Latitudinal variation of spectral optical thickness and columnar size distribution of the El Chichon stratospheric aerosol layer

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Abstract
Multiwavelength solar radiometer observations of directly transmitted solar radiation have been obtained at 10 discrete wavelengths between 0.440 and 2.233 μm during an airborne latitudinal survey in late April and early May 1983. These observations have been used to derive the spectral optical thickness of the El Chichon stratospheric aerosol layer from 68°N to 56°S. The stratospheric optical thicknesses 1 year after the eruptions of El Chichon were still greatly enhanced over background levels for the stratosphere. The maximum optical thickness of 0.14 (0.670‐μm wavelength) was found at 50°N, rather than at 20°N as was found during an airborne latitudinal survey in late October and early November 1982. Columnar aerosol size distributions have been inferred from the 1982 and 1983 data sets by numerically inverting the aerosol optical thickness measurements as a function of wavelength. In May 1983, latitudes north of 5°S had bimodal stratospheric aerosol size distributions, while latitudes south of 5°S had monomodal size distributions. During October 1982, when optical thickness measurements were obtained only from 0.440 to 0.871 μm, the size distributions showed a strong latitudinal variation but were everywhere monomodal. For both dates the northern hemisphere distribution indicated a significantly higher ratio of large particles to small particles than the southern hemisphere distribution. The addition of the near‐infrared wavelengths in the May 1983 experiment increased the accuracy of the size distribution determinations at the large‐particle end of the spectrum, as anticipated, but was also found to increase the accuracy at the small‐particle end.