Laboratory Insights into the Diel Cycle of Optical and Chemical Transformations of Biomass Burning Brown Carbon Aerosols

Transformations of biomass burning brown carbon aerosols (BB-BrC) over their diurnal lifecycle are currently not well studied. In this study, the aging of BB tar proxy aerosols processed by NO 3 • under dark conditions followed by the photochemical OH • reaction and photolysis were investigated in tandem flow reactors. The results show that O 3 oxidation in the dark diminishes light absorption of wood tar aerosols, resulting in higher particle single-scattering albedo (SSA). NO 3 • reactions augment the mass absorption coefficient (MAC) of the aerosols by a factor of 2-3 by forming secondary chromophores, such as nitroaromatic compounds (NACs) and organonitrates. Subsequent OH • oxidation and direct photolysis both decompose the organic nitrates (ONs, representing bulk functionalities of NACs and organonitrates) in the NO 3 • -aged wood tar aerosols, thus decreasing particle absorption. Moreover, NACs degrade faster than organonitrates by photochemical aging. The NO 3 • -aged wood tar aerosols are more susceptible to photolysis than to OH • reactions. The photolysis lifetimes for the ONs and for the absorbance of the NO 3 • -aged aerosols are on the order of hours under typical solar irradiation, while the absorption and ON lifetimes toward OH • oxidation are substantially longer. Overall, nighttime aging via NO 3 • reactions increases the light absorption of wood tar aerosols and shortens their absorption lifetime under daytime conditions.