Atmospheric brown carbon (BrC) is an important contributor to the radiative forcing of climate by organic aerosols. Because of the molecular complexity of BrC compounds and their dynamic transformations, it is challenging to predictively understand BrC optical properties. Photochemical reactions (photolysis, OH radical and O<sub>3</sub> reactions) tend to diminish light absorption and hence lower the warming effects of biomass burning BrC. Night-time aging and the resulting the optical properties of BrC aerosols are less known. In this talk we will present laboratory studies on night-time NO<sub>3</sub> radical reactions with tar aerosols from wood pyrolysis. The study shows that the optical properties of BrC change because of transformations driven by reactions with the NO3 radical that form new absorbing species and lead to significant absorption enhancement over the ultraviolet−visible (UV-vis) range. The overnight aging increases the mass absorption coefficients of the BrC by a factor of 1.3−3.2 between 380 nm and 650 nm. Night-time aging of BrC aerosols represents an important source of secondary BrC. Nitrated organic compounds, particularly nitroaromatics, were identified as the main products that contribute to the enhanced light absorption in the secondary BrC.

Formation of secondary brown barbon in biomass burning aerosol proxies through NO3 radical reactions

ACS Fall 2021

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