High-pressure flow study of the reactions OH+NOx->HONOx: Errors in the falloff region

We present data for the rate constant of the reaction NO2 + OH --> HONO2 in nitrogen from 2 to 600 torr at 300 K. This is the first application of our high-pressure flow technique to a pressure-dependent reaction. The pressure range in this experiment overlaps the ranges covered by traditional discharge flow studies and flash photolysis studies, allowing exploration of the transition between the low-pressure and intermediate pressure regimes. The measured rate constants are in excellent agreement with previously published values; however, current recommendations systematically overestimate the room temperature rate constant by between 10 and 30% in the pressure range 20 - 700 torr. A reanalysis of all available data yields a new recommendation in good agreement with mast of the data over the entire observational pressure range. This analysis includes an explicit treatment of the collisional efficiencies of different bath gases and includes the extreme broadening of the pressure falloff curve caused by the very strong HO-NO2 bond. We also report room temperature results for the reaction NO + OH over a more limited pressure range (4 to 75 torr). Our data agree with the currently recommended rate constants over this range.