DARE (Dedicated Aerosol Retrieval Experiment) is a study to design an instrument for accurate remote sensing of aerosol properties from space. DARE combines useful properties of several existing instruments like TOMS, GOME, ATSR and POLDER. It has a large wavelength range, 330 to 1000 nm, to discriminate between aerosol types. The wide swath will enable daily global coverage of the earth at a pixel size of 5 x 5 km<sup>2</sup> (nadir). The instrument will have three viewing angles, looking forward, at nadir and backward. These angles will facilitate the separation of atmospheric and surface contributions to the satellite signal. Multiple views will also help to determine the height and thickness of aerosol layers. Full polarization information will be measured for at least one of the viewing angles and at many of the available wavelengths. Polarization helps to separate surface/atmosphere signals and it contains valuable information on the shape of aerosol particles. Cloud detection will be enhanced by adding thermal infrared detectors at the same spatial resolution. Simulations and tests were performed to optimize the current design of the instrument. The expected performance of DARE in comparison to other instruments will be discussed.
At present there is an increasing interest in remote sensing of aerosols from space because of the large impact of aerosols on climate, earth observation and health. TNO has performed a study aimed at improving aerosol characterisation using a space based instrument and state-of-the-art aerosol retrieval algorithms, based on requirements for up-to-date regional and global aerosol transport models. The study has resulted in instrument specifications and a concept design for aerosol detection from space. Based on the study the main requirements for a dedicated aerosol spectrometer are: a spectral range from 330-1000 nm with a spectral resolution from 2 nm (UV) to > 30 nm (NIR), observation in at least 3 polarisation directions (Stokes parameters) over a field of view (FOV) in swath directions of > 114 degrees and observation in at least 3 viewing directions (backwards, nadir, forward). The spectrometer design is a prism imaging spectrometer using a single detector array to measure the complete spectra for 2 polarisation directions. In this way the requirements for each viewing direction can be met with only 2 detector arrays. The system has a modular set-up, which makes the implementation of, for example, a change in the number of observation directions very simple. The basic requirements to discriminate between aerosol types are currently only met POLDER, that combines multiple view angles with polarization. The DARE concept shows an attractive potential for the development of next generation aerosol sensors.
To retrieve aerosol properties from the radiance measured at the top of the atmosphere in clear sky conditions, the contributions of the surface and the various atmospheric constituents need to be separated. This is easiest done over dark surfaces, i.e. with a negligible reflectance that can relatively easily be accounted for. This principle has been used for the retrieval of aerosol optical depth (AOD) over water from satellite observations in the near-infrared. The AOD at wavelengths in the UV can be determined both over water and over land, using the same principle of a dark surface. For longer wavelengths, the dual view provided by
ATSR-2 is used to separate the aerosol reflectance from the surface contribution. Single and dual-view algorithms have been developed by TNO-FEL and tested for the US east coast and over Europe. Currently the algorithms are extended with other aerosol types and tested versus data over the Indian Ocean (INDOEX area) and South Africa (SAFARI experiment). The initial results indicate that the AOD can be retrieved within reasonable limits. Apart from the ATSR-2, algorithms aimed at the determination of aerosol optical depth and composition are developed for AATSR and SCIAMACHY (ENVISAT) and OMI (EOS-TERRA).