Rate Constant Measurements for the Reaction of HO₂ with O₃ from 200 to 300 K Using a Turbulent Flow Reactor

The rate constant for the reaction of HO 2 with O 3 was measured using a turbulent flow reactor with tunable diode laser absorption detection of HO 2 . Two separate methods were used to determine k 1 from the pseudo-first-order decay of HO 2 in excess O 3 . The isotopic labeling method used H 18 O 2 in excess 16 O 3 to avoid reformation of the reactant. The OH scavenger method used trifluorochloroethylene to remove the OH product and prevent re-formation of HO 2 . The turbulent flow reactor allowed measurements at temperatures between 297 and 197 K at total pressures from 80 to 175 Torr, spanning a wide range of stratospheric conditions. The temperature-dependent rate constant is given by the three-term expression k 1 ( T ) = {(103 ± 57) exp[−(1323 ± 160)/ T ] + 0.88} × 10 -15 cm 3 molecule -1 s -1, reflecting its non-Arrhenius behavior at low temperatures. These results demonstrate that the catalytic destruction of lower stratospheric O 3 by HO x radicals (OH and HO 2 ) may proceed faster than current models predict. The rate constant at 295 K is (2.0 ± 0.2) × 10 -15 cm 3 molecule -1 s -1 .