Radiative Effects of Dust - Mineral Dust: A Key Player in the Earth System - page 265

C ext
r 2

Q ext D (11.1)

Here, C ext is the single-particle extinction cross section in m 2 , which is analogous
to the shadow the aerosol particle casts on incident radiation. r is the radius of
the particle and Q ext is the non-dimensional extinction efficiency, which relates
how efficiently the particle extinguishes radiation relative to its size. Scattering
and absorption efficiencies can be defined in similar ways. Other metrics for
extinction can be derived from C ext . The mass-specific extinction coefficient (MEC),
measured in m 2 g 1 , is a commonly cited property of aerosols (can be derived from
measurements of mass and extinction), representing how efficient a given particle is
at extinguishing radiation per unit mass. It is defined as

3Q ext
4r aer ;

MEC D (11.2)

where aer is the density of the aerosol particle (note that the MEC is also simply the
C ext divided by the product of the parcel density and volume). A similar property,
the aerosol extinction coefficient, ext ,isdefinedas

ext D NC ext ; (11.3)

where N is the number of particles per cubic metre of air and ext is measured
in m 1 . The vertically integrated value of the extinction coefficient is equal to the
commonly used non-dimensional aerosol optical depth (AOD or ),

Z ext d z

D (11.4)

which represents the total amount of radiation extinguished by aerosol in an atmo-
spheric column. AOD is commonly measured by ground-based sun photometers,
retrieved by satellite products and calculated by aerosol models. (Note that the dust
load can also be indicated by visibility measurements, but the correlation with AOD
is only moderate - see Sect. 11.5 for more details.) All of the above properties
( C ext , Q ext , ext ,MECand) are typically reported at certain values of œ - usually
around 550 nm for solar wavelengths since this is close to the wavelength of peak
intensity of the solar spectrum and many instruments measure extinction in this
range. However, we emphasise that these properties can vary markedly over the
range of incident wavelengths, œ.
An important diagnostic of scattering and absorption measurements is the single-
scattering albedo ( SSA ), ! 0 , the ratio of scattering to extinction at a particular
wavelength, defined as

sca
ext

! 0 D : (11.5)