Photochemistry of Nitrous Acid (HONO) and Nitrous Acidium Ion (H 2 ONO + ) in Aqueous Solution and Ice

We have examined the photochemistries of two N(III) species, nitrous acid (HONO) and nitrous acidium ion (H2ONO+), in solution and ice. Although the light absorption spectra for these two species are very similar, their photochemical efficiencies are quite different: the *OH (and NO) quantum yield for HONO is approximately 8 times greater than that of H2ONO+ at 274 K. The temperature dependent expressions for the *OH (and NO) quantum yields are In(phi(HONO --> *OH) = (7.14 +/- 0.57) - (2430 +/- 160)/T and In(phi(H2ONO+ --> *OH) = (3.16 +/- 0.67) - (1890 +/- 180)/T. The temperature dependence for H2ONO+ includes both solution and ice data (255-283 K), suggesting that its ice photochemistry is occurring in a quasi-liquid environment. The quantum yields for HONO and H2ONO+ are independent of wavelength, in contrast to NO2-. On the basis of the pH dependence of N(III) photolysis, our results are consistent with recently reported pKa values of 1.7 for H2ONO+ and 2.8 for HONO. Using our results in a kinetic model of nitrogen chemistry illustrates that the fluxes of HONO and NO(x) from sunlit snow can be explained by nitrate photolysis and are pH dependent because of a competition between HONO evaporation and N(III) reactions on ice grains.