BALTIC SEAENVIRONMENT PROCEEDINGS No. 59 - Helcom

average concentrations of nitrogen in precipitation in the north and the south part of the
Baltic Sea ate shown in Figure 2.
The HELCOM Recommandation 14/l also requires the participating institutions to forward
descriptions of the methods used for sampling and analysis. These descriptions are to be
updated every 3 years and they are part of a common quality assurance activity which also
calls for additional measurements of marine tracers (section D in table 2).
The quality assurance also consists of international field intercomparisons generally carried
out in conjunction with ECE_EMEP and/or PARCOM (Helsinki Commission 1992). EGAP
has at the most recent meeting in May 1993 asked these two organisations to participate in
yet another exercise concerning the trace metals in precipitation.
Models
The atmosphere is the most efficient medium for transport of pollution from sources to
receptor. Airborne pollutants can within a few days travel over distances of 5000 km or
more. Air pollution knows no borders.
As a result North European marginal seas such as the North Sea and the Baltic are signifi-
cantly affected by atmospheric long range transport of man made emissions of pollutants.
Therefore the results from the monitoring programme of EGAP which is limited to the
Baltic Sea cannot alone give a complete picture of the state of air pollution and its origin
in remote sources. This knowledge is neccessary if control strategies for the reduction of
the atmospheric deposition fluxes of pollutants to the Baltic Sea are to be developed.
The only way of delineating the atmospheric transport pathways and hence the emitter-
receptor relationship is through numerical modelling. With modem computer technology it
is now possible to carry out calculations that not only cover a whole continent such as
Europe but also include the complex physical and chemical processes that control the
transport, transformation and deposition of air pollutants.
For long range transport of atmospheric pollutants and long term averages of concentration
and deposition fields, Lugrungian models have proved to be an appropriate tool. The mass
balance for pollutants is calculated for air parcels following the motion along air trajec-
tories in the lower few thousand metres of the atmosphere.
The EMEP model of the Meteorological Synthezing Centre-W was one of the first in this
category to be developed for operational use (Zversen et al., 1989, 1990). It is a one-layer
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