Condensed-Phase Products in Heterogeneous Reactions: N2O5, ClONO2, and HNO3 Reacting on Ice Films at 185 K

Abstract

Heterogeneous reactions are important in a wide variety of chemical processes. In many cases reactions on a surface will change both the physical and chemical characteristics of the surface, which in turn will change the surface reactivity toward further gas/surface collisions. As a case study of relevance to the atmosphere, we have investigated the reactions of the NOy species ClONO2, N2O5, and HNO3 on thin ice films 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 (physical versus chemical) of the surface reaction product are not known. These reactions were studied using a Knudsen cell reactor to probe heterogeneous reaction rates, mass spectrometry to identify gas-phase reactants and products, and FTIR reflection−absorption spectroscopy to probe 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 conditions, a supercooled liquid with a composition slightly more dilute than 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 supercooled H2O/HNO3 liquid layer at 185 K with a reactive uptake coefficient of γ = 0.003 ± 0.002 and γ = 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 nitric acid dihydrate (NAD) surfaces representative of type Ia PSCs. HNO3 reacted with the supercooled liquid layer with γ > 0.02. When H2O vapor pressures were decreased to below the ice frost point, the supercooled H2O/HNO3 liquid layer became more concentrated in HNO3 as H2O preferentially desorbed. Only during desorption when stoichiometric ratios of 3:1 or 2:1 H2O:HNO3 were obtained did the supercooled liquid layer crystallize to NAT or NAD, respectively. These results suggest that water-ice particles in the polar stratosphere may be initially coated with a supercooled H2O/HNO3 liquid layer and that heterogeneous nucleation of NAT on ice from either the gas phase or the H2O/HNO3 supercooled liquid phase is slow. The implications of a supercooled H2O/HNO3 liquid layer on ice will be discussed in the context of polar ozone depletion.