Antarctic stratospheric ice crystals

Abstract

Ice crystals were replicated over the Palmer Peninsula at approximately 72øS on six occasions during the 1987 Airborne Antarctic Ozone Experiment. The sampling altitude was between 12.5 and 18.5 km (45-65 thousand ft pressure altitude) with the temperature between 190 and 201 K. The atmosphere was subsaturated with respect to ice in all cases. The collected crystals were predominantly solid and hollow columns. The largest crystals were sampled at lower altitudes where the potential temperature was below 400 K. While the crystals were larger than anticipated, their low concentration results in a total surface area that is less than one tenth of the total aerosol surface area. The large ice crystals may play an important role in the observed stratospheric dehydration processes through sedimentation. Evidence of scavenging of submicron particles further suggests that the ice crystals may be effective in the removal of stratospheric chemicals. 1. 16,449 16,450 GOODMAN ET AL ' ANTARCTIC STRATOSPHERIC ICE CRYSTALS The computed interception efficiencies [Wong et al., 1955; Leto and Farlow, 1979; Ranz and Wong, 1952; Dunn and Renken, 1987] for wire and disk surfaces are better than 90% for spherical particles larger than 2/xm, at 19-km altitude and 200 m/s. There are several other factors that can influencecollection efficiency. First, particle shape can significantly affect inertial impaction because of variable drag forces, and second, it is assumed that each particle that is intercepted adheres to the surface. At low velocities, solid particles tend to stick, while at higher velocities, bouncing may become statistically significant. Bouncing depends on several parameters, including incident velocity, incident angle, particle size, particle surface geometry and orientation, particle and surface material properties, surface contamination, temperature, and electrostatic forces. While detailed theoretical

Structure of ice crystallized from supercooled water

“…Although evidence from diffraction measurements is absent, crystals grown from water vapor at approximately 200 K can have a cubic habit (35) and 25% of crystals in the Antarctic stratosphere were solid cubes during one campaign (36). In addition, a halo at 28°from the sun has been photographed and is consistent with the rare presence of octahedral crystals of ice I c in the atmosphere (37).…”

Physical properties of stratospheric clouds during the Antarctic winter of 1995

“…In fact, under the common (and simplifying) assumption of PSC formation via heterogeneous nucleation, typical stratospheric cooling rates [Murphy and Gary, 1995] would lead to the activation and growth of all the ambient aerosols [WoJSy et al, 1990]. Conversely, observations often reveal the presence of mixed phase PSC, where only a small fraction of ambient particles selectively nucleated into crystals, the others remaining supercooled droplets [Goodman et al, 1989;Dye et al, 1992Dye et al, , 1996Hofmann et al, 1989]. In this respect it ought to be remembered that observed substantial denitfification without large dehydration [e.g., Rindand et al, 1996] takes place only if ambient HNO 3 condenses into a small fraction of ambient aerosols, which can then grow large enough for fast settling [Salawitch et al, 1989].…”

Production of Ice in Tropospheric Clouds: A Review

“…Evidence for cubic ice in the atmosphere includes photographs of a 28° halo (Riikonen et al 2000) and sampled crystals that had an unmistakable cubic morphology (Goodman et al 1989). Like supercooled water, the cubic form of ice is not a stable phase (Eisenberg and Kauzmann 1969), which may have consequences for ice in the atmosphere.…”