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

Abstract

A leading source of uncertainty in predicting the climate and health effects of secondary org. aerosol (SOA) particles is to assess how their phys. and chem. properties change over their atm. lifetimes. In this regard, the evolution of the distribution of ever-present SOA dicarboxylic acid (DCAs) homologues driven by atm. oxidants provides a universal and sensitive probe of both aerosol age and atm. oxidative power along the aerosol trajectory. The mechanism of DCAs atm. degrdn., however, is still uncertain. Here, we report direct evidence on the oxidn. of DCAs initiated by gas-phase OH-radicals, •OH(g), obtained in novel expts. 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-R_n-COOH (R_n = C_2H_4, C_3H_6, C_4H_8, C_5H_(10), and C_6H_(12)) aq. microjets to ∼10 ns •OH(g) pulses from the 266 nm laser flash photolysis of O_3(g)/O_2(g)/H_2O(g) mixts. yielded an array of more reactive closed- shell, viz. HOOC- R_n(-H)(OOH) – COO-, HOOC-R_n(-2H) (=O) – COO-, HOOC-R (-H) (OH) - COO, detectable radical, viz. HOOC-R_n(-H) (OO•) – COO-, and shorter- chain O=C(H) – R_n-1-COO-/HOOC-R_n-1-COO- species. We infer that OH attack on interfacial DCAs leads, in part, to their degrdn. into shorter-chain carbonyls O=C(H) – R_n-1-COO- en route to oxalic acid the via selective β-scission of intermediate HOOC-C(O•) H-R_n-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 compn. worldwide.

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