Studies of reactions of importance in the stratosphere. III. Rate constant and products of the reaction between ClO and HO2 radicals at 298 K

Abstract

The rate constant for the radical–radical reaction ClO+HO2→HOCl+O2 was measured at 298 K by the discharge flow technique using mass spectrometry for detection of the HOCl product at m/e=52. The ClO radical was generated by reacting ozone with chlorine atoms produced in a microwave discharge, and the concentration of ClO determined by measuring the decrease in ion current due to Cl2+ at m/e=70 upon activation of the discharge. This method was found to be in agreement with a nitric oxide titration of ClO and with the stochiometric conversion of ClO to NO2 by reaction with a large excess of NO followed by absolute calibration for NO2 at m/e=46. Two reactions were used to generate the hydroperoxyl radical: (1) H+O2+M→HO2+M, and (2) Cl+H2O2→HCl +HO2. The rate constant k1 was found to be independent of pressure over the range 2–6 Torr, the result being k1= (4.5±0.9) ×10−12 cm3 molecule−1 s−1, where the error includes our estimate of the maximum possible systematic error. An upper limit of 2% for the branching ratio to the alternative products of this reaction, HCl+O3, was established by attempting to detect ozone as a reaction product. For these measurements the reactions Cl+ClOCl→Cl2+ClO and Cl+OClO →2ClO were used to generate the ClO radical in the absence of ozone. No other reaction products could be identified in the mass spectrum.