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Hydroxyacetone (HA) is an atmospheric oxidation product of isoprene and other organic precursors that can form brown carbon (BrC). Measured bulk aqueous-phase reaction rates of HA with ammonium sulfate, methylamine, and glycine suggest that these reactions cannot compete with aqueous-phase hydroxyl radical oxidation. In cloud chamber photooxidation experiments with either gaseous or particulate HA in the presence of the same N-containing species, BrC formation was minor, with similar mass absorption coefficients at 365 nm (<0.05 m2 g−1). However, rapid changes observed in aerosol volume and gas-phase species concentrations suggest that the lack of BrC was not due to slow reactivity. Filter-based UHPLC/(+)ESI-HR-QTOFMS analysis revealed that the SOA became heavily oligomerized, with average molecular masses of ∼400 amu in all cases. Oligomers contained, on average, 3.9 HA, 1.5 ammonia, and 1.6 other small aldehydes, including, in descending order of abundance, acetaldehyde, glycolaldehyde, glyoxal, and methylglyoxal. PTR-ToF-MS confirmed the production of these aldehydes. We identify C17H26O5, C10H22O9, C15H27NO7, C17H23NO5, and C18H32N2O9 as potential tracer ions for HA oligomers. We hypothesize that efficient oligomerization without substantial BrC production is due to negligible N-heterocycle (e.g., imidazoles/pyrazines) formation. While HA photooxidation is unlikely a significant atmospheric BrC source, it may contribute significantly to aqueous SOA formation. 

Abstract. Alpha-dicarbonyl compounds are believed to form browncarbon in the atmosphere via reactions with ammonium sulfate (AS) in clouddroplets and aqueous aerosol particles. In this work, brown carbon formationin AS and other aerosol particles was quantified as a function of relativehumidity (RH) during exposure to gas-phase glyoxal (GX) in chamberexperiments. Under dry conditions (RH < 5 %), solid AS,AS–glycine, and methylammonium sulfate (MeAS) aerosol particles brown withinminutes upon exposure to GX, while sodium sulfate particles do not. When GXconcentrations decline, browning goes away, demonstrating that this drybrowning process is reversible. Declines in aerosol albedo are found to be afunction of [GX]2 and are consistent between AS and AS–glycineaerosol. Dry methylammonium sulfate aerosol browns 4 times more than dryAS aerosol, but deliquesced AS aerosol browns much less than dry AS aerosol.Optical measurements at 405, 450, and 530 nm provide an estimatedÅngstrom absorbance coefficient of -16±4. This coefficient andthe empirical relationship between GX and albedo are used to estimate anupper limit to global radiative forcing by brown carbon formed by 70 ppt GXreacting with AS (+7.6×10-5 W m−2). This quantity is< 1 % of the total radiative forcing by secondary brown carbonbut occurs almost entirely in the ultraviolet range.