Combined Smog Chamber/Oxidation Flow Reactor Study on Aging of Secondary Organic Aerosol from Photooxidation of Aromatic Hydrocarbons

Secondary organic aerosol (SOA) is a significant component of atmospheric fine particulate matter (PM2.5), and their physicochemical properties can be significantly changed in the aging process. In this study, we used a combination consisting of a smog chamber (SC) and oxidation flow reactor (OFR) to investigate the continuous aging process of gas-phase organic intermediates and SOA formed from the photooxidation of toluene, a typical aromatic hydrocarbon. Our results showed that as the OH exposure increased from 2.6 x 10(11) to 6.3 x 10(11) molecules cm(-3) s (equivalent aging time of 2.01-4.85 days), the SOA mass concentration (2.9 +/- 0.05-28.7 +/- 0.6 mu g cm(-3)) and corrected SOA yield (0.073-0.26) were significantly enhanced. As the aging process proceeds, organic acids and multiple oxygencontaining oxidation products are continuously produced from the photochemical aging process of gas-phase organic intermediates (mainly semi-volatile and intermediate volatility species, S/IVOCs). The multigeneration oxidation products then partition to the aerosol phase, while functionalization of SOA rather than fragmentation dominated in the photochemical aging process, resulting in much higher SOA yield after aging compared to that in the SC. Our study indicates that SOA yields as a function of OH exposure should be considered in air quality models to improve SOA simulation, and thus accurately assess the impact on SOA properties and regional air quality.

Automated summary

Secondary organic aerosol (SOA) is an important component of atmospheric fine particulate matter (PM2.5), and its physicochemical properties change significantly during the aging process. By studying the continuous aging process of gas-phase organic intermediates and SOA formed from the photooxidation of toluene, it was found that the SOA mass concentration and corrected SOA yield were significantly enhanced as the OH exposure increased. Organic acids and multiple oxygen-containing oxidation products were continuously produced during the aging process, and functionalization of SOA dominated in the photochemical aging process. This study suggests that SOA yields as a function of OH exposure should be considered in air quality models to accurately assess their impact on SOA properties and regional air quality.