Jpd. Abbatt, INTERACTION OF HNO3 WITH WATER-ICE SURFACES AT TEMPERATURES OF THE FREE TROPOSPHERE, Geophysical research letters, 24(12), 1997, pp. 1479-1482
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-i
ce, 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 s
maller 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 t
he 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 temperat
ures. The uptake is largely irreversible, with only 20-->25% of the ad
sorbed HNO3 desorbing from the film when the exposure to gas-phase HNO
3 is stopped. These experiments suggest that adsorption to cirrus clou
d surfaces may be an important scavenging process of HNO3 in the tropo
sphere.