Non-spherical particles in the antarctic polar stratosphere-increase in particulate content and stratospheric water vapor budget

The back-scattering coefficient and depolarization ratio of antarctic stratospheric aerosols were observed by a lidar at Syowa Station (69°00’S, 39°35’E) in 1983 in order to understand the nature of polar stratospheric aerosols. Balloon measurement was also made on 3 June to obtain the number concentration of the particles. The balloon data showed a large concentration of particles with radius larger than 0.15 μm (15 particles/cm³ in the lower stratosphere) and a size distribution which was rich in smaller size particles (N(r > 0.15 μm)/ N(r> 0.25 μm)= 12 ∼ 80, where N(r>0.15 μm) and N(r>0.25 μm) are the number concentrations of particles with radius larger than 0.15 μm and 0.25 μm, respectively). During early June, the back-scattering coefficient rapidly increased, but the depolarization ratio was at a low level, at most 0.15. After mid-June, the depolarization ratio increased as the winter progressed, and this suggests that most of the stratospheric particles had non-spherical shapes (possibly ice crystal particles) in mid-winter.

The lidar measurement showed that the stratospheric particles layer was transported downward at the rate of 0.8 mm/s during winter. The mass of water transported from the stratosphere to the troposphere is estimated to be about 5 × 10⁷ ton per winter season if we assume that the settling rate of the layer is due to gravitational sedimentation of ice crystal particles. If it is due to downward air motion carrying smaller crystals, this is reduced to 5 × 10⁴ tons.