The pre-sunrise and post-sunset differences in the tropical urban aerosol distributions have been studied by conducting coordinated experiments using a continuous wave, bistatic Argon ion lidar and a spectroradiometer at the Indian Institute of Tropical Meteorology, Pune, India during 1997-2000. The results of the study indicate higher aerosol concentration in the air layers close to the ground, and lower concentration aloft on all the days of observations. Further, the concentrations are found to be greater in the early-morning (pre-sunrise) hours and lower in the late-evening (post-sunset) hours during the winter season and vice versa during the pre-monsoon season. These deviations are considered to be due to the convective activity and associated turbulent mixing during the pre-monsoon and close-to-ground and elevated haze layer formations during the winter months over the experimental station. (C) 2003 Elsevier Science Ltd. All rights reserved.