Uptake of Nitric Acid on Ice at Tropospheric Temperatures: Implications for Cirrus Clouds

The uptake of nitric acid by thin ice films at temperatures from 209 to 220 K was studied using a Knudsen cell flow reactor and FTIR reflection absorption spectroscopy (FTIR−RAS). Using the measured HNO 3 uptake along with the measured ice surface areas, equilibrium coverages (ϑ) and initial uptake efficiencies (γ) for HNO 3 on ice were determined. A negative temperature dependence was observed for both the ϑ and γ data. The initial uptake efficiencies ranged from γ = 7 × 10 -3 at T = 209 K to γ = 3 × 10 -3 at T = 220 K. Using P HNO 3 = 8 × 10 -7 Torr, the uptake of HNO 3 saturated at around one monolayer (ML) at T = 213 K and decreased with increasing temperature. At temperatures below T = 213 K, multiple layers of HNO 3 were adsorbed using P HNO 3 = 8 × 10 -7 Torr. Using P HNO 3 = 1.7 × 10 -6 Torr, temperature dependent uptake was also observed, however the transition from monolayer to multilayer uptake occurred at a higher temperature than when using a lower nitric acid pressure. A multilayer Frenkel−Halsey−Hill (FHH) model was applied to our temperature dependent data and was used to facilitate comparisons with other laboratory and field studies obtained at higher and lower nitric acid partial pressures. Our coverage values at T > 213 K are quantitatively lower than previous laboratory results by Abbatt, however extrapolations of our model to data by Arora et al. agree within a factor of 2. We have also compared our modeled coverages to field observations by Weinheimer et al. and Meilinger et al. of HNO 3 uptake on lee-wave and cirrus clouds, respectively. Modeled coverages fit both sets of data well but lie slightly above the Meilinger et al. data.