" The unit of measure for total ozone. If you were to take all
the ozone in a column of air stretching from the surface of the
earth to space, and bring all that ozone to standard temperature (0
deg. Celsius) and pressure (1013.25 millibars, or one atmosphere),
the column would be about 0.3 centimeters thick. Thus, the total
ozone would be 0.3 atm-cm.
The radiance is defined as the radiant energy emitted by the unit surface of a given target, per unit of time, in a given direction, per unit solid angle. Units: Watts m-2 ster-1. The monochromatic radiance is defined relative to a given wavelength and a unitary wavelength interval.
The irradiance is defined as the summation of the normal component of the radiance, relative to a given surface over a hemisphere. Units: Watts m-2. The monochromatic irradiance is defined relative to a given wavelength and a unitary wavelength interval.
The monochromatic irradiance from a "black body" surface is known as the Planck's law:
The blackbody radiation is isotropic i.e. the radiance is independent of the direction.
These are two adimensional variables defined as ratios to the blackbody radiation incident over a given surface; therefore they represent fractions of energy. A given wavelength and a unitary wavelength interval are considered.
The Kirchoff's law states the equality between emissivity and absorptivity.
A grey body is a body whose emissivity/absorptivity are
independent of the
These are two adimentional variables, defined similarly to the
Two common sources of radiation, relative to a given atmospheric
In particular in space measurements the reflectivity is defined as:
The extinction of a radiation beam, passing through a given atmospheric layer can be due either to absorption or to scattering processes.
The variation of the radiation intensity is proportional to the intensity of the incident radiation; to the depth of the layer (considering a beam incident normally to the surface); and to an extinction coefficient, Ke , which can be written as the sum of an absorption and a scattering coefficient.
This is known as the Beer or the (Bougner) law:
a unitary wavelength interval.
It refers to the redistribution of the energy of a radiation
It is a fraction of absorbing gas molecules, usually referred to the unit of mass.
Typical units are: m2 Kg-1. It depends on the gas temperature/pressure conditions.
It is a fraction of scattering molecules, usually referred to the unit of mass.
Considering the extinction of the solar radiation, through an atmospheric layer, the optical depth can be defined on the basis of the Beer law as:
where is the solar irradiance
incident at the top of the atmosphere;
Given an atmospheric layer, the transmissivity can be
Therefore it is a multiplying factor between the monocromatic irradiance incident at the top of the layer and its value at the bottom of it
In the radiative transfer theory it is introduced in order to distinguish among various scattering regimes. It is defined as:
It is typical of the atmospheric gas molecules scattering the solar radiation. The scattered energy is inversely proportional to the power 4 of the wavelength of the incident radiation. It also depends on the refractive index of the scatterers. The colour blue of the sky results from such a phenomenon.
Scattering of molecules/particles occurring roughly when
It is typical of atmospheric aerosols and particulate, scattering the solar radiation.
The scattered energy has an oscillating behavior, depending on
Inelastic light scattering by air molecules (e.g. NO2, O2). The ring effect source spectrum has to be taken into account in the retrieval of atmospheric trace gases by the DOAS technique.
The Ring effect is thought to
be due to the rotational Raman scattering and to the vibrational
Raman scattering as well.
It is an indicator of solar activity, computed as the integral of the area beneath the h and k peaks of the solar irradiance spectrum (near 280 nm), divided by a reference intensity derived from the wings nearby the above mentioned peaks. The peaks originate from the solar chromosphere and show time variations (e.g. over 27 and 11 years time intervals), whereas the wings originate from the photosphere (below) and are relatively stable.
Solar Backscattering Ultra Violet (SBUV) technique
The SBUV technique, for stratospheric ozone measurements,
The DOAS technique, for atmospheric trace gas retrieval, is based on a filtering procedure, of the measured atmospheric spectra (divided by a reference spectrum), aimed at eliminating the broad spectral features, whereas retaining the narrow spectral features. The former is due to scattering by air molecules and aerosols, as well as to absorption by aerosols and clouds. The latter are due to trace gas absorption. The filtered spectra are then correlated to the gas species absorption cross section, as derived by laboratory measurements. The advantage of this technique is that it does not relay on absolute radiance measurements.