Kinetics and Mechanism of HO2 Uptake on Solid NaCl

The interaction of HO 2 radicals with solid NaCl has been investigated. The uptake coefficient γ was measured using a coaxial reactor with a movable central rod covered with NaCl. The radicals were detected at low concentrations (∼4 × 10 10 molecule/cm 3 ) by matrix isolation ESR and at high concentrations (∼5 × 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 γ does not depend on the HO 2 concentration. The γ value measured on NaCl agrees well with our previous γ value for nonreactive uptake by NH 4 NO 3 in this temperature range. Furthermore, in the temperature range from 331 to 335 K, the γ value decreases sharply at low HO 2 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 HO 2 is of comparable importance to the homogeneous loss, particularly in the evening, when the HO 2 concentration is low.