Jonas Hamperl

Jean-Baptiste Dherbecourt
email: jean-baptiste.dherbecourt@onera.fr

Myriam Raybaut

Julien Totems

Rosa Santagata

Jean- Michel Melkonian

Antoine Godard

Patrick Chazette

Cyrille Flamant

Demonstration of range-resolved detection of stable water isotopologues by differential absorption lidar

We present range-resolved measurements of the stable water isotopologues H2 16 O and HD 16 O in the lower troposphere enabled by a ground-based differential absorption lidar based on a high-energy parametric laser source.

Introduction

Observations of stable water isotopologues in the lower atmosphere can provide valuable insights into the variety of processes governing the global hydrological cycle. The provision of such data with sufficient vertical resolution in the lower troposphere (0-3 km) has the potential to significantly increase the accuracy in the predictions made by atmospheric general circulation models [START_REF] Steen-Larsen | Evaluating the skills of isotope-enabled general circulation models against in situ atmospheric water vapor isotope observations[END_REF]. For this purpose, differential absorption lidar (DIAL) is a promising technique which has been successfully demonstrated for water vapor and other greenhouse gases [START_REF] Orr | Infrared LIDAR Applications in Atmospheric Monitoring[END_REF]. In order to quantify the relative abundance in HD 16 O (D), the method requires collocated measurements of both the main isotopologue H2 16 O and HD 16 O with an equivalent relative precision, which has not been demonstrated so far despite recent progress on multiple wavelengths DIALs [START_REF] Wagner | Multi-frequency differential absorption LIDAR system for remote sensing of CO2 and H2O near 1.6 μm[END_REF], and the proven capability of some approaches to probe isotopic water vapor lines [START_REF] Yu | Photon-counting distributed free-space spectroscopy[END_REF]. In this paper we present a DIAL instrument operating at 1.98 µm, for which calculations indicate that range-resolved measurements of D should be achievable [START_REF] Hamperl | Differential absorption lidar for water vapor isotopologues in the 1.98 µm spectral region: sensitivity analysis with respect to regional atmospheric variability[END_REF]. We show first results for the water isotopologues H2 16 O and HD 16 O along a horizontal line of sight.

Lidar setup

The DIAL transmitter (Fig. 1) is based on a doubly-resonant nested-cavity optical parametric oscillator (NesCOPO) [START_REF] Godard | Nested Cavity Optical Parametric Oscillators -A Tunable Frequency Synthesizer for Gas Sensing[END_REF] combined with an optical parametric amplification (OPA) stage pumped by a Nd:YAG laser delivering 12 ns pulses at a repetition rate of 150 Hz. The NesCOPO provides single-frequency emission tunable in a 1 nm window around 1982.5 nm suitable for addressing H2 16 O and HD 16 O absorption lines (Fig. 2). The OPA stage consists of three high-aperture (5x5 mm²) PPKTP crystals [START_REF] Zukauskas | 5 mm thick periodically poled Rb-doped KTP for high energy optical parametric frequency conversion[END_REF]. For enhanced conversion efficiency, the idler beam is filtered prior to the last crystal. With pump energy pulses of 70 mJ, output energies at 1.98 µm are in the range of 6-9 mJ. Pulse-to-pulse wavelength monitoring is realized with a commercial wavemeter (High Finesse WS6 IR). Wavelength control and on/off switching is accomplished by adjusting the NesCOPO cavity length using piezoelectric transducers. The amplified signal beam passes a beam expander resulting in a divergence full-angle of 300 µrad. The lidar receiver consists of a 10-inch Newtonian telescope which focuses the backscattered radiation onto a 900 µm-multimode fiber. The resulting full field-of-view angle is 1.2 mrad. The fiber output is imaged onto a commercial InGaAs amplified photodiode (300 µm diameter) with a 3.5 MHz electronic bandwidth.

First demonstration of water isotopologues detection by DIAL

DIAL tests along a horizontal path were conducted on 16 April 2021 under relatively dry conditions (ca. 2.5 g/kg water vapor mixing ratio) in the suburban Paris region. Water vapor H2 16 O and the isotopologue HD 16 O were addressed at 1982.93 nm and 1982.47 nm, respectively (Fig. 2). The two-way differential optical depth Δτ is calculated as the logarithm of the ratio of off-line to on-line return signals. The mixing ratio of each isotopologue at a range R is then proportional to the slope of Δτ within a range gate ΔR evaluated by a least-squared regression. The HD 16 O retrieval requires a correction due to the H2 16 O absorption contribution (Fig. 2). The upper panels of figure 3 show the recorded on/off lidar signals expressed as signal-to-detection-noise ratio (SNR), with a dominant noise contribution from the transimpedance amplifier (1.3 pW Hz -1/2 noise-equivalent power). The lower panels of figure 3 show the retrieved volume mixing ratios (VMR). This DIAL experimental demonstration on water isotopologues is a first important step toward range-resolved D quantification in the vertical direction. For this purpose, future work will focus on the characterization of errors and biases as well as on increasing the power performance of the transceiver for higher sensitivity.

Aknowledgments

This research has received funding from the EU H2020 research and innovation program under grant agreement Nr. 821868, and from the French Agence Nationale pour la Recherche (ANR) under grant Nr. ANR-16-CE01-0009.

Fig. 1 .

 1 Fig. 1. Lidar setup Fig. 2. Water vapor absorption lines and DIAL on/off wavelengths

Fig. 3 .

 3 Fig. 3. Raw (no energy normalization) averaged DIAL signals over 30 min and retrieved VMR for H2 16 O (a) and HD 16 O (b).