Atmospheric oxidation of new 'green' solvents part II: methyl pivalate and pinacolone

Abstract. Lab-based experimental and computational methods were used to study the atmospheric degradation of two promising “green” solvents: pinacolone, (CH₃)₃CC(O)CH₃ and methyl pivalate, (CH₃)₃CC(O)OCH₃. Pulsed laser photolysis coupled to pulsed laser induced fluorescence was used to determine absolute rate coefficients (in 10⁻¹² cm³ molecule⁻¹ s⁻¹) of k₁(297 K) = (1.2 ± 0.2) for OH + (CH₃)₃CC(O)CH₃ (R₁) and k₂(297 K) = (1.3 ± 0.3) for OH + (CH₃)₃CC(O)OCH₃ (R₂), in good agreement with one previous experimental study. Rate coefficients for both reactions were found to increase at elevated temperature, with k₁(T) adequately described by k1(297 – 485 K) = 2.1 × 10⁻¹² exp(−200/T) cm³ molecule⁻¹ s⁻¹. k₂(T) exhibited more complex behaviour, with a local minimum at around 300 K. In the course of this work, k₃(295 – 450 K) for the well-characterised reaction OH + C₂H₅OH (ethanol, R₃) were obtained, in satisfactory agreement with the evaluated literature. UV-vis. spectroscopy experiments and computational calculations were used to explore (CH₃)₃CC(O)CH₃ photolysis (R₄). Absorption cross sections for (CH₃)₃CC(O)CH₃, σ₄(λ) in the actinic region were larger and the maximum was red-shifted compared to estimates used in current state-of-science models. As a consequence, we note that photolysis (R₄) is likely the dominant pathway for removal of (CH₃)₃CC(O)CH₃ from the troposphere. Nonetheless, large uncertainties remain as quantum yields ϕ₄(λ) remain unmeasured. Lifetime estimates based upon (R₁) and (R₄) span the range 2–9 days and are consequently associated with a poorly constrained Photochemical Ozone Creation Potential estimate (POCP_E). In accord with previous studies, (CH₃)₃CC(O)CH₃ did not absorb in the actinic region, allowing for straightforward calculation of an atmospheric lifetime of ≈ 9 days and a small POCP_E ≈ 11.