Photolysis of biomass burning organic aerosol, 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 solar spectrum. Large uncertainties remain in our parameterization of this warming, in part due to a lack of knowledge about atmospheric lifetimes for the chromophores (the light absorbing structures in BrC molecules). An important removal pathway includes chemical transformations that fragment the chromophore, thus removing its ability to absorb visible light. However, the photochemical loss rates measured in the laboratory do not generally match what is observed in ambient measurements. There are also different amounts of photo-resistant BrC, which is a fraction of the mixture that does not rapidly bleach. An important BrC source in the atmosphere is biomass burning and the overall photochemical decay rates for these emissions are important to quantify to improve our parameterizations of their radiative effects. Here we show results for laboratory studies of FIREX filter samples probing the role of water vapor in photolysis of aerosol particles irradiated on a filter. Kinetic analysis of photo-bleaching in aqueous solutions demonstrates that an intermediate photolysis rate should be included to improve predictions for BrC lifetimes in the atmosphere.