TRO-pico : Instruments

Pico-SDLA H2O, CO2 and CH4

With the technical support of the French “Division Technique” of INSU (Institut National des Sciences de l’Univers), in Meudon, the Infrared laser sondes Pico-SDLA has been developed to measure in situ water vapour, CO2, and methane under small balloons (P.I., Georges Durry, GSMA; master builder ,DT-INSU, head of Pico-SDLA: Nadir Amarouche). Pico-SDLA is a smaller/lighter version of the former TDL balloonborne SDLA and micro-SDLA. Pico-SDLA benefits from the last generation of antimonide laser diode that emits in the 2.6 µm region for H2O and CO2). There are three versions of the instrument, one for each measured species. The laser beam propagates over a one-meter path for H2O (and 0,5 m for CO2) and is absorbed in situ by ambient water vapor or CO2 molecules. concentration is deduced from the Beer-Lambert law and a molecular model. The inversion method, as well as the detection technique is the same as for the SDLA and micro-SDLA spectrometers. The instrumental error on the UTLS concentration is typically about 5% and the time resolution is 20 ms per spectrum, that is 800 ms in case of mean of 40 spectra). In the TRO-pico configuration, the total weight of Pico-SDLA H2O or pico-SDLA CO2 is of about 8 kg. The wavelength of laser is 2.63 µm for the H2O version and 2.68 for the CO2 version.
The IR laser sonde pico-SDLA-CH4 is based on the same principle as for the H2O or the CO2 version. The only difference is from the 3.3 µm laser source used in the 2012 version of the instrument: It is based on difference frequency generation (DFG) that emits at 3.3 µm from two DFB (Distributed Feedback) diodes that emits at 1 and 1.5 µm respectively, from which the two beams are conducted toward a non-linear crystal. At the output of the crystal there is an emission at 3.3 µm. The optical path for this version of the instrument is 4 m with an optical cell of 2 m and a single reflexion on a mirror. A new version was used in the 2013 campaign that is based on a new diode which emits directly at 3.3 µm without the need of the DFG. The corresponding optical path was 3 m and the weight of the instrument was 10 kg instead of the 15 kg of the DFG version.

FLASH-B

The FLASH-B (FLuorescence Advanced Stratospheric Hygrometer for Balloon) instrument is a light-weighted Lyman-alpha hygrometer (~2 kg). The instrument is based on the fluorescent method, which uses the photodissociation of H2O molecules at a wavelength <137 nm followed by the measurement of the fluorescence of excited OH radicals. The source of Lyman-alpha radiation (121.6 nm) is a hydrogen discharge lamp, while the detector of OH fluorescence at 308–316 nm is a Hamamatsu R647-P photomultiplier run in photon counting mode with a narrow band interference filter selecting the fluorescencespectral region. The intensity of the fluorescent light sensed by the photomultiplier is directly proportional to the water vapour mixing ratio under stratospheric conditions (10–150 hPa) with small oxygen absorption (3% at 50 hPa). The H2O measurement range is limited to pressures lower than ~300 hPa due to strong Lyman-alpha absorption in the lower troposphere. Measurements are only made at nighttime. Its mass lighter than 2 kg allows flights under 1.2 kg rubber balloons during the TRO-pico campaign.

COBALD

COBALD is a lightweight backscatter sonde developed at ETH Zurich as a successor to the Wyoming backscatter sonde of Rosen and Kjome (1991). With a total weight of approximately 550 g including batteries, the instrument can be flown on operational weather balloons. COBALD measures molecular, aerosol and cloud particle backscatter in the atmosphere from the ground to the level of balloon burst. Two LEDs with 250 mW optical power each emit light at wavelengths of 455 and 870 nm. To register the backscattered light, a photodiode is placed between the LEDs, and the associated optics establishes an overlap region at distances larger than 0.5 m in front of the instrument. So far, the instrument is designed for applications during night-time only as solar radiation saturates the detector. Backscatter by molecules and aerosols contribute to the measured signal, whose separation is achieved following Rosen and Kjome (1991). The molecular number density is determined from temperature and pressure recorded simultaneously by the hosting radiosonde. Together with certain conservative assumptions on aerosol loading in regions of clean air, the normalization of the backscatter signal yields the backscatter ratio (BSR) defined as the ratio of the total – aerosol and molecular – to molecular signal. Analogously to the BSR the aerosol backscatter ratio is ABSR = BSR – 1. The two different wavelengths allow definition of the colour index (CI) as the ratio of the ABSR at 870 nm divided by the ABSR at 455 nm. Information on particle size can be obtained from the CI subject to certain assumption on particle size distribution, shape and refractive index.

Mini-SAOZ

The balloon-borne SAOZ UV-visible spectrometer (Système d’Analyse par Observations Zénithales), developed by Service d’Aéronomie in the early 90s and flown since then more than 125 times on short duration balloons as well as 15 times on long duration Infra-Red Montgolfier around the world for studying stratospheric ozone, NO2, OClO, BrO, H2O and aerosols at all latitudes.
The mini-SAOZ is a smaller/lighter (less than 12 kg) version of SAOZ, developed in 2009 based on more advanced technology, allowing more flexible and relatively cheap flights on small balloons. The measurements carried out during the ascent of the balloon and by solar occultation from float altitude around 30 km at sunset or sunrise. The instrument is a small 716 g Czerny-Turner spectrometer of 75 mm focal length and 0.8 nm spectral resolution in the 300-800 nm range. The sensor is 2048 x 14 pixels CCD detector linked by an optical fiber to an optical head of 5°/+45° elevation and 360° azimuth field of view on the top of the payload. The using of small polyethylene 500/1500 m3 balloons, easy to launch, the mini-SAOZ allows flexible flights next to tropical thunderstorms.
The species retrieval procedure makes uses of the DOAS (Differential Optical Absorption Spectroscopy) applied to the ratio between each spectrum and reference spectrum measured immediately after reaching float altitude around 30 km at the smallest possible sun zenith angle for minimizing the residual absorption along the line of sight. After wavelength alignment on Fraunhofer solar lines, slant column amount of each species are derived by least squares correlation with laboratory absorption cross-sections in an iterative process from which vertical profiles of the various species are retrieved by an onion peeling inversion scheme.