Studies of reactions of importance in the stratosphere. VI. Temperature dependence of the reactions O+NO2→NO+O2 and O+ClO→Cl+O2

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Abstract
The reactions of two free radicals, nitrogen dioxide and chlorine monoxide, with oxygen atoms, O+NO2→NO+O2 and O+ClO→Cl+O2, have been investigated in a fast flow system over the temperature range 220–387 K, at a total pressure of 2.3 Torr. Reaction rates were measured under psuedo‐first‐order conditions with either ClO or NO2 in large excess and the decay of oxygen atoms monitored by adding nitric oxide at the end of the flow tube and detecting the O+NO+M chemiluminescence. The reactions are of interest, because they are thought to be the rate determining steps in two catalytic chains which destroy odd oxygen (O and O3) in the stratosphere. Measurements of the rate constants over the stated temperature interval yielded the Arrhenius expressions kO+NO2=(6.58±0.52)×10−12 exp[(142±23)/T] and kO+ClO=(2.61±0.60)×10−11 exp[(97±64)/T]. Thus, both reactions display small negative activation energies. In addition, experiments showed no pressure effect on the reaction rate in the range 2.3–5.3 Torr and at a temperature of 230 K. Addition of a partial pressure of 1 Torr of oxygen also had no effect on the rate constant at 230 K, suggesting that ClO ⋅ O2 formation is of little or no significance under stratospheric conditions.