Geoscience Reference
In-Depth Information
in lower SSA values for each refractive index tested and lower MEC values across
the spectrum, except around 9.5 m. The spectral locations of the peaks in MEC
and SSA can be important for remote sensing applications, which measure radiances
across specific wavelength bands (e.g. Brindley et al. 2012 ), and in determining the
magnitude of the LW direct aerosol effect, including effects on land and sea surface
temperatures.
11.2.4
Measurements of Single-Scattering Albedo
As discussed earlier in this chapter, the SSA is crucial for determining the sign of
radiative forcing due to aerosol. In contrast to most other aerosol types, the SSA
for dust increases, or is almost constant, with wavelength due to the contribution
from large particles. This spectral contrast can be used to distinguish dust from
other types of aerosol. However, it is a troublesome quantity to measure. Table 11.1
shows the range of SSA found for dust in the existing literature, and it is not trivial
to account for this wide range. Many factors, including source of dust, age since
uplift, size distribution and measurement technique, can contribute. SSA calculated
using scattering theory and the full size distribution, including the coarse mode, tend
to be considerably lower than those reported as derived from airborne nephelometer
measurements. In our experience, much of this difference can be accounted for, if it
is assumed that the coarse mode particles do not enter the measurement apparatus
and therefore that the scattering measured by most airborne nephelometers at least
is only due to the sub-micron component of dust (Ryder et al. 2013 ). Sokolik and
Toon ( 1999 ) demonstrate that the SSAs for a variety of mineral particles with radius
0.7 m are up to 0.15 lower than the values for particles with radius 0.5 mand
up to 0.25 lower than those for particles with radius 0.25 m. The SSAs reported
from sun photometers such as AERONET tend to be lower than those from the other
methods, although the reason for this is unclear.
Regardless of the absolute accuracy of one method compared to another, when
the same methods are used, the SSA of dust can be seen to vary between locations.
For example, Su and Toon ( 2011 ) show higher SSA close to sources in the
Gobi Desert compared to source regions in the Sahara, whilst Koven and Fung
( 2006 ) show SSA variations within the latter region. Mineral composition of dust
is discussed in detail in Chap. 2 , but of particular interest here is the study by
Moosmüller et al. ( 2012 ). Considering dust from a range of locations in the Middle
East and Africa, they showed that the SSA of the accumulation mode at 405 nm
was substantially lower than that at 870 nm and, at both wavelengths, is linearly
correlated with the iron content. However, particularly in Asia, the dust is often
mixed with other aerosol, and since this usually has an absorbing component
(contributed by soot particles), this too can lead to a reduction in SSA.
Search WWH ::




Custom Search