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 under dark conditions followed by the photochemical OH reaction and photolysis were investigated in tandem flow reactors. The results show that O oxidation in the dark diminishes light absorption of wood tar aerosols, resulting in higher particle single-scattering albedo (SSA). NO 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-aged wood tar aerosols, thus decreasing particle absorption. Moreover, NACs degrade faster than organonitrates by photochemical aging. The NO-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-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 reactions increases the light absorption of wood tar aerosols and shortens their absorption lifetime under daytime conditions.