CONDENSED-PHASE PRODUCTS IN HETEROGENEOUS REACTIONS - N2O5, CLONO2, AND HNO3 REACTING ON ICE FILMS AT 185 K

Heterogeneous reactions are important in a wide variety of chemical pr ocesses. In many cases reactions on a surface will change both the phy sical and chemical characteristics of the surface, which in turn will change the surface reactivity toward further gas/surface collisions. A s a case study of relevance to the atmosphere, we have investigated th e reactions of the NOy species ClONO2, N2O5, and HNO3 on thin ice film s representative of water-ice polar stratospheric clouds (type II PSCs ). Although these species are known to produce HNO3 upon reacting with the ice surface, the phase, composition, and state of adsorption (phy sical versus chemical) of the surface reaction product are not known. These reactions were studied using a Knudsen cell reactor to probe het erogeneous reaction rates, mass spectrometry to identify gas-phase rea ctants and products, and FTIR reflection-absorption spectroscopy to pr obe the phase and composition of the condensed phase. Under ice frost point conditions at 185 K, each NOy species reacted with ice to form a metastable supercooled H2O/HNO3 liquid layer. Although a crystalline 3:1 H2O:HNO3 hydrate is most thermodynamically stable under these cond itions, a supercooled liquid with a composition slightly more dilute t han 3:1 H2O:HNO3 continued to grow throughout the NOy exposure period. This product composition is similar to that expected for liquid type Ib PSCs in the atmosphere. ClONO2 and N2O5 reacted with the supercoole d H2O/HNO3 liquid layer at 185 K with a reactive uptake coefficient of gamma = 0.003 +/- 0.002 and gamma = 0.0007 +/- 0.0003, respectively. These measured rate coefficients are about 2 orders of magnitude lower than the corresponding reaction rates on pure ice but are comparable to those measured on crystalline nitric acid trihydrate (NAT) or nitri c acid dihydrate (NAD) surfaces representative of type Ia PSCs. HNO3 r eacted with the supercooled liquid layer with gamma > 0.02. When H2O v apor pressures were decreased to below the ice frost point, the superc ooled H2O/HNO3 liquid layer became more concentrated in HNO3 as H2O pr eferentially desorbed. Only during desorption when stoichiometric rati os of 3:1 or 2:1 H2O:HNO3 were obtained.did the supercooled liquid lay er crystallize to NAT or NAD, respectively. These results suggest that water-ice particles in the polar stratosphere may be initially coated with a supercooled H20/HNO3 liquid layer and that heterogeneous nucle ation of NAT on ice from either the gas phase or the H2O/HNO3 supercoo led liquid phase is slow. The implications of a supercooled H2O/HNO3 l iquid layer on ice will be discussed in the context of polar ozone dep letion.