AquaVIT-4 : Participating instruments

Pico-Light H2O (GSMA/DT-INSU, PI: Pr. G. Durry, co-PI: Dr. M. Ghysels-Dubois, PM: Nadir Amarouche)

It is the lightweight successor of Pico-SDLA H2O (Durry et al., 2008b; Ghysels et al., 2016). Pico-SDLA H2O was tested against other hygrometers, both in flight and in an atmospheric simulation chamber (Behera Abhinna K. et al., 2018; Berthet et al., 2013; Durry et al., 2008a; Fahey et al., 2014; Ghysels et al., 2016; Korotcenkov, 2018). Pico-Light H2O is a tunable diode laser spectrometer which allows to measure in situ water vapor mixing ratio using direct absorption spectroscopy over a 1-m optical path in ambient air. It has been developed since 2017 by DT-INSU (Division Technique de l’INSU, UAR 855, Meudon, France) and GSMA (UMR CNRS 7331, Reims, France). The in situ mixing ratio is retrieved from the processing of recorded atmospheric spectra of two water vapor lines: the 413←414 H216O line at 3802.96561 cm-1(troposphere), and the 202←101H216O line at 3801.41863 cm-1(upper troposphere and stratosphere). The novelty of Pico-Light H2O lies in its new electronics, lighter weight (2.7 kg), simpler mechanical structure, and improved energy management. The dramatic weight reduction made it possible to fly the instrument from a small rubber weather balloon. Such a hygrometer launched on small balloons could make frequent measurements in difficult meteorological conditions and at low cost.

Pico-Light H2O has been tested in flight three times. One time during the StratoScience 2018 CNES balloon campaign from Timmins, CA and two times from the Aire-sur-l’Adour CNES balloon facility (France) in 2019. The mean relative difference between Pico-Light and MLS v4.2 is 7.5% between 38 hPa and 100 hPa. Between 68 hPa and 146 hPa over Aire-sur-l’Adour, the mean difference is 10.8 %, similar to the mean bias reported in the same altitude range by (Yan et al., 2016) (mean bias: 11%). For altitudes higher than 19.6 km (<56 hPa), the mean bias between Pico-Light and MLS is larger than the one reported by (Yan et al., 2016), probably due to a moderate contamination of Pico-Light due to outgassing (see the discussion in section 6.2). For pressure levels greater than 100 hPa, differences can reach 80% due to the coarse vertical resolution of MLS.

The measurement precision is of 277 ppbv for an integration time of 8 ms (unitary spectra) and 130 ppbv for an integration time of 1s.

Figure 1: a) Picture of Pico-Light H2O under rubber balloon on February 19, 2019 from the CNES Aire-sur-l’Adour balloon facility (southern france). b) Details of the hygrometer.

References :

1- Durry, G., Amarouche, N., Joly, L. et al. Laser diode spectroscopy of H2O at 2.63 μm for atmospheric applications. Appl. Phys. B 90, 573–580 (2008).

2- Ghysels, M., Riviere, E. D., Khaykin, S., Stoeffler, C., Amarouche, N., Pommereau, J.-P., Held, G., and Durry, G.: Intercomparison of in situ water vapor balloon-borne measurements from Pico-SDLA H2O and FLASH-B in the tropical UTLS, Atmos. Meas. Tech., 9, 1207–1219,, 2016.

3- Ghysels, M., Durry, G., Amarouche, N., Samake, J.-C., Frérot, F., and Rivière, E. D.: A lightweight balloon-borne mid-infrared hygrometer to probe the middle atmosphere: Pico-Light H2O. Comparison with Aura-MLS v4 and v5 satellite measurements, Atmos. Meas. Tech. Discuss. [preprint],, in review, 2020.

Frost-point hygrometer from NOAA/ESRL (PI: Dr. D. Hurst)

he NOAA frost point hygrometer (Hall et al., 2016; Hurst et al., 2011a, 2014 ) is based on the chilled mirror technique in which the mirror temperature is rapidly adjusted to maintain a stable layer of frost on the mirror. When the frost layer is stable, the frost layer is in equilibrium with the water vapor in the air flowing over the mirror, and the mirror temperature is equal to the frost point temperature. Well-established empirical relationships exist between the temperature of (water) ice and the vapor pressure above it. The partial pressure of water vapor in the air flowing over the mirror is directly determined from the frost point temperature, and with simultaneous measurements of the atmospheric pressure, the water vapor mixing ratio (mole fraction) is easily calculated.

FPH NOAA is one of the most tested hygrometer in the atmosphere over the world. NOAA has been using this hygrometer over 40 years (representing over 500 flights) to perform soundings of water vapor on a monthly basis over 3 launch sites (2 of them have been added in 2004 and 2010) : Boulder (CO, USA), New Zeland and Hawaï. In this frame, the NOAA team has participated to numerous inter-comparison campaigns in flight (Hall et al., 2016; Hurst et al., 2011a, c; Jensen et al., 2011; Kley et al., 2000), in simulation chamber (previous AquaVIT (Fahey et al., 2014)) and in the frame of satellite validations (Aura MLS) (Jensen et al., 2011; Hurst et al., 2014, 2016).

Measurements from FPH NOAA have been used to study the trend in stratospheric water vapor over Boulder (Hurst et al., 2016, 2011b). The dataset from FPH NOAA has been used, together with satellite observations, as an input of scientific analysis on mid latitude stratospheric water vapor trend. 

Figure 2 : Left: Vertical profiles of water vapor during the ascent (blue) and descent (green) of the frost-point hygrometer of NOAA ESRL (USA). Right top: Picture of the hygrometer together with a ozone sonde. Right bottom: Principle of the FPH NOAA.

SAWPhy hygrometer (LMD, PI: Dr. A. Hertzog):

SAWfPHY is a frost-point hygrometer developed at LMD (UMR CNRS 8539). The technique relies on the detection of a frost layer on the surface of a cooled mirror, leading to the determination of the air frost point temperature. Common frost point hygrometer use optical techniques to detect the frost layer. SAWfPHY uses the interaction between the frost layer and acoustic wave propagating at the surface of a quart1- Vladimir Yushkov, Valery Astakhov, and Serafim Merkulov “Optical balloon hygrometer for upper-troposphere and stratosphere water vapor measurements”, Proc. SPIE 3501, Optical Remote Sensing of the Atmosphere and Clouds, (18 August 1998); crystal. This technique, allows to detect layers 10 to 100 times thinner than with optical methods. The hygrometer is in operation since 2019 and is still under development. The detection principle of the instrument has been tested under open stratospheric balloon in Kiruna on April 2010 and March 2011. It is currently participating to the Stratéole-2 equatorial balloon campaign.

Figure 3: Picture of the SAWPhy hygrometer

FLASH Lyman-α hygrometer (LMD/CAO, PI: Dr. S. Khaykin)

It is a compact and lightweight version of the FLASH (Fluorescence Lyman-Alpha Stratospheric Hygrometer) instrument developed at the Central Aerological Observatory in Russia specifically for balloon-borne measurements of water vapor in the upper troposphere and lower stratosphere, between 5  – 300 hPa (Yushkov et al., 1998). The instrument senses water vapor by measuring the intensity of fluorescence from OH radicals that have been photodissociated from water molecules exposed to Lyman-alpha radiation (121.6 nm) as proposed by (Khaykin et al., 2013)Bertaux and Dellanoy (1978) at Service d’Aeronomie. The Lyman-alpha is produced by an onboard hydrogen lamp, whereas the fluorescence signal at 308–316 nm is measured using a photomultiplier tube in photon-counting mode. The intensity of the fluorescence is directly proportional to the water vapor mixing ratio at stratospheric conditions. FLASH-B uses a coaxial open-path optical layout, in which the measurement volume is located outside the instruments, 2–3 cm away from the lens of the instrument. To reduce the background light and to avoid saturation of the photomultiplier tube, FLASH-B can only be operated at night, at a solar zenith angle (SZA) greater than 92 deg. FLASH-B is installed in the payload with the lens facing downward. The measurements during balloon ascent above 90 mbar are subject to contamination due to outgassing from the instrument walls and balloon envelope (as for other hygrometers), whereas the descent measurements are entirely free from contamination. (

FLASH-B has a significant flight heritage on sounding balloons in the tropics (Khaykin et al., 2009) and in the Arctic (Khaykin et al., 2013), as well as in a previous long-duration balloon experiment (Lykov et al., 2014). The performance of FLASH-B has been extensively documented both in laboratory intercomparisons (AQUAVIT-I, (Fahey et al., 2014) and through collocated balloon soundings with frost-point, tunable diode laser, and Lyman-alpha hygrometers (Ghysels et al., 2016; Khaykin et al., 2013; Vömel et al., 2007), yielding mean relative deviations of less than 2.4 % in the lower stratosphere.

Prior to delivery, the FLASH-B instrument is calibrated in a stratospheric simulation chamber at constant pressure (50 hPa) and temperature (−40 °C) over a wide range of mixing ratios (1–100 ppmv) against the reference dew point hygrometer MBW 373L. The detection limit for a 4 s integration time is of the order of 0.1 ppmv, while the accuracy is limited by the calibration error amounting to 4 %. The typical precision in the stratosphere is 5 %–6 %, whereas the total uncertainty is less than 10 % throughout the stratosphere.

Figure 4: FLASH-B without thermal protective layers.

References :

1- Vladimir Yushkov, Valery Astakhov, and Serafim Merkulov “Optical balloon hygrometer for upper-troposphere and stratosphere water vapor measurements”, Proc. SPIE 3501, Optical Remote Sensing of the Atmosphere and Clouds, (18 August 1998);

QCLAS hygrometer (EMPA, PI: Dr. B. Tuzson)

The Quantum-Cascade Laser Absorption Spectrometer (QCLAS)is a compact instrument for balloon-borne measurements of water vapor in the upper troposphere and lower stratosphere (UTLS), developed at Empa. The instrument is based on direct laser absorption spectroscopy in the mid-IR spectrum (wavelength 6 μm),and relies on a segmented circular multipass cell (optical path length 6 m) that was specifically developed to meet the stringent requirements posed by the harsh environmental conditions of the UTLS. Quick response and minimal interference by water vapor outgassing from surfaces are achieved by an open-path approach. An elaborate thermal management system ensures excellent internal temperature stability despite of outside temperature variations of up to 80 K. With a total weight of 3.9 kg, the QCLAS is a fully independent system, operating autonomously for the duration of a balloon flight. The first two successful test flights were performed in December 2019 from the meteorological observatory of Lindenberg (Germany).

Figure 5 : Picture of the QCLAS hygrometer (EMPA, Switzerland).

References :

1- Graf, M., Emmenegger, L., and Tuzson, B.: Compact, circular, and optically stable multipass cell for mobile laser absorption spectroscopy, Opt. Lett., 43, 2434–2437,, 2018.

2- Graf, M., Scheidegger, P., Kupferschmid, A., Looser, H., Peter, T., Dirksen, R., Emmenegger, L., and Tuzson, B.: Compact and lightweight mid-infrared laser spectrometer for balloon-borne water vapor measurements in the UTLS, Atmos. Meas. Tech., 14, 1365–1378,, 2021.