Open Path Measurement of Atmospheric Transmission Spectra in the Region of 3000–3450 nm Using Tunable Mid Infrared Lidar

This paper reports the open path measurement of atmospheric transmission in the spectral region of 3000–3450 nm using a mid infrared tunable lidar in New Delhi (28.7 o N, 77.1 o E), India. Results on the measurement of atmospheric transmission in New Delhi are reported for the first time, to the best of our knowledge. A tunable infrared lidar system has been developed in-house, which transmits laser radiation with the wavelength interval of 1 nm in the above spectral band into the atmosphere. A 200-mm-diameter Cassegrain receiver telescope with mercury cadmium telluride (MCT) detector is used to collect the backscattered radiation from the atmosphere. The reflected laser radiation received from the tower located at a distance of 400 m is used to generate the transmission spectrum. Data recorded during the daytime measurements have been considered in this paper. Transmission spectra measured over several days showed a similar pattern, which confirmed the repeatability of the system performance. The measured spectra are also compared with the SkyCalc transmission model generated using the online database. Preliminary analysis indicated that these spectra had good agreement in general with a small shift in wavelength region. We have also analyzed the HITRAN database line-by-line and identified the possible major absorbing molecules in this band. The sources of air pollutants that absorb in this band and also discussed.