A Model for the Spectral Dependence of Aerosol Sunlight Absorption

Abstract

Sunlight-absorbing aerosols, e.g., black and brown carbon (BC and BrC), have a potentially large, but highly uncertain contribution to climate warming. The spectral dependence of the aerosol absorption in the visible and near-UV regime is almost universally well-described with a heuristic power law, where the exponent is termed the absorption Angstrom exponent. However, the underlying physicochemical causes for this relation are unknown. Here, a model is presented that predicts the emergence of the power law spectral dependence and unifies the absorption behavior of BC and BrC. Building on the theory of light-absorption in amorphous materials, the interaction between multiple functional groups upon absorption is predicted to be a key feature for this broad spectral dependence. This aerosol amorphous absorption model is in agreement with recent empirical findings and provides a conceptual basis for the additional research needed to better constrain the optical properties of light-absorbing aerosols and the...