Brown carbon: A significant atmospheric absorber of solar radiation

Abstract

Several recent observational studies have shown organic carbon aerosols to be a significant source of absorption of solar radiation. The absorbing part of organic aerosols is referred to as brown" carbon (BrC). Using a global chemical transport model and a radiative transfer model, we estimate for the first time the enhanced absorption of solar radiation due to BrC in a global model. The simulated wavelength dependence of aerosol absorption, as measured by the absorption Ångström exponent (AAE), increases from 0.9 for non-absorbing organic carbon to 1.2 (1.0) for strongly (moderately) absorbing BrC. The calculated AAE for the strongly absorbing BrC agrees with AERONET spectral observations at 440-870 nm over most regions but overpredicts for the biomass burning-dominated South America and southern Africa, in which the inclusion of moderately absorbing BrC has better agreement. The resulting aerosol absorption optical depth increases by 18% (3%) at 550 nm and 56% (38%) at 380 nm for strongly (moderately) absorbing BrC. The global simulations suggest that the strongly absorbing BrC contributes up to +0.25 W m-2 or 19% of the absorption by anthropogenic aerosols, while 72% is attributed to black carbon, and 9% is due to sulfate and non-absorbing organic aerosols coated on black carbon. Like black carbon, the absorption of BrC (moderately to strongly) inserts a warming effect at the top of the atmosphere (TOA) (0.04 to 0.11 W m-2), while the effect at the surface is a reduction (-0.06 to -0.14 W m-2). Inclusion of the strongly absorption of BrC in our model causes the direct radiative forcing (global mean) of organic carbon aerosols at the TOA to change from cooling (-0.08 W m-2) to warming (+0.025 W m-2). Over source regions and above clouds, the absorption of BrC is higher and thus can play an important role in photochemistry and the hydrologic cycle. © 2013 Author(s).