Tropospheric and lower stratospheric aerosol backscatter profiles were obtained with an airborne backscatter lidar at 9.25-μm wavelength during the NASA Global Backscatter Experiment (GLOBE) airborne field campaigns in November 1989 and May/June 1990. The range of latitudes extended from 70 ¿N to 62 ¿S over the Pacific Ocean basin. The data provide evidence that the tropics are an effective sink for aerosol particles in the lidar-active size range, most likely through efficient wet deposition processes. A reduction of planetary boundary layer (PBL) thickness and aerosol mass density was observed within the tropical regions of enhanced cumulus convection. PBL thickness maxima were consistently observed in the southern hemisphere at subtropical and high latitudes. The downward transport of volcanic aerosol from the February 1990 Kelut eruption into the upper troposphere was observed in the southern hemisphere during the May/June 1990 period. A springtime enhancement of aerosol in the middle troposphere, due to convection and transport of surface material, was observed in both hemispheres, although the seasonal enhancement was much more dramatic in the northern hemisphere due to the influence of the Asian continental source. Above the PBL the observed springtime enhancement was in the form of extensive layers, with the high-altitude layers reaching the midlatitude tropopause near 140 ¿E longitude.¿ 1997 American Geophysical Union