Sohan L. Jain

Radio & Atmospheric Sciences Division Radio, National Physical Laboratory, New Delhi - 110 012, INDI

The earth’s environment is constantly changing. Human activities have been influencing the global atmosphere since the beginning of the industrial era, causing shifts from its natural state. For example, the release of chlorofluorocarbons has significantly increased the rate of destruction of protective ozone by influencing chemical reactions in the polar stratosphere. Also the measurements have shown that tropospheric ozone is increasing gradually due to anthropogenic activities. Surface ozone is a secondary pollutant, its concentration in lower troposphere depends upon its precursors (CO, CH₄, non methane hydrocarbons, NOx) as well as weather and transport phenomenon. The surface ozone exceeding the ambient air quality standard is health hazard to human being, animal and vegetation. Thus the atmospheric trace gases play an important role in many processes such as biosphere-troposphere interaction, the chemistry of troposphere and the troposphere – stratosphere exchange. Therefore the monitoring of various other minor constituents including ozone, green house gases and aerosols in the atmosphere has an important role in the understanding of physics, chemistry and dynamics of the stratosphere and troposphere.

The discovery of laser in early 1960’s and its rapid development led almost immediate first application in atmospheric probing. With the availability of tunable laser sources with adequate energy and spectral purity remote monitoring of atmosphere using lidars has now been actively pursued. Keeping this in view a highly sophisticated and ultra high spectral resolution Laser Heterodyne System with 1 GHz acousto-optic spectrometer as a back-end and a differential absorption lidar (DIAL) using a tunable CO₂ laser have been designed and developed at National Physical Laboratory, New Delhi, to monitor various minor constituents in the atmosphere. The Laser Heterodyne system has been utilized to get vertical profiles of ozone at Antarctica as well as at NPL, New Delhi. Recently a micro pulse lidar (MPL) with facility to measure depolarization ratio has been set up to get vertical profiles of aerosols and to characterize aerosols in the atmosphere. In the present communication the salient features of the laser based experimental systems developed for atmospheric probing in general and those developed (DIAL, MPL and LHS) at NPL in particular along with the results obtained will be presented.