HETEROGENEOUS INTERACTIONS OF CLONO2 AND HCL ON NITRIC-ACID TRIHYDRATE AT 202-K

Using a low-pressure flow tube coupled to a mass spectrometer, reaction probabilities (gamma's) for ClONO2 + H2O --> HOCl + HNO3 (1) and ClONO2 + HCl --> Cl2 + HNO3 (2) have been measured on nitric acid trihydrate (NAT) films at 202 K for reactant partial pressures in the 10(-6) Torr range. When the water vapor pressure over the NAT film approaches that of ice (P(H2O) almost-equal-to 1.5 x 10(-3) Torr, H2O-rich NAT), gamma-1 = 0.002 +/- 0.001 and gamma-2 > 0.2. For lower water partial pressures (P(H2O) almost-equal-to 2 x 10(-4) Torr), characteristic of HNO3-rich NAT, the gamma's decrease by 2 orders of magnitude. For HCl partial pressures of 5 x 10(-6) Torr, the experiments indicate that H2O-rich Nat films take up HCl in amounts similar to those taken up by water-ice surfaces (almost-equal-to 1 x 10(15) molecules/cm2); HNO3-rich NAT films take up 2 orders of magnitude less HCl (<1 x 10(13) molecules/cm2). At high HCl partial pressures (> 1 x 10(-4) Torr), very much greater uptake by both H2O-rich and HNO3-rich films is observed, indicating that the NAT films melt under these conditions.