S.L. Jain, B.C. Arya and Arun Kumar

Radio and Atmospheric Science Division, National Physical Laboratory, New Delhi – 110012

 

The measurement of various minor constituents is of great significance to understand physics, Chemistry and dynamics of the atmosphere. The natural and anthropogenic activities affect the chemical composition of the atmosphere and have important implications for life support system on the mother planet, the Earth. Therefore regular monitoring of various trace species in the atmosphere plays an important role. The Differential Absorption LIDAR (DIAL) has been proved to be an important tool for remote sensing of trace species in the atmosphere. In view of this a differential absorption lidar system using a tunable CO₂ laser has been designed and developed at National Physical Laboratory, New Delhi, to monitor various minor constituents in the atmosphere. The system is capable of real time measurement of trace species in the atmosphere. In this system a tunable CO₂ laser is used which has a good spectral coincidence between laser emission lines and the absorption lines of many gases of interest. A pair of CO₂ laser wavelengths one ‘ON’ and another ‘OFF’ the absorption line of the gas of interest is transmitted in the atmosphere and reflected back by a reflector to a receiver located near the transmitter. A liquid nitrogen cooled HgCdTe detector has been used to detect the signal. The system has been used to monitor water vapour, surface ozone, ammonia and ethylene concentration in the atmosphere. It is found that very low concentration of ammonia was present in the atmosphere when the humidity was high. The ammonia and relative humidity in the atmosphere was found to be anti-co-related. This may be attributed to high solubility of Ammonia in water and is removed from the atmosphere by aqueous aerosols. The Ammonia was found to be varying from 8 ppb to 63 ppb on different days at NPL, New Delhi. It is also observed that Some times ethylene concentration was more than 20 ppb while that of surface ozone was more than 100 ppb which is a health hazard. In this paper salient features of experimental set up and results obtained will be presented in detail.