Concentration-Dependent Photochemical Evolution of Primary Brown Carbon under Irradiation

SITHUMI LIYANAGE, Colton Calvert, Holly Anthony, Elijah Schnitzler, Oklahoma State University

     Abstract Number: 54
     Working Group: Aerosol Chemistry

Abstract
Brown carbon (BrC) components of biomass burning organic aerosol (BBOA) influence climate directly, by absorbing and scattering solar radiation, and indirectly, by enhancing cloud formation. The absorption of solar radiation contributes to warming, but the extent of this effect depends on the optical properties of the aerosol, which can vary throughout its atmospheric residence time. Primary BrC undergoes various atmospheric aging processes through irradiation and oxidation that can alter its chemical and optical properties. Overall, this aging generally leads to the whitening of organic aerosols; however, irradiation can also lead to an initial increase in the visible light absorption of BrC species. Here, we address this variability for light-driven processing by examining the evolution of primary BrC optical properties under irradiation while varying the initial concentration of BrC to modulate between these two scenarios. BrC aerosol was generated from eastern red cedar sapwood, and prepared across a range of concentrations from thin films to diluted solutions. Samples were exposed to irradiation, from above, in a solar simulator. Importantly, all samples appeared the same color from above, using sample thicknesses inversely proportional to mass concentration, such that the number of photons absorbed at time zero was the same for all samples. The resulting changes in light absorption were monitored in terms of UV-Vis absorbance at regular intervals. Results indicate that higher concentrations of BrC favor an initial darkening followed by a gradual whitening, while lower concentrations favor a uniform whitening. Changes in physicochemical properties (i.e., viscosity) are explored in terms of reactive uptake of ozone. These results emphasize the significance of concentration in the atmospheric lifetime and optical evolution of BrC, which has consequences for modeling its impact on climate change and visibility.