Effect of Ammonium Salts on the Decarboxylation of Oxaloacetic Acid in Atmospheric Particles

Abstract

Oxaloacetic acid (OAA) is a 3-oxocarboxylic acid formed from the oxidation of succinic acid. OAA and other 3-oxocarboxylic acids experience a decarboxylation reaction in aqueous solutions, which can be catalyzed by ammonium and amines. This catalysis has not been studied under atmospherically relevant conditions despite previous interest in OAA in the atmosphere. To address this, 1 mM solutions of OAA were prepared with varying concentrations of ammonium sulfate, ammonium bisulfate, ammonium chloride, and sodium sulfate to simulate various atmospheric conditions. The extent of the decarboxylation was monitored using UV–visible absorption spectroscopy. OAA’s uncatalyzed decarboxylation lifetime was around 5 h. Under moderately acidic conditions representative of aerosol particles (pH = 3–4), the decarboxylation rate increased linearly with ammonium concentration up to about 2.7 M, after which additional ammonium had no effect. The effective lifetime of OAA reduced to approximately 1 h under these conditions. Density functional theory calculations support the proposed catalytic mechanism, predicting the free energy barrier height for decarboxylation to be approximately 21 kcal/mol lower after OAA has reacted with ammonium. In more acidic solutions (pH < 1), OAA’s decarboxylation was suppressed, with lifetimes of tens of hours, even in the presence of ammonium. A comparison of the decarboxylation rate with the expected rate of oxidation by OH suggests that decarboxylation will be the dominant loss mechanism for OAA, and presumably other 3-oxocarboxylic acids, in aerosol particles and cloud/fog droplets. This result explains why OAA is hard to detect in field measurements even though it is a known oxidation product of succinic acid.

Radical-Initiated Brown Carbon Formation in Sunlit Carbonyl–Amine–Ammonium Sulfate Mixtures and Aqueous Aerosol Particles

“…Unique peaks that were seen only after chamber exposure to methylamine and sunlit clouds were assigned to proposed molecular structures containing 3–6 double bonds and appear to form from precursors including methylamine, acetaldehyde/acetyl radicals, and/or HA. The formation of the unique C 10 H 10 O 4 m / z 217.0500 product also appears to involve oxidation followed by decarboxylation, which can be catalyzed by ammonium salts . Chamber exposure increased the weighted average number of conjugated double bonds per molecule from 1.0 (nonconjugated) to 3.0, again dominated by the production of C 13 H 20 N 2 O 6 ( m / z 301.1410) with its four double bonds, three of which are conjugated.…”

“…The formation of the unique C 10 H 10 O 4 m/z 217.0500 product also appears to involve oxidation followed by decarboxylation, which can be catalyzed by ammonium salts. 73 Chamber exposure increased the weighted average number of conjugated double bonds per molecule from 1.0 (nonconjugated) to 3.0, again dominated by the production of C 13 H 20 N 2 O 6 (m/z 301.1410) with its four double bonds, three of which are conjugated. In summary, exposure to methylamine, sunlight, and cloud processing results in the conversion of MG oligomers into more conjugated product molecules that incorporate more nitrogen (both methylamine and ammonia) and photolysis products (acetyl radicals and HA).…”

Stability of α-Pinene and d-Limonene Ozonolysis Secondary Organic Aerosol Compounds Toward Hydrolysis and Hydration

“…For example, phthalic anhydride was found to be taken up into the particle phase and then to add water to form phthalic acid . Nucleophilic reactions can occur during the hydrolysis of lactones, resulting in ring opening products. , There have been numerous studies on organonitrates, which have been shown to hydrolyze in humid conditions, especially in the presence of acids. − Dicarbonyls, such as glyoxal and methylglyoxal, can easily partition into the aqueous phase and undergo hydration, resulting in oligomer formation. − Oxaloacetic acid has been shown to decarboxylate with a lifetime of 5 h in pure water and 1 h in ammonium sulfate aerosols . However, not all potentially hydrolyzable compounds undergo hydrolysis due to kinetic limitations.…”