Kinetics of the reaction of CF3CHO with OH between 204 K and 361 K

Abstract

Trifluoroacetaldehyde (CF₃CHO) is formed in the atmosphere by the oxidation of a number of fluorinated, organic compounds of anthropogenic origin. The reaction of CF₃CHO with the OH radical is a potential source of atmospheric trifluoroacetic acid (TFA) which is a highly persistent, water-soluble compound that may accumulate in aquatic ecosystems and for which uncertainty about its sources, fate, and potential ecological impact persists. In light of growing concerns about the impact of TFA, we present the first study of the temperature dependence of the rate coefficient for the title reaction over the atmospherically relevant temperature range of 204 K to 361 K. Rate coefficients were determined using pulsed laser photolysis–pulsed laser induced fluorescence (PLP–PLIF) and direct concentration measurements via Fourier Transform Infrared (FTIR) spectroscopy, as well as relative rate experiments in an atmospheric simulation chamber using ethane (C₂H₆) as a reference compound. The rate coefficient (k₁) obtained with PLP–PLIF at room temperature is (5.8 ± 0.5) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹. The temperature dependence is described by the expression k₁(T) = (3.8 ± 0.2) × 10⁻¹³ × (T/300)² × exp[(131 ± 16)/T]. The relative-rate experiments showed that the rate coefficient obtained can be significantly biased by reactions of the CF₃O radical with CF₃CHO and/or C₂H₆ and also reactions of CF₃CHO with HO₂. Based on the expression of k₁ given above, the lifetime of CF₃CHO with respect to reaction with the OH radical varies from 22 days at the surface (T ∼ 300 K) to 30 days in the upper troposphere (T ∼ 220 K).