Some reactions of the OH radical, which can be initiated by flash photolysis, have been studied using resonance absorption to follow how the OH concentration varies with time. The reaction OH + NO₂(+M)→ HNO₃+ M (4), has been investigated at 300 K and 416 K, and at total pressures (of He) between 20 and 300 Torr. In this range, it is in the transition region between second and third order kinetics and the variation of the rate with total pressure is compared to the behaviour predicted by RRKM theory. Association of OH with NO, OH + NO + M → HNO₂+ M, (5), has been studied, but only in its third order range, where k₅= 4.1 (±0.6 )× 10^–31 cm⁶ molecule^–2 s^–1 at 300 K and with M = He; the activation energy =–6.7 (± 2.0) kJ/mol. Some measurements have also been made on the bimolecular reactions, OH + HNO₃→ H₂O + NO₃(11) and O + HNO₃→ OH + NO₃. (12) At 300 K, k₁₁= 1.3 (± 0.5)× 10^–13 cm³ molecule^–1 s^–1 and k₁₂ < 1.3 × 10^–14 cm³ molecule^–1 s^–1.

The importance of these results for several complex chemical systems is discussed and work on the steady-state photolysis of NO₂—HNO₃ mixtures is re-evaluated.