A high resolution technique for remotely sensing aerosol sulfate composition has been developed, based on the ratio of aerosol backscatter measured at 9.1 and 10.6 &mgr;m wavelengths with two continuous wave CO2 lidars. This is demonstrated using data from the NASA GLObal Backscatter Experiment (GLOBE) over the Pacific Ocean in 1990. Results indicate changes from sulfuric acid with some ammoniation in clean conditions and presence of dust with ammoniated sulfates in continental plumes. Lidars provide good estimates of backscatter ratio with ~5 second sample times (~1 km spatial resolution) in aerosol concentrations as low as ~10-2 &mgr;g/m3. ¿ American Geophysical Union 1995