The European-scale atmospheric transport model MSCE-HM was developed for operational modelling of heavy metal (HM) transboundary pollution within the EMEP region in order to provide countries-participants of the Convention on Long-range Transboundary Air Pollution with information on atmospheric heavy metal pollution.
MSCE-HM model is used for the following purposes:
- evaluation of atmospheric transport and deposition of HMs on regional (EMEP) and national (individual countries) scale
- evaluation of transboundary transport of HMs
- modeling of ecosystem-dependent depositions for evaluation of critical load exceedances (in cooperation with CCE)
- assessment of temporal and spatial trends
- projection of future levels of HM contamination and trends under various emission scenarios
The model scheme of heavy metal behaviour
The EMEP/MSC-E regional model of heavy metals airborne pollution (MSCE-HM) is a three-dimensional Eulerian-type chemical transport model driven by off-line meteorological data. The model considers heavy metal emissions from anthropogenic and natural sources, transport in the atmosphere, chemical transformations (of mercury only) both in gaseous and aqueous phases, and deposition to the surface (see figure below). The model computation domain is defined on the polar stereographic projection and covers the standard EMEP region by a regular grid with 5050 km spatial resolution at 60°N. For national-scale applications finer resolution is used (e.g., 55, 1010 km). Detailed description of the model is available in [Travnikov and Ilyin, 2005].
The vertical structure of the model is formulated in the sigma-pressure coordinate system. The model domain consists of 15 irregular sigma-layers and has a top at 100 hPa.
The atmospheric advection and the vertical transport are described in the model using mass conservative and monotone Bott`s advection scheme [Bott, 1989a; 1989b, 1992]. An implicit treatment of the vertical eddy diffusion is chosen in order to avoid restrictions on the integration time step because of possible sharp gradients of the pollutant mixing ratio.
Vertical grid structure of the model domain.
Such heavy metals as lead and cadmium and their compounds are characterized by very low volatility. It is assumed in the model that these metals (as well as some others (nickel, chromium, zinc etc.) are transported in the atmosphere only in the composition of aerosol particles. It is believed that their possible chemical transformations do not change properties of their carrying particles with regard to removal processes. On the contrary, mercury transformations in the atmosphere include transitions between the gaseous, aqueous and solid phases, chemical reactions in the gaseous and aqueous environment.
Model description of removal processes includes dry deposition and wet scavenging. The dry deposition scheme is based on the resistance analogy approach [Wesely and Hicks, 2000] and allows taking into account deposition to different land cover types (forests, grassland, water surface etc.). Dry deposition of particles to vegetation is described using the theoretical formulation by Slinn  and fitted to experimental data [Ruijgrok et al., 1997; Wesely et al., 1985]. The parameterization of dry deposition to water surfaces is based on the approach suggested by Williams  taking into account the effects of wave breaking and aerosol washout by seawater spray. The model distinguishes in-cloud and sub-cloud wet scavenging of particulate species and highly soluble reactive gaseous mercury based on empirical data. Besides, the precipitation rate is scaled for convective precipitation according to Walton et al.  to take into account fractional coverage of a grid cell with precipitating clouds.
Model evaluation includes comparison of modelled concentrations and deposition with measurements, sensitivity studies and comparison of modelling results with the results obtained by other regional-scale transport models (so-called imtercomparison studies). Evaluation of modelling results against measurements is carried out every year, and is described in annual reports. Examination of model sensitivity and estimation of model-related uncertainties of calculated deposition and concentrations is described in MSC-E technical report [Travnikov and Ilyin, 2005]. Detailed information on the model intercomparison studies is available in technical reports and in peer-reviewed papers. MSCE-HM model has been reviewed at the EMEP/TFMM Workshop held in Moscow in October 2005. The Workshop concluded that the MSCE-HM model is suitable for the evaluation of the long-range transboundary transport and depositions of heavy metals in Europe (ECE/EB.AIR/GE.1/2006/4).
Along with MSCE-HM model there is ongoing development of the global multiscale modelling approach for heavy metals at MSC-E. The Global EMEP Multi-media Modelling System (GLEMOS) is being developed to evaluate HM pollution at different scales: global, regional, and local. The GLEMOS model is intended to substitute the MSCE-HM model in near future.
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