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One uncertainty in evaluating particulate matter (PM) modelling results is caused by mismatches among regulated, measured, and modelled particle diameters. Current PM regulations are based on the aerodynamic diameter (EPA, 2004). PM measurements also target the aerodynamic diameter in order to address regulatory concerns. However, in air quality models, particles are often modelled on the basis of the Stokes diameter. To evaluate model performance using size–resolved measurement data, it seems logical that modelling results should be given in the aerodynamic diameter.

The impact of the Chernobyl power-plant accident in 1986 gave a major impetus to dispersion modelling activities around that time, especially in those countries directly impacted by the radioactive cloud. In the United Kingdom, the greatest contamination occurred in upland areas across the western half of the country where intense convective rainfall had intercepted the plume (most notably, in NW Wales, Cumbria and SW Scotland). Significant quantities of radionuclides were deposited locally in these upland grassland environments. The Met Office provided specialist forecasts during the incident (based essentially on trajectory techniques); however no operational long-range dispersion model was available for use at that time. Hence central government sanctioned us with developing an emergencyresponse modelling capability to provide detailed predictions of the transport and deposition of radioactive materials that might arise from any similar events in the future. The Met Office Nuclear Accident ModEl (abbreviated to NAME) was in use by 1988 with a major upgrade (NAME II) operational from 1994.

The National Oceanic and Atmospheric Administration (NOAA), in cooperation with the Environmental Protection Agency, is developing an Air Quality Forecasting Program that will eventually result in an operational Nationwide Air Quality Forecasting System. The initial phase of this program, which couples NOAA’s Eta meteorological model with EPA’s Community Multiscale Air Quality (CMAQ) model, began operation since May of this year and has been providing forecasts of hourly ozone concentrations over the northeastern United States.

Research on the numerical prediction of chemically speciated gas and particle phase components of the atmosphere has been driven by public health studies linking both gases and particles to adverse health effects. Three dimensional airquality models containing detailed chemical and physical processes for gas and particle formation (Meng et al., 1998; Dennis et al., 1996; Ackermann et al., 1998) provide a means of linking and describing the complex non-linear processes leading to these adverse air-quality health outcomes.

The dispersion of trace gases in the atmosphere depends on the state of the atmospheric boundary layer (ABL) and one of the most important parameters characterizing it is the intensity of turbulence within the ABL. Hence, the reliability of the atmospheric dispersion models depends on the way turbulent parameters are calculated and related to the structure of the ABL

There is a growing need for meteorological data in urban areas in support of air pollution research and management, but measurement poses substantial challenges. Most densely-developed sites make it impossible to conform to the standard WMO Guidelines for site selection and instrument exposure (WMO, 1996) due to obstruction of airflow and radiation exchange by buildings and trees, unnatural surface cover and waste heat and water vapour from human activities. New guidelines (Oke, 2004) to assist in this task form the basis of the first part of this paper. Here emphasis is on those variables of greatest use in air pollution applications. Valid and repeatable results can be obtained despite the heterogeneity of cities, but it requires careful attention to principles and concepts specific to urban areas. Guidelines must be applied intelligently and flexibly, rigid ‘rules’ have little utility. It is necessary to consider exposures over non-standard surfaces at non-standard heights, splitting observations between more than one location, or being closer to buildings or anthropogenic heat and vapour sources than is normal WMO recommended practice.

The problems associated with urban atmospheric pollution have received increasing attention in the past decades. The residence time of a pollutant within a street canyon is a very important factor that affects both the study of hot spots and the chemical reactions that take place within the street canyon. An example for such chemical reactions are the photocatalytic reactions that take place on the wall surfaces of a street canyon covered with TiO2, like the ones studied in the PICADA project within EC’s 5th Framework Programme. In the frame of this project, a field experiment took place in Guerville, France, which was designed in order to evaluate the performance of translucent TiO2 coatings and also validate the microscale model MIMO which is suitable for street canyon applications. On this basis, a 3D study of the effect of the street canyon length on the flow field and the dispersion of pollutants within street canyons was undertaken. During this study, several numerical simulations for street canyons of the same height and width and therefore aspect ratio but different lengths have been performed, using MIMO as well as the commercial CFD code CFX TASCflow (URL 1). Additionally, a 2D simulation for a street canyon of the same aspect ratio was performed and the results obtained for the residence time and the flow field were compared.

Dispersion models require information on the turbulence characteristics in the planetary boundary layer (PBL). This information is most often extracted from either meteorological measurements or from numerical (prognostic or diagnostic) models, and the requested turbulence parameters are then estimated using a PBL pre-processor.

In this work an experience of using Lagrangian Particle model Spray (in AriaIndustry package from Arianet, Italy) for risk assessment at Krško Nuclear Power Plant (NPP) in Slovenia is described. Krško NPP is placed in a semi-open basin sourrounded with very complex terrain. The area is characterised by very low wind speeds and thermal inversions.

While most regulation and research of air pollution episodes are focused on Europe, transport of air pollutants from Europe to Northern and Western Turkey hasn’t been studied sufficiently, although unusually high air pollution episodes occur in these regions. As an example during 05-12 of January, 2002, it has been identified from observational data that particulate matter levels were almost 4-5 times (364 μg/m3) higher than the World Health organization standards for Europe (70 μg/m3). For the same period, other pollutants were significantly higher than the standards as well.