The ambient air quality framework directive 96/62/EC (FWD) of the European Commission provides an EU-wide framework for national, regional and local measures to improve or maintain air quality. According to the directive, the Member States have to assess air quality throughout their territory. Besides direct measurements, the use of other techniques, such as air quality modeling, is required in regions where measurements do not give sufficient information for an air quality assessment.

This paper summarizes the features of the urban/regional scale model, REM3/CALGRID, and presents the first results of a REM3/CALGRID application to assess European-wide O3 and PM10 concentration distributions.

Rather than creating a completely new model, the urban-scale photochemical model CALGRID (Yamartino et al., 1992 and 1996) and the regional scale model REM3 (Stern, 1994; Hass et al., 1997) were used as starting point for the new urban/regional scale model, REM3/CALGRID. The premise was to design a Eulerian grid model of medium complexity that can be used on the regional, as well as the urban, scale for short-term and long-term simulations of oxidant and aerosol formation.

The new model includes the following additional features:

· A generalized horizontal coordinate systems, including latitude-longitude coordinates;

· A vertical transport and diffusion scheme that correctly accounts for atmospheric density variations in space and time, and accounts for all vertical flux components when employing either dynamic or a fixed layers;

· Updated releases of the SAPRC-93 and CBM-IV photochemical reaction schemes including Carter's (1996) 1-product isoprene scheme and SO2 oxidation to SO4;

· An equilibrium aerosol module that treats the thermodynamics of the inorganic sulfate, nitrate, ammonium aerosols and water (Schaap, 2000);

· A simple wet scavenging module based on precipitation rates;

· A new methodology to eliminate errors totally from operator-split transport and ensure correct transport fluxes, mass conservation, and that a constant mixing ratio field remains constant;

· Inclusion of the recently improved and highly-accurate, monotonic advection scheme developed by Walcek (2000) and Walcek and Aleksic (1998). This fast and accurate scheme has been further modified in this study to exhibit even lower numerical diffusion for short wavelength distributions; and

· An emissions data interface for long-term applications, that enables on-the-fly calculation of hourly anthropogenic and biogenic emissions, and greatly facilitates emissions reduction scenario studies.

The model was run on a PC for all of 1999 for a modeling domain that covers Western and Central Europe with a horizontal resolution of 0.25° latitude and 0.5° longitude. The atmosphere was divided into four vertical layers: a 20 m thick surface layer and three, time- and space-varying layers, with two equal-thickness layers below the mixing height and one above the mixing height and extending to the domain top at 2500m.

A first evaluation of the model's performance shows an excellent capability to reproduce O3 time-series at many monitoring stations in Europe and, in spite of the simplified treatment of aerosol formation, an ability to reproduce most of the observed secondary inorganic aerosol concentration characteristics seen in Dutch measurements. PM10, the particle target species of the EU's directive, is generally underestimated, primarily because of incompleteness in the particle emissions inventory. Attempts to reduce this problem by estimating the contribution of missing PM10 sources, have been quite successful for the Dutch stations used in the evaluation.

*This project was funded by the FRG's Umweltbundesamt. The authors would like to thank Dipl.-Met. A. Graff from the Umweltbundesamt for his support.