Kinetics and mechanism of HO2 uptake on solid NaCl

The interaction of HO2 radicals with solid NaCl has been investigated. The uptake coefficient gamma was measured using a coaxial reactor with a movable central rod covered with NaCl. The radicals were detected at low concentrations (similar to4 x 10(10) molecule/cm(3)) by matrix isolation ESR and at high concentrations (similar to5 x 10(11) molecule/cm(3)) by titration with NO followed by gas-phase EPR detection. In the temperature range from 243 to 295 K, the apparent activation energy of gamma does not depend on the HO2 concentration. The gamma value measured on NaCl agrees well with our previous gamma value for nonreactive uptake by NH4NO3 in this temperature range. Furthermore, in the temperature range from 331 to 335 K, the gamma value decreases sharply at low HO2 concentrations. A combined Eley-Rideal and Langmuir-Hinshelwood mechanism with reasonable parameter values can explain the observations; such a mechanism is also consistent with the observation of an inhibition effect of water vapor on the uptake coefficient. The conclusion is that, in the coastal troposphere, the heterogeneous loss of HO2 is of comparable importance to the homogeneous loss, particularly in the evening, when the HO2 concentration is low.