Atmospheric oxidation of new "green" solvents - Part 2: 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 molec.-1 s-1) of k1(297 K) Combining double low line (1.2 ± 0.2) for OH + (CH3)3CC(O)CH3 (Reaction R1) and k2(297 K) Combining double low line (1.3 ± 0.2) for OH + (CH3)3CC(O)OCH3 (Reaction 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) Combining double low line 2.1 × 10-12 exp(-200/T) cm3 molec.-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) was obtained for the well-characterised reaction OH + C2H5OH (ethanol; Reaction R3), in satisfactory agreement with the evaluated literature. UV-Vis spectroscopy experiments and computational calculations were used to explore cross-sections for (CH3)3CC(O)CH3 photolysis (Reaction R4), while (CH3)3CC(O)OCH3 showed no sign of absorption over the wavelengths of interest. 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 (methyl ethyl ketone (MEK) values) used in current state-of-science models. As a consequence, we note that photolysis (Reaction R4) is likely the dominant pathway for removal of (CH3)3CC(O)CH3 from the troposphere. Nonetheless, large uncertainties remain as quantum yields (λ) remain unmeasured. Lifetime estimates based upon Reactions (R1) and (R4) span the range 2-9 d and are consequently associated with a poorly constrained estimated photochemical ozone creation potential (POCPE). In accord with previous studies, (CH3)3CC(O)OCH3 did not absorb in the actinic region, allowing for straightforward calculation of an atmospheric lifetime of ≈ 9 d and a small POCPE≈ 11.