[CF-metadata] One new aerosol standard name

alison.pamment at stfc.ac.uk alison.pamment at stfc.ac.uk
Thu Dec 2 07:25:59 MST 2010


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.pamment at stfc.ac.uk
Chilton, Didcot, OX11 0QX, U.K.

> -----Original Message-----
> From: cf-metadata-bounces at cgd.ucar.edu [mailto:cf-metadata-
> bounces at cgd.ucar.edu] On Behalf Of alison.pamment at stfc.ac.uk
> Sent: 10 November 2010 07:46
> To: cf-metadata at cgd.ucar.edu; olivier.boucher at metoffice.gov.uk;
> michael.schulz at lsce.ipsl.fr; michael.schulz at cea.fr; taylor13 at 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.pamment at stfc.ac.uk
> Chilton, Didcot, OX11 0QX, U.K.
> 
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