Abstract
Abstract. Secondary organic aerosol (SOA) is transformed after its initial formation, but this chemical aging of SOA is poorly understood. Experiments were conducted in the Carnegie Mellon environmental chamber to form secondary organic aerosol (SOA) from the photo-oxidation of toluene and other small aromatic volatile organic compounds (VOCs) in the presence of NOx under different oxidizing conditions. The effects of the oxidizing condition on organic aerosol (OA) composition, mass yield, volatility, and hygroscopicity were explored. Higher exposure to the hydroxyl radical resulted in different OA composition, average carbon oxidation state (OSc), and mass yield. The OA oxidation state generally increased during photo-oxidation, and the final OA OSc ranged from −0.29 to 0.16 in the performed experiments. The volatility of OA formed in these different experiments varied by as much as a factor of 30, demonstrating that the OA formed under different oxidizing conditions can have a significantly different saturation concentration. There was no clear correlation between hygroscopicity and oxidation state for this relatively hygroscopic SOA.
Highlights
Secondary organic aerosol (SOA) is produced when gasphase precursors are oxidized, forming lower volatility products that partition to the condensed phase
We investigated the relationship between oxidation, volatility and hygroscopicity of SOA formed from the photooxidation of toluene and other small aromatic volatile organic compounds (VOCs) under a variety of oxidation conditions
The TD was held at the same temperature during the photo-oxidation periods to observe changes in volatility during this period, and the temperature in the TD was varied during the dark period to obtain a thermogram
Summary
Secondary organic aerosol (SOA) is produced when gasphase precursors are oxidized, forming lower volatility products that partition to the condensed phase. Without the correct representation of SOA production and evolution mechanisms, modeling attempts often lead to underestimations of ambient mass loadings (Heald et al, 2005; Volkamer et al, 2006). The large uncertainty in SOA concentrations predicted by chemical transport models (CTMs) demonstrates the need for experimental data on the multi-generation oxidation reactions or “aging” that lead to changes in mass loadings and physicochemical properties of SOA. Several computational studies have highlighted the importance of incorporating extended chemical mechanisms but obtaining corresponding relationships between chemical aging and physiochemical properties of the SOA, such as its volatility, in atmospherically relevant systems (Cappa and Wilson, 2012; Shrivastava et al, 2013)
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