Field deployment of a 4320-nm quantum cascade laser-based TDLS system to compare the background CO2 levels in Mt. Abu with foreground measurements in Gandhinagar, India

We report on the design, development, and field deployment of a 10-mW, 4320-nm distributed-feedback quantum cascade laser-based tunable diode laser spectroscopy (TDLS) system in India for in situ measurement of atmospheric CO<inf>2</inf>. The portable system was deployed at Mount Abu (24.5926° N, 72.7156° E), a hill station in western India, to carry out week-long measurements of background atmospheric CO<inf>2</inf> using direct detection. The mean mole fraction was estimated to be 396 ± 8 ppm. The system was then deployed in Gandhinagar (23.2156° N, 72.6369° E), the capital of the state of Gujarat, to make foreground measurements over the next week. The mean mole fraction at this location was 503 ± 27 ppm. The difference between the background levels in Mount Abu and foreground levels in Gandhinagar is evident. The detection limit of the system, as measured from an Allan variance analysis, was determined to be 260 ppb for an integration time of 58 s and a path length of 20 cm, which is sufficient for such measurements. Another compact and light-weight TDLS system was also deployed for water vapor measurement. It consisted of a 1392.54-nm distributed feedback laser driven by custom electronics and a digital signal processor to carry out waveform generation, data acquisition, and postprocessing tasks.