We have used the GAMETAG Pacific midtropospheric aerosol data set to calculate aerosol optical extinction coefficients (&sgr;) at two wavelengths (0.55 μm and 1 μm) and volume backscatter coefficients (β) at wavelengths (1 μm, 9.11 μm, 9.25 μm, and 10.6 μm). At an altitude of 5 km over the Pacific, northern hemispheric mean values of β for 10.6 μm are near 10-10 m-1 sr-1 at an altitude of 5 km, with southern hemispheric values approximately an order of magnitude lower. The 9.11 μm values are roughly a factor of 3 higher than the 10.6 μm values; 9.25 μm values are approximately the same as 9.11 μm values. For our data, averaging times of 5--10 min are necessary for the calculated β values to be representative of β values as seen by a satellite lidar system. Under the assumptions of this study the molecular form of the sulfate aerosol is not a major determining factor in the calculated β values at 10.6 μm but could be significant at 9.11 μm. A study of relationships among the optical parameters indicates that visible and near-visible values of β and &sgr; may be useful in predicting 9.11- and 10.6 μm backscatter, so that short wavelength aerosol data bases from satellites and Nd-YAG lidars may be used in extending the data based on direct backscatter measurements at CO2 wavelengths. ¿American Geophysical Union 1991