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.

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.