Nitrate near the Surface of Polycrystalline and Dendritic Ice

Abstract:

The nitrate anion is an important participant in polar atmospheric chemistry as a photolytic source of NO_x to the atmosphere. Nitrate is also measured in ice cores as a potential record of NO_x in the past. A complete picture of the location, concentration, and availability of nitrate in snow and ice in Earth’s environment is currently lacking, but is essential to modelling air-snow chemical interactions. In this laboratory study, nitrate near the bottom surface of polycrystalline ice was observed with attenuated total reflection infrared spectroscopy (ATR-IR). Upon freezing millimolar solutions of sodium nitrate (NaNO₃), magnesium nitrate (Mg(NO₃)₂), and nitric acid (HNO₃), the concentration of nitrate near the surface at -18 to -1°C was enhanced by several orders of magnitude. The nitrate concentration near the surface is close to values predicted by the bulk phase diagrams at warm temperatures. At colder temperatures, however, the concentration is less than predicted, even after annealing. In order to more closely approximate snow, ice was deposited on a cold ATR crystal from the gas phase with concurrent or subsequent exposure to gas-phase nitric acid. The potential advantages and limitations of ATR-IR in studying dendritic ice in the laboratory will be discussed.