Organosulfates of isoprene, alpha-pinene, and beta-pinene have recently been identified in both laboratory-generated and ambient secondary organic aerosol (SOA). In this study, the mechanism and ubiquity of organosulfate formation in biogenic SOA is investigated by a comprehensive series of laboratory photooxidation (i.e., OH-initiated oxidation) and nighttime oxidation (i.e., NO3-initiated oxidation under dark conditions) experiments using nine monoterpenes (alpha-pinene, beta-pinene, d-limonene, l-limonene, alpha-terpinene, gamma-terpinene, terpinolene, Delta(3)-carene, and beta-phellandrene) and three monoterpenes (alpha-pinene, d-limonene, and l-limonene), respectively. Organosulfates were characterized using liquid chromatographic techniques coupled to electrospray ionization combined with both linear ion trap and high-resolution time-of-flight mass spectrometry. Organosulfates are formed only when monoterpenes are oxidized in the presence of acidified sulfate seed aerosol, a result consistent with prior work. Archived laboratory-generated isoprene SOA and ambient filter samples collected from the southeastern U.S. were reexamined for organosulfates. By comparing the tandem mass spectrometric and accurate mass measurements collected for both the laboratory-generated and ambient aerosol, previously uncharacterized ambient organic aerosol components are found to be organosulfates of isoprene, alpha-pinene, beta-pinene, and limonene-like monoterpenes (e.g., myrcene), demonstrating the ubiquity of organosulfate formation in ambient SOA. Several of the organosulfates of isoprene and of the monoterpenes characterized in this study are ambient tracer compounds for the occurrence of biogenic SOA formation under acidic conditions. Furthermore, the nighttime oxidation experiments conducted under highly acidic conditions reveal a viable mechanism for the formation of previously identified nitrooxy organosulfates found in ambient nighttime aerosol samples. We estimate that the organosulfate contribution to the total organic mass fraction of ambient aerosol collected from K-puszta, Hungary, a field site with a similar organosulfate composition as that found in the present study for the southeastern U.S., can be as high as 30%.

Detailed organic analysis of natural aerosols from the Amazonian rain forest showed considerable quantities of previously unobserved polar organic compounds, which were identified as a mixture of two diastereoisomeric 2-methyltetrols: 2-methylthreitol and 2-methylerythritol. These polyols, which have the isoprene skeleton, can be explained by OH radical-initiated photooxidation of isoprene. They have low vapor pressure, allowing them to condense onto preexisting particles. It is estimated that photooxidation of isoprene results in an annual global production of about 2 teragrams of the polyols, a substantial fraction of the Intergovernmental Panel on Climate Change estimate of between 8 and 40 teragrams per year of secondary organic aerosol from biogenic sources.

Recent work in our laboratory has shown that the photooxidation of isoprene (2-methyl-1,3-butadiene, C 5 H 8 ) leads to the formation of secondary organic aerosol (SOA). In the current study, the chemical composition of SOA from the photooxidation of isoprene over the full range of NO x conditions is investigated through a series of controlled laboratory chamber experiments. SOA composition is studied using a wide range of experimental techniques: electrospray ionization-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry, high-resolution mass spectrometry, online aerosol mass spectrometry, gas chromatography/mass spectrometry, and an iodometric-spectroscopic method. Oligomerization was observed to be an important SOA formation pathway in all cases; however, the nature of the oligomers depends strongly on the NO x level, with acidic products formed under high-NO x conditions only. We present, to our knowledge, the first evidence of particle-phase esterification reactions in SOA, where the further oxidation of the isoprene oxidation product methacrolein under high-NO x conditions produces polyesters involving 2-methylglyceric acid as a key monomeric unit. These oligomers comprise ∼22-34% of the high-NO x SOA mass. Under low-NO x conditions, organic peroxides contribute significantly to the low-NO x SOA mass (∼61% when SOA forms by nucleation and ∼25-30% in the presence of seed particles). The contribution of organic peroxides in the SOA decreases with time, indicating photochemical aging. Hemiacetal dimers are found to form from C 5 alkene triols and 2-methyltetrols under low-NO x conditions; these compounds are also found in aerosol collected from the Amazonian rainforest, demonstrating the atmospheric relevance of these low-NO x chamber experiments.

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Highly oxygenated compounds assigned to be oxidation products of α‐pinene have recently been observed in substantial concentrations in ambient aerosols. Here, we confirm the unknown α‐pinene tracer compound with molecular weight (MW) 204 as the C8‐tricarboxylic acid 3‐methyl‐1,2,3‐butanetricarboxylic acid. Its gas and liquid chromatographic behaviors and its mass spectral characteristics in electron ionization and negative ion electrospray ionization perfectly agree with those of a synthesized reference compound. The formation of this compound is explained by further reaction of cis‐pinonic acid involving participation of the OH radical. This study illustrates that complex, multi‐generation chemistry holds for the photooxidation of α‐pinene in the presence of NOx.

In the present study, we have characterized in detail the MS(2) and MS(3) fragmentation behaviors, using electrospray ionization (ESI) in the negative ion mode, of previously identified sulfated isoprene secondary organic aerosol compounds, including 2-methyltetrols, 2-methylglyceric acid, 2-methyltetrol mononitrate derivatives, glyoxal and methylglyoxal. A major fragmentation pathway for the deprotonated molecules of the sulfate esters of 2-methyltetrols and 2-methylglyceric acid and of the sulfate derivatives of glyoxal and methylglyoxal is the formation of the bisulfate [HSO(4)](-) anion, while the deprotonated sulfate esters of 2-methyltetrol mononitrate derivatives preferentially fragment through loss of nitric acid. Rational interpretation of MS(2), MS(3) and accurate mass data led to the structural characterization of unknown polar compounds in K-puszta fine aerosol as organosulfate derivatives of photooxidation products of unsaturated fatty acids, i.e. 2-hydroxy-1,4-butanedialdehyde, 4,5- and 2,3-dihydroxypentanoic acids, and 2-hydroxyglutaric acid, and of alpha-pinene, i.e. 3-hydroxyglutaric acid. The deprotonated molecules of the sulfated hydroxyacids, 2-methylglyceric acid, 4,5- and 2,3-dihydroxypentanoic acid, and 2- and 3-hydroxyglutaric acids, showed in addition to the [HSO(4)](-) ion (m/z 97) neutral losses of water, CO(2) and/or SO(3), features that are characteristic of humic-like substances. The polar organosulfates characterized in the present work are of climatic relevance because they may contribute to the hydrophilic properties of fine ambient aerosol. In addition, these compounds probably serve as ambient tracer compounds for the occurrence of secondary organic aerosol formation under acidic conditions.

SummaryThere are 9 pages in this Supporting Information, including 1 table, 4 figures, 5 schemes, and 10 references. S1. Aerosol samplesThe α-pinene SOA samples used for the time course analysis (Fig. S1) were obtained from an ozonolysis experiment carried out at the IfT in the 9 m 3 Teflon smog chamber (S1). Briefly, α-pinene ozonolysis was performed in the presence of acidic seed particles (0.03 M (NH 4 ) 2 SO 4 /H 2 SO 4 ). No OH scavenger was used in this experiment. The relative humidity and temperature of the chamber were around 50% and 23 ºC. The initial concentrations of α-pinene, ozone, and seed particles were 100 ppb, 60 ppb, and 20,000 cm -3 , respectively. A more detailed description of the experimental procedure is given in (S1). Samples were collected using a condensation-growth and impaction system (C-GIS). Details about the C-GIS sampling system are reported in a previous study (S2). S2. Preparation of standardsTerpenylic acid was prepared from homoterpenyl methyl ketone as reported by Baeyer (S3) (Scheme S1). 5 g of cis-pinonic acid (Aldrich, cis-3-acetyl-2,2-dimethylcyclobutaneacetic acid) was dissolved in 60 g of H 2 SO 4 (50%) at 100 °C and left for 30 min, resulting in a brown mixture with yellow-green fluorescence. Subsequently, 150 mL of water was added to the mixture, the mixture was further saturated with (NH 4 ) 2 SO 4 and the organic phase was extracted with CHCl 3 . The resulting extract was dried over Na 2 SO 4 , the solvent was evaporated to yield homoterpenyl methyl ketone, and the product was re-crystallized from water. In a subsequent step, homoterpenyl methyl ketone was oxidized as reported by Wallach (S4) (Scheme S1). 3 g of homoterpenyl methyl ketone was dissolved in a 0.93 M KOH solution (60 ml), and 300 mL of 0.158 M KMnO 4 solution was added to the solution within 20 min. After filtration of the brown precipitate, the solution was acidified with 10% H 2 SO 4 and repeatedly extracted with diethyl ether. The extract was dried over Na 2 SO 4 and the solvent was evaporated to yield terpenylic acid.The preparation of diaterpenylic acid acetate is given in the supporting information of Iinuma et al. (S5).

Abstract. In the present study, we have characterized the structure of a higher-molecular weight (MW) 358 α-and β-pinene dimeric secondary organic aerosol (SOA) product that received ample attention in previous molecular characterization studies and has been elusive. Based on mass spectrometric evidence for deprotonated molecules formed by electrospray ionization in the negative ion mode and chemical considerations, it is suggested that diaterpenylic acid is a key monomeric intermediate for dimers of the ester type. It is proposed that cis-pinic acid is esterified with the hydroxyl-containing diaterpenylic acid, which can be explained through acid-catalyzed hydrolysis of the recently elucidated lactone-containing terpenylic acid and/or diaterpenylic acid acetate, both first-generation oxidation products. To a minor extent, higher-MW 358 and 344 diester products are formed containing other terpenoic acids as monomeric units, i.e., diaterpenylic acid instead of cis-pinic acid, and diaterebic acid instead of diaterpenylic acid. It is shown that the MW 358 diester and related MW 344 compounds, which can be regarded as processed SOA products, also occur in ambient fine (PM 2.5 ) rural aerosol collected at night during the warm period of the 2006 summer field campaignCorrespondence to: M. Claeys (magda.claeys@ua.ac.be) conducted at K-puszta, Hungary, a rural site with coniferous vegetation. This indicates that, under ambient conditions, the higher-MW diesters are formed in the particle phase over a longer time-scale than that required for gas-toparticle partitioning of their monomeric precursors in laboratory α-/β-pinene ozonolysis experiments.