Kinetics of the reactions of OH radicals with C2H2 and CO

Absolute rate constants for the reactions of OH radicals with C2H2 (k1) and CO (k2) have been determined using a flash photolysis–resonance fluorescence technique. Rate constants k1 were determined at room temperature over the pressure range 25–400 torr total pressure of argon, and over the temperature range 298–422 °K at ∼200 torr total pressure of argon. k1 was pressure dependent below ∼200 torr total pressure of argon, but was in the high pressure kinetic region at total pressures ?200 torr. The Arrhenius expression in the high pressure region was k1=1.91×10−12e−(620±400)/RT cm3 molecule−1 sec−1. Rate constants k2 were determined at 299±1 °K with argon and SF6 as diluent gases over the pressure range 25–643 torr total pressure. With argon as the diluent gas k2 showed, within experimental error, little or no pressure dependence as reported earlier from this laboratory. However, with SF6 as the diluent gas k2 increased from (1.53±0.16) ×10−13 cm3 molecule−1 sec−1 at 25 torr total pressure to (3.43±0.35) ×10−13 cm3 molecule−1 sec−1 at 604 torr total pressure. This observation of a pressure dependence for SF6 is in agreement with other recent literature data for M=SF6, M=N2+O2 and M=H2. This pressure dependence of k2 has obvious implications regarding the conversion of CO to CO2 and the concentrations of the OH radical in the troposphere.