Stepwise Oxidation of Aqueous Dicarboxylic Acids by Gas-Phase OH Radicals

A leading source of uncertainty in predicting the climate and health effects of secondary organic aerosol (SOA) is how its composition changes over their atmospheric lifetimes. Because dicarboxylic acid (DCA) homologues are widespread in SOA, their distribution provides an ideal probe of both aerosol age and the oxidative power of the atmosphere along its trajectory. Here we report, for the first time, on the oxidation of DCA(aq) by center dot OH(g) at the airwater interface. We found that exposure of aqueous HOOC-Rn-COOH (R-n = C2H4, C3H6, C4H8, C5H10, and C6H12) microjets to similar to 10 ns center dot OH(g) pulses from the 266 nm laser photolysis of O-3(g)/O-2(g)/H2O(g) mixtures yields the corresponding (n-1) species O-C(H)-Rn-1-COO-/HOOC-Rn-1-COO-, in addition to an array of closed-shell HOOC-R-n(-H)(OOH)-COO, HOOC-R-n(-2H)(=O)-COO-, HOOC-R-n(-H)(OH)-COO-, and radical HOOC-R-n(-H)(OO center dot)-COO species. Oxalic and malonic acids, which are shown to be significantly less hydrophobic and reactive than their higher homologues, will predictably accumulate in SOA, in accordance with field observations.