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McKelvey School of Engineering

Brown School

EECE Seminar - Rachel O'Brien

Monday, January 24, 2022 | 11:30 AM - 12:30 PM

Rachel O'Brien, Assistant Professor
Department of Chemistry
William & Mary 

Photolysis of atmospheric organic aerosol: chemical transformations and photo bleaching

Abstract: Organic aerosol particles can influence the climate through either directly absorbing or scattering solar radiation or by acting as nuclei for cloud droplets. Some aerosol particles are dominantly scattering while others contain organic molecules that can absorb solar radiation in the visible region, termed brown carbon (BrC). We still have large uncertainties in the magnitude of these climate effects and a better understanding of the removal rates for the particle mass and absorption properties (i.e. color) is needed. One removal process is photolysis, where absorption of solar radiation leads to fragmentation of the organic molecules and the loss of particle mass and/or color. However, the photolysis rates and the overall extent of mass that can be removed via direct photolysis in laboratory experiments does not match what is used in models and often differs from ambient measurements. In this talk, I will combine results from work in our lab looking at photolysis of biogenic secondary organic aerosol as well as BrC from biomass burning organic aerosol to evaluate gaps in our ability to predict the observed ambient removal rates. By probing complex mixtures from recent biomass burning experiments (e.g. FIREX samples), I will demonstrate that our current measured rates in the laboratory are overestimated and that a slower photolysis rate, as well as a potential gas-phase oxidation rate, should be used to predict the role of photolysis on organic aerosol lifetime in the atmosphere.   

Bio: Rachel O’Brien is an Assistant Professor of Chemistry at William & Mary and she earned her Ph.D. from UC Berkeley studying oligomeric materials in ambient secondary organic aerosol. She worked as a postdoc at LBNL imaging aerosol particles with soft x-rays as well as at MIT where she characterized aerosol aging with an Aerosol Mass Spectrometer. Rachel’s research group focuses on characterizing organic aerosol particles and organic films on indoor surfaces by studying the chemical composition and the products formed during atmospheric aging. Future research in her lab will also involve field and laboratory experiments looking at the role volatile chemical products pay in OA formation as well as investigations targeting indirect photolysis as a removal mechanism for OA in the atmosphere. 

Event Type



McKelvey School of Engineering


Science & Technology



Energy, Environmental & Chemical Engineering
Event Contact

Kim Coleman: kcole@wustl.edu

Speaker Information

Rachel O'Brien, Assistant Professor
Department of Chemistry
William & Mary 

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