The CEL-Biscarosse site

For tower measurements at the in-flow CO2 measurements will be performed at CEL-Biscarosse. The site is surrounded by a lot of small sandhills (50-70m above sea level –asl-) covered with pines. CEL recommends the installation of the instruments on the HF tower. It is a 50 m high concrete tower, located on a hill about 70 m asl. There are two platforms located at 47m and about 40m. It is located at 2km from the Atlantic Ocean. In terms of facility for our experiment at this point the situation is satisfying:

• air inlet may be fixed to the tower and there is a cable track from the top to the base of the tower;
• electricity is available and there is enough power for both CARIBOU (5kW) and FTIR (6kW);
• the CARIBOU may be installed within the tower together with 5 high pressure tanks;
• telephone connections may be installed;
• the FTIR container (~10 tons) may be installed on firm ground close to the tower;
• daily access to the site is possible for a couple of well identified scientists;
• flying over the tower with the Piper Aztec is possible, even so there are of course a lot of restrictions.

Meteorological parameters are measured on another pylon closer from the sea. Data can be made available daily during the intensive campaign. Vertical profiles of wind speed and direction are also measured every 15 min with a SODAR, up to 1 km high. These data can be used to detect the elevation of the atmospheric boundary layer (if it is lower than 1km).

figure II.2.1-a : Location of Centre d'Essai des Landes (CEL) at Biscarosse

figure II.2.1-b : The HF Tower and surroundings.

The Marmande site

At the Marmande site (see location in section II.1), the CO2 profile is measured at five levels up to 20m. This site is representative of the core of the domain, i.e. it is located in the middle of the area, between the western (Atlantic ocean shoreline) and eastern (Toulouse city) edges. Whereas the Biscarrosse measurements represents the initial CO2 concentration for the incoming airmass (assuming westerlies), the Marmande ones analyses the airmass which has travelled for several hours above the continent. However, it must be kept in mind that, while the Biscarrosse system is located on a 50m tower, itself built on the top a 70m hill, and can therefore be considered as representative of boundary-layer conditions (above surface layer), the Marmande profile is measured over a flat, low-altitude field in the valley of the Garonne river, and would probably not overpass the top of the surface layer, and thus is influenced by the local conditions. Furthermore, a PP Systems CIRAS-SC IRGA is used to analyse the sample air for H₂O and CO₂. This analyser, which is thermostated and compensates for cell pressure and temperature, and is calibrated twice a day, allows to measure CO2 concentration with an absolute accuracy better than 1ppm.