The measured cloud volume density profiles were compared
to equilibrium volumes computed for nitric acid trihydrate (NAT) and liquid
aerosols. The measurements on March 4 are confidently identified
as NAT, and isentropic back-trajectories show that conditions were favorable
for ice formation roughly 11 hours upwind of the observed cloud.
These results support the suggestion that ice formation could be a necessary
precursor to NAT nucleation. A HALOE profile showing NAT detected
in the stratosphere above England is shown in Figure 1. The HALOE
volume densities are compared to model predictions of NAT and LTA volume
based on coincident radiosonde temperatures, HALOE H2O,
and coincident HNO3 from MLS. The observed cloud
top altitude (25.5 km) is closest to the NAT calculation, and from
cloud top down to about one vertical resolution below (23.5 km) the average
measured-calculated volume differences are 19% for NAT and 131% for LTA.
Considering the shape and magnitude of the cloud volume profile,
the NAT model provides the most convincing reproduction of the HALOE observations.
This comparison is robust, and the NAT identification holds for a
variety of changes to HNO3, H2O,
|Figure 1. HALOE sunrise measurements on March 4, 1996. The HALOE cloud profile was at 53.8°N, 359.3°E, 6:48 UT, and the HALOE cloud-free profile was at 53.9°N, 335.2°E, 8:24 UT. The coincident MLS HNO3 observation is for March 3, 11:50 UT at 55.1°N, 7.8°E. (a) The measured cloud volume density (diamonds, error bars are ±50%) compared with volumes calculated for LTA and NAT using the average temperature profile in Figure 1b. The indicated range of calculated volumes is for varying the measured HNO3 by ±50%. Also shown are the HALOE H2O (ppmv) and MLS HNO3 (ppbv) profiles used for the volume calculations, and the HALOE cloud free profile (pluses) used to derive background H2SO4. (b) Temperatures from four radiosonde stations on March 4 at 12 UT, the average of these soundings, and TNAT and Tice computed using the H2O and HNO3 in Figure 1a.|