Gas phase rate coefficients and activation energies for the reaction of butanal and 2-methyl-propanal with nitrate radicals

Rate coefficients have been determined for the reaction of butanal and 2-methyl-propanal with NO3 using relative and absolute methods. The relative measurements were accomplished by using a static reactor with long-path FTIR spectroscopy as the analytical tool. The absolute measurements were made using fast-flow-discharge technique with detection of NO3 by optical absorption. The resulting average coefficients from the relative rate experiments were kNO(3)298 = (1.0±0.1)×10-14 and kNO(3)298 = (1.2±0.2)×10-14 (cm3 molecule-1 s-1) for butanal and 2-methyl-propanal, respectively. The results from the absolute measurements indicated secondary reactions involving NO3 radicals and the primary formed acyl radicals. The prospect of secondary reactions was investigated by means of mathematical modeling. Calculations indicated that the unwanted NO3 radical reactions could be suppressed by introducing molecular oxygen into the flow tube. The rate coefficients from the absolute rate experiments with oxygen added were and kNO(3)298 = (1.2±0.1)×10-14 and NO(3)298 = (0.9±0.1)×10-14 (cm3 molecule-1 s-1) for butanal and 2-methyl-propanal. The temperature dependence of the reactions was studied in the range between 263 and 364 K. Activation energies for the reactions were determined to 12±2 kJ mole-1 and 14±1 kJ mole-1 for butanal and 2-methyl-propanal, respectively.