Urban air quality studies from long-term aerosol laser radar measurements at a tropical station

Monitoring of urban atmospheric aerosols and trace gases using the lidar techniques has been in progress at the Indian Institute of Tropical Meteorology (IITM), Pune (18°32'N, 73°51'E, 559 m AMSL), India since 1985. The Argon-ion lidar facility, being fully computer-controlled, offers all the benefits of on-line acquisition and processing of data while retaining the very valuable graphical representation in the familiar two- and three-dimensional views of aerosol and trace gas characteristics. Over 1000 lidar-derived profiles (covering the altitude range between 20 m and 1380 m) of aerosol number density obtained during night clear-sky conditions over the 12-year period (October 1986 through September 1998) have been used to build the climatology of mixing depth, stable layer, and associated ventilation coefficients and aerosol layer structures in the atmospheric boundary layer. The analysis of long-term data indicates variations in mixing depth from about 200 m to 550 m, and the associated ventilation coefficients reveal relatively higher values during the pre-monsoon (March-May) indicating better air quality due to larger mixing depths and stronger transport winds, and lower values during the south-west monsoon (June-September) and winter (December-February) seasons indicating low dispersal of pollutants or poorer air quality over the experimental station. Though the ventilation coefficients are low during the monsoon months, the effect of air pollution is considered to be negligible due to the effects of cloud scavenging and rain washout. But the low coefficients during the winter late evenings exhibit higher pollution potential at the station. Thus, the above results emphasize the importance of lidar for air quality measurements and pollution potential forecast in urban regions in general, and industrial regions in particular