Light Absorption by Charge Transfer Complexes in Brown Carbon Aerosols

Recently, it has become apparent that a fraction of the organic species in ambient aerosols absorbs ultraviolet visible light with a potential impact on climate. It is believed that this light-absorbing, sometimes called brown, carbon originates from biomass and biofiiel burning and could be formed through secondary processes in particles or clouds. Here, we identify for the first time charge transfer (CT) complexes 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 (300-600 nm) observed for water-extracted ambient particulate matter collected in Athens, GA. Corresponding fluorescence emission spectra with broad, overlapping long-wavelength tails are consistent with efficient energy transfer among a near continuum of such coupled excited states. We postulate that 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. These findings imply that light absorption by organic aerosols is governed by a combination of independent as well as interacting chromophores and that both must be included in an accurate representation of aerosol optical properties.