Influence of temperature on the chemical removal of 3-methylbutanal, trans-2-methyl-2-butenal, and 3-methyl-2-butenal by OH radicals in the troposphere

Absolute rate coefficients for the gas-phase reactions of OH radical with 3-methylbutanal (k1), trans-2-methyl-2-butenal (k2), and 3-methyl-2-butenal (k3) have been obtained with the pulsed laser photolysis/laser-induced fluorescence technique. Gas-phase concentration of aldehydes was measured by UV absorption spectroscopy at 185 nm. Experiments were performed over the temperature range of 263–353 K at total pressures of helium between 46.2 and 100 Torr. No pressure dependence of all ki (i = 1–3) was observed at all temperatures. In contrast, a negative temperature dependence of ki (i.e., ki increases when temperature decreases) was observed in that T range. The resulting Arrhenius expressions (±2σ) are: k1(T) = (5.8 ± 1.7)×10−12 exp{(499 ± 94)/T} cm3 molecule−1 s−1, k2(T)=(6.9 ± 0.9)×10−12 exp{(526 ± 42)/T} cm3 molecule−1 s−1, k3(T)=(5.6 ± 1.2)×10−12 exp{(666 ± 54)/T} cm3 molecule−1 s−1.The tropospheric lifetimes derived from the above OH-reactivity trend are estimated to be higher for 3-methylbutanal than those for the unsaturated aldehydes. A comparison of the tropospheric removal of these aldehydes by OH radicals with other homogeneous degradation routes leads to the conclusion that this reaction can be the main homogeneous removal pathway. However, photolysis of these aldehydes in the actinic region (λ > 290 nm) could play an important role along the troposphere, particularly for 3-methyl-2-butenal. This process could compete with the OH reaction for 3-methylbutanal or be negligible for trans-2-methyl-2-butenal in the troposphere.