Laboratory studies of heterogeneous reactions on ice and nitric acid-doped ice surfaces representative of polar stratospheric clouds and cirrus clouds

Heterogeneous reactions on ice particles play an important role in the formation of the Antarctic ozone hole and in the chemistry of cirrus clouds and airplane condensation trails. Nonetheless, the phase, composition, and state of adsorption of surface products are largely unknown. To this end, atmospherically relevant molecules were studied on thin ice and HNO3 /H2O films. FTIR reflection absorption infrared spectroscopy was used to probe the condensed phases, and a Knudsen cell reactor was used to measure heterogeneous reaction rates.; N2O5 and ClONO2 reacted rapidly on ice at 185 K to form a supercooled ∼3:1 H2O:HNO3 liquid layer over the ice surface. Reaction efficiencies over the supercooled liquid layer were gamma = 0.0007 +/- 0.0003 for N2O5 and gamma = 0.0003 +/- 0.002 for ClONO2. Although nitric acid trihydrate (NAT) was most thermodynamically stable, the supercooled H 2O/HNO3 liquid never crystallized to NAT when at the ice frost point. These results suggest that polar stratospheric cloud (PSC) ice particles may be coated with a supercooled H2O/HNO3 liquid layer over the surface. Therefore, water-ice PSCs may be most accurately modeled with reaction rates over supercooled H2O/HNO3 liquids.; The reaction of ClONO2 on crystalline H2O/HNO 3 films was studied at 185 K. As the relative humidity increased from 5 to 130%, reaction efficiencies increased from 0.0004 to 0.007 and progressively water-rich surface layers formed. The availability of surface water largely controls the observed kinetics of this reaction.; The interaction of HNO3 and HCl with ice showed a monolayer and a multilayer uptake regime from 180--205 K delineated by the ice/liquid coexistence curve for each species. HBr and HI showed continuous uptake on ice with the formation of amorphous H2O/HX layers. All species are expected to show monolayer uptake under cirrus cloud conditions.; Finally, the uptake of methanol on ice was examined from 120--200 K. Methanol surface coverages at a partial pressure of 2 x 10 -7 Torr decreased from a full monolayer at 138 K to less than 0.001 monolayer at 175 K, suggesting that methanol will not be removed by cirrus cloud ice particles.