The Angstrom exponent
[Gregory L. Schuster, Oleg Dubovik and Brent N. Holben (2006): "Angstrom exponent and bimodal aerosol size distributions". ''Journal of Geophysical Research: Atmospheres'', volume 111, issue D7, article D07207, pages 1-14. ][Itaru Sano (2004): "Optical thickness and Angstrom exponent of aerosols over the land and ocean from space-borne polarimetric data". ''Advances in Space Research'', volume 34, issue 4, pages 833-837. ] or Ångström exponent
[D. A. Lack1 and J. M. Langridge (2013): "On the attribution of black and brown carbon light absorption using the Ångström exponent".
''Atmospheric Chemistry and Physics'', volume 13, issue 20, pages 10535-10543. ][Ji Li, Chao Liu, Yan Yin, and K. Raghavendra Kumar (2016): "Numerical investigation on the Ångström exponent of black carbon aerosol". ''Journal of Geophysical Research: Atmospheres'', volume 121, issue 7, pages 3506-3518. ] is a parameter that describes how the
optical thickness
In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material.
Thus, the larger the optical depth, the smaller the amount of transmitted radiant power through ...
of an
aerosol
An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
typically depends on the
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of the light.
Definition
In 1929, the
Swedish
Swedish or ' may refer to:
Anything from or related to Sweden, a country in Northern Europe. Or, specifically:
* Swedish language, a North Germanic language spoken primarily in Sweden and Finland
** Swedish alphabet, the official alphabet used by ...
physicist
Anders K. Ångström found that the optical thickness of an aerosol depends on the wavelength of light according to the
power law
In statistics, a power law is a Function (mathematics), functional relationship between two quantities, where a Relative change and difference, relative change in one quantity results in a proportional relative change in the other quantity, inde ...
:
where
is the optical thickness at wavelength
, and
is the optical thickness at the reference wavelength
.
[Anders Ångström (1929): "On the Atmospheric Transmission of Sun Radiation and on Dust in the Air". ''Geografiska Annaler'', volume 11, issue 2, pages 156–166. ][ The parameter is the Angstrom exponent of the aerosol.
]
Significance
The Angstrom exponent is inversely related to the average size of the particles in the aerosol: the smaller the particles, the larger the exponent. For example, cloud droplets are usually large, and thus clouds have very small Angstrom exponent (nearly zero), and the optical depth does not change with wavelength. That is why clouds appear to be white or grey.
This relation can be used to estimate the particle size of an aerosol by measuring its optical depth at different wavelengths.
Determining the exponent
In principle, if the optical thickness at one wavelength and the Angstrom exponent are known, the optical thickness can be computed at a different wavelength. In practice, measurements are made of the optical thickness of an aerosol layer at two different wavelengths, and the Angstrom exponent is estimated from these measurements using this formula. The aerosol optical thickness can then be derived at all other wavelengths, within the range of validity of this formula.
For measurements of optical thickness and taken at two different wavelengths and respectively, the Angstrom exponent is given by
:
The Angstrom exponent is now routinely estimated by analyzing radiation measurements acquired on Earth Observation
Earth observation (EO) is the gathering of information about the physical, chemical, and biological systems of the planet Earth. It can be performed via remote-sensing technologies (Earth observation satellites) or through direct-contact sensors ...
platforms, such as AErosol RObotic NETwork
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthr ...
, or AERONET.
See also
* Langley extrapolation
Langley extrapolation is a method for determining the Sun's irradiance at the top of the atmosphere with ground-based instrumentation, and is often used to remove the effect of the atmosphere from measurements of, for example, aerosol optical thick ...
References
IPCC Third Assessment Report, has extensive coverage of aerosol-climate interactions
* Kuo-nan Liou (2002) ''An Introduction to Atmospheric Radiation'', International Geophysics Series, No. 84, Academic Press, 583 p, {{ISBN, 0-12-451451-0.
External links
Angstrom coefficient page at NASA GSFC
AERONET: an international network of sunphotometers measuring aerosol properties
Spatial distributions of the Angstrom coefficient
as derived from MISR Misr or MISR may refer to:
* Misr, the romanized Arabic name for Egypt
* misr, singular of Arabic ''amsar'', which were early Arabic "garrison towns"
* Misr (domain name), a top-level Internet domain name
* Misr, a variant of the AKM assault rifle ...
.
Scattering, absorption and radiative transfer (optics)
Atmospheric radiation
Visibility