Atmospheric absorption by brown carbon aerosol may play an important role in global radiative forcing. Brown carbon has a strongly wavelength-dependent absorption cross section that is greatest in the ultraviolet spectral region. We have developed a new laboratory instrument to measure aerosol optical extinction as a function of wavelength for in situ aerosol, using cavity enhanced spectroscopy over the 360–420 nm spectral region. We retrieve refractive indices as a function of wavelength from the measured extinction cross sections.<br/><br/>One proposed mechanism to produce brown carbon aerosol is the reaction of ammonia or amino acids with carbonyl products in secondary organic aerosol (SOA). We have generated SOA in situ by reacting biogenic alkenes (alpha-pinene, limonene, and alpha-humulene) with excess ozone, humidifying the resulting aerosol, and reacting the humidified aerosol with gaseous ammonia. We report the complex refractive indices for these aerosol, and the atmospheric conditions under which this mechanism will produce significant aerosol absorption.<br/><br/>Recent field measurements in the southeastern U.S. during summer 2013 allow us to further quantify the processes that contribute to brown carbon aerosol absorption.

Laboratory and field studies of brown carbon aerosol in the near-ultraviolet spectral region

249th National Meeting (2015)

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