The paper describes the results of aerosol investigations in the free troposphere of the Northern Hemisphere. The data have been obtained on the spatial distribution of the Arctic haze and background aerosol in the northern and middle latitudes of Russia, North America, Canada, and North Atlantic. The aerosol is studied using a lidar placed onboard the Cyclone IL–18 aircraft- laboratory. The data are presented on the vertical distribution of scattering and depolarization coefficients. It has been found that the profiles of the scattering coefficients for the Arctic haze in different regions of the Arctic undergo strong fluctuations, however, their mean values differ only slightly. The results of summer investigations as well as the measurements made in winter and spring in the regions without the Arctic haze have shown that the tropospheric aerosol has fine-cellular and stratified structure. The aerosol volume scattering and depolarization coefficients are within the range (0.8–8.4)•10–5 m–1 and 0.01–0.15, respectively. These characteristics significantly depend on atmospheric pollution and geographical position of the measurement region relative to industrial regions as well as on the meteorological conditions. In particular, they depend on the direction of motion of air masses and temperature distribution.