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.
Highlights
Formation of secondary organic aerosol (SOA) in the atmosphere has become the emerging topic in atmospheric science and chemistry in the recent decades
The Potential Aerosol Mass Oxidation Flow Reactor (PAM oxidation flow reactor (OFR)) used in the current study is a Penn State Potential Aerosol Mass (PAM), which is developed by the Pennsylvania State University
The current study examined the reaction mechanism of the photooxidation and ageing of toluene using the PAM OFR
Summary
Formation of secondary organic aerosol (SOA) in the atmosphere has become the emerging topic in atmospheric science and chemistry in the recent decades. It has been shown in previous studies that SOA is associated with air pollution and haze events [1], which affects the global climate forcing by serving as a cloud condensation nuclei and participating in heterogeneous chemical reactions [2,3]. Understanding the formation mechanism of SOA is crucial to both the study of atmospheric processes and public health. In spite of the number of studies, the understanding of the formation processes of SOA in the atmospheric is still far from complete. Model predictions based on the experimental yield of SOA generally underestimate the mass loading of SOA in ambient measurements [11,12]
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