Rate of Gas Phase Association of Hydroxyl Radical and Nitrogen Dioxide

The reaction of OH and NO2 to form gaseous nitric acid (HONO2) is among the most influential in atmospheric chemistry. Despite its importance, the rate coefficient remains poorly determined under tropospheric conditions because of difficulties in making laboratory rate measurements in air at 760 torr and uncertainties about a secondary channel producing peroxynitrous acid (HOONO). We combined two sensitive laser spectroscopy techniques to measure the overall rate of both channels and the partitioning between them at 25 degrees C and 760 torr. The result is a significantly more precise value of the rate constant for the HONO2 formation channel, 9.2 (+/- 0.4) x 10(-12) cm(3) molecule(-1) s(-1) (1 SD) at 760 torr of air, which lies toward the lower end of the previously established range. We demonstrate the impact of the revised value on photochemical model predictions of ozone concentrations in the Los Angeles airshed.