Predicted Concentrations and Optical Properties of Brown Carbon from Biomass Burning over Europe.

Black carbon (BC) and brown carbon (BrC) are light-absorbing carbonaceous aerosol components that can contribute to radiative forcing and thus affect the climate. In this study, we focus on the modification of aerosol optical properties associated with BrC emissions from biomass burning. BrC is simulated with the introduction of three new species in the three-dimensional chemical transport model PMCAMx-SR, two primary-absorbing (inert and reactive BrC) species, and one "photobleached" BrC species. 10% of the emitted BrC is assumed to be inert, and the rest to be reactive and able to undergo photobleaching. The scattering and absorption coefficients of the aerosol for different wavelengths are estimated by using Mie theory. BrC causes a 5-15% increase of the aerosol optical depth at 550 nm in regions affected by fires and can increase light absorption by up to 12% compared to when there is no BrC. During major biomass burning events, the absorption of BrC can reach up to 13% of that of BC at 550 nm and 25% at 440 nm. The ability of the model to reproduce measured absorption is improved when BrC is added to the light-absorbing components.