Brown carbon (BrC) slowly formed from glyoxal+ammonium sulfate (AS) and methylglyoxal+AS mixtures in the dark has been experimentally shown to photobleach within minutes. This has led to a general assumption that any atmospheric brown carbon formed by carbonyl + reduced nitrogen (Maillard) reactions overnight will be rapidly destroyed at sunrise. In this work we tested this assumption by reacting glyoxal, methylglyoxal, glycolaldehyde or hydroxyacetone in aqueous mixtures with reduced nitrogen species at pH 4-5 in the dark and in sunlight (&gt;350 nm) for at least three 4 h intervals. The absorbance of fresh carbonyl+AS mixtures decreased when exposed to sunlight, and no BrC formed, as expected from previous work. However, the addition of either methylamine or glycine changed this behavior: with amines present, absorbance increased in sunlit samples at comparable rates as in the dark. Hydroxyacetone+amine+AS aqueous mixtures generally browned faster in sunlight than in the dark, especially if small amounts of HOOH were added, which indicates a radical-initiated oligomerization mechanism that forms brown carbon is involved. The absorbance of most amine-containing reaction mixtures did not decline even after 12 h of sunlight exposure, suggesting that radical-initiated brown carbon formation outpaces photobleaching in these experiments. In suspended aerosol experiments, aqueous aerosol particles generated from glyoxal and methylglyoxal+methylamine+AS mixtures were aged for 1 h in sunlight (&gt;300 nm) or indoors before filtration, extraction, and absorbance measurements. The glyoxal-containing aerosol particles browned more in sunlight than indoors, with or without added HOOH, unlike bulk solutions. Together these results indicate that when amines are present, radical-initiated browning processes successfully compete with photobleaching, and are enhanced in aqueous aerosol particles relative to bulk liquid solutions.

Radical-initiated brown carbon formation in sunlit carbonyl amine ammonium sulfate bulk mixtures and aqueous aerosol particles

ACS Fall 2021

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