The space-time variability of aerosol inhomogeneities provides unique information on atmospheric behavior needed for climate and environmental research and operational programs. An additional indirect forcing from aerosols results from their involvement in nucleation and growth of cloud droplets, reducing droplet size and thereby potentially influencing cloud albedo. These studies have particular significance over tropics where the convective and dynamical processes associated with high-altitude thunderstorms greatly affect the vertical distributions of aerosols and pre-cursor gases. As the anthropogenic share of the total aerosol loading is quite substantial over many parts of the world, it is essential to monitor the aerosol features systematically over longer time scales. Such observations are very important for understanding the coupling processes that exist between physico-chemical, radiative, dynamical and biological phenomena in the Earth's environment, and provide valuable input information for modeling and simulation studies of climate and air quality. The multi-year aerosol number density data acquired during October 1986-September 2000 with a computer-controlled lidar at the Indian Institute of Tropical Meteorology (IITM), Pune, an urban station in India have been utilized to investigate (i) climate variability, (ii) cloud macro-physical parameters and (iii) environmental pollution. The results reveal a long-term trend in aerosol loading, single and multiple layer clouds with low cloud-base during the south-west monsoon months, and high pollution potential during winter late evenings. The trends in aerosol loading and air quality are found to be changing from year to year depending upon meteorological parameters (precipitation in particular). Some of these parameters have also been compared with co-located complementary facilities such as solar radiometers. In order to enlarge the scope of these studies, a dual polarization micro pulse lidar (DPMPL) has been installed at IITM recently to investigate the cloud composition, and aerosol-cloud-climate interactions. The initial results obtained from this state-of-the-art lidar system showed interesting features in the time evolution of nocturnal (stable) boundary layer which have strong bearing on air pollution potential over the experimental station. The complete details of the lidar systems used in the above studies together with discussion of salient results are presented in this paper.
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.
A high-spectral resolution radiometer and a Volz sunphotometer have been in operation to monitor solar irradiance in the visible region 0.4 micrometers 0.7 micrometers , and at five discrete wavelengths covering the visible and near IR regions, respectively, since November 1993 at the Indian Institute of Tropical Meteorology, Pune, India. Using the spectroradiometer-derived spectral variation of total optical depth in the Chappuis-band, ozone content in the atmospheric column was estimated by following the multiple regression method. Total ozone content in the atmosphere was estimated by following the multiple regression method. Total ozone content in the atmosphere was also determined by using the differential absorption of solar radiation at two wavelengths in and around the Chappuis-band from the sunphotometer observations. The observations carried out on about 200 cloud-free days spread over the period from February 1993 to May 1996 were used in the study. A comparison of total ozone, thus retrieved by the Chappuis- band method differed from the Dobson spectrophotometer measurements by about +/- 20 percent on the most stable days. These differences can be attributed to a combination of large aerosol optical depths, diurnal variation of aerosol optical depth, the deviation from the assumed power law relationship coefficients. The ozone optical depths inferred experimentally from the Chappuis-band method have been used to determine more accurate aerosol optical depths as compared to those routinely to those routinely obtained by using model ozone vertical profiles.
Conference Committee Involvement (2)
Lidar Remote Sensing for Environmental Monitoring XV
4 April 2016 | New Delhi, India
Lidar Remote Sensing for Environmental Monitoring VII