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The Effects of Ammonium on the Atmospheric Aging of 3-Oxodicarboxylic Acids
ALEXANDRA KLODT, Jorge Fernandez, Michael Olsen, Sergey Nizkorodov, University of California, Irvine
Abstract Number: 46
Working Group: Aerosol Chemistry
Abstract
3-oxocarboxylic acids are observed less often in field-collected SOA than in laboratory-generated SOA. This can be explained by 3-oxocarboxylic acids’ ability to undergo a decarboxylation reaction. These molecules are often water soluble, and exist in aerosols alongside hygroscopic salts, such as ammonium sulfate. This work studied the effects ammonium can have on the decarboxylation reaction of a representative 3-oxocarboxylic acid, oxaloacetic acid (OAA). The decarboxylation of OAA to pyruvic acid was monitored using UV-vis spectroscopy, and pseudo-first order rate constants were calculated for 1 mM OAA solutions containing varying concentrations of ammonium sulfate, ammonium bisulfate, ammonium chloride, and acidified sodium sulfate (to make the pH more comparable to the ammonium sulfate condition). OAA’s decarboxylation reaction was determined to be catalyzed by ammonium over a broad range of conditions found in the atmosphere. In weakly acidic solutions, pH’s near 3 to 4, increasing ammonium concentrations linearly increased the calculated rate constant until 2.7 M ammonium, after which additional ammonium had no further effect. However, at pH’s less than 2, the decarboxylation reaction was much slower, even in the presence of ammonium. Based on these results, 3-oxocarboxylic acids will have lifetimes near 5 hours in cloud/fog water, between 1 and 5 hours in aerosols which have been neutralized by ammonia, and nearly 40 hours in highly acidic particles, even if they are partly neutralized by ammonia.