The absorbance of UV/visible light by atmospheric aerosol particles (e.g. soot and brown carbon) has important implications for climate change. Remediation of this material could be one short-term solution to slow atmospheric warming. The primary source of brown carbon is biomass burning, however, a second non-combustion source is found in aqueous phase reactions in cloud water. Bulk-phase absorptivity was measured for Maillard reaction products generated with systems of amines and small aldehydes, including methylamine, ammonium sulfate, glyoxal and methylglyoxal. The effects of cloud processing were simulated by comparing the absorptivity of liquid samples to those that were allowed to evaporate prior to absorbance and total organic carbon measurements. The extent of photobleaching was quantified by measuring the absorptivity of samples after exposure to UV/visible light. Chemical composition of Maillard products was characterized using an Aerosol Chemical Speciation Monitor. Electron-impact mass spectra were used to determine the extent to which large fragments and families of fragments correlate to absorptivity. The spectra of these model brown carbon systems were also compared to ambient and laboratory-generated particles in order to determine how cloud processing and photobleaching transform these compounds. Preliminary results indicate that systems with methyglyoxal had higher absorptivity than all other systems and that evaporation served to increase brownness. The mass spectra show significant formation of larger molecules (m/z ≥ 100) in methylamine systems with little evidence for similar compounds in ammonium sulfate systems. However, ammonium sulfate systems did contain a higher proportion of larger fragments (m/z ≥ 100) after evaporation to dryness. Finally, in both methylamine and ammonium sulfate systems, glyoxal appeared to result in a higher percentage of large fragments than methylgloxal. Therefore, chromophore formation may be intricately linked to the formation of higher MW compounds but does not appear to be directly correlated with oligomer chain length.

Chemical and optical properties of model brown carbon systems characterized by UV/visible spectroscopy and aerosol mass spectrometry

253rd National Meeting (2017)

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