ChBE Seminar Series: Photolysis of atmospheric brown carbon
Speaker: Rachel O'Brien, Assistant Professor, Dept of Chemistry @ William & Mary, Williamsburg, Va.
Title: Photolysis of atmospheric brown carbon: chemical transformations and photo bleaching
Brown carbon (BrC) in aerosol particles and cloud droplets can contribute to climate warming by absorbing solar radiation in the visible region of the spectrum. Large uncertainties remain in our parameterization of this warming, due in part to a lack of knowledge about sources and about atmospheric lifetimes for the chromophores (light absorbing structures in the BrC molecules). One important removal pathway is photolysis, where absorption of solar radiation leads to the fragmentation or removal of the chromophore. The photolysis rates for different types of BrC chromophores can vary by orders of magnitude and the impact of the molecule’s environment (i.e., dry particles, gas-phase, aqueous cloud droplet, etc.) is not well understood. Large differences in the photo bleaching behavior of different systems between the aqueous phase and dry particles are observed. Some of these differences are due to condensed phase radical driven processes and some are likely due to the presence of secondary photochemistry occurring in aqueous solutions. There are different types of photo-recalcitrant organic material, some of which can retain a brown color, even over days of direct photolysis. Altogether, this work shows that a better parameterization of the photo bleaching rates for atmospheric brown carbon will be obtained as our understanding of the lifetimes of the different types of chromophores across environmentally relevant conditions is improved.
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.