Design and Validation of a Vehicle-Mounted Near-IR If Wavelength Modulation Spectroscopy System for On-Road Measurements of Ambient Water Vapour in Gandhinagar-Ahmedabad, India

We report the first set of vehicle-mounted measurements of ambient water vapour concentration in Ahmedabad, India obtained over a two-month period using a compact and portable 1392 nm tunable laser-based wavelength modulation spectroscopy (WMS) system. The water vapour absorption transition line at 1391.672 nm was selected for the measurements. A calibration-free $\boldsymbol{R}_{\boldsymbol{1f}}/\mathbf{\Delta} \boldsymbol{I}_{\boldsymbol{1}}$ WMS technique recently developed by our group was used for reliable and accurate estimation of the mole fraction. The single pass detection limit was found from an Allan variance analysis to be 37 ppm for a 10.3 cm long open-path measurement (equivalent to 3.81 ppm-m) for an integration time of 22 s. The system was fitted with a GPS unit, mounted on a car and driven around Ahmedabad covering 77 km everyday to automatically generate a spatial distribution of ambient water vapour along the route. These measurements demonstrate the system's suitability for city-wide measurements of greenhouse gases and NOx emissions from vehicles.