysical Research – Atmospheres 99,18779–18787.

Ng, N.L., Kwan, A.J., Surratt, J.D., Chan, A.W.H., Chhabra, P.S., Sorooshian, A., Pye, H.T.O., Crounse, J.D., Wennberg, P.O., Flagan, R.C., Seinfeld, J.H.,2008. Secondary organic aerosol (SOA) formation from reaction ofisoprene with nitrate radicals (NO3). Atmospheric Chemistry andPhysics Discussions 8, 3163–3226.

Pandis, S.N., Paulson, S.E., Seinfeld, J.H., Flagan, C., 1991. Aerosol formationin the photooxidation of isoprene and b-pinene. AtmosphericEnvironment 25A, 997–1008.

Sato, K., Klotz, B., Taketsugu, T., Takayanagi, T., 2004a. Kinetic measure-ments for the reactions of ozone with crotonaldehyde and its methylderivatives and calculations of transition-state theory. PhysicalChemistry Chemical Physics 6, 3969–3976.

Sato, K., Klotz, B., Hatakeyama, S., Imamura, T., Washizu, Y., Matsumi, Y.,Washida, N., 2004b. Secondary organic aerosol formation duringthe photooxidation of toluene: dependence on initial hydrocarbonconcentration. Bulletin of the Chemical Society of Japan 77, 667–671.

Sato, K., Hatakeyama, S., Imamura, T., 2007. Secondary organic aerosolformation during the photooxidation of toluene: NOx dependenceof chemical composition. Journal of Chemical Physics A 111,9796–9808.

Schkolnik, G., Falkovich, A.H., Rudich, Y., Maenhaut, W., Artaxo, P., 2005.New analytical method for the determination of levoglucosan, poly-hydroxy compounds, and 2-methylerythritol and its application tosmoke and rainwater samples. Environmental Science & Technology39, 2744–2752.

Singh, H.B., 1995. Composition, Chemistry, and Climate of theAtmosphere. Van Nostrand Reinhold, New York.

K. Sato / Atmospheric Environment 42 (2008) 6851–6861 6861

Surratt, J.D., Murphy, S.M., Kroll, J.H., Ng, N.L., Hildebrandt, L.,Sorooshian, A., Szmigielski, R., Vermeylen, R., Maenhaut, W.,Claeys, M., Flagan, R.C., Seinfeld, J.H., 2006. Chemical composition ofsecondary organic aerosol formed from the photooxidation ofisoprene. Journal of Physical Chemistry A 110, 9665–9690.

Szmigielski, R., Surratt, J.D., Vermeylen, R., Szmigielska, K., Kroll, J.H.,Ng, N.L., Murphy, S.M., Sorooshian, A., Seinfeld, J.H., Claeys, M.,2007. Characterization of 2-methylglyceric acid oligomers insecondary organic aerosol formed from the photooxidation of

isoprene using trimethylsilylation and gas chromatography/iontrap mass spectrometry. Journal of Mass Spectrometry 42,101–116.

Tuazon, E.C., Atkinson, R., 1990. A product study of the gas-phase reactionof isoprene with the OH radical in the presence of NOx. InternationalJournal of Chemical Kinetics 22, 1221–1236.

Xia, X., Hopke, P.K., 2006. Seasonal variation of 2-methyltetrols inambient air samples. Environmental Science & Technology 40,6934–6937.