Dear All, I haven't received any replies to my request for advice on the equivalence of the terms "attenuation" and "extinction" in relation to the effects of atmospheric aerosol on optical radiation. However, I've been speaking to colleagues at RAL and looking at the literature to see how these terms are used.
My colleagues assure me that the two terms are used with equivalent meaning and they independently made the same point as Olivier that "attenuation" tends to be the favoured term when discussing radio/radar wavelengths and "extinction" is the term of choice at optical wavelengths. I looked at Wikipedia and a number of papers (see references below) and what I have gleaned is that "attenuation" in its broadest sense simply means the reduction over distance of energy propagating through a medium. The term can equally apply to electromagnetic waves, sound waves and even seismic waves. When applied to electromagnetic radiation passing through the earth's atmosphere, the mechanisms by which attenuation occurs are absorption, scattering, or most often a combination of the two, due to the presence of gas molecules and aerosol particles. The term "extinction" means the combined effects of absorption and scattering. I think the upshot of all this is that, although attenuation can occur due to a single mechanism e.g. absorption, the term itself is an umbrella for all possible mechanisms. Therefore, for electromagnetic radiation, "extinction" and "attenuation" have identical meanings. Something that struck me quite clearly from the literature is that, particularly when reference is made to attenuation/extinction coefficients, the two terms are used interchangeably. Indeed the AMS Glossary says "attenuation coefficient-(Also called extinction coefficient, especially in reference to optical frequencies.) ... The attenuation coefficient is defined through Bouguer's law as dL/L = -(gamma) ds where L is the monochromatic radiance at a given wavelength, gamma is the attenuation coefficient, and ds is a differential increment of pathlength. So defined, the attenuation coefficient is the same as the volume extinction coefficient and has dimensions of inverse length." The name under consideration is volume_extinction_coefficient_in_air_due_to_ambient_aerosol; m-1. Based on the above, I believe it is correct to note the equivalence of extinction and attenuation in the explanation. Furthermore, the explanations of the existing optical_thickness names which also note the equivalence of the two terms are correct. volume_extinction_coefficient_in_air_due_to_ambient_aerosol is accepted for inclusion in the standard name table. References: AMS Glossary: http://amsglossary.allenpress.com/glossary/search?id=attenuation-coeffic ient1 Wikipedia: http://en.wikipedia.org/wiki/Attenuation Aerosol extinction contribution to atmospheric attenuation in infrared wavelengths. John A. Hodges, 1972, Applied Optics vol 11 no. 10, 2304-2310. Modelled infrared extinction and attenuation performance of atmospherically disseminated high aspect ratio nanoparticles. Paul G. Appleyard, 2007, Journal of Optics A: Pure and Applied Optics vol 9, 278-300. Best wishes, Alison ------ Alison Pamment Tel: +44 1235 778065 NCAS/British Atmospheric Data Centre Fax: +44 1235 446314 Rutherford Appleton Laboratory Email: alison.pamm...@stfc.ac.uk Chilton, Didcot, OX11 0QX, U.K. > -----Original Message----- > From: cf-metadata-boun...@cgd.ucar.edu [mailto:cf-metadata- > boun...@cgd.ucar.edu] On Behalf Of alison.pamm...@stfc.ac.uk > Sent: 10 November 2010 07:46 > To: cf-metadata@cgd.ucar.edu; olivier.bouc...@metoffice.gov.uk; > michael.sch...@lsce.ipsl.fr; michael.sch...@cea.fr; taylo...@llnl.gov > Subject: [CF-metadata] One new aerosol standard name > > Dear All, > > There is just one remaining CMIP5 output quantity for which there is no > existing standard name. The quantity is a 3D field of atmospheric > extinction coefficient at 550 nm due to ambient aerosol. The quantity > has been requested by Michael Shulz and Olivier Boucher. It may be > calculated at each model level as the extinction optical thickness due > to the aerosol contained in that layer alone divided by the thickness > of > the model layer. The units are m-1. Some models may do the calculation > the other way round and compute extinction optical thickness by > multiplying the aerosol mass concentration by a mass extinction > coefficient and integrating over the depth of the layer. > > The proposed name is > volume_extinction_coefficient_in_air_due_to_ambient_aerosol; m-1. > > There is a question regarding the meaning of "extinction" as opposed to > "attenuation" because it has been suggested that their equivalence > should be noted in the explanation of the name. However, it is not > entirely clear whether the two really are equivalent and advice from > radiation experts would be most welcome. In the following I have copied > parts of the preliminary email discussion that have given rise to this > question. > > Initially, I suggested that the standard name for this quantity should > be > volume_optical_attenuation_coefficient_in_air_due_to_ambient_aerosol > by analogy with the existing standard names > volume_attenuation_coefficient_of_downwelling_radiative_flux_in_sea_wat > e > r volume_beam_attenuation_coefficient_of_radiative_flux_in_sea_water. > The explanations of both the existing names contain the sentence > "Attenuation is sometimes called extinction". Also, all the > optical_thickness name explanations refer to path integrals of "volume > scattering/absorption/attenuation" coefficients. > > However, Olivier wrote that: > > you're right that "attenuation coefficient" is the terminology in > electromagnetic > > theory (and maybe it is used by the ocean people). However although > we > use the term > > attenuation in Beer's law or in expression like "attenuated > backscatter signal" it is > >very rarely used as "attenuation coefficient" in atmospheric optics. > The standard term > > is "extinction coefficient" or "mass/volume extinction coefficient" > and I would prefer > > if we stick to this with a note that it is sometimes called > "attenuation coefficient" > > (even though attenuation is sometimes taken to mean just absorption" > when for > > aerosols/molecules scattering is usually more important than > absorption...). > > Michael expressed agreement with Olivier and suggested that we call the > name > volume_extinction_coefficient_in_air_due_to_ambient_aerosol > and include Olivier's note in the explanation. > > Karl Taylor is also content to use "extinction" in the name, but has > queried the distinction between "attenuation" and "extinction" because > it is important that the standard name explanation be correct: > > Do these coefficient apply to both direct beam and scattered > radiation? And I think it > > needs to be > > clear how this differs from attenuation. In the case of direct beam > radiation: > > > > 1. I thought attenuation was the fraction absorbed+back-scattered > (but > attenuation didn't > > include what was scattered forward, as well the portion of the direct > beam radiation that > > passed through unaffected). > > > > 2. I thought extinction was only the fraction absorbed. > > > > Olivier responded to this by saying: > > Extinction is scattering plus absorption. There is no ambiguity on > this. > > > > Attenuation refers to the direct beam so includes forward scattering > as well. > > Karl wrote: > > O.K., great, then use of extinction is unambiguous but before noting > its possible > > equivalence with attenuation, we should decide whether it truly is or > if "attenuation is > > sometimes taken to mean just absorption". > > Olivier replied: > > For me attenuation means the same as extinction but wikipedia says it > sometimes means only > > absorption. That could be sloppy language when the medium is not > scattering much, just guessing. > > Please can anyone advise on whether attenuation and extinction are one > and the same? > > Best wishes, > Alison > > ------ > Alison Pamment Tel: +44 1235 778065 > NCAS/British Atmospheric Data Centre Fax: +44 1235 446314 > Rutherford Appleton Laboratory Email: > alison.pamm...@stfc.ac.uk > Chilton, Didcot, OX11 0QX, U.K. > > -- > Scanned by iCritical. > _______________________________________________ > CF-metadata mailing list > CF-metadata@cgd.ucar.edu > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata -- Scanned by iCritical. _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata