Chemistry Reference
In-Depth Information
results in different ways. This will affect the absolute concentrations that have been
measured. Putaud et al. [
2
] extensively describe the accuracy of the analytical
techniques common in Europe. While ion chromatography (SIA) performs well
(
10%), an accurate determination of EC, OC and MD remains a challenge. EC
concentrations by various techniques differ by a factor of 4, and OC by 30%. The
uncertainty in the determination of mineral dust amounts can reach 100% for reasons
mentioned above.
Only PM10 is discussed as data on PM2.5 are much less available. In our region,
the average mass contribution of PM2.5 to PM10 is about 60-70%, and the relative
distribution of the different chemical parts in PM2.5 usually resembles that of
PM10. All components are present in both the fine and coarse fraction. Whereas
SIA, EC and OM dominate more in the fine fraction, SS and MD contribute more to
the coarse mode.
<
2.3 Chemical Composition
Figure
1
shows the chemical distributions for the urban and regional background
sites in the three PM10 data sets selected here. Common characteristics can be
observed: always, the major constituent is the SIA, followed by the carbon-
containing components (EC+OM). Smaller, but non-negligible, contributions arise
from MD and SS. At least 81% of the particulate mass was explained in the mass
balances. The highest closure was seen at the urban background site Borgerhout
(87%) which seems due to relatively a high amount of carbonaceous material.
Relatively, there is more SIA present at the rural sites: 42-44% versus 34-41% at
urban sites. However, differences between urban and rural sites are not very large
stressing the role of SIA in building up background levels [
31
,
32
]. In north-western
Europe, agricultural activities (like livestock and soil fertilising) frequently take
place yielding high emissions of ammonia in rural areas. Combined with the NOx
emissions from intensified traffic and SO2 emissions from industry, the formation of
ammonium nitrate and ammonium sulphate aerosols is favoured [
12
].
The sum of EC and OM contributes another 20-27% (at urban sites) and
18-24% (rural) to the PM mass, with OM dominating EC at most sites. Like in
the case of SIA, differences between rural and urban sites appear modest which
is probably caused by a considerable natural (biogenic) input. A study by ten
Brink et al. [
13
] on the presence of
14
C in PM filter samples revealed that at least
64% of the organic carbon measured at an urban Dutch site was contemporary
(i.e. emissions from biogenic material and biomass wood combustion).
A gradient for sea salt is observed as expected. Near the North Sea (Houtem
(rural background), Schiedam (urban background)), the marine contribution can be
as high as 16-18%. Further inland, some 6-10% is measured. In Germany, it
declines to some 4%. On average, the sea-salt contribution in this region is a
substantial 10% and is due to the dominance of transport of clean marine air from
the West diluting anthropogenic emissions onshore and transporting the pollution
further eastward over the European continent.
