Chemical Composition of Gas and Particle Phase Products of Toluene Photooxidation Reaction under High OH Exposure Condition

Abstract

In the current study, the photooxidation reaction of toluene (C7H8) was investigated in a Potential Aerosol Mass Oxidation Flow Reactor (PAM OFR). The hydroxyl radical (OH) exposure of toluene in the PAM OFR ranged from 0.4 to 1.4 × 1012 molec cm−3 s, which is equivalent to 3 to 12 days of atmospheric oxidation. A proton transfer reaction-mass spectrometer (PTR-MS) and a scanning mobility particle sizer (SMPS) were used to study the gas-phase products formed and particle number changes of the oxidation reaction in PAM OFR. The secondary organic aerosol (SOA) formed in the PAM OFR was also collected for off-line chemical analysis. Key gas-phase reaction products of toluene, including glyoxal, methyl glyoxal, unsaturated carbonyl compounds, and benzaldehyde, were identified by the PTR-MS. Second generation products, including acetic acid, formaldehyde, formic acid, and acetaldehyde, were also detected. By comparing the mass spectrums obtained under different OH exposures and relative humidity (RH), changes in the two parameters have minimal effects on the composition of gas-phase products formed, expect for the spectrum obtained at OH exposure of 0.4 × 1012 cm−3 s and RH = 17%, which is slightly different from other spectrums. SMPS results showed that particle mass concentration increases with increasing OH exposure, while particle number concentration first increases and then decreases with increasing OH exposure. This result probably suggests the formation of oligomers at high OH exposure conditions. Off-line chemical analysis of the SOA sample was dominated by C4 diacids, including malic acid, citramalic acid, and tartaric acid. The well-known toluene SOA marker 2,3-Dihydroxy-4-oxopentanoic acid, as well as 2,3-dihydroxyglutaric acid, which has not been identified in previous toluene photooxidation experiments, were also detected in the SOA sample. Our results showed good agreements with the results of previous smog chamber studies of toluene photooxidation reaction, and they suggested that using PAM OFR for studies of oxidation reaction of different VOCs can be atmospherically relevant.

Nontarget Tandem High-Resolution Mass Spectrometry Analysis of Functionalized Organic Compounds in Atmospherically Relevant Samples

“…The assignment of functional groups in the two SOA samples was also compared with those of the toluene and isoprene SOA compounds with tentative molecular structures proposed by previous papers − obtained under similar NO x conditions. Our results identified at least one functional group correctly for 25 toluene SOA products and 23 isoprene SOA products (see the list in Table S6).…”

Four- and Five-Carbon Dicarboxylic Acids Present in Secondary Organic Aerosol Produced from Anthropogenic and Biogenic Volatile Organic Compounds

“…In the toluene-derived SOA sample, meso-tartaric acid was detected with a signal intensity comparable to that of DL-tartaric acid (Figure 2). Lau et al [18] detected tartaric acid in toluene SOA, but they did not distinguish diastereomers. To the best of our knowledge, this is the first report of the identification of meso-tartaric acid in toluene-derived SOA.…”

“…Methyl tartaric acid is present in both SOA produced in a chamber and in PM 2.5 collected in the United States and Poland [16]. C4-C5 multifunctional organic acids, such as 2,3dihydroxypentanedioic acid (DHPDA) [17,18], malic acid [18], and tartaric acid [18] are known to be present in SOA from toluene. These small dicarboxylic acids may also be produced by different SOA precursors.…”