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ABSTRACT
We demonstrate a standoff system based on quartz-enhanced photoacoustic spectroscopy technique for the concentration measurements of atmospheric ozone and methane. The technique is a modified version of Photoacoustic spectroscopy. Two primary features of this technique are the employment of a tunable quantum cascade laser and a resonant quartz-crystal tuning fork detector. Both of these features facilitate simultaneous sensing of multiple molecular species. External-cavity quantum cascade laser having a spectral range from 7 to 10 micron is used. Diurnal concentration variations of methane and ozone are estimated for open-path up to 25 m. The ambient methane and ozone concentration maxima were observed to have values of 3.5 parts per million by volume and 140 parts per billion by volume, respectively. Finite-element mesh-based software is used to simulate the Eigen frequency of the tuning fork sensor. High-resolution transmission molecular spectroscopic database of atmospheric gases and the real-time gas concentration data from the Delhi Pollution Control Committee have been used as references.
Acknowledgment
The authors are thankful to Sh. Hari Babu Srivastava, Director LASTEC, and Dr. M N Reddy for their encouragement and support for carrying out this work. The authors wish to thank Ms. Usha Kumari, Neha Bhardwaj, Umesh Sah, and Gaurav Chabra for the help during the experiments. The authors are also thankful to Dr. Devinder Pal Ghai and Ms. Gagan Preet for their help in writing this article.