Nitrate photolysis proceeds via two major channels at illumination wavelengths above 290 nm: NO 3 -+ hν (+H + ) f NO 2 + • OH (1) and NO 3 -+ hν f NO 2 -+ O( 3 P) (2). A recent study determined the quantum yield of reaction 1 on ice by measuring NO 2 production, but suggested their values might be lower bounds because of incomplete recoveries of NO 2 . We measured the quantum yield of pathway 1 using an alternate approach, i.e., by following the formation of • OH. Our quantum yields for • OH (Φ OH ) at 263 K were independent of nitrate concentration and illumination wavelength (λ > 300 nm), but were dependent upon pH. Values of Φ OH decreased from (3.6 ( 0.6) × 10 -3 at pH 7.0 to (2.1 ( 0.8) × 10 -3 at pH 2.0, where the listed pH values are those of the sample solution prior to freezing. Temperature dependence experiments (239-318 K; pH 5.0) showed that values of Φ OH in ice pellets and aqueous solutions were both well described by the same regression line, ln(Φ OH ) ) ln(Φ 1 ) ) -(2400 ( 480)(1/T) + (3.6 ( 0.8) (where errors represent (1σ), suggesting that the photolysis of nitrate on ice occurs in a "quasi-liquid layer" rather than in the bulk ice. Our ice quantum yields between 268 and 240 K are 3-9 times higher, respectively, than Φ 1 values determined previously in ice. Applying our quantum yields to past field experiments indicates that nitrate photolysis can account for the flux of NO x from sunlit snow in the Antarctic and at Summit, Greenland, but that nitrate was only a minor source of the snowpack NO x measured during the Alert 2000 campaign in the Canadian Arctic. Additional calculations show that the photolysis of nitrate on cirrus clouds in the upper troposphere is a minor source of NO x that cannot account for the apparent underestimation of the ratio of NO x /HNO 3 in current numerical models.

Hydrogen peroxide (HOOH) in ice and snow is an important chemical tracer for the oxidative capacities of past atmospheres. However, photolysis in ice and snow will destroy HOOH and form the hydroxyl radical (*OH), which can react with snowpack trace species. Reactions of *OH in snow and ice will affect the composition of both the overlying atmosphere (e.g., by the release of volatile species such as formaldehyde to the boundary layer) and the snow and ice (e.g., by the *OH-mediated destruction of trace organics). To help understand these impacts, we have measured the quantum yield of *OH from the photolysis of HOOH on ice. Our measured quantum yields (Phi(HOOH --> *OH)) are independent of ionic strength, pH, and wavelength, but are dependent upon temperature. This temperature dependence for both solution and ice data is best described by the relationship ln(Phi(HOOH --> *OH)) = -(684 +/- 17)(1/T) + (2.27 +/- 0.064) (where errors represent 1 standard error). The corresponding activation energy (Ea) for HOOH (5.7 kJ mol(-1)) is much smaller than that for nitrate photolysis, indicating that the photochemistry of HOOH is less affected by changes in temperature. Using our measured quantum yields, we calculate that the photolytic lifetimes of HOOH in surface snow grains under midday, summer solstice sunlight are approximately 140 h at representative sites on the Greenland and Antarctic ice sheets. In addition, our calculations reveal that the majority of *OH radicals formed on polar snow grains are from HOOH photolysis, while nitrate photolysis is only a minor contributor. Similarly, HOOH appears to be much more important than nitrate as a photochemical source of *OH on cirrus ice clouds, where reactions of the photochemically formed hydroxyl radical could lead to the release of oxygenated volatile organic compounds to the upper troposphere.

The uptake of HCl in water ice and nitric acid ice films has been investigated in a flow reactor interfaced with a differentially pumped quadrupole mass spectrometer. These studies were performed under experimental conditions that may mimic the polar stratosphere. The HCl uptake in ice films at 188 and 193 K was determined to be in the range of 8.7 X 1013 to 1.8 X 1015 molecules/cm2 (if the geometric area of the flow reactor, 290 cm2, was used in the calculation) when HCl partial pressures of 7 X 10-8 to 6 X 1 0 6 Torr were used. On the basis of a model which accounts for the total surface area of the films, the true surface density could be a factor of 25 lower than that calculated by the geometric area. A slightly higher uptake was observed at the lower temperature of 188 K. The uptake of HC1 in ice was significantly enhanced by using an HCl partial pressure greater than 1 X Torr. The observation was found to be consistent with the formation of the hexahydrate or the trihydrate of HC1 according to the phase diagram of the HCl/H20 system. The uptake of HCl in nitric acid ice at 188 K was determined to be in the range of 8.0 X 1013 to 5.3 X lOI4 molecules/cm2 at a HCl partial pressure of 4.5 X lo-' Torr. Measurement of both H N 0 3 and H2O vapor pressures was made to positively identify the formation of nitric acid trihydrate (NAT) surface according to the phase diagram of the HNOs/ H2O system. The HCl uptake in NAT is comparable to that in water ice in the present experiment, but significantly smaller than the previously reported values by Mauersberger and his co-workers. Implications of these results for the heterogeneous chemistry of the polar ozone depletion are briefly discussed.

We have studied the photolysis of nitric acid (HNO(3)) in the gas phase at 253 and 295 K, on aluminum surfaces at 253 and 295 K, and on ice films at 253 K, by using 308 nm excimer laser photolysis combined with cavity ring-down spectroscopy. We monitored both the ground-state NO(2) and the electronically excited NO(2), NO(2)*, produced from the HNO(3) photolysis. NO(2)* + OH is a predominant photolysis pathway (if not the only photolysis pathway) from the gas-phase photolysis of HNO(3) at 308 nm. The NO(2)* quantum yields from the HNO(3) photolysis on aluminum surfaces are 0.80 +/- 0.15 at 295 K and 0.92 +/- 0.26 at 253 K, where errors quoted represent 2sigma measurement uncertainty. The corresponding NO(2)* quantum yield from the HNO(3) photolysis on ice films is 0.60 +/- 0.34 at 253 K. The 308 nm absorption cross sections of HNO(3) on Al surfaces and on ice films have been directly measured. Absorption cross sections of HNO(3) on Al surface at 308 nm are (4.19 +/- 0.17) x 10(-18) and (4.23 +/- 0.45) x 10(-18) cm(2)/molecule at 253 and at 295 K, whereas the corresponding absorption cross section of HNO(3) on ice films is (1.21 +/- 0.31) x 10(-18) cm(2)/molecule at 253 K (errors quoted represent 2sigma measurement uncertainty). Atmospheric implications of the results are discussed.