Competing Photochemical Effects in Aqueous Carbonyl/Ammonium Brown Carbon Systems

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.