MEDITERRANEAN
ATMOSPHERIC MERCURY CYCLE SYSTEM
(MAMCS)
The overall goal of the MAMCS project was to improve our understanding of the mechanisms influencing the dynamics of mercury in the Mediterranean Sea region including:
emissions from natural and anthropogenic sources,
atmospheric transport and deposition to water and terrestrial receptors,
chemical and physical transformations of Hg0 and other Hg species in the atmosphere with changing meteorological conditions.
The modelling effort at the framework of
MAMCS includes the state-of- the art meteorological and dispersion models
chemical-physical transformation models, dry and wet deposition models and
mercury emission inventory (MEI) database. In order to calibrate and validate the performance, environmental
parameters (e.g. ambient concentrations
of atmospheric mercury, deposition, fluxes, meteorological parameters) were
measured at five sites during four field campaigns.
At the framework of MAMCS project two atmospheric numerical modelling system, namely RAMS and SKIRON/Eta, have been for development and simulation of the mercury physicochemical processes. The parallel development and implementation of the mercury processes in two atmospheric models allows the intercomparison of the results and consequently the more accurate development of the mercury modelling system. Both RAMS and SKIRON/Eta are 3-D full-physics limited-area models and they have similar capabilities. They can be used for high-resolution simulations and in this way they can represent satisfactorily regional and mesoscale features. Mesoscale and large-scale precipitation processes are important for the wet deposition of mercury. Also, both atmospheric models include highly accurate turbulence schemes. This is important since the dry deposition of mercury is strongly dependent on turbulence near the surface. Any uncertainties related to wet and dry removal processes were tested extensively.
RAMS
includes a detailed cloud microphysical scheme and it has the capability of
two-way interactive nesting. Sensitivity tests indicated that a very detailed
cloud microphysical scheme is not absolutely necessary for the mercury removal
processes because in this study emphasis is given on regional and synoptic
scale features. SKIRON/Eta is an accurate model and is appropriate for long
period simulations since it does not require significant computer resources.
The elemental, reactive and particulate mercury were taken
into account in the model development. The model used the most detailed and
accurate emission inventory created during the project. Moreover, it treated
physico-chemical processes, atmospheric reactions, transformations, removal
processes and especially the aqueous phase chemistry and
gas-to-solid partitioning of elemental mercury.
Four different scenarios of mercury transport and
dispersion, one for each season, have been analysed at the framework of the
MAMCS project. Model simulations using RAMS and SKIRON/Eta were performed. The
main results of this study are presented below:
The Mediterranean Sea region is not only
affected by mercury released in its vicinity but also from air masses enriched
in mercury from stations in northern and northeastern Europe. This suggests
that local and remote emissions must be taken into account in mercury studies
in the Mediterranean. This is particularly important for elemental mercury,
which can be transported in long-ranges before deposition.
In
general, a satisfactory agreement is evident between observations and model
output. However, a systematic model evaluation is difficult unless some other
controlling factors, like emission inventories and observation quality, are not
improved significantly.
Higher
values of mercury concentration over the Mediterranean Sea Region were usually
observed during the summer period. This is attributed to the synoptic
conditions prevailing during the warm period of the year. The dominant flow is
from north to south during summer promoting the transfer of pollutants from
Europe to the Mediterranean region. Moreover, the summer period in the
Mediterranean is characterised by anticyclonic circulations, which are known to
be associated with large-scale subsidence and no rain. Therefore, the lower
troposphere is usually stably stratified and consequently wet and dry
deposition processes are suppressed. The synoptic conditions prevailing during
winter lead to higher concentrations of mercury over northern Europe than over
the Mediterranean sea basin due to the dominance of quick removal processes
over Europe. The annual wet deposition values of particulate and reactive
mercury (HgP and Hg2) were one order of magnitude higher
than the dry ones. The annual dry and wet deposition amounts of elemental
mercury adsorbed are comparable. Also, they are about 4 orders of magnitude
smaller than those of Hg2 and HgP.
The
deposition patterns showed that the largest amounts of mercury are deposited in
eastern Europe and in the Mediterranean region, especially in its eastern part.
Taking into account that the vast majority of the mercury sources is located
over central and northwest Europe, two main paths of transport are indicated.The one path is from central to eastern
Europe and the other is from Europe towards the Mediterranean sea, namely from
north to south. This may have important negative implications not only for the
fish and agricultural production of the nearby countries but also for the
population directly exposed to mercury.
The difficulties in measuring the wet and dry
deposition of mercury make the deposition patterns estimated by the model very
useful. The models are also helpful in estimating the mercury concentration due
to the lack of reliable and consistent measuring methods. A well-developed
numerical model is also much cheaper than a dense observation network that is
required for high-resolution estimations of the concentration and deposition.
From this aspect the developed models should be considered as very useful tools
for studying the mercury processes and therefore be used by policy makers.
This
study focused on the regional and synoptic transport of mercury. The
representation of mesoscale features was not in the aims of this research. This
is the main reason that a relatively coarse resolution of about 50 km was used
here. Very high resolution simulations with a grid spacing of about 5-10 km are
required in order to represent mesoscale phenomena. This kind of simulations
could resolve mesoscale transport and could provide
an understanding of the effects of local versus remote sources. The significant computer resources required for very high
resolution simulations will be available in the near future, and this will
allow the study of the mercury cycle on the meso-scale.
Despite
the significant modelling effort devoted so far, there is still need for
further development. There is a need for a better representation of the
gas-phase chemistry and the gas-to-particle conversions. A more accurate and
systematic way of measuring the various mercury species and a better
understanding of the air-water interactions is necessary.
Fig. 1: "Annual" wet deposition HgP (ng/m2). From the SKIRON/Eta model.
Fig. 2: "Annual" dry deposition of HgP (ng/m2). From the SKIRON/Eta model.
Fig. 3: "Annual" dry deposition of adsorbed Hg0 (ng/m2). From the SKIRON/Eta model.
Fig. 4: "Annual" wet deposition of adsorbed Hg0 (ng/m2). From the SKIRON/Eta model.
Fig. 5: "Annual" dry deposition of Hg2 (ng/m2). From the SKIRON/Eta model.
Fig. 6: "Annual" wet deposition of Hg2 (ng/m2). From the SKIRON/Eta model.
Fig. 7: Observations of surface concentration of Hg0 (ng/m3) in Porte Palo (Sicily)
in May 1999. Times in LST.
Fig. 8: Hg0 (ng/m3) concentration in Sicily (1-18 May). Thin line: SKIRON/Eta
output, bold line: 6-hour moving average of the observations of Fig. 7.
Fig. 9: Hg0 (ng/m3) concentration in Neve Yam (1-18 May). Thin line: SKIRON/Eta
output, bold line: observations.