Radiative Effects of Dust - Mineral Dust: A Key Player in the Earth System - page 264

In the short-wave (SW) part of the spectrum (also called “solar” wavelengths),
dust is mainly a scattering aerosol - an increase of dust therefore normally results
in an increase in solar radiation scattered back to space and a decrease in the direct
beam reaching the surface. However, there is also some absorption of SW by the dust
which offsets to some extent the increased scattering at the top of the atmosphere.
This effect is modified by the underlying surface, for example, more additional SW
is scattered back to space by dust blowing over the ocean compared to dust over the
desert surface itself that already scatters a fair proportion of SW back to space. In the
long-wave (LW) part of the spectrum, dust can absorb infrared (IR) radiation being
emitted by the warm surface and re-emit this radiation in all directions, including
back towards the surface. Thus, the LW flux leaving the top of the atmosphere
(TOA) is decreased and the LW flux arriving at the surface is increased - similar
to the greenhouse effect of increased carbon dioxide. The TOA effect is enhanced
and the surface effect is reduced if the dust layer is elevated and cold.
In order to quantify the radiative impacts of dust, it is necessary to know several
things about both the dust itself and the surrounding environment. Firstly, we must
be able to describe the so-called optical properties as a function of wavelength (these
are the extinction coefficient, the single-scattering albedo and phase function, all
defined below) - these are dependent on the size, shape and composition of the dust,
which are in turn dependent on the dust source and transport processes (see Chap. 2 ) .
Secondly, the vertical profile of dust is crucial for assessing the impact on heating
rates through the atmospheric column; it is this that determines the impact on, for
example, stability and convection. Finally, the radiative impact of the dust does not
solely depend on the properties of the dust itself, but also strongly on the underlying
surface. For example, highly scattering dust present over a dark ocean will have a
big impact on radiation balance at the TOA, whereas the same dust over the highly
reflective sand seas of the Sahara will have a much smaller effect in the SW.
In this chapter we will first define and discuss the optical properties of dust
and illustrate their sensitivity to key physical properties of the dust itself like
particle size or shape (Sect. 11.2 ). We will then review recent measurements of dust
optical properties and the corresponding perturbation to the radiative flux efficiency
(Sect. 11.3 ). To conclude we will discuss the implications of our knowledge of dust
radiative impact for satellite retrievals (Sect. 11.4 ) and its inclusion in weather and
climate models (Sect. 11.5 ).

11.2 Optical Properties of Dust

11.2.1 Definition of Optical Properties

Mineral dust (and other atmospheric aerosols) is able to interact with both solar and
terrestrial electromagnetic radiation. Particles can both scatter radiation in different
directions and absorb it, re-emitting it as thermal energy. The sum of scattering and
absorption of energy at a particular wavelength is known as extinction and can be
expressed by