Determining Activity Coefficients of SOA from Isothermal Evaporation in a Laboratory Chamber

Nonideal molecular interactions in aerosol particles influence the partitioning of semivolatile organic compounds (SVOCs). However, few direct measurements exist that determine activity coefficients (gamma), which quantify nonideality, for individual organic compounds in different atmospheric mixtures. By measuring the isothermal evaporation of SOA formed from multifunctional SVOCs in an environmental chamber, this study determines the mole-fraction-based gamma of the bulk SOA in a variety of pre-existing chemically complex particles, approximated as if the bulk SOA behaved like a single SVOC. These multicomponent mixtures contain molecular structures commonly found in the atmosphere. When treated as a single liquid phase, gamma of the bulk SOA tends to increase from similar to 1 to similar to 5 as the organic seeds and the SOA have more differing polarities. A high computed gamma value of 74 for a wet ammonium sulfate-SOA system indicates phase separation. The gamma for some individual species was also quantified based on gas-phase measurements and AIOMFAC model predictions. However, the bulk SOA gamma cannot be explained by the simplified speciated SVOC-seed interactions, suggesting incomplete compositional understanding of the bulk SOA. These results demonstrate a method to quantify nonideal behavior and show that it can occur in multicomponent mixtures and therefore influence OA formation, evolution, and lifetimes.

Automated summary

Nonideal molecular interactions in aerosol particles affect the distribution of semivolatile organic compounds (SVOCs). This study quantifies the activity coefficients (gamma) of individual organic compounds in different atmospheric mixtures by measuring the isothermal evaporation of secondary organic aerosols (SOA) formed in an environmental chamber. The results show that the gamma of bulk SOA in complex atmospheric mixtures can be influenced by varying polarities of organic seeds and SOA.