PROGRESS IN FURTHER DEVELOPMENT
OF MSCE-HM AND MSCE-POP MODELS
(implementation of the model review recommendations)

EMEP/MSC-E Technical Report 4/2006

A. Gusev, I. Ilyin, L.Mantseva, O.Rozovskaya, V. Shatalov, O. Travnikov

ABSTRACT

This technical report reflects the progress in further development of MSC-E models of long-range transport of heavy metals (HMs) and persistent organic pollutants (POPs). The MSCE-HM and MSCE-POP models have been reviewed at the EMEP Task Force on Measurements and Modelling meeting in Zagreb and TFMM Workshop on model review in Moscow in 2005. It was concluded that the models are suitable for the evaluation of the long-range transboundary transport and deposition of HMs and POPs in Europe. Along with that the TFMM Workshop in Moscow has recommended to continue further improvement of modelling approaches for HMs and POPs.

It was recommended at the workshop that MSC-E consider the following issues.

General requirements:
· Validation of meteorological fields generated by MM5;
· Inclusion of a shallow lowest model layer;
· Description of emission processes that are driven by meteorology such as resuspension and volatilisation from soils;
· Extension of the HM and POP modelling domain to the global scale and employing meteorological fields with 1°x1° resolution.

Specific requests for HM modelling:
· extension of the MSCE-HM model to the consideration of other elements and heavy metals, including Ni, As, Cu, Cr, Zn and Se;
· Improvement of deposition processes description;
· Investigation of mercury dry deposition to forests ;
· Further research and improvement of the description of mercury chemical transformations in the atmosphere.

Specific requests for POP modelling:
· degradation of POPs in particle-bound and gaseous phase in the atmosphere including photodegradation;
· seasonal dependence of soil volatilisation;
· values of concentrations at boundaries of the EMEP grid;
· seasonal variations of emissions;
· application of the MSCE-POP model to screening of a wider range of POPs for their potential environmental significance;
· inverse modelling using passive sampling campaign data;
· investigation of the potential influence of climate change on the fate and behaviour of POPs.

Following these recommendations MSC-E has started its work on further improvement of MSC-E modelling approach for HMs and POPs.

The verification of meteorological fields, generated by meteorological driver MM5, has been performed. Different parameterisations of atmospheric processes in MM5 were used and the obtained meteorological data were compared with ECMWF re-analysis. In Chapter 1 preliminary results of meteorological data verification generated by MM5 are presented. More detailed information is given in the Annex A of the report.

Parameterisation of emissions of metals to the atmosphere driven by meteorological processes has been developed. There is a number of natural mechanisms responsible for emission of aerosol-bound heavy metals to the atmosphere. In particular, they include emission with wind-blown dust and sea-salt aerosol. Since human activity has led to significant increase of concentrations of heavy metals in soils, compared to pre-industrial times, the meteorologically-driven emissions include both natural component and re-emission of previously deposited matter from anthropogenic sources, and further in the report will be called "natural and historical emission". Brief description of these approaches is presented in Chapter 2.

According to the recommendations of TFMM, pilot parameterisations for arsenic, nickel, chromium, zinc, copper and selenium were elaborated. Besides, new approach to model natural and historical emissions of these "new" heavy metals together with lead and cadmium was developed. A number of model simulations aimed at evaluation of model performance after introduction of these changes were carried out. The simulations were performed on the base of two emission data sets. The first one includes data officially reported by Parties to the UNECE for 2000. For countries which did not report their national data, emission expert estimates of TNO were used [van der Gon et al., 2005]. The second one represents emission expert estimates for 2000 produced in the framework of ESPREME project [http://espreme.ier.uni-stuttgart.de/data.html]. The results of the numerical tests are described in Chapter 3.

With respect to the refinement of MSCE-POP model parameterisation essential attention at current stage of work was given to the harmonisation of physical-chemical properties of selected POPs and the improvement of process parameterisations used in the model. Further refinement of physical-chemical properties of POPs considered in modelling activities (PCDD/Fs, PAHs, PCBs, lindane and HCB) has been performed. Significant part of these improvements concerns the values of three partition coefficients: octanol/water partition coefficient, air/water partition coefficient or dimensionless Henry constant, and octanol/air partition coefficient. The values of these three coefficients should be harmonised as they are actually not independent. The description of adjustment procedure and the results of its application for the harmonisation of POP physical-chemical parameters are presented in the first section of Chapter 4. Updated model parameterisations for PCDD/Fs, PAHs, PCBs, lindane and HCB are given in the Annex B.

In order to improve the agreement of computed and observed seasonal variations of POP concentrations in the atmosphere a number of modifications was introduced into the MSCE-POP model. In course of the comparison of model results with measurements it was found that the model considerably underestimates seasonal variations of B[a]P air concentrations for a number of monitoring sites. Possible reasons of this disagreement could be connected with the neglecting of B[a]P degradation in particle-bound phase and underestimation of seasonal variations of B[a]P emissions. The possibility of improving the agreement between model results and measurements for B[a]P by the refinement of degradation process description and usage of different scenarios of seasonal variations of emission is considered in the second section of Chapter 4.

Additional modification of MSCE-POP model parameterisation is connected with the description of POP partitioning in soil. The processes of POP absorption in soil and volatilisation to the atmosphere are described in the model taking into account the effect of temperature variations. However the parameterisation of POP partitioning in soil previously used in the model did not take into account temperature dependence of octanol/water partition coefficient KOW. It is expected that temperature dependence of KOW can affect the behaviour of POPs in soils and, as a consequence, the rate of POP volatilisation to the atmosphere. The sensitivity analysis of POP behaviour in soils with respect to KOW is presented below in the second section of Chapter 4.

Further development of EMEP/MSC-E modelling approach for POPs has been continued by improving the consistency of hemispheric and regional scale modelling for POPs. The developed modelling approach permits to evaluate POP pollution levels for European region and to provide estimates of transboundary fluxes between European countries accounting also for the contributions of non-European emission sources and of re-emission of POPs accumulated in environmental compartments. The later contributions for many of POPs can be significant as they are characterized by considerable long-range transport potential and essential residence time in the environmental media where they can be accumulated during a long period of time. Implemented modelling approach is based on the nesting of regional and hemispheric scale modelling using MSCE-POP model. The description of the approach is given in Chapter 5. The Annex C presents POP emission data used for modelling. A special attention is given to its computational aspects, compatibility of the input data used for hemispheric and regional modelling, and evaluation of transboundary transport of POPs between the European countries.

In close cooperation with the Task Force on POPs MSC-E has continued the activities on evaluating new substances by criteria of the long-range transport potential and overall persistence contributing to the preparatory work for the review of the POP Protocol. In particular, model evaluation of the Long-Range Transport Potential (LRTP) and overall persistence for a number of substances was carried out for PentaBDE, endosulfan, dicofol hexachlorobutadien (HCBD) pentachlorobenzene (PeCBz) and polychlorinated naphthalenes (PCN). On the basis of these calculations a technical report has been prepared by MSC-E [Vulykh et al., 2006] and delivered to the Task Force on POPs for the support of the work on peer review of new substances dossiers that may be proposed by Parties for inclusion into annexes to the Protocol.

In cooperation with national experts from the Parties to the Convention MSC-E has continued the work on POP model intercomparison study. In the current year the second stage of the study devoted to the comparison of mass balance estimates, calculated deposition and concentration fields of POPs in different environmental compartments and a number of sensitivity studies is completed. The updated results of this stage are presented in the revised EMEP/MSC-E Intermediate Technical Report 5/2006 "POP Model Intercomparison Study. Stage II. Comparison of mass balances estimates and sensitivities". At present the third stage of the intercomparison is ongoing. This stage is aimed at the comparison of model predictions of long-range transport potential and overall persistence of 14 reference pollutants made by participating models. This activity assumes the application of these models, including MSCE-POP model, to screening of a wide range of POPs for their potential environmental significance.

The TFMM Workshop on MSC-E models review has pointed out a number of long-term strategic issues important for the evaluation of European region pollution by HMs and POPs. MSC-E has initiated implementation of these tasks and will continue to work in these directions in close co-operation with TFMM. It is planned to provide the detailed information on the modifications made and modelling results obtained using updated MSCE-HM and MSCE-POP model versions to the next TFMM.