Rate coefficients have been determined for the reaction of butanal and 2-methylpropanal with NO 3 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 NO 3 by optical absorption. The resulting average coefficients from the relative rate experiments were and k
for butanal and 2-methyl-propanal, respectively. The results from the absolute measurements indicated secondary reactions involving NO 3 radicals and the primary formed acyl radicals. The prospect of secondary reactions was investigated by means of mathematical modeling. Calculations indicated that the unwanted NO 3 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 and 298 Ϫ14 298 k ϭ (1.2 Ϯ 0.1) ϫ 10 ϭ (0.9 Ϯ 0.1) ϫ NO NO 3 3
(cm 3 molecule Ϫ1 s Ϫ1 ) for butanal and 2-methyl-propanal. The temperature dependence Ϫ14 10 of the reactions was studied in the range between 263 and 364 K. Activation energies for the reactions were determined to 12 Ϯ 2 kJ mole -1 and 14 Ϯ 1 kJ mole -1 for butanal and 2-methylpropanal, respectively.