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.

The contribution of organic aerosol from biomass burning is poorly constrained, and the lack of consensus regarding its overall contribution to global radiative forcing leads to significant uncertainties in climate modeling. Identification of potential brown carbon chromophores from common biomass burning emissions may reduce this uncertainty. Biacetyl (BA) is found in emissions from industry and biomass burning from various ecosystems and shares structural similarities with other small carbonyls that react with ammonium sulfate (AS) to produce brown carbon compounds. Like previous carbonyl + AS studies, the BA + AS system results in the formation of hundreds of different products; these were separated and identified using supercritical fluid chromatography–tandem mass spectrometry, isotopic substitution experiments, and comparisons to standards. Kinetic information was obtained through spectral decomposition of experimentally measured UV–visible absorbance data. Theoretical TDDFT calculations were utilized to extract more information on the light absorbance of identified products and to determine how these individual chromophores would contribute to the light absorbance of organic aerosol. This information could provide insight into unknown organic aerosol behavior by furthering our understanding of the reactivity of a common biomass burning emission product like biacetyl.

Carbonyl-containing volatile organic compounds (CVOCs) have been identified in a variety of atmospherically relevant aqueous aerosol conditions and can contribute significantly to total secondary organic aerosol mass. While dark chemistry has been extensively studied for several CVOC-containing reaction systems, the chemistry of these same compounds under irradiated conditions is not as well understood. We present time-resolved UV− visible measurements and inferred kinetic rate constants for CVOC/ ammonium sulfate (AS) aerosol mimic solutions exposed to direct, broadband radiation for periods of up to 24 h. Glycolaldehyde/AS solutions were observed to monotonically decrease in chromophoricity over irradiated periods. Glyoxal/AS solutions demonstrated a rise and subsequent fall in absorbance while irradiated. Methylglyoxal/AS and hydroxyacetone/AS solutions demonstrated multiple increases and decreases in chromophoricity at different peak locations. The chemical speciation of these CVOC/AS mixtures show that higher molecular-weight oligomer compounds are not photostable; their disappearance is accompanied by the formation of both larger and smaller photochemical products, which can form under a variety of time scales within the same reaction system. The observation of photochemically driven browning phenomena in addition to photobleaching implies that more nuanced approaches are necessary to accurately capture aqueous aerosol chemistry under daytime conditions.

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.