4366
J. Phys. Chem. B 1999, 103, 4366-4376
Effect of HNO3 and HCl on HDO Diffusion on Crystalline D2O Ice Multilayers Frank E. Livingston and Steven M. George* Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0215 ReceiVed: August 7, 1998; In Final Form: March 1, 1999
HDO diffusion kinetics on ultrathin (25-200 BL, 90-730 Å) HNO3- and HCl-dosed crystalline D2O ice multilayers were investigated using a combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth profiling. Crystalline hexagonal D2O ice multilayers were grown epitaxially on a Ru(001) single-crystal substrate. HDO diffusion into the ice bulk was measured along the c-axis of crystalline ice by monitoring the HDO and D2O LITD signals during depth profiling by isothermal multilayer desorption. The HDO diffusion rate into HNO3-dosed D2O ice films was ∼30-70 times slower than that of HDO diffusion into pure D2O ice. The measured HDO diffusion coefficients at initial HDO and DNO3 coverages of 0.5-3.0 bilayers (BLs) ranged from D ) 2.5((0.3) × 10-18 cm2/s at T ) 150 K to D ) 1.5((0.2) × 10-15 cm2/s at T ) 173 K. Arrhenius analysis yielded diffusion kinetic parameters of Do ) 71.9 ( 9.2 cm2/s and EA ) 13.2 ( 1.4 kcal/mol. In contrast, the HDO diffusion rate into HCl-dosed D2O ice films was ∼10-20 times faster than that of HDO diffusion into pure D2O ice. The measured HDO diffusion coefficients at initial HDO and DCl coverages of 0.3-5.0 BL varied from D ) 9.8((0.7) × 10-17 cm2/s at T ) 146 K to D ) 3.5((0.2) × 10-14 cm2/s at T ) 161 K. Arrhenius analysis yielded diffusion kinetic parameters of Do ) 2.4((0.1) × 1012 cm2/s and EA ) 19.0 ( 0.3 kcal/mol. Measurements of HDO surface diffusion were also conducted using LITD probing to monitor the relaxation of a HDO coverage gradient on the basal (001) face of the ice multilayer. HDO surface diffusion on HNO3- and HCl-dosed ice multilayers was not observed within the resolution of the LITD experiment at 140 K. The lack of measurable HDO surface diffusion is consistent with HDO diffusion into the ice bulk at the surface diffusion temperatures. The diffusion kinetics predicted at stratospheric temperatures of T ) 180-210 K indicate that H2O molecules readily diffuse into pure crystalline ice on the millisecond to microsecond time scale. In the presence of HNO3 and HCl, the H2O surface residence times are considerably increased and decreased, respectively. The effect of HNO3 and HCl on the H2O surface residence time may influence heterogeneous atmospheric chemistry by altering absorption rates into ice cloud particles.
I. Introduction Reactions on ice surfaces and in the ice bulk have received significant attention for their importance in atmospheric chemistry,1-5 meteorology,6-12 and climatology.13-16 Heterogeneous chemical processes on polar stratospheric ice clouds (PSCs) have been shown to play a critical role in the seasonal depletion of ozone in the Antarctic.1,2,4,5,17-20 In particular, stratospheric ice particles convert relatively stable forms of reservoir chlorine (ClONO2, HCl) into photochemically labile chlorine species (Cl2, HOCl) by such heterogeneous reactions as ClONO2 + HCl f Cl2 + HNO3. The active chlorine molecules can then be photolyzed to produce Cl radicals that can catalytically destroy ozone. Polar stratospheric clouds are classified by type and are composed of either concentrated solutions of nitric acid (Type I PSC) or water-ice (Type II PSC).4,21-23 Microphysical processes on cirrus and convective ice clouds can also have a direct and dramatic impact on the Earth’s climate.13-16 For example, the chemistry and lifetimes of cirrus ice particles can markedly influence the global radiation budget13-16 as well as the glaciation of lower-level supercooled clouds.24-26 The lower-level supercooled ice crystals can further affect atmospheric conditions by triggering storm development.24-26 Atmospheric ice surfaces are highly dynamic at stratospheric conditions.27,28 The H2O desorption kinetics predict that H2O
desorbs from crystalline H2O ice at rates of approximately 1 × 1016-1 × 1018 molecules/(cm2s) or ∼10-1100 BL/s at typical polar stratospheric temperatures from 180 to 210 K.27,28 One bilayer (BL) on the basal plane surface of hexagonal ice corresponds to 1.06 × 1015 H2O molecules/cm2.29 H2O molecules also readily diffuse into the crystalline ice bulk on the microsecond to millisecond time scale at stratospheric temperatures.30-33 The dynamic nature of the ice surface may influence the kinetics and mechanisms of the important heterogeneous reactions on PSCs and alter fundamental microphysical processes on cirrus (anvil) ice cloud particles.27,34 The presence of atmospheric species, such as HNO3 and HCl, may significantly perturb H2O desorption and diffusion on crystalline ice. Recent isothermal laser-induced thermal desorption (LITD) measurements of H2O desorption kinetics from HNO3- and HCl-dosed crystalline ice multilayers demonstrated that the absolute H2O desorption rates are severely altered in the presence of acid impurities.35 For example, the H2O desorption rates from HNO3-dosed and HCl-dosed ice were ∼5 times smaller and ∼2 times larger, respectively, than H2O desorption rates from pure ice over the temperature range from T ) 150-171 K.35 Consequently, the presence of atmospheric acids can affect the formation and stability of ice clouds and influence the time period for heterogeneous ice chemistry. The measurements of H2O self-diffusion in crystalline ice are fairly numerous.36-46 H2O bulk diffusion in macroscopic
10.1021/jp9833294 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/12/1999
Crystalline D2O Ice Multilayers natural and artificial crystalline and polycrystalline ice has been measured using isotopic tracer techniques.37-44 The isotopic tracer studies have examined the diffusion rate of H218O, D2O, and T2O in ice at temperatures of T ) 238-273 K near the ice melting point. X-ray topography experiments on the growth of interstitial-type dislocation loops and dipoles on the surface of crystalline H2O ice have been used to determine indirectly the H2O diffusion coefficients in ice.45,46 Nuclear magnetic resonance (NMR) spectroscopy has also been utilized to measure H2O self-diffusion in the “quasiliquid layer” on micron-sized (
