Fourier transform infrared studies of model polar stratospheric cloud surfaces: Growth and evaporation of ice and nitric acid/ice

Abstract

Fourier Transform Infrared (FTIR) surface studies have been used to probe the microphysical properties of nitric acid trihydrate (NAT) and ice films representative of type I and II polar stratospheric clouds (PSCs), respectively. The infrared spectra of NAT and nitric acid monohydrate (NAM) are obtained by dosing calibrated 3:1 and 1:1 mixtures of water and nitric acid onto a support at −90°C. The spectra, along with previously reported spectra for NAT and NAM, are used to monitor growth, transformation, and evaporation of ice and NAT films. Under our experimental pressure and temperature conditions, NAT is observed to grow at temperatures around 5°C higher than the ice frost point. This is in agreement with the conclusions drawn by Hanson and Mauersberger (1988a,b) from vapor pressure studies over ice and NAT solids. At temperatures 2°C warmer than NAT appearance, we observe growth of a film recently assigned by Ritzhaupt and Devlin (1990) as nitric acid dihydrate (NAD). Because it is formed at the highest temperatures, it is possible that NAD plays a role in PSC formation. However, the differences between the pressures used in the present study and atmospheric pressures, as well as uncertainties in measurements of absolute temperature, could explain NAD formation in our experiment that may not occur in the atmosphere. FTIR surface studies have also been used to probe the condensed products that result from the interaction of ice with gaseous nitric acid. The experiments indicate that upon initial exposure of ice to 1.8×10−6 torr of HNO3, a layer of ice is quantitatively converted to NAT. However, conversion of ice to NAT does not proceed indefinitely, but rather the system reaches saturation. For longer exposures or higher HNO3 pressures, NAM becomes the dominant nitric acid containing species on the surface. Evaporation studies were performed to test the feasibility of a recent denitrification mechanism proposed by Wofsy et al. (1990). The results indicate that ice coated with 0.02 μm of NAT evaporates at a temperature about 4°C higher than uncoated ice. This is consistent with the mechanism for stratospheric denitrification proposed by Wofsy et al. (1990).