Nucleation and Growth of HNO3.3H2O Particles in the Polar Stratosphere

Abstract

Growth of nitric acid trihydrate (NAT) particles on background stratospheric aerosols is examined for an isolated air parcel cooled at a uniform rate. The free energy barrier against nucleation and rates of cooling are varied over a range of probable values. During the process of nucleation, the saturation ratio of HNO3 vapor reaches a maximum value between 2 and 15, corresponding to supercooling by 1–4 K. Significant supersaturation may be maintained after nucleation due to the small surface area of NAT available for vapor deposition.

If cooling rates exceed 0.5-1 K day^–1, small (<2 μm radius) particles of NAT are produced. A major fraction of the available condensation nuclei is activated and removal of HNO3 by gravitational settling is slow. If cooling rates are less than 0.5–1 K day^–1, the number of aerosols that nucleate is reduced, leading to differential growth of large (>2 μm radius) NAT particles. Gravitational settling of NAT particles could result in removal of HNO<sub>3<./sub> on time scales close to one week.

Observations of 5 μm radius particles in clouds at temperatures above the water frost point may reflect condensation of NAT on ice particles that fall through a column of air as it is cooled. Rapid condensation of HNO₃ on ice particles is promoted by the high supersaturation attained during nucleation and maintained during subsequent cooling. This process provides a mechanism for irreversible removal of HNO₃.