A comparison between high-altitude horizon photography and direct sampling of El Chichon cloud particles

Perturbations to the visible radiation by the El Chichon aerosol layers in the stratosphere have been observed using in situ time-lapse photography of the earth horizon from a balloon gondola. The measurements were performed on May 18, 1982, from Laredo, Texas (27.3°N, 99.5°W), during a sounding performed with optical particle counters that measured aerosol size distributions in six different size ranges. The results of the microdensitometry of the photographic film agree well with the corresponding quantities determined from the optical counter data by applying Mie theory for spherical particles. The optical thickness of the stratosphere from a height of about 18.8 km to the top of the atmosphere, averaged over visible wavelengths, was found to be on the order of 0.18. At a height close to the maximum reached by the balloon (27 km) the residual optical thickness due to aerosol particles was found to be on the order of 0.7 × 10−3. The isodensity contours in the stratosphere described the main pattern of the intensity distributions of multiply scattered sunlight in the vicinity of the earth horizon. This pattern, observed at relatively high solar elevation, indicated that both the upward and downward intensities, besides depending on the particle characteristics and on their vertical distribution, also depend on variations of the general distribution in the underlying cloud coverage.