Photochemical transformations in ice: implications for the fate of chemical species
Post-depositional photochemical alterations in snowpacks and sea ice may affect the chemical records in polar caps and the chemistry of the polar atmospheric boundary layer. Although it is known that UV-induced photochemistry actually occurs in ice matrices, quantitative information on such processe...
| Summary: | Post-depositional photochemical alterations in snowpacks and sea ice may affect the chemical records in polar caps and the chemistry of the polar atmospheric boundary layer. Although it is known that UV-induced photochemistry actually occurs in ice matrices, quantitative information on such processes is still lacking. With new methods for determining the light absorption by chromophores embedded in packed ice, this study investigates the rates and products of the photodegradation of 4-nitrophenol and nitrate in ice.
A quantum yield (Φ_(ice)) of (2.3 ± 0.4) x 10^(-4) was obtained for the photochemical degradation of 4-nitrophenol over the wavelength range of 300 to 370 nm in ice pellets (pH 5.6). Five reaction products were positively identified: hydroquinone, benzoquinone, 4-nitrosophenol, nitrate, and nitrite. Indirect evidence suggests the formation of organic polymers. These results are similar to those found for 4-nitrophenol photolysis in aqueous solutions, indicating that comparable mechanisms operate in both phases.
Upon irradiation (λ = 313 ± 15 nm) of NO_3^- doped ice layers, the formation of NO_2(g) and NO_2^- was observed. The yield for both products increased with temperature over the range 248 - 268 K; with values of Φ_(NO_2-) ~ (4.8 ± 1.5) x 10^(-3) and Φ'(NO_2) (1.2 ± 0.9) x 10^(-3) at 263 K, 10 mM KNO_3. The formation of NO_2^- during the photolysis of NO_3^- in ice pellets has apparent activation energy, E_a, of 5.8 kcal mole^(-1). This E_a is similar to the water cage-effect for supercooled water. Φ_(NO_2) showed a much stronger temperature dependence (E_a ~ 10 kcal mole^(-1)); This can be interpreted as the probability of the product NO_2 escaping into the gas-phase, before it is photolyzed into NO.
These results suggest that, under our experimental conditions, the photochemical transformations occur within the quasi-liquid layer, which behaves as a supercooled solution.
The experimental data for Φ'_(NO_2), coupled with snow absorptivity data, lead to a predicted NO_2 fluxes in reasonable agreement with recent measurements in Antarctic snow under solar illumination. NO_3^- photolysis within snowpacks may also be a significant source for OH radicals, which may further react and cause chemical changes in important species, such as H_2O_2 and H_2CO and CH_3CHO.
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