ABSTRACT
Direct sunlight absorption by trace gases can be used to quantify them and investigate atmospheric chemistry. In such experiments, the main optical apparatus is often a grating or a Fourier transform spectrometer. A solar tracker based on motorized rotating mirrors is commonly used to direct the light along the spectrometer axis, correcting for the apparent rotation of the Sun. Calculating the Sun azimuth and altitude for a given time and location can be achieved with high accuracy but different sources of angular offsets appear in practice when positioning the mirrors. A feedback on the motors, using a light position sensor close to the spectrometer, is almost always needed. This paper aims to gather the main geometrical formulas necessary for the use of a widely used kind of solar tracker, based on two 45° mirrors in altazimuthal set-up with a light sensor on the spectrometer, and to illustrate them with a tracker developed by our group for atmospheric research.
ABSTRACT
We present a sophisticated radiative transfer code for modeling outgoing IR radiation from planetary atmospheres and, conversely, for retrieving atmospheric properties from high-resolution nadir-observed spectra. The forward model is built around a doubling-adding routine and calculates, in a spherical refractive geometry, the outgoing radiation emitted by the Earth and the atmosphere containing one layer of aerosol. The inverse model uses an optimal estimation approach and can simultaneously retrieve atmospheric trace gases, aerosol effective radius, and concentration. It is different from existing codes, as most forward codes dealing with multiple scattering assume a plane-parallel atmosphere, and as for the retrieval, it does not rely on precalculated spectra, the use of microwindows, or two-step retrievals. The simultaneous retrieval on a broad spectral range exploits the full potential of current state-of-the-art hyperspectral IR sounders, such as AIRS and IASI, and should be particularly useful in studying major pollution events. We present five example retrievals of IASI spectra observed in the range from 800 to 1200 cm(-1) above dust, volcanic ash, sulfuric acid, ice particles, and biomass burning aerosols.
