Unveiling the Mixing Behavior of Secondary Organic Aerosols from Isoprene and Trimethyl Benzene

YUZHI CHEN, Rahul Zaveri, Alla Zelenyuk, Claire E. Moffett, Gregory W. Vandergrift, Swarup China, John Shilling, Pacific Northwest National Laboratory

     Abstract Number: 264
     Working Group: Aerosol Chemistry

Abstract
Secondary organic aerosol (SOA) contributes a large fraction of atmospheric aerosol, significantly impacting climate, air quality, and human health. SOA formation is traditionally described by semivolatile partitioning theory, which typically assumes that different SOA types form a well-mixed condensed phase on a short time scale. However, this assumption has been challenged by several recent studies showing that a well-mixed phase is not always formed, even between fresh and aged SOA of the same type. The oxygen-to-carbon ratio (O:C) has been widely used to characterize the oxidation state of SOA and can be linked to various SOA properties, including phase separation. Here, we investigate the ability of O:C to predict whether equilibrium partitioning and ideal mixing are achieved between two different types of SOA. We conduct two types of chamber experiments: (1) co-condensation experiments, where isoprene and isotopically labeled trimethyl benzene (TMB) were oxidized simultaneously to form SOA; and (2) sequential condensation experiments, where isoprene SOA was formed in the presence of pre-existing TMB SOA, and vice versa. We find that in both scenarios, an equilibrium partitioning model significantly overpredicted the SOA mass yield, indicating isoprene SOA and TMB SOA did not form a well-mixed phase within the experiment time scale of 4-5 hours, despite their similar O:C. To further elucidate the mixing state of the binary SOA mixture, we employed miniSPLAT, a single particle spectrometer, for depth-profiling, enabling us to analyze the morphological distribution of TMB and isoprene SOA within aerosol particles. Utilizing isotopically labeled TMB allows us to distinctly differentiate between isoprene SOA and TMB SOA and thus unravel the extent of mixing or phase separation. Complementary offline analysis of the SOA filter samples using nanospray desorption electrospray ionization mass spectrometry (nanoDESI-MS) provides molecular-level insights into the SOA mixture composition.