Dynamic response of light absorption by PM2.5 bound water-soluble organic carbon to heterogeneous oxidation

Abstract

Along with greenhouse gases, worldwide biomass-burning events add substantial amounts of a complex optical system consisting of elemental carbon and organic particulate matter to the atmosphere. These particulate matters significantly affect the Earth’s radiation balance as well as perturb the precipitation pattern. Considerable attention has been given to the impact of physicochemical transformation or aging on the optical properties of aqueous solution (a homogeneous medium) of light-absorbing fractions of the organic particulate matter. However, their heterogeneous photochemical aging remains largely unexplored. This experimental study explores heterogeneous absorbance photo-bleaching kinetics of the water-soluble organic-carbon (WSOC) intrinsic to PM2.5 from rice straw smoldering smoke. In the wavelength region of 280–400 nm, the absorption ability decreased consistently with aging while a characteristic initial increase of absorption was observed in the 400–500 nm intervals, which again decreased with further aging. The dynamic light absorptivity of the WSOC may not be related to the corresponding mass concentrations. The relative quantum efficiency of the heterogeneous aging of these WSOC was estimated to be (0.01‒0.3) %. The kinetic results suggest that their half-life could be nearly a week under ambient solar radiation. Copyright © 2019 American Association for Aerosol Research.