Effects of Photolysis on the Transformation of Brown Carbon in Atmospheric Cloud Water

Abstract

The effect of organic species in atmospheric aerosols with a wavelength absorbance dependence, referred to as brown carbon (BrC), on climate change has been less thoroughly studied than those of other contributors, resulting in BrC being a disproportionately large source of climate model uncertainty.1 Multiple factors cause this, including BrC’s relatively high reactivity and its transformation through aqueous reactions in cloud water droplets cycling through expansion and evaporation. BrC absorbance is also sensitive to aerosol pH, which varies with composition, temperature and humidity. In this work, an experimental procedure was developed wherein samples are exposed to UV radiation and then analyzed with spectrophotometry and fluorometry. This will enable characterization of the transformation of brown carbon in cloud water exposed to solar radiation. Also included are pH cycling steps that will enable measurements of sample absorbance spectra at different atmospherically relevant pH’s. The described procedure will be used to analyze bulk cloud water samples obtained from the summit of Whiteface Mountain, NY. Results recorded with a BrC containing trial material, Suwannee River Natural Organic Matter, are discussed. This work also uses Whiteface Mountain sample composition data in combination with historical air mass trajectories and meteorological data to estimate the absorbance of BrC containing air masses prior to sample collection. Data from fire hazard maps were also gathered for future model refinement. Overall, the work of this thesis project is part of a long-term effort to increase understanding of how BrC changes in the atmosphere and reduce uncertainty in estimates of global radiative forcing.