Kawa et al. [1992] using in-situ aircraft data suggested that Nitric Acid
Trihydrate (NAT) is the composition of Polar Stratospheric Clouds (PSCs)
in the Arctic, in agreement with Antarctic findings. PSCs in the Arctic
are not observed to form, however, until the saturation ratio of HNO
with respect to NAT is greater than 10, in marked contrast to the Antarctic,
where PSCs are observed to form under conditions of HNO
saturation of
1 and above. Dye et al. [1992] analyzed particle size and volume measurements
from a forward scattering spectrometer probe, model 300, during January and
February 1989 in the Airborne Arctic Stratospheric Experiment and showed that
in the Arctic a major fraction of sulfuric acid droplets remain liquid until
temperatures down to at least 193 K. They suggest that homogeneous freezing
of the sulfuric acid droplets might occur near 190K and play a role in the
formation of PSC's. This suggestion is in line with theoretical calculations
of the freezing rate of sulfuric acid solution drops under stratospheric
conditions (Jensen et al. [1991]) and laboratory studies by Ohtake [1993].
The first evidence of NAT particles in Dye et al.'s study occurred at
saturation ratios of HNO
with respect to NAT of 1 (about 195oK)
as an enhancement of the large particles on the tail of the sulfuric
acid droplet size distribution. In agreement with Kawa [1992], the
major increases in number and volume associated with the main NAT cloud
were not seen in these Arctic investigations until 191-192 K, which
corresponds to an apparent saturation ratio of HNO
with respect
to NAT of about 10. They also show that a decrease in the number of
large particles was observed in regions of the airmass which were
denitrified, consistent with the idea that denitrification may be
the result of preferential nucleation and growth of NAT on only the
largest sulfate particles, which then sediment out of the airmass.