UPTAKE OF HCL IN WATER ICE AND NITRIC-ACID ICE FILMS

The uptake of HCl in water ice and nitric acid ice films has been inve stigated in a flow reactor interfaced with a differentially pumped qua drupole mass spectrometer. These studies were performed under experime ntal conditions that may mimic the polar stratosphere. The HCl uptake in ice films at 188 and 193 K was determined to be in the range of 8.7 X 10(13) to 1.8 X 10(15) molecules/cm2 (if the geometric area of the flow reactor, 290 cm2, was used in the calculation) when HCl partial p ressures of 7 X 10(-8) to 6 X 10(-6) Torr were used. On the basis of a model which accounts for the total surface area of the films, the tru e surface density could be a factor of 25 lower than that calculated b y the geometric area. A slightly higher uptake was observed at the low er temperature of 188 K. The uptake of HCl in ice was significantly en hanced by using an HCl partial pressure greater than 1 X 10(-5) Torr. The observation was found to be consistent with the formation of the h exahydrate or the trihydrate of HCl according to the phase diagram of the HCl/H2O system. The uptake of HCl in nitric acid ice at 188 K was determined to be in the range of 8.0 X 10(13) to 5.3 X 10(14) molecule s/cm2 at a HCl partial pressure of 4.5 X 10(-7) Torr. Measurement of b oth HNO3 and H2O vapor pressures was made to positively identify the f ormation of nitric acid trihydrate (NAT) surface according to the phas e diagram of the HNO3/H2O system. The HCl uptake in NAT is comparable to that in water ice in the present experiment, but significantly smal ler than the previously reported values by Mauersberger and his co-wor kers. Implications of these results for the heterogeneous chemistry of the polar ozone depletion are briefly discussed.