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 modelling 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 (3.9 ± 1.7 and 3.0 ± 1.3 TgC yr⁻¹ from biomass burning and biofuel) and secondary (5.7 TgC yr⁻¹ from aromatic oxidation) sources. We evaluate the model by comparing the results with observed absorption by water-soluble OC in surface air in the United States, and with single scattering albedo observations at Aerosol Robotic Network (AERONET) sites all over the globe. The model successfully reproduces the seasonal variations of observed light absorption by water-soluble OC, but underestimates the magnitudes, especially in regions with high secondary source contributions. Our global simulations show that BrC accounts for 21 % of the global mean surface OC concentration, which is typically assumed to be scattering. We find that the global direct radiative effect of BrC is nearly zero at the top of the atmosphere, and consequently decreases the direct radiative cooling effect of OC by 16 %. In addition, the BrC absorption leads to a general reduction of NO₂ photolysis rates, whose maximum decreases occur in Asia up to −8 % (−17 %) on an annual (spring) mean basis. The resulting decreases of annual (spring) mean surface ozone concentrations are up to −6 % (−13 %) in Asia, indicating a non-negligible effect of BrC on photochemistry in this region.