Color of brown carbon: A model for ultraviolet and visible light absorption by organic carbon aerosol

We recommend ultraviolet and visible absorption spectra to represent particular types of atmospheric organic particles. Spectra of liquids and particles can be compared using the absorption coefficient of bulk material divided by material density. Reported absorption by absorbing organic aerosol from combustion is greater than that of organic material isolated by humic acid extraction. We examine ultraviolet and visible spectra of 200 organic compounds, concluding that visible absorption may be attributable to n -> pi* electronic transitions in a small fraction of oxygenated compounds. Absorption spectra can be communicated using the band-gap and Urbach relationships instead of the absorption Angstrom exponent. Water-soluble atmospheric aerosol has a band- gap of about 2.5 eV; insoluble aerosol may have a lower band- gap and higher absorption. Although different types of organic carbon may exhibit a continuum in absorption, there is a sharp distinction between the most-absorbing organic carbon and black carbon.