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Abstract
Light-absorbing organic material, or "brown carbon", can significantly influence the effect that aerosols have on climate, yet, the mechanism of light absorption in brown carbon aerosols is still poorly understood. In this work, charge transfer (CT) complexes are identified as a significant source of light absorption by organic compounds in aerosols. A dense manifold of these complexes, formed from interactions between alcohol and carbonyl moieties, accounts for approximately 50% of the absorption observed for water-extracted ambient particulate matter. Most likely a wide variety of CT complexes are formed as a result of a supramolecular association of self-assembling, smaller molecules, just as they are in natural humic substances, and that this gives rise to absorption that extends to red wavelengths where no other potential chromophores absorb. Bulk optical measurements were performed with difference extractions solvents, before and after chemical reduction with NaBH4, pH adjustments, and photobleaching in a laboratory and with solar irradiation. Additional indices quantified the extent of similarities between brown carbon aerosols and natural humic substances. These findings imply that light absorption by organic aerosols is governed by an interaction model, with independent and interacting chromophores both contributing to optical absorption and fluorescence. Furthermore, the environment of brown carbon chromophores can affect light absorption, and should be measured or controlled to best estimate its effect on global climate.