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Dobson unit

" 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.
To make the units easier to work with, the "Dobson Unit" is defined to be 0.001 atm-cm. Our 0.3 atm-cm would be 300 Dobson Units (DU)."

Radiance

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.

Irradiance

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.

Monochromatic Emissivity

Monochromatic Absorptivity

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.

Grey body

A grey body is a body whose emissivity/absorptivity are independent of the
wavelength from a given surface.

Monocromatic Reflectivity

Monochromatic Transmittivity

These are two adimentional variables, defined similarly to the emissivity/absorptivity,
relative to a given surface/layer, with reference to a given radiation source.

Two common sources of radiation, relative to a given atmospheric layer,
are the solar and the terrestrial radiation (i.e. emitted by the earth surface).

In particular in space measurements the reflectivity is defined as:

 image   When the earth radiance is isotropically distributed the reflectivity is
equal to the albedo and the surface is said to be of Lambertian type.  

Beer law

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:

 image

where K e is referred to the unit of mass and imageis the layer density.
The Beer law is usually formulated relative to a given wavelength and
a unitary wavelength interval.

Scattering

It refers to the redistribution of the energy of a radiation beam,
incident over a given target, in all directions.  

 image

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.

 image

It is a fraction of scattering molecules, usually referred to the unit of mass.

 image

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:

 image

where image is the solar irradiance incident at the top of the atmosphere;
image is the solar zenith angle;
imageand image refer, respectively, to a given vertical height and to the top of the atmosphere, and
image is the optical thickness at a given wavelenght.

 image

Given an atmospheric layer, the transmissivity can be defined,
on the basis of the Beer law, as:

 image Therefore it is a multiplying factor between the monocromatic irradiance incident at the top of the layer imageand its value at the bottom of it image   

image 

In the radiative transfer theory it is introduced in order to distinguish among various scattering regimes. It is defined as:

 image
image

Rayleigh scattering

 image

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.

Mie scattering

 Scattering of molecules/particles occurring roughly when image  
(When image the radiation behaviour is described by the geometric optics.)

It is typical of atmospheric aerosols and particulate, scattering the solar radiation.

The scattered energy has an oscillating behavior, depending on the particle
size parameter. Forward scattering is prevailing over backward scattering.
It results in the reddish/bluish colour of the particles.

Ring effect

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.
The latter leads to the so-called "Fraunhofer ghosts" in the earth radiance spectra.
They appear as displacements of the Fraunhofer lines, present in the solar radiation.

Fraunhofer lines  

Fraunhofer ghosts

Mg II index

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,
is based on the observations (nadir) of the solar backscattered light, at different wavelengths (at least 2),
for which there is significantly different ozone absorption.
Therefore absolutely calibrated radiance measurements are needed.
The accuracy of the measurements, from the presently available radiometers, depends on the diffuser plate stability and on its Bidirectional Reflectance Distribution Function (BRDF) knowledge.  

Differential Optical Absorption Spectroscopy (DOAS) technique

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.

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