1407 - Illustrate the interaction of e.m. radiation in the thermal infrared with the atmosphere understanding specifc characteristics of radiative transfer in this specific spectral region.
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Illustrate the interaction of e.m. radiation in the thermal infrared with the atmosphere understanding specifc characteristics of radiative transfer in this specific spectral region.
When we talk about “thermal infrared (or terrestrial) radiation” we commonly refer to the energy emitted from the Earth-atmosphere system. Trapping of thermal infrared radiation by atmospheric gases is typical of the atmosphere and is therefore called the “atmospheric effect”. The atmospheric effect is sometimes referred to as the “greenhouse effect” because in a similar way glass, which covers a greenhouse, transmits short-wave solar radiation, however absorbs long-wave thermal infrared radiation. Imagine a beam of radiation travelling through a small section of air. The air is made up of changing concentrations of different species, with all molecules absorbing and emitting thermal radiation at different rates. As the radiation travels through different layers of the atmosphere, the intensity of radiation will constantly be modified by both absorption and emission processes as described by the Schwarzschild's equation. In case of a sensor on board of a satellite, the net radiation measured would be that which is attenuated through each layer (as small increments of absorption and emission) from the surface to the top of the atmosphere plus the radiation emitted directly from the atmosphere. In this case, this process can be described by the radiative transfer equation (RTE).
The equation of radiative transfer simply says that as a beam of radiation travels through the atmosphere, it loses energy to absorption, gains energy by emission, and redistributes energy by scattering. Many radiative transfer codes exist which are able, i.e. on the basis of known properties of the atmosphere, to computed the effect of the atmosphere on the thermal infrared radiation providing atmospheric transmittance (absorption), atmospheric scattering and atmosphere path emission. Commonly, in satellite remote sensing, the thermal infrared region is defined as the region of the electromagnetic spectrum comprised between 8 and 14 micron. In an atmosphere free of particles (aerosols due to phenomena like fires, volcanic eruption, dust storm, etc.) the thermal infrared radiation is mainly affected by triatomic gases like water vapor, carbon dioxide and ozone.