A LOW-TEMPERATURE INFRARED STUDY OF THE REACTIONS OF THE STRATOSPHERIC NOY RESERVOIR SPECIES DINITROGEN PENTOXIDE WITH WATER ICE, 80-160 K

The low-temperature chemistry of thin films of water ice and the impor tant stratospheric NOy species dinitrogen pentoxide have been investig ated in order to spectroscopically characterize one of the principal h eterogeneous reactions that occurs on polar stratospheric cloud partic les (PSCs). This contributes to stratospheric dentrification. Using re flection-adsorption infrared spectroscopy, we have observed the format ion of both covalent and ionic forms of solid N2O5 on both ice and the clean substrate. Thermal evolution experiments suggest that the solid covalent phase, which can only be formed at the lowest temperatures ( T<100K), is metastable with respect to the ionic phase. However, we ob serve no reaction between the solid ionic form and the pure ice film a t T<170K. Reaction does occur above 140K between the ice film and gas- phase covalent N2O5 to form a surface layer of hydroxonium ions, solva ted nitrate ions, and molecular nitric acid. Annealing to 160K introdu ces more water into the surface layer by diffusion and produces a furt her reaction of the excess NO2+ and results in the formation of more m olecular nitric acid. Some of the water reacts with molecular nitric a cid to form an amorphous hydrate (H2O)(n).H3O+NO3-. From this, we conc lude that only N2O5 in the gas-phase covalent form reacts readily with ice and consequently that the reaction in the stratosphere is likely to involve the interaction between the gas phase and the ice surface a nd not a solid-solid interfacial one. The reaction of gas-phase N2O5 w ith a nitrate-containing ice surface and the competition between surfa ce species for excess water has important implications for different N 2O5 chemistry on type I (NAT) and type II (ice) PSC particles.