Atmospheric fate of methyl pivalate: OH/Cl-initiated degradation and the roles of water and formic acid

Abstract

Environmental contextOxygenated volatile organic compounds can lead to the formation of tropospheric ozone, and thus have an impact on climate and human health. Methyl pivalate is one such compound, but the way it breaks down in the atmosphere is not well understood. We investigate the oxidative degradation of methyl pivalate, and show that harmful peroxyacyl nitrates and organic nitrates are the major products. AbstractThe atmospheric degradation mechanism and dynamics of methyl pivalate (MP) by OH radicals and Cl atoms are explored. The rate constants, computed using variational transition-state theory over the range of 200–2000 K at the CCSD(T)/6-311++G(d,p)//B3LYP/6-311G(d,p) level, are all in agreement with the experimental data. The alkyl radicals, which are formed from the reactions of OH or Cl with MP, can react with O2 and NO to produce the peroxyacyl nitrates, organic nitrates, and alkoxy radicals. The atmospheric evolution mechanisms for the (CH3)3CCOOCH2O•, •OCH2(CH3)2CCOOCH3, and •O(CH3)2CCOOCH3 radicals are also clarified. The OH- and Cl-determined atmospheric lifetimes and the global warming potentials (GWPs) of MP are shown to be low, suggesting that its environmental impact can be ignored. The Arrhenius expressions of kOH = 3.62 × 10−23T3.80exp(522.66/T) and kCl = 1.76 × 10−15T1.79exp(−55.89/T) cm3 molecule−1 s−1 are fitted within 200–2000 K. Compared with the OH/Cl-initiated degradation of (CH3)3CCOOCH3, the auto-decomposition reaction of (CH3)3CCOOCH3 → (CH3)2C=CH2 + HCOOCH3 may be more important at the high temperature range of 1500–2000 K. Moreover, the results show that the water and formic acid molecules can promote the degradation of MP. This study is helpful for evaluating the atmospheric implications of gaseous MP.