Optical of the atmosphere

McCartney, E.J., Optics of the Atmosphere Scattering by Molecules and Particles, Wiley and Sons, 1976. 

Let s consider the influence of gases and particles on the optical properties of the atmosphere. Reduction in visibility is caused by the following interactions in the atmosphere light scattering by gaseous molecules and particles, and light absorption by gases and particles (2). 

By its random nature, turbulence does not lend itself easily to modelling starting from the differential equations for fluid flow (Navier-Stokes). However, a remarkably successful statistical model due to Kolmogorov has proven very useful for modelling the optical effects of the atmosphere. 

This first step makes necessary a correction of the atmosphere aberrations by means of an adaptive optics or at the minimum a tip tilt device. If the turbulence induces high aberrations the coupling efficiency is decreased by a factor VN where N is the number of spatial modes of the input beam. Note that tilt correction is also mandatory in a space mission as long as instabilities of the mission platform may induce pointing errors. Figure 10 (left) illustrates the spatial filtering operation. This function allows a very good calibration of... 

Just as in the case for the hydrosphere, the atmosphere participates in all of the major biogeochemical cycles (except for phosphorus). In turn, the chemical composition of the atmosphere dictates its physical and optical properties, the latter being of great importance for the heat balance of Earth and its climate. Both major constituents (O2, H2O) and minor ones (CO2, sulfur, nitrogen, and other carbon compounds) are involved in mediating the amounts and characteristics of both incoming solar and outgoing infrared radiation. 

Here B is the world average burden of anthropogenic sulfate aerosol in a column of air, in grams per square meter. The optical depth is then used in the Beer Law (which describes the transmission of light through the entire vertical column of the atmosphere). The law yields I/Iq = where I is the intensity of transmitted radiation, Iq is the incident intensity outside the atmosphere and e is the base of natural logarithms. In the simplest case, where the optical depth is much less than 1, (5 is the fraction of light lost from the solar beam because of... 

The error of this approximation is less than 0.01% for relevant global mean temperatures (see the Appendix). The thickness of the atmospheric layers, which is increasing with the altitude, is correlated to the optical depth of the... 

The solar radiation incident on the atmosphere and the Earth s surface represents the largest external energy contribution. The optical irradiation at the upper boundary layers of the atmosphere is 4,435 kJ/cm2/year, of which around 1,108 kJ/cm2/year reaches the Earth s surface. 

Running in parallel to spectacular space-based astronomy, optical telescopes and radiotelescopes have progressed in a quite breathtaking manner thanks to the new techniques of interferometry and active and adaptive optics. Telescopes perched on mountain peaks, such as the CFHT (Canada-France-Hawaii Telescope) and Keck on Mauna Kea in Hawaii and the VLT on Cerro Parana in Chile, and radiotelescopes set out like windmills in Puerto Rico, Sologne (in the French Alps), The Netherlands and Spain, gather photons able to cross the layers of the atmosphere without major alteration, whilst spectrographs then dissect the radiation into its finest detail. 

GOME is a nadir sounding spectrometer which observes the up-welling radiance from the top of the atmosphere and the extra-terrestrial solar irradiance between 240 and 790 nm. The resolution of the measurements is chosen to be suitable for the application of the differential optical absorption spectroscopy (DOAS) technique, which was developed for long-path measurements and zenith sky observations (e.g. Platt and Pemer, 1980 Mounter a/., 1987 Eisinger etal., 1997). 

Accomplishment of the complex observational experiment LACE-98 made it possible to obtain extensive information about atmospheric aerosol (aircraft measurements of the size distribution and number density of fine aerosols, coefficients of aerosol absorption, backscattering and depolarization, chemical composition of aerosol, as well as surface observations of the spectral optical thickness of the atmosphere, coefficients of extinction and backscattering). Fiebig et al. (2002) compared the observational data on optical parameters obtained from the results of numerical modeling for total H2S04 aerosol near the tropopause as well as for the ammonium sulfate/soot mixture in the remainder of the air column (Osborne et al., 2004). 

As part of some international and national projects, many measurements of atmospheric optical thickness were carried out. By using passive satellite sensors, estimates can be averaged vertically over a surface pixel. Therefore, to get a deeper understanding of the optical thickness of atmospheric layers, aircraft measurements are made which give the vertical distributions both of tropospheric aerosols and other characteristics of the atmosphere. Among successful airborne experiments we should highlight the ITOP, SHADE, and SAMUM experiments. These experiments made it possible to study the transformation of aerosols during the distant transport of smoke and desert dust. 

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