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.