Effect of HNO₃ and HCl on HDO Diffusion on Crystalline D₂O Ice Multilayers

HDO diffusion kinetics on ultrathin (25−200 BL, 90−730 Å) HNO 3 - and HCl-dosed crystalline D 2 O ice multilayers were investigated using a combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth profiling. Crystalline hexagonal D 2 O 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 D 2 O LITD signals during depth profiling by isothermal multilayer desorption. The HDO diffusion rate into HNO 3 -dosed D 2 O ice films was ∼30−70 times slower than that of HDO diffusion into pure D 2 O ice. The measured HDO diffusion coefficients at initial HDO and DNO 3 coverages of 0.5−3.0 bilayers (BLs) ranged from D = 2.5(±0.3) × 10 -18 cm 2 /s at T = 150 K to D = 1.5(±0.2) × 10 -15 cm 2 /s at T = 173 K. Arrhenius analysis yielded diffusion kinetic parameters of D o = 71.9 ± 9.2 cm 2 /s and E A = 13.2 ± 1.4 kcal/mol. In contrast, the HDO diffusion rate into HCl-dosed D 2 O ice films was ∼10−20 times faster than that of HDO diffusion into pure D 2 O 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 cm 2 /s at T = 146 K to D = 3.5(±0.2) × 10 -14 cm 2 /s at T = 161 K. Arrhenius analysis yielded diffusion kinetic parameters of D o = 2.4(±0.1) × 10 12 cm 2 /s and E A = 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 HNO 3 - 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 H 2 O molecules readily diffuse into pure crystalline ice on the millisecond to microsecond time scale. In the presence of HNO 3 and HCl, the H 2 O surface residence times are considerably increased and decreased, respectively. The effect of HNO 3 and HCl on the H 2 O surface residence time may influence heterogeneous atmospheric chemistry by altering absorption rates into ice cloud particles.