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The radical terminating, termolecular reaction between OH and NO2 exerts great influence on the NOy/NOx ratio and O-3 formation in the atmosphere. Evaluation panels (IUPAC and NASA) recommend rate coefficients for this reaction that disagree by as much as a factor of 1.6 at low temperature and pressure. In this work, the title reaction was studied by pulsed laser photolysis and laser-induced fluorescence over the pressure range 16-1200 mbar and temperature range 217-333 K in N-2 bath gas, with experiments at 295 K (67-333 mbar) for O-2. In situ measurement of NO2 using two optical absorption set-ups enabled generation of highly precise, accurate rate coefficients in the fall-off pressure range, appropriate for atmospheric conditions.

We found, in agreement with previous work, that O-2 bath gas has a lower collision efficiency than N-2 with a relative collision efficiency to N-2 of 0.74. Using the Troe-type formulation for termolecular reactions we present a new set of parameters with k(0)(N-2) = 2.6 x 10(-30) cm(6) molecule(-2) s(-1), k(0)(O-2) = 2.0 x 10(-30) cm(6) molecule(-2)s(-1), m = 3.6, k(infinity) = 6.3 x 10(-11 )cm(3) molecule(-1) s(-1), and F-c = 0.39 and compare our results to previous studies in N-2 and O-2 bath gases.