The BOUSSOLE project is made of three basic and complementary elements, namely (1) monthly maintenance cruises, (2) a permanent optics mooring, and (3) a coastal Aerosol Robotic Network (AERONET) station.

Each of these 3 segments is designed to provide specific measurements of various parameters at different and complementary spatial and temporal scales. When combined together, they should provide a comprehensive time series of near-surface (0-200 m) ocean and atmosphere inherent optical properties (IOPs) and apparent optical properties (AOPs), needed to accomplish several objectives :

• Performing the vicarious radiometric calibration of satellite ocean color sensors, i.e., the simulation of the top-of-the-atmosphere (TOA) total radiances recorded by the sensor in various spectral bands, including the visible and near infrared (NIR) domains.

• Performing the validation of the Level-2 geophysical products that are produced from the observations of these satellite ocean color sensors. This includes mostly the normalized water-leaving radiances, the pigment concentrations and the aerosol optical thickness and types.

• Performing both of these operations on a long-term basis, i.e., at least for the duration of the MERIS mission, for which the BOUSSOLE project has been primarily set up.

• Making progress in several domains of ocean optics and bio-optics, as already stated in introduction.

A basic description of the three elements and the operations carried out there is given in Monthly maintenance cruises, The mooring site and The coastal site.

From July 2001 to December 2003, the BOUSSOLE optics cruises were planned with a duration of 3 consecutive days distributed on a near-monthly basis within the budget of 30-31 sea days per year aboard the research vessel Tethys II. In these first two and a half years, some cruises were unsuccessful in data collection due to bad weather preventing ship departure or scientific work for the entire three days. For this reason, for 2004 the cruise duration during the seasons with characteristically unsettled conditions, October to April, were increased to up to five days with a subsequent reduction in the overall number of cruises per year. The same logic is followed since 2005, yet without reduction of the number of cruises (i.e., 12 cruises per year).

The standard optics day commences and ends with a 400m CTD rosette profile. The rosette is equipped with 11 Niskin bottles and an AC9 in the position of the twelfth bottle. Additional sensors are the CTD + O2 sensors, a CDOM fluorometer, a chlorophyll fluorometer and a backscattering sensor. For at least one of the CTD casts each day, the bottles are fired at selected depths during the ascent of the CTD from depth throughout the range surface to 400m. The actual depths of the water collections are decided during the descent of the CTD after observation of the fluorescence profile, thus optimizing the representation of any features in the water column. For other casts, triplicate samples are collected from 5 and 10m. In the summer season, when the optics day, defined as the period during which the sun is at an angle greater than 20 degrees above the horizon, is longer or when there are ideal satellite matchup conditions, an additional CTD profile is performed around solar noon or in time with a SeaWiFS or MODIS pass. The seawater samples of 2.8 L are filtered through Whatman GF-F filters using a low pressure vacuum and stored in liquid nitrogen. Back in the laboratory, the filters are later analyzed by HPLC and spectrophotometry for pigments and particulate absorption, respectively.

Multispectral upwelling and downward irradiances profiles, E_u(z, λ) and E_d(z, λ), respectively (z indicates depth and λ is wavelength), are performed with the objectives of providing synchronous in situ SeaWiFS and MERIS calibration and validation profiles, characterization of the light field throughout the day at the BOUSSOLE site and for providing a supportive data set to data from the BOUSSOLE buoy. During these profiling sessions, multiple profiles are performed with the SPMR, if possible, to improve the quality of the data by replicates. The first SPMR session of the day begins after the first CTD profile and continues ideally until the end of the so-called optics day, before the final CTD profile. A gimbaled 4π PAR sensor positioned on the foredeck and operated from the CTD computer serves as a light field stability indicator during SPMR profiling.

For the satellite passes, whenever possible, SPMR profiling is performed within 1 hour of satellite overhead passes of SeaWiFS and MERIS and around solar noon. Optimal conditions for these measurements are low humidity, blue skies and flat, calm sea surface. If the sky is clear and sea conditions are calm, SIMBADA measurements are performed consecutively where possible with SPMR profiles. If sea conditions are poor but the sun is clear of clouds, SIMBADA sun photometer measurements can be made at intervals throughout the day to measure atmospheric optical thickness.

When the sea surface conditions are calm, a low-volume (minimal shadow) surface float is used to suspend the SPMR in a vertical position with the E_u sensor submerged approximately 20 cm below the surface. The SPMR is held in this surface position for a duration of at least three minutes and is released when the light field is expected to be stable. This decision is a judgement call based on sky observation and monitoring of the PAR sensor. To start the descent, an electronic trigger mechanism is fitted to the surface float which can be controlled from inside the lab. Multiple descents ideally will be started in this way and the data will be used to assess near-surface upwelled radiance (L_u) extrapolation model calculations.

For each cruise, at the end of the optics measurements on site, there will be one CTD and IOP transect between the BOUSSOLE site and the Port of Nice, France. This transect consists of five fixed locations on-route from BOUSSOLE to Nice and a final optional station close to Nice, to be used if the oceanic front appears to be exceptionally close to the coast, a factor which is decided upon in situ by observing the thermosalinograph during transit. The aim is to have a representative profile of the water column on either side of the boundary to the Ligurian current. The approximate time of day that this transect is performed is kept similar for each cruise, whenever possible, to minimize the influence of diurnal variability.

A study of the spatial variability at the BOUSSOLE Site is performed once on each cruise. A fixed square mile quadrilateral grid based on GPS positions is covered by the ship at a speed of 7-8 kts. During transit around this grid, water is continuously being pumped from an inlet beneath the ship’s hull, and directed to the thermosalinograph and a fluorometer. Samples are collected at three equidistant points along this grid, for subsequent HPLC analyses, in order to convert dimensionless fluorescence measurements into chlorophyll concentrations.

On other uninterrupted transits between Nice and BOUSSOLE when there is a high sun angle and clear skies, SIMBADA sun photometer measurements are taken at 30 minute intervals (approximately 5 nmi) to characterize variability in atmospheric optical thickness between the Cap Ferrat sun photometer site and the BOUSSOLE site.

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The mooring site is equipped with a new type of buoy, specifically designed for collecting radiometric and optic data. The aim there is twofold :

(1) to get a near-continuous sampling at the surface (depths < 10 meters), in order to efficiently support the validation of the geophysical parameters that are derived from the ocean color remote-sensing observations of a variety of satellite sensors.

(2) to get a high-resolution data set of radiometric and optic quantities, in order to support fundamental work in marine optics, e.g., bidirectionality of the radiance field emerging from the ocean, short-term changes of optical properties, response of optical properties after strong environmental forcings (red rains, storms), effect of wind-induced bubbles, near-surface behavior of radiometric quantities, etc.

The satellite sensors that have been or are being supported include MERIS from the European Space Agency (ESA), SeaWiFS and MODIS-A (on the Aqua spacecraft) from the National Aeronautics and Space Administration (NASA), plus the third Polarization and Directionality of the Earth Reflectance (POLDER-3) sensor aboard the Polarisation et Anisotropie des Réflectances au sommet de l’Atmosphère, couplées avec un Satellite d’Observation emportant un Lidar (PARASOL) satellite, from the French Centre National d’Etudes Spatiales (CNES).

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