The Atmospheric Chemistry of the Acetonoxy Radical

The chemistry of the acetonoxy radical, CH 3 C(O)CH 2 O, formed in the atmospheric degradation of acetone, was studied by a combination of experimental and theoretical methods. In an environmental chamber study conducted over the temperature range 225−298 K, acetonoxy radical chemistry was shown to be dominated by decomposition, CH 3 C(O)CH 2 O → CH 3 C(O) + CH 2 O. No evidence was found for a reaction of this species with O 2, CH 3 C(O)CH 2 O + O 2 → CH 3 C(O)CHO + HO 2, even at 225 K in the presence of 1 atm O 2 . In a theoretical ab initio and statistical kinetics investigation, the barrier to CH 3 C(O)CH 2 O decomposition was found to be 6−7 kcal/mol. Using SSE theory and RRKM-based master equation analysis, it was determined that about 80% of the CH 3 C(O)CH 2 O radicals formed in the CH 3 C(O)CH 2 O 2 + NO reaction have sufficient energy to decompose “promptly” under tropospheric conditions. On the basis of TST theory and allowing for falloff, the dissociation rate of thermalized CH 3 C(O)CH 2 O radicals was found to be on the order of 5 × 10 7 s -1 at 1 atm and 300 K and 5 × 10 5 s -1 at 0.2 atm and 220 K. The results confirm that the acetonoxy reaction with O 2 is always negligible in the troposphere, consistent with the experimental observations. As part of this study, the rate coefficient for reaction of Cl with acetone ( k 2 ) was measured by a relative rate technique, and a value of k 2 = (3.1 ± 0.5) × 10 -11 exp(−815 ± 150/ T ) cm 3 molecule -1 s -1 is reported.