As determined in situ by impactor samplers flown on an ER‐2 at 16.5‐ to 20.7‐km pressure altitude and on a DC‐8 at 9.5‐ to 12.6‐km pressure altitudes, the 1991 Pinatubo volcanic eruption increased the particle surface area of stratospheric aerosols up to 50‐fold and the particle volume up to 2 orders of magnitude. Particle composition was typical of a sulfuric acid‐water mixture at ER‐2 altitudes. Ash particles coated with sulfuric acid comprised a significant fraction of aerosol at DC‐8 altitudes. Mie‐computed light extinction increased up to 20‐fold at midvisible and >100‐fold at near‐IR wavelengths. The optical thickness measured through the aerosol layer by an autotracking Sun photometer aboard a DC‐8 aircraft at 10.7‐ to 11.3‐km pressure altitudes shows a spectral shape that is similar to the Mie‐calculated spectral extinction at ER‐2 altitudes. Surface area distributions calculated by inversion of spectral optical depth measurements show characteristics that are similar to the mean surface area distribution resulting from 35 in situ measurements.

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As determined by impactor samplers flown on ER‐2 and DC‐8 aircraft, black carbon aerosol (BCA) mass loadings in the stratosphere average 0.6 nanograms per standard cubic meter, or 0.01% of the total aerosol. Upper tropospheric BCA increases to 0.3%. Low stratospheric concentration is commensurate with present commercial air traffic fuel consumption, given the following assumptions: the BCA emissions are 0.1 grams per kilogram of fuel consumed, 10% of route mileage is above the tropopause, and average BCA stratospheric residence time is about one year. Taking BCA into account, the stratospheric single scatter albedo is ≈0.99. Using parameters for planned supersonic commercial aircraft, whose emissions will be predominantly in the stratosphere, we show that such traffic will double stratospheric BCA concentration. This would reduce the aerosol single scattering albedo by one percent, and double the BCA surface area that is available for heterogeneous chemistry.

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