10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Growth Kinetics and Size Distribution Evolution of Viscous Secondary Organic Aerosol
RAHUL ZAVERI, John Shilling, Alla Zelenyuk, Jiumeng Liu, David Bell, Emma L. D'Ambro, Cassandra Gaston, Joel A. Thornton, Alexander Laskin, Peng Lin, Jacqueline Wilson, Richard Easter, Jian Wang, Allan Bertram, Scot T. Martin, John Seinfeld, Douglas Worsnop, Pacific Northwest National Laboratory
Abstract Number: 1022 Working Group: Aerosol Modeling
Abstract Low bulk diffusivity inside viscous semisolid atmospheric secondary organic aerosol (SOA) can prolong equilibration timescale, but its broader impacts on aerosol growth and size distribution dynamics are poorly understood. We present here quantitative insights into the effects of bulk diffusivity on the growth and evaporation kinetics of SOA formed under dry conditions from photooxidation of isoprene in the presence of a bimodal aerosol consisting of Aitken (ammonium sulfate) and accumulation (isoprene or α-pinene SOA) mode particles. Aerosol composition measurements and evaporation kinetics indicate that isoprene SOA is composed of several semivolatile organic compounds (SVOCs), with some reversibly reacting to form oligomers. Model analysis shows that liquid-like bulk diffusivities can be used to fit the observed evaporation kinetics of accumulation mode particles, but fail to explain the growth kinetics of bimodal aerosol by significantly under-predicting the evolution of the Aitken mode. In contrast, the semisolid scenario successfully reproduces both evaporation and growth kinetics, with the interpretation that hindered partitioning of SVOCs into large viscous particles effectively promotes the growth of smaller particles that have shorter diffusion timescales. This effect has important implications for the growth of atmospheric ultrafine particles to climatically-active sizes.