The first atmospheric aerosol backscatter measurements at the rare isotope CO2 wavelength of 9.1 &mgr;m were obtained in the troposphere over the Pacific Ocean in May-June 1990. A distinct aerosol backscatter background mode was found in both hemispheres with modal value ~10-10 m2kg-1sr-1 (backscatter normalized by air density), remarkably consistent with ground-based and airborne backscatter climatologies collected at other wavelengths, locations, and times. ¿ American Geophysical Union 1996