Uptake of gas-phase nitric acid to ice at low partial pressures: evidence for unsaturated surface coverage

The adsorption of gas-phase nitric acid onto water-ice surfaces at temperatures between 200 and 239 K has been studied over short time scales using a coated-wall flow tube coupled to a chemical ionization mass spectrometer. The nitric acid partial pressures used were between 10(-8) hPa and 10(-6) hPa, making this the first systematic study under partial pressure conditions present in the upper troposphere. Whereas previous findings using this technique have shown that the surface coverages are saturated at 2 to 3 x 10(14) molecules cm(-2) (referenced to the geometric surface area of the ice film) when partial pressures are larger than about 10(-7) hPa, the principal finding from this study is that the surface coverages are in the unsaturated regime at lower partial pressures. A conventional Langmuir adsorption isotherm describes the uptake in a quantitative manner while dissociative Langmuir isotherms that have been used in the past to model this process do not. The unsaturated surface coverages are strongly temperature dependent, in agreement with a number of field measurements of the nitric acid (or NOy) component of cirrus cloud particles. These laboratory results match those in the field better than do those measured at significantly higher partial pressures but, nevertheless, they still indicate somewhat greater uptake, particularly at higher temperatures.