Daytime tropospheric loss of hexanal and trans-2-hexenal: OH kinetics and UV photolysis

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. 5 Absolute absorption cross sections (σ λ) were obtained using a recently built system operating in the UV region. This work represents the first reported σ λ for trans-2-hexenal. The obtained σ λ allowed the estimation of the photolysis rates (J) of hexanal and trans-2-hexenal 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 10 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 15 troposphere. The loss rate due to the OH chemical removal was compared with the estimated photolysis rates. Our results show that OH-reaction and UV photolysis are the main loss processes for these aldehydes in the troposphere. For hexanal, both processes compete across the troposphere, however, UV photolysis can contribute up to 70% to the overall loss of trans-2-hexenal.