OPAR (Reunion Island Atmospheric Physics Observatory) is a scientific structure dedicated to atmospheric observations in the framework of the OSU-R (Observatoire des Sciences de l’Univers de la Réunion). OPAR is composed of three measurements sites located on Reunion Island (55.5°E, 21°S) in the Southern-western part of Indian Ocean: Gillot Airport (10 m ASL), Saint Denis University (80 m ASL) and the Maïdo facility (2160 m ASL).
The OPAR structure has been officially created in February 2003, but the instrumental setup at Reunion Island started to be installed at the beginning of the 1990s, with stratospheric Rayleigh lidar measurements. Due to the increasing interest in atmospheric measurements at this latitude, some other lidar systems (tropospheric and stratospheric ozone lidars, Doppler lidar, tropospheric aerosols) have been implemented later.
Till October 2012, all the instruments were installed at Saint Denis de la Reunion, on a campus a few meters above sea level and within the atmospheric boundary layer. In order to increase the number, quality and diversity of observations of atmospheric dynamical and chemical parameters started in the nineties, a new facility was built at the top of the Maïdo mountain, on the western part of Reunion Island. It became the main instrumented facility of OPAR in October 2012.
Technical presentation of the Maïdo Facility
The total surface of the plot of land is 6600 m2 including the road access, scientific container areas, parking, building, electrical substation, and outsides. The surface of the building is 600 m2 including, 173 m2 for the lidar space, 129 m2 for other scientific rooms (FTIR, Micro Wave Radiometer, in situ measurements), 300m2 for bedrooms, meeting room, and storage and ancillaries (water plant, power supply, secondary diesel power supply unit). A total of 164 m2 of scientific areas are available on the roof, enabling the installation of measurement heads above the scientific labs. Two specific experimental container areas for 12 m sea containers (40 feet) are equipped with water, electricity and local area network, enabling an easy plug in of experiments for campaigns. The access road is sized for big container trucks. During the design studies, most rooms have been designed taking into account the properties of the instruments and some space has been reserved for future instruments. Two dedicated radio links and optical fibers are connecting the station to phone and data link networks.
Stratospheric ozone lidar
The stratospheric ozone LIDAR is in operation at the Maïdo facility since February 2013. This lidar is devoted to stratospheric ozone measurements (17-45 km) using the Differential Absortion technology. Both wavelengths are generated with two different lasers:
* A Neodyme YAG laser generates the OFF non absorbed wavelength 355 nm. The fundamental frequency is 1064 nm. The 355 nm wavelength corresponds to the third harmonic, obtained when the laser beam crosses two KDP crystals at the output of the laser head. The laser pulse frequency is 30 Hz and the output power is nearly 5 Watts.
* An exciplexe laser generates the ON absorbed wavelength. This laser is a high pressure pulsed laser. The amplifying medium consists of a mixture of rare gazes (Neon and Xenon) and halogen gaz (Hydrogen Chlorure). An electrical discharge in the gas generates some chemical reactions which produces a complex halogen molecule (Xenon Chlorure). The radiative deactivation generates stimulated emission at 308 nm.
Receiving optics consist of four 500 mm diameter, at the focal points of which ar our optical fibers. Backscattered signals are focused on the cores of these fibers, then transmitted by the fibers to a spectrometer. Each beam is then divided in two parts (8% and 92% of the energy) by a glass membrane. Each beam is then detected by photomultipliers and digitized by the electronics. This fast counting system (400 Mhz bandwidth), digitizes the temporal signal with a 1 µs resolution (150 m spatial resolution). A mechanical chopper enables the control of the laser firing rate and the obturation of the backscattered signal in the lower layers. The electronic obturation system has been put into service since 2002 to enable a better selection of measurements in the high altitude layers.
Temperature-Water vapour lidar
The temperature-water vapour lidar is in operation at the Maïdo facility since October 2012. This LIDAR is devoted to temperature measurements (25-95 km) and to wator vapour measurements (2.5-20 km).
The first lidar at Reunion Island was a Rayleigh–Mie, operating since 1994. This system was based on a Nd:YAG laser emitting at 532 nm, and a reception system composed by a mosaic of 4 parabolic mirrors, with a diameter of 500 mm each and optical fibers at their focus points to collect the backscattered light. This system was successively upgraded in 1998 with DIAL channels to produce tropospheric ozone profiles and Raman channels in 2002 to produce water vapor profiles. The actual Raman water vapor lidar system is an upgrade of the receiving optics of the existing Rayleigh–Mie lidar system in operation since 1994. It is dedicated to water vapor measurements in the UTLS and to the measurements of the middle and upper atmosphere temperature using Rayleigh scattering. The light source of this lidar consists in two Quanta Ray Nd:Yag lasers. The system is designed to work at 532 or 355 nm. Pulses of both lasers can be synchronized, and coupled through polarization cubes. The backscattered signal is collected by a 1.2 m diameter telescope that was previously used at Biscarrosse (France) for Rayleigh and Raman measurements and that was refurbished in 2011. A narrow field of view of 1 mrad can be used to reduce as much as possible sky background and detector noise. Contrary to the lidar system used at Reunion Island University before 2012, the current system uses a set of lenses and mirrors instead of optical fibers to transfer backscattered signals to the optical ensemble, in order to avoid a systematic bias in water vapor measurements due to fluorescence in fiber-optic cables. Regarding the photon detector, we use, in a first step, new Hamamatsu R7400-03g or 20g (depending on the wavelengths) mini-PMTs and data acquisition consists in the use of LICEL PR 10–160 transient recorders in photo-counting. Coaxial geometry for emission and reception provides parallax effects to be avoided, extends measurement down to the ground and facilitates the alignment. We defined and built an integrated and removable support for a calibration lamp to complement the calibration with total water vapor column measurements from a collocated GPS instrument to use the hybrid technique. Both possible emitted wavelengths combined with a set of permanently installed detection boxes working both in the visible and in the UV enable different operating modes.
Tropospheric ozone lidar
The tropospheric ozone LIDAR is in operation at the Maïdo facility since January 2013. This lidar is devoted to tropospheric ozone measurements (4-18 km) using the Differential Absortion technology.
This lidar system is another upgrade of the existing Rayleigh–Mie lidar system. The emission part of this system consists of a wavelength pair (289 and 316 nm) obtained by Raman shifting of the fourth harmonic of the Nd:Yag laser in a high pressure deuterium cell. The energy at 266 nm is 40 mJ pulse−1. The laser frequency is 30 Hz and the beam diameter 10 mm. The length and diameter in/out of the Raman cell are respectively 1500, 20 and 55 mm. The beam is expanded in a divergence optimizer system located after the Raman cell. The output diameter and divergence of the emitted beam are 30 mm and 0.25 mrad. Regarding the reception system, we use the 4 telescope mosaic used before. The signal collected is transmitted with 1.5mm diameter optical fibers. The spectral separation of 289 and 316 nm beams is obtained with a spectrometer formed by a Czerny–Turner holographic grating. The altitude of the Maïdo Mount being 2200 m.a.s.l., the transfer of the tropospheric ozone DIAL system from the university (80 m.a.s.l.) to this location is positive concerning the upper limit of the profile, but it will also increase the lower limit from 3–4 to 5–6 km, i.e. over the lower limit of the free troposphere corresponding to the trade wind inversion. In order to compensate this, we add a smaller 200 mm diameter telescope, with a commutation from one mode to the other by switching the optical fibers at the entrance of the spectrometer.
OPAR, Reunion Island University
15 av. R. Cassin
97715 St Denis Messag. cedex 9
Phone: 00262 2 62 93 86 64
Observatory for atmospheric physics in La Réunion website
Baray J.-L., Y. Courcoux, P. Keckhut, T. Portafaix, P. Tulet, J.-P. Cammas, A. Hauchecorne, S. Godin-Beekmann, M. De Mazière, C. Hermans, F. Desmet, K. Sellegri, A. Colomb, M. Ramonet, J. Sciare, C. Vuillemin, C. Hoareau, D. Dionisi, V. Duflot, H. Vérèmes, J. Porteneuve, F. Gabarrot, T. Gaudo, J.-M. Metzger, G. Payen, J. Leclair de Bellevue, C. Barthe, F. Posny, P. Ricaud, A. Abchiche, and R. Delmas, Maïdo observatory: a new high-altitude station facility at Reunion Island (21° S, 55° E) for long-term atmospheric remote sensing and in situ measurements, Atmos. Meas. Tech., 6, 2865–2877, 2013