March 9, 2020
2:00 – 3:30pm
Rodin Auditorium, Green Hall
Rachel O’Brien, assistant professor
environmental analytical chemistry
Department of chemistry
william & mary university
HOST: DR. MARTIN
Photolysis of organic aerosol: mass loss and chemical transormations
Abstract: Secondary organic aerosol can be removed from the atmosphere via chemical loss processes or physical ones, like rainout. Large uncertainties remain in our parameterization of these losses, hindering our ability to accurately model atmospheric SOA concentrations. Recent work has demonstrated that photolytic aging may be a significant mechanism for SOA mass loss. However, in chamber experiments with a-pinene SOA we find that mass loss stops and that more than 50% of the mass remains at longer time periods. We term this material “photo-recalcitrant SOA”. In-depth characterization of photo-bleaching on filters shows loss of carbonyl chromophores and an increase in carboxylic acid functional groups. Concurrent oligomerization reactions are also observed, with the formation of new accretion products (likely esters and anhydrides) after approximately 4 days of aging. When this photo-recalcitrant SOA material is dissolved in water, it does not contribute to the formation of OH radicals via secondary photo-chemistry. Altogether, our work shows that fresh SOA generated in the laboratory has very different chemical reactivity compared to photo-bleached SOA. Given that this photo-bleaching takes place over ~1-2 days in the atmosphere, photo-recalcitrant SOA may be a significant fraction of ambient SOA.
Bio: Rachel O’Brien, Assistant Professor of Chemistry at William & Mary, earned her Ph.D. from UC Berkeley studying oligomeric material in ambient SOA. She worked as a postdoc at LBNL imaging aerosol particles with soft x-rays and at MIT where she characterized aerosol aging with an AMS. At William & Mary, Rachel’s research focuses on characterizing organic aerosol particles and organic films on indoor surfaces by studying the chemical composition and the products formed during atmospheric aging.