Chemistry Reference
In-Depth Information
originated from the same source located in the Western Saharan region. However, the
Sahara dust particles are usually strongly diluted by mixing with marine and urban
aerosols as they are moving from the Sahara desert towards continental Greece [
42
].
The transported desert dust particles normally lie in the coarse size fraction and
thus, the PM
2.5
/PM
10
ratio is relatively low (
<
0.4) differing significantly from the
typical values in polluted urban areas of the Eastern Mediterranean Basin [
16
].
During these dust outbreaks the contribution of mineral dust to PM
10
mass
concentrations might reach 60-80%, while the average mineral dust contribution
to PM
10
ranges from 13% to 19% [
31
,
43
].
The mineralogical composition of Sahara dust particles shows the predominance
of aluminosilicates (clays). Illite is also present in many cases while quartz particles
are rare. Scanning Electron Microscopy (SEM) results on dust composition
transported over different regions in the Mediterranean Basin have shown that
Al-rich clay minerals such as illite and kaolinite are very common in PM
10
for
Cyprus and dominant for Crete. Dust particles are also very rich in calcium which is
distributed between calcite, dolomite and sulphates and Ca-Si particles (e.g.
smectites) whereas iron oxides are often detected [
43
].
5.2 Road Dust Resuspension
As previously mentioned, in the big urban centres of Eastern Mediterranean Basin
the concentrations of PM
10
are frequently above the threshold limit values. How-
ever, the quantification of vehicles exhaust emissions suggests that solely the latter
cannot possibly explain the observed high concentrations.
The PM generated by traffic can be divided into two categories according to its
mode of formation. Fuel combustion is the primary mechanism by which particles
are formed; however, there are a number of other processes, involving mechanical
abrasion and corrosion, which can also result in PM being released directly to the
atmosphere. These processes include: tyre wear, brake wear, clutch wear and road
surface abrasion. The abrasion and corrosion processes can also lead to the deposi-
tion of particles on the road surface. The material which collects on the road
surface, referred to as 'road dust', may also contain exhaust particles, and material
from a range of sources that are not related to road transport (e.g. mineral and
biogenic material, and material from industrial/domestic activity). Road dust may
subsequently be suspended or resuspended in the atmosphere as a result of tyre
shear, vehicle-generated turbulence, and the action of the wind. In central and
northern Western Europe, frequent precipitation (causing dust washout and
immobilisation) and lower dust deposition reduce the amount of road dust available
for resuspension but this does not occur that frequent in the Mediterranean Basin.
Consequently the relevance of the resuspended road dust is estimated to be high for
the Mediterranean region.
Research studies on the quantification of the road dust contribution on ambient
PM levels are scarce for Eastern Mediterranean countries. In addition, emission
