Heterogeneous reaction of NO3 with ice and sulfuric acid solutions: Upper limits for the uptake coefficients

Using a Knudsen cell reactor, we have studied the uptake kinetics of the nitrate radical, NO3, on ice and on sulfuric acid solutions. The experiments on ice were carried out over the temperature range 170 < T/K < 200. Experiments with liquid sulfuric acid were performed over the concentration range 60-95 wt % H2SO4. Nitrate radical is detected by laser-induced fluorescence. Measures had to be taken to characterize the fluorescence quenching by water vapor, for which we found a bimolecular rate constant of (6.9 +/- 0.5) x 10(-10) cm(3) molecule(-1) s(-1) Also, the rate of NO3 disappearance on the walls of the Teflon-coated reactor is determined and accounted for in the kinetic analysis. We find that, within the detection limit of our apparatus, there is no interaction between NO3 and these surfaces, allowing us to report the upper limit for the uptake coefficient of gamma < 10(-3).