Brown carbon in aerosol remains a significant source of error in global climate modeling due to its complex nature and limited product characterization. Though significant efforts have been made in the previous decade to identify the major lightabsorbing brown carbon chromophores formed through the reactions of carbonylcontaining compounds with ammonium, substantial work is still required to identify the main absorbing species resulting from reactions of glyoxal, glycolaldehyde, and hydroxyacetone with ammonium sulfate (AS). Using tandem mass spectrometry and 15 N experiments to confirm proposed structures and support their mechanistic pathways, compelling evidence is provided for the formation of pyrazines and imidazoles in the glyoxal + AS, glycolaldehyde + AS, and hydroxyacetone + AS systems. Through density functional theory calculations, the N-containing oligomers and aromatic heterocycles formed within these reaction systems are shown to contribute to brown carbon light absorption, thus holding significant relevance toward accurately predicting their effects on global climate.

Abstract. Atmospheric particles contain thousands of compounds with many different functional groups and a wide range of polarities. Typical separation methods for aqueous atmospheric systems include reverse-phase liquid chromatography or derivatization of analytes of interest followed by gas chromatography. This study introduces supercritical fluid chromatography–mass spectrometry as a separation method for the methylglyoxal–ammonium sulfate reaction mixture (a proxy for aqueous atmospheric aerosol mimics). Several column compositions, mobile-phase modifiers, and column temperatures were examined to determine their effect on separation and optimum conditions for separation. Polar columns such as the Viridis UPC2™ BEH column combined with a mobile-phase gradient of carbon dioxide and methanol provided the best separation of compounds in the mixture and, when coupled to an electrospray ionization tandem mass spectrometer, allowed for detection of several new masses in the methylglyoxal–ammonium sulfate reaction mixture as well as the possible identification of several isomers. This analysis method can be extended to other aqueous aerosol mimics, including the mixtures of other aldehydes or organic acids with ammonium or small amines.

Abstract. Atmospheric particles contain thousands of compounds with many different functional groups and a wide range of polarities. Typical separation methods for aqueous atmospheric systems include reverse-phase liquid chromatography or derivatization of the analytes of interest followed by gas chromatography. These methods can be time-consuming and do not easily separate highly polar aqueous molecules. This study uses supercritical fluid chromatography-mass spectrometry to separate the methylglyoxal-ammonium sulfate reaction mixture as a proxy for aqueous atmospheric aerosol mimics. Several column compositions, mobile phase modifiers, and column temperatures were examined to determine their effect on separation and the optimum conditions for separation in a minimal amount of time and sample preparation. Polar columns such as the Viridis UPC2 BEH column combined with a mobile phase gradient of carbon dioxide and methanol provided the best separation of compounds in the mixture. This separation method can be extended to analyze other aqueous atmospheric systems, including the mixtures of other aldehydes or organic acids with ammonium or small amines.

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