The total aerosol content in the vertical atmospheric column was measured in Minsk (Belarus) and Belsk (Poland) with a multiwavelength aerosol lidar and a sun-scanning spectral radiometer. Three-dimensional five-day backward trajectories were calculated using wind field data of the GidroMetCenter of the Republic Belarus supplemented with the vertical wind velocity data and the data on the wind field in the atmospheric boundary layer. It was found that the most powerful aerosol source areas lie beyond the Belarus and Poland territories. They are situated south-east, south, and south-west from the monitoring stations. The revealed sources are close to the expert EMEP emission data. On the average, south territories affect mostly the atmospheric situation at the monitoring sites. About 60% of aerosols in Minsk and about 50% of aerosols in Belsk are of transborder origin.