Current scenario of ambient carbon dioxide levels at multiple locations in urban Ahmedabad revealed by a 2004 nm tunable diode laser spectroscopy system

This paper reports the design and deployment of a portable tunable diode laser spectroscopy system for in situ measurement of atmospheric carbon dioxide. Recent measurements made at three busy locations in urban Ahmedabad in India reveal that the mean values of mole fraction of carbon dioxide are significantly higher than the global average of 406.05 ppm. This is most likely due to the huge amount of automobile emissions in these congested areas and poor regulation of vehicular emissions. The measurements use a calibration-free second harmonic wavelength modulation spectroscopy (2/WMS) technique developed in our group. A narrow linewidth tunable vertical cavity surface-emitting laser (VCSEL) is used to selectively interrogate the carbon dioxide transition at 2003.5 nm. The mole fraction of carbon dioxide is extracted in real-time from least-squares fit of a simulated 2/ WMS line shape to the experimentally obtained 2/ WMS signal. The calibration-free nature of measurements eliminates the need for calibrated gas mixtures that are required in other techniques. This made it possible to deploy the system in the field.