Atmospheric Oxidation Mechanism of Methyl Pivalate, (CH3)3CC(O)OCH3

Abstract

Flash photolysis−resonance fluorescence techniques were used to measure the rate constant for the reaction of OH radicals with methyl pivalate, (CH3)3CC(O)OCH3 over the temperature range 250−370 K. The rate constant exhibited a weak temperature dependence, increasing at both low and high temperature from a minimum value of approximately 1.2 × 10-12 cm3 molecule-1 s-1 near room temperature. The UV absorption spectrum of methyl pivalate was measured between 160 and 500 nm at room temperature. Smog chamber/FTIR techniques were used to study the Cl atom and OH radical initiated oxidation of (CH3)3CC(O)OCH3 in the presence of NOx in 700 Torr of N2/O2 diluent at 296 K. Relative rate techniques were used to measure k(Cl+(CH3)3CC(O)OCH3) = (4.1 ± 0.5) × 10-11, k(Cl+(CH3)3CC(O)OCH2Cl) = (1.8 ± 0.3) × 10-11, and k(Cl+(CH3)3CC(O)OC(O)OH) = (1.7 ± 0.2) × 10-11 cm3 molecule-1 s-1. The reaction of Cl atoms with (CH3)3CC(O)OCH3 was found to proceed (11 ± 3) % via H-abstraction at the −OCH3 site. The Cl atom initiated oxidation of (CH3)3CC(O)OCH3 in the presence of 15−600 Torr of O2 and 10−30 mTorr of NOx in 700 Torr total pressure of N2 diluent at 296 K gives HCHO, CO, acetone, CO2, and CH3OC(O)O2NO2 products. OH radical initiated oxidation of (CH3)3CC(O)OCH3 in air produces acetone in a yield of 51 ± 6%. Environmental chamber experiments were performed to quantify the effect of methyl pivalate on ozone formation under simulated atmospheric conditions. An expression representing the atmospheric oxidation mechanism of methyl pivalate in computer models of atmospheric chemistry is recommended.