Chemistry Reference
In-Depth Information
Mineral dust sources in urban areas include road dust resuspension and demolition
and construction activities. There is no apparent reason why the unknown part
should also be higher or lower at urban or rural sites. A possible cause may be the
amount of water attached to SIA which appears increased. Hygroscopic salts on
particles, like ammonium nitrate and ammonium sulphate in the fine fraction, and
sodium nitrate and sodium sulphate in the coarse fraction attract water erroneously,
increasing the PM mass.
The reason why SIA is higher in urban areas is less obvious as these are
secondary aerosols. The observed increment is predominantly caused by more
nitrate and sulphate. The reaction of nitric acid and sulphuric acid with the sea-
salt aerosol in a marine urbanised environment follows an irreversible reaction
scheme. In essence, the chloride depletion stabilises part of the nitrate and sulphate
in the coarse mode and may partly explain part of the observed increment. How-
ever, it also raises the question how to assign the coarse mode nitrate in the mass
closure. The sea salt and nitrate contributions cannot simply be added any more as
nitrate replaces chloride. Reduction of NO
x
emissions may cause a reduction of
coarse mode nitrate, which is partly compensated by the fact that chloride is not lost
anymore. A reduction would yield a net result of ((NO
3
-Cl)/NO
3
¼
)
27/62 times the nitrate reduction (where the numbers are molar weights of the
respective components), and this factor could be used to scale back the coarse
nitrate fraction in the chemical mass balance. A similar reasoning may be valid for
the anthropogenic sulphate in the coarse fraction. Corrections like these are uncom-
mon in current mass closure studies, and consequences will have to be explored in
more detail.
Important for the national and European air pollution policy is the question how
much of the measured particulate matter is of anthropogenic origin. It is this
fraction that can be targeted by national and European abatement strategies. A
pragmatic assessment of the natural versus anthropogenic contributions to PM was
given here. The estimation is that between 20% and 25% of PM10 is of natural
origin for countries like Germany, Belgium and the Netherlands. Hence, the
majority of PM in the north-western European region is of anthropogenic origin.
The uncertainty in such an analysis is considerable, and the result should be taken as
indicative.
A CTM was used to obtain a more detailed source apportionment for the
Netherlands. The model explains about 60% of the observed PM10 mass concen-
tration. Application of a dedicated source apportionment module showed that the
origin of the individual species may differ considerably. The most important
contributions to total PM10 mass in north-western Europe are associated with
agriculture, on-road and off-road transport as well as natural sources (sea salt).
Together these explain about 75% of the modelled mass. Secondary contributions
are derived from power generation, industrial processes and combustion as well as
households. Of the modelled part, 70-80% of PM10 over the Netherlands is
anthropogenic. The increase in source contribution going from low to high PM
levels is proportional for most sectors, except for agriculture and transport, which
become more important mainly due to the more than proportional rise in ammo-
nium nitrate concentrations. Sea-salt concentrations decline with rising PM10.
(62-35)/62
¼
