HETEROGENEOUS REACTIONS IN A STRATOSPHERIC BOX MODEL - A SENSITIVITY STUDY

Recent laboratory data concerning the reactions of HCl and HOx on/in s ulfuric acid aerosol (Hanson et al., 1994), N2O5 and ClONO2 hydrolysis on the frozen aerosol (Hanson and Ravishankara, 1993a) and the temper ature dependence of the HNO3 absorption cross section (Burkholder et a l., 1993) indicate that a reevaluation of the role of heterogeneous re actions in the chemical balance of the stratosphere is required. A che mical module prepared for a;three-dimensional (3-D) global chemistry t ransport model (CTM) and a general circulation model (GCM) has been us ed to carry out a sensitivity study of the effects of heterogeneous re actions on/in the sulfate aerosol and on the polar stratospheric cloud (PSC) particles. We present here results for the latitudes 60 degrees S, 70 degrees S and 75 degrees S at the 50-mbar level. Our findings i ndicate that (I) the new values of the HNO3 cross sections result in l ower mixing ratios for NOx and make ozone more vulnerable to catalytic destruction by ClOx; (2) the effect of the heterogeneous reactions OH + HNO3(a) --> H2O + NO3 and HO2 + HO2(a) --> H2O2 + O-2 are small in comparison with the same gas phase reactions and play a negligible rol e for the ozone balance; (3) the HCl reactions in the sulfuric acid ae rosol at 60 degrees S and 70 degrees S increase the chlorine activatio n up to 0.53 parts per billion by volume (ppbv) and 0.72 ppbv, respect ively, for liquid aerosol and Up to 0.87 ppbv for frozen aerosol at 70 degrees S for volcanic conditions and this results in considerable oz one depletion at these latitudes; (4) studying the ozone ''hole'' phen omenon, we have considered the different initial ratios of ClONO2/HCl, of N2O5, galactic cosmic rays (GCRs), and longer lifetimes for the PS C. We have speculated an existence of the reaction N2O5 + HCl(a) --> C lNO2 + HNO3.