Use of aerosol microphysical measurements to model IR backscatter in support of GLOBE

We have used the GAMETAG Pacific mid‐tropospheric aerosol data set to calculate aerosol optical extinction coefficients (σ) at two wavelengths (0.55 μm and 1 μm) and volume backscatter coefficients (β) at 4 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−1sr−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 σ 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 useful in extending the data base of direct backscatter measurements at CO2 wavelengths.