Interaction of HNO3 with water-ice surfaces at temperatures of the free troposphere

The uptake of gas-phase HNO3 by water-ice films has been measured in a low-temperature, coated-wall flow tube under conditions where water-ice, and not a hydrate of nitric acid, is thermodynamically stable. It is observed that there is uptake of HNO3 on the order of 1-->3x10(14) molecules per cm(2) of ice film on a short timescale, and a somewhat smaller uptake on a much longer timescale of 10's of minutes. The short timescale uptake is insensitive to the nitric acid partial pressure, varying by not more than a factor of two over a 20-fold variation in the partial pressure of HNO3 from 1.3 x 10(-7) to 3.1x10(-6) torr. This implies that the ice surface is saturated with HNO3 at these partial pressures. Also, the uptake is somewhat dependent on temperature over the range 208 to 248 K, with the largest uptake at the lowest temperatures. The uptake is largely irreversible, with only 20-->25% of the adsorbed HNO3 desorbing from the film when the exposure to gas-phase HNO3 is stopped. These experiments suggest that adsorption to cirrus cloud surfaces may be an important scavenging process of HNO3 in the troposphere.