Longer term aging of organic aerosol: photobleaching and chemical transformations

Rachel O'Brien,
College of William and Mary

"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."