Second European Stratospheric Arctic and Midlatitude Experiment campaign: Correlative measurements of aerosol in the northern polar atmosphere

The stratospheric aerosol layer in the Arctic winter was studied by three independent, height-resolving, optical techniques close in space and time on February 2, 1994, above northern Scandinavia. The balloon-borne Radiometre Ballon (RADIBAL) experiment measured altitude profiles of the radiance and polarization of scattered sunlight at two wavelengths: a ground-based lidar measured vertical backscatter ratio profiles at one wavelength and the satellite-borne Polar Ozone and Aerosol Measurement (POAM) II solar occultation instrument measured atmospheric extinction at nine wavelengths. From the RADIBAL data the mode radius and variance of the aerosol size distribution are derived as well as the particle refractive index. This size distribution is used to convert the lidar backscatter ratio to an extinction coefficient. The POAM II aerosol extinction coefficients are derived under the assumption that the spectral dependence of the aerosol optical depth follows a quadratic law at all altitudes. This quadratic dependence is used to deduce the mode radius and variance of the aerosol size distribution and to interpolate the POAM data to the wavelengths of the other two instruments. In the common altitude range of measurements errors and the temporal and spatial variability of the aerosol layer. The geographic area of measurements was outside the polar vortex on that day. The effective aerosol particle radius decreases slightly with increasing altitude from 0.40 mu m at 16 km to 0.25 mu m at 22 km and the effective variance ranges from 0.15 to 0.25. The mean refractive index is 1.44 at 850 nm, which is compatible with a 75%-sulfuric acid-water aerosol. The aerosol number densities decrease from 5 to 1 cm(-3) over the 16- to 22-km altitude range and the surface area density decreases from 5 to less than 1 mu m(2) cm(-3) over the same altitude range.