The atmospheric oxidation of hydroxyacetone: Chemistry of activated and stabilized CH₃C(O)CH(OH)OO• radicals between 252 and 298 K

The products of the Cl-atom-initiated oxidation of hydroxyacetone (HYAC, CH₃C(O)CH₂OH) have been examined under conditions relevant to the earth's lower atmosphere. Over the range of temperatures studied (252-298 K), in the absence of NO_x, methylglyoxal (CH₃C(=O)CH=O, MGLY) was formed with a primary yield >84% (96 ± 9% at 298 K), while in the presence of elevated NO_x, MGLY and formic acid were both formed as major primary products. In contrast to a previous study, acetic acid was not identified as a major primary product under the conditions studied. The results are quantitatively interpreted from a consideration of the formation of a stabilized CH₃C(O)CH(OH)OO• radical, either in a ≈50% yield from the addition of O₂ to CH₃C(O)CH•(OH) or in 100% yield from the addition of HO₂ to MGLY. At high temperature and low NO_x, decomposition of the stabilized CH₃C(O)CH(OH)OO• radical to MGLY is favored, while lower temperatures and conditions of high NO_x favor bimolecular reactions of the stabilized radical, with subsequent production of formic acid. Analysis of the data allows for a semiquantitative determination of K₃ = (2.9 ± 0.4) × 10⁻¹⁶ cm³ molecule⁻¹, for the HO₂ + MGLY ↔ CH₃C(O)CH(OH)OO• equilibrium process at 298 K and a roughly order of magnitude increase in K₃ at 252 K.