Chemistry of fog waters in California's Central Valley: 2. Photochemical transformations of amino acids and alkyl amines

Abstract Although amino compounds are seemingly ubiquitous in atmospheric particles and deposition, little is known of their fate in the troposphere. We report here on the fate of 21 amino acids and alkyl amines in fog waters from Davis, California, illuminated with simulated sunlight or monochromatic light. In all experiments four amino acids – histidine (His), methionine (Met), tryptophan (Trp), and tyrosine (Tyr) – consistently decayed, with half-lives that ranged from ∼1 h (Met) to ∼23 h (Tyr) in midday, winter-solstice sunlight at Davis, CA ( solar zenith angle =62°) . Half-lives for the remaining amino compounds examined were typically >45 h in our experiments. Reactions with photoformed hydroxyl radical ( · OH ) and singlet molecular oxygen (O 2 ( 1 Δ g ) or 1 O 2 ∗ ) accounted for essentially all of the loss of His and Tyr, the less reactive of the four amino acids that consistently decayed, but were minor sinks for the more reactive compounds (Met and Trp). Additional experiments revealed that methionine sulfoxide (MetSO) was formed with a yield of 58–88% during the oxidation of methionine, suggesting that the ratio of MetSO to Met might be a useful chemical marker for the age of atmospheric particles and drops. Other products expected from the transformation of amino compounds include ammonia, organic acids, and possibly mutagenic nitrosoaromatics. To complement our laboratory experiments, we also calculated rates of transformations of amino acids in near-neutral pH fog drops under ambient conditions. These calculations reveal that ozone should be a major sink for amino acids and that half-lives for many amino acids in ambient fog drops will be much shorter than those determined in our photochemistry experiments. Overall, our results indicate that reactions in atmospheric condensed phases will transform amino nitrogen compounds (including free amino acids as well as proteins and peptides) and, consequently, increase the bioavailability of nitrogen in atmospheric deposition.

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