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

Abstract

The uptake of gas‐phase HNO 3 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 HNO 3 on the order of 1 → 3 ×10 14 molecules per cm² 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 HNO 3 from 1.3 × 10 −7 to 3.1 × 10 −6 torr. This implies that the ice surface is saturated with HNO 3 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 HNO 3 desorbing from the film when the exposure to gas‐phase HNO 3 is stopped. These experiments suggest that adsorption to cirrus cloud surfaces may be an important scavenging process of HNO 3 in the troposphere.