Photochemistry of nitrous acid (HONO) and nitrous acidium ion (H2ONO) 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.