Environmental Engineering Reference
In-Depth Information
2.3 Desert Surfaces of the Planet
Earth
s large deserts are a major permanent source of nanoparticles which air
currents lift in the atmosphere. The first observations about dust storms carrying
dust from the Gobi Desert to the Yellow Sea coast were made in the fifth year of Di
Xin of the Shang Dynasty (about 1150 BC) [
25
].
Dust storms seem to be the most important source of nanoparticles in the
atmosphere. The long-range migration of both mineral dust and anthropogenic
pollutants on the continents has been the focus of recent investigations which
have revealed that about 50 % of aerosols in the troposphere are minerals originat-
ing from deserts [
26
] (Figure
2.2
).
The chemical composition of very fine desert sand dust varies with origin and the
anthropogenic activities developed in the adjacent areas that the air currents cross.
Analytical determinations of the particulate matter composition during a dust
storm, performed in China and South Korea, reveal high silicon concentrations,
aluminium, calcium and iron traces (Table
2.2
). Many toxic substances like heavy
metals (Hg and Cd) and PAHs (Polynuclear Aromatic Hydrocarbons) usually
produced by coal burning have also been identified, but without specifying their
origin in the stratospheric particle clouds which were the subject of the experiments
[
31
,
32
].
Recent simultaneous determinations conducted in the area around Xian, a city
located at 400-m altitude, and at the summit of Mount Hua (2,060 m) during a
massive atmospheric dust transport from the Gobi desert, have revealed sulphate,
nitrate and ammonium ions of anthropogenic origin, besides a high carbon and
organic nitrogen concentration, probably resulted from the atmospheric microbio-
logical activity. The size of the wind-carried particles varied between 80 and
1,000 nm.
In the past decades, the type and behaviour of aerosols originating in Sahara
have been studied intensively both in the Sahel region, in several archipelagos in
the Atlantic Ocean, and Southern Europe.
Saharan dust contribution to PM
10
was determined by gravimetric evaluation of
Al, Si, Fe, Ti, nns (non-sea salt) Ca, nssNa and nss K oxides [
33
]. In most cases, the
microcrystal content accounts for over 75 % of the oxide content identified as PM
through measurements taken in Italy during Saharan-type dust storms, for several
years. According to the multiannual means, the microcrystal content in Southern
Italy atmosphere, expressed as PM
10
, is 5.42
'
g/m
3
, reaching a value as high as
μ
g/m
3
when air currents are strong. The amount of microcrystal aerosols and
their contribution to PM
10
indicate non-significant season dependence.
Optical measurements of particles, taken in parallel with composition measure-
ments, show that within the 400-600-nm range, dust contains mostly soluble
compounds of Fe, K and Co.
An estimation of the ionic balance of the atmosphere indicates that, on the one
hand, the reactions involving anthropogenic acids and microcrystal particulates can
play a major role in the sedimentation of nanoparticle systems; on the other hand,
67.9
μ
