Light Extinction in the Atmosphere

Not all solar radiation approaching earth's surface can reach the ground. Atmosphere weakens light intensity in several ways, usually classified as absorption and scattering and summed up as atmospheric light extinction.

Absorption takes place when photons are stopped by atmospheric gases like O2, N2, H2O, CO2, SO2, O3, N2O, and aerosols, which are liquid droplets, solid particles, or a

Transmittance 0.4 0.6

Figure 1 Light extinction in a medium following Lambert-Beer law. Value of kextinction coefficient is 0.2 m~1 (e.g., extinction by water in a coastal sea, so medium length has been plotted as depth).

Figure 1 Light extinction in a medium following Lambert-Beer law. Value of kextinction coefficient is 0.2 m~1 (e.g., extinction by water in a coastal sea, so medium length has been plotted as depth).

mixture of both. They can have anthropogenic (e.g., combustion, mechanic origin) or natural source (e.g., volcanoes, meteors). Photon energy is taken up by gases and particles and can be transformed into heat or radiated.

Photons are scattered when they are deviated without energy loss from their original path by diffusion phenomena due to gases and aerosols, and then no longer reach the ground. Atmospheric scattering is usually divided into Rayleigh scattering, due to particles with diameter smaller than 1/10 of radiation wavelength (mostly oxygen and nitrogen for visible radiation) and Mie scattering, caused by particles with diameter up to 10 times the radiation wavelength. Above this length, a nonselective scattering takes place and geometrical optics laws apply.

These extinction phenomena are usually modeled using Lambert-Beer law. Ignoring Earth curvature effects, it is possible to account for solar radiation reaching the atmosphere with a solar zenith angle 0 different than zero, because the extinction path can be shorter or longer with dependency on hour, day, and latitude.

Jout Jin e where L is the height from the ground to which Iin incoming radiation is referring, and m is the optical air mass, defined as the relative length of the path that light has to travel through the atmosphere to reach the ground. Air mass takes the value of one at zenith, and for small zenith angles (up to 60°) it can be computed with good approximation from

For larger zenith angles, for which effects of refraction, air density, curvature, nonuniform vertical distribution of substances, and temperature are not negligible, semiem-pirical formulas derived from field values are used.

The k coefficient can be split into a sum k = X k

to account for absorption and scattering by aerosols, pollution gases, water vapor, and atmospheric gases. Usually, extinction in the visible range due to gases is small compared to the one by suspended particles in inhabited regions.

Light extinction in the atmosphere is strongly wavelength dependent and shorter wavelength radiations are decreased more. For example, greenhouse gases like H2O, CO2, N2O, and CH4 intensely absorb infrared radiation, while the radiation reaching the ground is mainly composed of ultraviolet and visible radiation.

Also, k can be computed as the sum of different contributions, like water, suspended solids (e.g., detritus), dissolved material, and living material (e.g., phytoplank-ton biomass). For example, one can consider contributions from water and dissolved matter (kw-diss) and particulate (kpart) matter k kw—diss ^ kpart kpart can also be written as a function of particulate matter concentration C:

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