There are two main processes that take place when incoming and outgoing electromagnetic radiation interacts with the atmosphere: light scattering and light absorption. While greenhouse gases such as CO2, CH4, N2O, and water vapor absorb a wide range of infrared radiation in rather broad bands, other trace gases such as O3, NO2, and HCHO have distinct narrow band absorption structures in the UV and visible range (Gottwald and Bovensmann, 2012).
Besides the importance of atmospheric radiative transport for the greenhouse effect (climate change) and for the determination of the efficiency of photochemical reactions
36
in the atmosphere, accurate knowledge of atmospheric radiative transport is essential for the retrieval of trace gases, aerosols, and cloud parameters from remote sensing data (Burrows et al., 2011).
The usual input parameters used for the computation of radiative transfer are the solar spectral input, extinction optical depth (the sum of the layer optical depths due to molecular absorption, Mie and Rayleigh scattering), single-scattering albedo, and phase function of scattered radiation (Ricchiazzi et al., 1998).
1.4.2.1. The solar spectrum
All energy used for physical and biological processes within the atmosphere and biosphere comes from the Sun. Hydrogen is converted to helium inside the Sun by nuclear fusion under enormous pressure and high temperatures. Large quantities of heat are transported by convection to the Sun’s surface. The superheated gas on the Sun’s surface emits electromagnetic radiation, which travels through space to the top of the Earth’s atmosphere within about 8.5 minutes.
Fig. 1.6: The Sun’s spectral irradiance (in terms of power of the Sun’s electromagnetic radiation received per unit area) at the top of the atmosphere (solid blue) and at the surface (dotted magenta) (from Fröhlich and Lean, 2004)
37 The radiative output of the Sun was initially believed to be constant and thus, expressed as the solar constant. However, long-term observations from space beginning in the late 1970s have shown that the radiative output of the Sun changes in time, from minutes to decades (Fröhlich and Lean, 2004).
One quantity commonly related to radiation transfer is the irradiance, which is the radiant flux (defined as the radiated energy per time) received per unit area, expressed in units of W m-2 (Platt and Stutz, 2008).
In the simplest case, full transmission of the solar irradiance without any absorption and/or scattering processes would take place in the absence of an atmosphere. A surface albedo of one (perfect reflection of a white surface) would mean that no loss of the incoming irradiance due to absorption occurs, and all of the light is reflected back to space. Assuming this to be the case, the solid blue line (the incoming irradiance at the top of atmosphere) in Fig. 1.6 would not change. In reality, both the composition of the atmosphere and the fact that the Earth’s surface is not a perfect reflector, lead to a decreased solar irradiance when reaching the Earth’s surface (dotted magenta line).
1.4.2.2. Scattering, absorption, and emission of photons
As soon as the amount of energy (in the form of radiation) emitted by the Sun reaches the top of the atmosphere and enters the atmosphere, many interaction processes between radiation and atmosphere occur (Platt and Stutz, 2008; Burrows et al., 2011).
Scattering processes in the Earth’s atmosphere can be subdivided into elastic (Rayleigh and Mie scattering) and inelastic scattering (Raman scattering). While Rayleigh scattering due to air molecules is simply demonstrated by the blue color of cloud free skies, Mie scattering due to aerosol particles leads to the white and grey color of cloudy skies. Both Rayleigh and Mie scattering are elastic scattering processes without any changes in photon energy. In contrast, the inelastic Raman scattering occurs when the scattering molecule changes its state of excitation during the scattering process. The transfer of the photon’s energy to the molecule produces vibration and/or rotation of the molecule.
38
Because of the strong interaction with absorbers at particular wavelength intervals (see Fig. 1.6), radiation is removed from the radiation field and converted into some other form of energy (e.g. heat). While the absorption of solar UV radiation is an important prerequisite for life on land, the absorption of IR radiation is a key process in the climate system. The absorption processes in the atmosphere are used for the detection of trace gases by remote sensing techniques (see Sect. 1.5). Although the absorption of solar radiation is relatively low in the visible part of the electromagnetic spectrum (see Fig. 1.6), the narrow band absorption structures of NO2 can be used for retrieval of tropospheric NO2 amounts (Richter, 2006).
Thermal emission of air molecules or aerosol particles, which cannot exceed the emission from a black body for the temperature of the atmosphere, takes place at infrared wavelengths. However, it can be neglected in the visible part of the electromagnetic spectrum and thus, the observed spectral signatures can be directly related to absorption spectra of atmospheric constituents (Burrows et al., 2011).
For the retrieval of trace gases from remote sensing techniques, the effective light path within the atmosphere is simulated by radiative transfer models, which is usually achieved by invoking so called airmass factors (AMFs). The light path is determined by the geometry (e.g. viewing angle of the instrument and solar zenith angle), surface reflection, and scattering from air molecules, aerosol particles, and clouds. For weak absorbers, it is assumed that the effective light path is independent of the amount and vertical distribution of the absorber itself.
As the vertically integrated trace gas concentration or vertical column density (VCD) is the more typical quantity, the AMF is applied to convert the slant column density (SCD):
𝐴𝑀𝐹 =
𝑆𝐶𝐷39
1.4.2.3. The radiative transfer model SCIATRAN
The radiative transfer model (RTM) GOMETRAN was developed within the scope of the Global Ozone Monitoring Instrument (GOME) project for the simulation of backscattered radiation from the atmosphere and reflected radiation from the Earth’s surface in the spectral range 240-800 nm (Rozanov et al., 1997; Burrows et al., 1999). With the objective of the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) instrument to detect additional trace gases, the successor RTM SCIATRAN was extended to allow simulations in the larger spectral window and in limb viewing direction (the instrument looks at the edge of the atmosphere) (Rozanov et al., 2005). Besides a number of new capabilities, SCIATRAN also allows for the calculation of AMFs for ground-based and satellite-based measurements, which are used for the retrieval of tropospheric trace gas columns (see Sects. 2.2.1, 3.2.1, and 6.3.1.5).