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Regional and Global Scale Modeling 


Regional Chemistry Transport Models (RCTMs) and Global Chemical Transport Models (GCTMs) are being developed to support research on the biogeochemical cycle of mercury and also to support Hg policy development.

Chemical Transport Models are commonly used to simulate the emission, transport, transformation and eventual deposition of gaseous pollutants. Modern CTMs include not only gas phase components of the atmosphere but also aerosols, and recently the way in which atmospheric chemistry can feedback to meteorology, eg. cloud formation, radiative transfer has also been considered.

Most of these models are concerned with climate issues, with the long-range transport of hazardous compounds or with air-quality. However to date relatively few models have taken mercury into consideration, although it must be said that mercury modelling research has intensified significantly in the last ten years.

The CNRM modelling group makes use of two on-line models, one regional and one global. On-line models, as opposed to off-line models, simulate meteorological phenomena and the chemical composition of the atmosphere in the same time-step. This approach differs from off-line models in which the atmospheric chemistry and transport simulation make use of pre-calculated meteorological fields which are then interpolated in space and time to the modelling domain. This interpolation can lead to inaccuracies, which was why on-line models were developed. The calculation times required by on-line models is greater than off-line models, however as computing power has increased this has become less and less of a problem.

Model simulations serve a number of purposes. Because measurements are performed only in specific locations and sometimes only at certain periods. Models are therefore used to estimate concentrations and deposition fluxes where there is no measurement data. In the case of mercury this is unfortunately particularly relevant due to poor measurement coverage in most of the Northern Hemisphere and an almost total absence of measurement data in the Southern Hemisphere. This situation is changing however (GMOS) and the Minamata Convention should encourage nations to increase their monitoring efforts.

             
Emissions of Hg(0) from the Mediterranean Sea (see below)
               

The goal of the model simulations in the case of mercury is to estimate the net input of mercury into ecosystems. This requires that emissions, natural, anthropogenic and legacy (re-emission of previously deposited mercury) are included. The models must also take into account atmospheric chemistry, the removal of mercury compounds via precipitation and dry deposition. Once models have been shown to reproduce measurement data with a reasonable degree of accuracy it is then possible to use them to investigate what the consequences of emission reduction scenarios would be in terms of mercury fluxes to ecosystems.

Mercury fluxes to and from oceans are extremely important in the global mercury cycle, the figures show the marine Hg emissions as calculated by the global model (the units are grams per square kilometre per year), and the emissions from the Mediterranean Sea mixing through the troposphere in the regional model.