Direct Radiative Forcing Due to Carbonaceous Aerosols in Biomass Burning Emissions

Most climate forcing calculations treat black carbon (BC) as the only carbonaceous particulate light-absorber. Numerous studies have shown that some organic aerosols (OA), mainly associated with biomass burning emissions, contain significant amounts of light-absorbing brown carbon (BrC). However, the light absorption properties of biomass burning OA are poorly constrained, complicating its representation in climate models. During the Fire Laboratory at Missoula Experiment (FLAME 4), we conducted experiments to characterize the light absorption properties of OA in emissions of globally important biomass fuels. We showed that the effective absorptivity of OA depends largely on burn conditions, not fuel type, and derived a parameterization that links OA absorptivity to the BC-to-OA ratio of the emissions (Nature Geoscience, DOI:10.1038/ngeo2220). Here, we utilize this parameterization to estimate the direct radiative effect (DRE) of carbonaceous aerosols in biomass burning emissions using a global chemical transport model (GEOS-Chem) and a column radiative transfer model (libRadTran). The simulations were performed for the year 2005. Monthly-averaged global aerosol concentrations, including BC, OA, inorganic sulfates and nitrates, sea salt, and mineral dust, were obtained from GEOS-Chem simulations. Concentrations of BC and OA from biomass burning emissions were determined by running two GEOS-Chem simulations, one with and one without biomass burning emissions. We attributed the difference in BC and OA concentrations between the two simulations to biomass burning, and could thus calculate the BC-to-OA ratio for biomass burning emissions. libRadTran was used (offline) to calculated DRE due to biomass burning carbonaceous aerosols at each GEOS-Chem grid-cell. Our results show that the global average DRE due to carbonaceous biomass burning emissions increases significantly if light-absorption by OA is considered (using our parameterization for OA absorptivity), compared to the case with non-absorbing OA.