We studied the interactions of glyoxylic acid, pyruvic acid and oxalic acid with ammonium and corresponding sodium salts in aqueous solutions simulating a dark and radical free atmospheric aqueous aerosol condition. Cleavage of a carbon-carbon bond in pyruvic acid and glyoxylic acid leading to the decarboxylation was observed in the presence of ammo¬nium salts but was not observed from oxalic acid. At the beginning of the reaction, the decarboxylation appeared to proceeding slower compare to the later stage of reaction. The empirical rate constants for decarboxylation in the reaction solutions were estimated using a 'quasi-steady state' model: (i) glyoxylic acid and ammonium sulfate was 3.3 (± 0.7)×10-8 M-1 s-1; (ii) glyoxylic acid and ammonium nitrate was 1.4 (± 0.3)×10-8 M-1 s-1; (ii) glyoxylic acid and ammonium chloride was 1.9 (± 0.2)×10 -8 M-1 s-1; and (iii) pyruvic acid and ammonium sulfate was 15.8 (± 0.4)×10-8 M -1 s-1. Negligible CO2 was observed in the experiments with the corresponding sodium salts indicating the ammonium ion or ammonia is facilitating the carbon-carbon bond cleavage leading to carboxyl fragmentation of the &agr;-oxo carboxylic acids. It was observed that pyruvic acid undergoes decarboxylation at least four times faster than that of glyoxylic acid under similar reaction conditions. This indicates that the structure of the acid plays an important role in the decarboxylation. In the case of pyruvic acid, the reaction is likely faster because of the inhibited hydration of the carbonyl moiety due to the inductive effect of the adjacent methyl group. A tentative set of reaction mechanisms is proposed involving nucleophilic attack by ammonia on the carbonyl carbon leading to fragmentation of the carbon-carbon bond between the carbonyl and carboxyl carbons. Similar carbon-carbon bond cleavage is anticipated for &agr;-dicarbonyl compounds, which are structurally similar to the &agr;-oxo carboxylic acids. In the absence of photolysis and under limited availability of OH radicals, the decay of pyruvic acid can be dominated by the reaction with ammonium sulfate and can be an order of magnitude higher than the loss via reaction with the OH radical. Under similar conditions the reactions with ammonium salts are likely be a major sink for &agr;-oxo carboxylic acids in the atmospheric aqueous phase
