Lidar measurements of aerosol column content in an urban nocturnal boundary layer

Lidar aerosol measurements made at Pune (lat. 18°32′N, long. 73°51′E, 559 m AMSL), India, a tropical urban station, during the nine-year period from October 1986 to September 1995 have been used to study the temporal variations in the aerosol-loading in the nocturnal boundary layer. There is a long-term increasing trend in the aerosol column content in the 50–1100 m layer that has been attributed to be due to the increasing anthropogenic activity around the lidar site. The seasonal variations in aerosol content show a maximum in the pre-monsoon month of May and a minimum in the SW monsoon month of July. The percentage contribution of the 50–200 m layer to the total loading in the 50–1100 m layer is about 41% which points to the predominant surface source of particulate matter. On a seasonal scale, there is a decrease of about 36% in the aerosol content from premonsoon (March-May) to monsoon (June-September) season and this decrease on year-to-year basis is directly related to the amount of rainfall received at the ground at Pune during the monsoon season. This effect has not changed appreciably over the years whatever be the increase in the aerosol loading. The temporal evolution of aerosol content in the nighttime showed a rapid decrease soon after sunset and a slower rate of decrease in the midnight hours. The study also showed that surface relative humidity and winds influence the temporal variations in the aerosol column content.