Comment on acp-2022-446

Abstract

The atmospheric chemistry of 2,2,5,5-tetramethyloxolane (TMO), a promising ‘green’ solvent replacement for toluene, was investigated in laboratory and computational experiments. Results from both absolute and relative rate studies demonstrated that the reaction OH + TMO (R1) proceeds with a rate coefficient k1(296 K) = (3.1 ± 0.4) × 10−12 cm3 molecule−1 s−1, a factor of three smaller than predicted by recent structure activity relationships. Quantum chemical calculations (CBSQB3-G4) demonstrated that the reaction pathway via the lowest-energy transition state was characterised by a hydrogen-bonded pre-reaction complex, leading to thermodynamically less favoured products. Steric hindrance from the four methyl substituents in TMO prevent formation of such H-bonded complexes on the pathways to thermodynamically favoured products, a likely explanation for the anomalous slow rate of (R1). Further evidence for a complex mechanism was provided by k1(294 – 502 K), characterised by a local minimum at around T = 340 K. An estimated atmospheric lifetime of ≈ 3 days was calculated for TMO, approximately 50 % longer than toluene, indicating that any air pollution impacts from TMO emission would be less localised. Relative rate experiments were used to determine a rate coefficient, k2(296 K) = (1.2 ± 0.1) × 10−10 cm3 molecule−1 s−1 for Cl + TMO (R2); together with the slow (R1) this may indicate an additional contribution to TMO removal in regions impacted by high levels of atmospheric chlorine. All results indicate that TMO is a less problematic volatile organic compound (VOC) than toluene.