A leading source of uncertainty in predicting the climate and health effects of secondary organic aerosol (SOA) particles is to assess how their physical and chemical properties change over their atmospheric lifetimes. In this regard, the evolution of the distribution of ever-present SOA dicarboxylic acid (DCAs) homologues driven by atmospheric oxidants provides a universal and sensitive probe of both aerosol age and atmospheric oxidative power along the aerosol trajectory. The mechanism of DCAs atmospheric degradation, however, is still uncertain. Here, we report direct evidence on the oxidation of DCAs initiated by gas-phase OH-radicals, ●OH(g), obtained in novel experiments where early-stage intermediates and products generated at the air-water interface were unambiguously identified in situ by online electrospray mass spectrometry under various conditions. We found that exposure of sub-millimolar HOOC-Rn-COOH (Rn = C2H4, C3H6, C4H8, C5H10, and C6H12) aqueous microjets to ~10 ns ●OH(g) pulses from the 266 nm laser flash photolysis of O3(g)/O2(g)/H2O(g) mixtures yielded an array of more reactive closed-shell, viz. HOOC-Rn(-H)(OOH)-COO-, HOOC-Rn(-2H)(=O)-COO-, HOOC-Rn(-H)(OH)-COO-, detectable radical, viz. HOOC-Rn(-H)(OO●)-COO-, and shorter-chain O=C(H)-Rn-1-COO-/HOOC-Rn-1-COO- species. We infer that OH attack on interfacial DCAs leads, in part, to their degradation into shorter-chain carbonyls O=C(H)-Rn-1-COO- en route to oxalic acid the via selective β-scission of intermediate HOOC-C(O●)H-Rn-1-COO- radicals into ●COOH. We confirmed that oxalic acid, in contrast with its higher homologues, by being less hydrophobic and lacking readily abstractable H-atoms, is unreactive toward ●OH under present conditions and, therefore, would accumulate in aged SOA, in accordance with field observations on SOA composition worldwide.

Cascade oxidation of atmospheric aerosol dicarboxylic acids by gas-phase OH-radicals

249th National Meeting (2015)

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