Range-resolved detection of boundary layer stable water vapor isotopologues using a ground-based 1.98 µm differential...

This paper presents a first demonstration of range-resolved differential absorption LIDAR (DIAL) measurements of the water vapor main isotopologue H₂ ¹⁶O and the less abundant semi-heavy water isotopologue HD¹⁶O with the aim of determining the isotopic ratio. The presented Water Vapor and Isotope Lidar (WaVIL) instrument is based on a parametric laser source emitting nanosecond pulses at 1.98 µm and a direct-detection receiver utilizing a commercial InGaAs PIN photodiode. Vertical profiles of H₂ ¹⁶O and HD¹⁶O were acquired in the planetary boundary layer in the suburban Paris region up to a range of 1.5 km. For time averaging over 25 min, the achieved precision in the retrieved water vapor mixing ratio is 0.1 g kg^-1 (2.5% relative error) at 0.4 km above ground level (a.g.l.) and 0.6 g kg^-1 (20%) at 1 km a.g.l. for 150 m range bins along the LIDAR line of sight. For HD¹⁶O, weaker absorption has to be balanced with coarser vertical resolution (600 m range bins) in order to achieve similar relative precision. From the DIAL measurements of H₂ ¹⁶O and HD¹⁶O, the isotopic abundance δD was estimated as -51‰ at 0.4 km above the ground and -119‰ in the upper part of the boundary layer at 1.3 km a.g.l. Random and systematic errors are discussed in the form of an error budget, which shows that further instrumental improvements are required on the challenging path towards DIAL-profiling of the isotopic abundance with range resolution and precision suitable for water cycle studies.