The surface specific nonlinear laser spectroscopy method second harmonic generation (SHG) is used to investigate the heterogeneously catalyzed hydrolysis of chlorine nitrate (ClONO2) on ice, a key reaction in stratospheric ozone depletion occurring in the presence of polar stratospheric cloud (PSC) ice particles formed during the polar winter. The reaction, yielding hypochlorous acid (HOCl) and nitric acid (HNO3), is studied directly and in real time on a single crystal basal ice (Ih) surface maintained under typical conditions of the polar stratosphere. The ice crystal is kept in equilibrium with its vapor pressure.; Polarization studies are consistent with the clean basal ice surface at 158K being 3m symmetric, in contrast to proposals by others that the surface is disordered. The symmetry is retained upon HNO3 adsorption; this observation disagrees with proposals by others that this could cause surface melting.; A SHG spectrum from 290 to 310 nm is obtained from HOCl on ice; this spectrum resembles the electronic spectrum of HOCl and serves as an identification tool for adsorbed HOCl. HOCl adsorption onto ice is instantaneous and occurs in registry with the underlying ice lattice. Measured isothermal rate constants for HOCl desorption from ice result in an activation energy for desorption of 36 +/- 2 kJ/mol.; When submonolayer amounts of ClONO2 are hydrolyzed on the ice surface, the SHG vs. time traces show no changes for hundreds of seconds, then a sigmoidal increase, and eventually a constant value. The SHG increase is related to the appearance of HOCl. Predosing experiments show that the delay times are due to autocatalysis, with the HOCl product being a possible autocatalyst. The HNO3 co-product, on the other hand, acts as a surface poison and inhibits HOCl desorption.; A molecular reaction mechanism, based on one proposed by Bianco and Hynes, is presented and discussed in light of the obtained experimental data, supporting ab initio calculations, and numerical solutions to a kinetic model giving special consideration to the role of HNO3. Our results show that during the surface processes leading to stratospheric ozone depletion, heterogeneous reaction times start to compete with PSC particle lifetimes.
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