Methods to Parameterize Brown Carbon, Distinguish Brown Carbon Absorption From Enhanced Black Carbon Absorption, and Assess the Stability of Brown Carbon to Photochemical Aging

Abstract:

We present results obtained during the Fire Lab at Missoula Experiment-4 (FLAME-4) with a combination of multi-wavelength photoacoustic and cavity ringdown spectrometers. It will be shown that the single scattering albedo and Angstrom exponent of biomass burning emissions can be better parameterized by the organic carbon to black carbon ratio than by the modified combustion efficiency. Two different methods to distinguish the contribution to aerosol absorption from brown carbon versus black carbon and enhanced black carbon absorption will be presented. One method is based on extending the absorption seen at 660 nm with an assumed Angstrom exponent while the other assumes a similar absorption enhancement (determined via thermal denuder) of black carbon at 660 and 405 nm. Potential errors and advantages of both methods will be discussed. Finally, chamber experiments that show degradation of brown carbon by photochemical oxidation will be presented along with a number of methods by which to assess the amount of brown carbon that is degraded.