Measurements of tropospheric aerosol volume backscatter coefficients at 10.6 μm wavelength were obtained with airborne continuous wave and ground-based pulsed CO2 Doppler lidars over the Colorado High Plains during a 20-day period in summer 1982. A peristent ''background'' layer was found between 6- and 10-km altitude, with a generally uniform backscatter mixing ratio of ~10-10 m2 kg-1 sr-1. The upper boundary of this background layer varied with the tropopause height; the lower boundary varied with the strength and diurnal cycle of convective mixing in the plantary boundary layer (PBL). For quiescent meteorological conditions the transition from the PBL to the background layer was usually very sharp, with backscatter decreases sometimes as large as 3 decades in ~70 m. Sharp gradients were also found at the boundaries of shallow (tens of meters) subvisible cirrus clouds. For less stable conditions, associated with vertical aerosol transport by deep cumuliform clouds, backscatter tended to decrease exponentially with altitude. ¿American Geophysical Union 1991