A global simulation of brown carbon: implications for photochemistry and direct radiative effect

Abstract. Recent observations suggest that a certain fraction of organic carbon (OC) aerosol effectively absorbs solar radiation, which is also known as brown carbon (BrC) aerosol. Despite much observational evidence of its presence, very few global modeling studies have been conducted because of poor understanding of global BrC emissions. Here we present an explicit global simulation of BrC in a global 3-D chemical transport model (GEOS-Chem), including global BrC emission estimates from primary (5.7 and 4.3 Tg C yr-1 from biomass burning and biofuel) and secondary (5.7 Tg C yr-1 from aromatic oxidation) sources. We evaluate the model by comparing the results with observed absorption by OC in surface air in the United States, and with single scattering albedo observations at AERONET sites all over the globe. The model successfully reproduces the observed seasonal variations, but underes…