Radiative Effects of Dust - Mineral Dust: A Key Player in the Earth System - page 266

This is useful as it tells us about what fraction of the extinguished radiation
is scattered by aerosol; conversely, the co-albedo (1 ! 0 ) tells us what fraction

is absorbed and therefore heats the aerosol-laden air. This can be important
in determining the sign of radiative forcing due to the aerosol at the TOA and for
affecting atmospheric circulation changes (see Chap. 13 ) . Dust is notable for having
a relatively low SSA compared to many aerosols, such as sulphate, which are more
scattering.
The direction in which radiation is scattered when it interacts with a particle is
also important in determining the magnitude of the radiative impact. This can be
described in a number of ways. The phase function , P (), where is the angle of
scatter relative to the incident beam, is the angular distribution of light intensity
scattered by the particle - this is the most complete description of the scattering
pattern and is important for satellite measurements of radiance at the TOA and
subsequent retrievals of dust and other quantities. The radiative transfer parts of
climate models are increasingly able to use the phase function (thanks to greater
computational power allowing codes to represent radiation transfer as more than just
up versus down). Knowledge of the phase function is also of critical importance for
satellite applications. Several more commonly reported parameters can be derived
from the phase function, including the asymmetry parameter , g , defined as the
intensity-weighted average of the cosine of the scattering angle. A value of g

D 1

denotes purely forward scattering, whilst a value of g 1 denotes light scattered

completely in the backward direction. Many simplified models of radiative forcing
and climate (where radiation is limited to an upward and downward flux) use the
asymmetry parameter, but to define it requires knowledge or calculation of the entire
phase function. Finally, the hemispheric backscatter ratio , b , is the fraction of the
scattered intensity that is redirected into the backward hemisphere of the scattering
particle. The backscattering ratio can be measured directly by a nephelometer and
can be related in some cases directly to g .

D

11.2.2 Methods of Characterising Dust Optical Properties

Many of the dust optical properties defined above can be measured directly
(e.g. scattering and extinction - and subsequently SSA, hemispherical backscatter),
whilst others must be calculated, or at least some model is required to retrieve their
values from measurements. The instruments most commonly used to measure dust
optical properties or integrated quantities such as optical depth include nephelome-
ters (scattering at one or multiple wavelengths, e.g. McConnell et al. 2008 ; Osborne
et al. 2008 ), sun photometers (AOD and via retrieval size distribution, refractive
index and single-scattering albedo; e.g. Dubovik et al. 2002 ) and lidar (e.g. Mona
et al. 2012 ).
The calculation of scattering and absorption properties of dust from radiances
requires knowledge or assumptions about particle size, shape and refractive index.
In many studies, dust particles are assumed to be spherical, and the scattering is