The ultraviolet (λ=250–370 nm) photolysis and the OH-initiated oxidation of hexanal and trans-2-hexenal, which are relevant atmospheric processes, have been investigated at room temperature and as a function of temperature (T=263–353 K), respectively. This kinetic study as a function of temperature is reported here for the first time. Absolute absorption cross sections (σ_λ) were obtained using a recently built system operating in the UV region. The obtained σ_λ allowed the estimation of the photolysis rates (J) across the troposphere. Kinetic measurements of the gas-phase reaction of hydroxyl radicals (OH) with hexanal and trans-2-hexenal were performed by using the laser pulsed photolysis/laser-induced fluorescence technique. Rate coefficients k_OH for both aldehydes were determined at temperatures between 263 and 353 K at 50 Torr in helium or argon bath gases. The temperature dependence of k_OH for both aldehydes was found to be slightly negative. The tropospheric lifetime of hexanal and trans-2-hexenal due to the chemical removal by OH radicals has been estimated across the troposphere. The loss rate due to the OH chemical removal was compared with the estimated photolysis rates. Our results show that OH-reaction is the main loss process for these aldehydes in the troposphere, although photolysis is not negligible for hexanal.

In recent years, ultra-low temperature chemical kinetic experiments have demonstrated that some gas-phase reactions are much faster than was previously thought. One example is the reaction between OH and CH₃OH, which has recently been found to be accelerated at low temperatures yielding CH₃O as its main product. This finding raised the question of whether or not the CH₃O observed in the dense core Barnard 1b could be formed by the gas-phase reaction of CH₃OH and OH. Several chemical models including this reaction and grain-surface processes have been developed to explain the observed abundance of CH₃O, but they have met with little success. Here, we report for the first time the rate coefficients for the gas-phase reaction of OH and CH₃OH down to a temperature of 22 K, which is very close to the temperature in cold interstellar clouds. Two independent experimental set-ups based on the supersonic gas expansion technique coupled to the pulsed laser photolysis laser-induced fluorescence technique were used to determine the rate coefficients in the temperature range 22–64 K. The temperature dependence obtained in this work can be expressed as k(22–64 K) = ( 3.6 ± 0.1 ) × 10 − 12 ( T / 300 K ) − ( 1.0 ± 0.2 ) cm³ molecule⁻¹ s⁻¹. Implementing this expression in a chemical model of a cold, dense cloud results in CH₃O/CH₃OH abundance ratios similar to or slightly lower than the value of ∼3 × 10⁻³ observed in Barnard 1b. This finding confirms that the gas-phase reaction between OH and CH₃OH is an important contributor to the formation of interstellar CH₃O. The role of grain-surface processes in the formation of CH₃O, although it cannot be fully neglected, remains controversial.