Infrared transmission spectra of the atmosphere above 11-km altitude were obtained in December 1982 and April/May 1983 from a NASA Convair 990 aircraft. The observations on the first flight series encompassed low and middle latitudes in the northern hemisphere, while the observations on the second series covered a wide range of latitudes in both hemisphere. We have analyzed these data to derive the optical depth of the El Chichon volcanic cloud at a wavelength of 8.2 μm. Comparison of these values with concurrent measurements by Dutton and DeLuisi of the cloud's optical depth at 0.5 μm leads to the ratio of the cloud's infrared to visible optical depth, R. We analyzed the observed spectra by ratioing pairs of spectra taken at similar latitudes but different air masses, and comparing these ratio spectra with synthetic ones. The synthetic spectra allow for absorption by gases (O3, H2O, CO2, CH4, and N2O) as well as extinction by the cloud particles. Extinction by the cloud particles is particularly important in the 8- to 9-μm region. We obtain an average value for R of 0.25¿0.05. Our data imply that concentrated sulfuric acid was the dominant source of aerosol extinction in the 8- to 12-μm window region of the Earth's atmosphere and that the volume modal particle radius equalled 0.6¿0.1 μ at the time of our observations. At these times, the volcanic particles produced a significant warming of the lower stratosphere and had a nontrivial impact on the radiation budget of the troposphere and ground in the northern hemisphere.