Abstract
Abstract. Studies of the volatility distribution of secondary organic aerosol (SOA) from aromatic compounds are limited compared with SOA from biogenic monoterpenes. In this study, the volatility distribution was investigated by composition, heating, and dilution measurements for SOA formed from the photooxidation of 1,3,5-trimethylbenzene in the presence of NOx. Composition studies revealed that highly oxygenated monomers (C9H14Ox, x = 4–7) and dimers (C18H26Ox, x = 8–12) are the major products in SOA particles. Highly oxygenated molecules (HOMs) with five or more oxygens were formed during photochemical aging, whereas dimers degraded during photochemical aging. HOMs with five or more oxygens may be produced from the photooxidation of phenol-type gaseous products, whereas dimers in the particle phase may be photolyzed to smaller molecules during photochemical aging. The results of composition, heating, and dilution measurements showed that fresh SOA that formed from 1,3,5-trimethylbenzene (TMB) photooxidation includes low-volatility compounds with <1 µg m−3 saturation concentrations, which are attributed to dimers. Similar results were reported for α-pinene SOA in previous studies. Low-volatility compounds with <1 µg m−3 saturation concentrations are not included in the volatility distributions employed in the standard volatility basis-set (VBS) approach. Improvements in the organic aerosol model will be necessary for the study of anthropogenic SOA as well as biogenic SOA.
Highlights
Secondary organic aerosol (SOA) is a major component of atmospheric fine particles (Zhang et al, 2007), affecting climate (IPCC, 2013) and human health (Dockery et al, 1993; Shiraiwa et al, 2017)
We investigated SOA formation under dry conditions only because the chamber facility was designed for dry use (Akimoto et al, 1979)
The first injection of methyl nitrite was aimed at completing the photooxidation of the remaining TMB, whereas the second injection was aimed at ensuring that the photooxidation of gaseous secondary products is promoted
Summary
Secondary organic aerosol (SOA) is a major component of atmospheric fine particles (Zhang et al, 2007), affecting climate (IPCC, 2013) and human health (Dockery et al, 1993; Shiraiwa et al, 2017). SOA formed from the photooxidation of aromatic hydrocarbons is predicted to have a 33 % contribution to the global SOA production by atmospheric-model calculations (Kelly et al, 2018). Morino et al (2015) reported that a VBS model still underestimates organic aerosol concentrations in urban environments, predictions in remote environments show better agreement with observations. The volatility distribution of SOA, a key property in the prediction of particle levels in VBS models, has been investigated by several experimental techniques. The yield curve analysis for laboratory data assumes gas-phase single-step oxidation of volatile organic compounds (VOCs) during laboratory experiments and gas– particle equilibrium.
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