P. Sahm

Numerical simulation of photochemical smog formation in Athens, Greece—A case study

High emission levels and the unfavourable topography are the main reasons for the alarming photochemical air pollution levels in Athens. An analysis of available air quality data proves that air pollution levels in Athens are largely affected by local wind circulation systems. The most frequent of these systems is dominated by the phenomenon of the sea breeze. Severe air pollution episodes occur, however, primarily under synoptic situations leading to stagnant conditions in the atmosphere over Athens. Photosmog formation in the Athens Basin is studied with the photochemical dispersion model MARS. The implicit solution algorithm incorporated in MARS is characterized by a variable time increment and a variable order. This solver allows avoiding unnecessary operator splitting by a coupled treatment of vertical diffusion and chemical kinetics. In this paper, MARS is used to analyse the situation on 25 May 1990, a day for which very high air pollution levels were reported in Athens. The simulation results elucidate the characteristics of a photosmog episode under stagnant conditions in Athens. In general, the model results reproduce satisfactorily the observed air pollution patterns.

Intercomparison of Numerical Urban Dispersion Models – Part I: Street Canyon and Single Building Configurations

Microscale Computational Fluid Dynamics (CFD) models havebecome an efficient and common simulation tool forassessment and prediction of air quality in urban areas.The proper validation of such a model is a crucialprerequisite for its practical application. Within theframework of the European research network TRAPOS a workinggroup on computational fluid dynamics modelling wasestablished and model intercomparison exercises werelaunched. Different Computational Fluid Dynamics Codes wereapplied for simulating the wind flow and pollutantconcentration patterns in several test cases. The aim ofthe present model intercomparison is (1) to assess andallocate the source of differences that appear whendifferent CFD codes using the same turbulence model areapplied to well defined test cases and (2) to improve theknowledge base for model development and application.Throughout the series of model applications coveringmanifold urban configurations, the overall agreementbetween the various models and experimental data is fair.In spite of quantitative differences between the variousnumerical results, the models are capable of reproducingthe flow patterns and dispersion characteristics observedin urban areas but they show significant differences forthe turbulent kinetic energy field that controls thedispersion of pollutants.

Urban air quality in larger conurbations in the European Union

Abstract The air quality in about 200 urban agglomerations within the European Union (EU) is calculated for a reference year (1995) and for the year 2010. Relatively simple, robust tools have been applied which allow for a generalisation of the results on the scale of the whole EU. The projected air quality gives information on the frequency and severity of exceedance of air quality objectives and on the fraction of EU urban population potentially exposed. The parameter calculated is the urban background concentration, which is representative for the concentration in most of the urban area, with the exception of places under direct influence of sources, such as streets. Pollutants considered are SO2, NO2, PM10, Pb, O3, CO, and benzene. In 2010 the urban background concentrations will decrease strongly in the set of 200 modelled cities. It is projected, however, that the agreed or proposed air quality standards will still be exceeded in the future. The most serious problems are exceedances of the short- and long-term objectives for PM10 and exceedance of the long-term objective for NO2.

High-resolution simulations of the wind flow and the ozone formation during the heilbronn ozone experiment

Abstract The Heilbronn ozone experiment was performed in the period 23–26 June 1994 in the state of Baden-Wurttemberg, Germany, with the intention to investigate if peak ozone concentrations during summer smog periods may be reduced by the aid of short term local scale interventions. Extensive meteorological and air quality measurements carried out during the experiment provide valuable information on the prevailing situation. In addition, high-resolution model simulations with the EUMAC Zooming Model (EZM) were performed to complete the picture at areas where no observations were available and to facilitate assessing the effectiveness of the emission reduction interventions. The observational data enable a very detailed evaluation of the simulation results. The results of both measurements and simulations reveal that the interventions lead to air quality improvements with regard to the primary pollutant concentrations but only to minor changes in peak ozone levels. The main conclusion from the Heilbronn ozone experiment is that concerted large-scale interventions to the primary pollutant sources are needed for decisive reductions of peak ozone concentrations in Central Europe.

The OFIS model: an efficient tool for assessing ozone exposure and evaluating air pollution abatement strategies

The Ozone Fine Structure (OFIS) model describes transport and chemical transformation processes in an urban plume at very low computational effort. This newly developed Eulerian model may be used for calculating ozone levels for individual situations as well as for deriving ozone exceedance statistics based on large-scale meteorological and long-range transport information over a longer time period. Results are presented from the application of the OFIS model to Stuttgart for the period April to September 1990. Compared to measurements, OFIS satisfactorily describes both the absolute maximum ozone values and cumulative AOT60 values in the area of interest. Results of the OFIS model allow conclusions regarding the influence of assumed ozone emission reductions on the urban scale to be drawn.

Wind flow and photochemical air pollution in Thessaloniki, Greece. Part I: Simulations with the European Zooming Model

Abstract The European Zooming Model (EZM) is applied to simulate the wind flow and the pollutant transport and transformation in the Greater Thessaloniki area (GTA) during a day characterised by weak synoptic forcing and a pronounced sea breeze circulation. Two chemical reaction mechanisms of different complexities, KOREM and EMEP, are used for these simulations. The evolution of the wind field and particularly the development of the sea/land breeze flow are investigated in relation to ozone concentration patterns. Comparison of model results to observations at a large number of locations showed the considerable skill of the model in predicting the major mesoscale features affecting the GTA: the sea breeze onset time and development, its inland penetration as well as its intensity. According to both observations and model results, ozone levels are kept at moderate levels. They are decisively affected by the break-up of the night-time inversion and the onset of the sea breeze. The compact mechanism KOREM is very sensitive to the VOC/NOx emission ratio. When this ratio is close to unity, i.e. inside urban areas, KOREM performs very well proving its appropriateness for regulatory use in such environments. The more sophisticated EMEP mechanism underestimates urban ozone levels but performs very well in rural areas remaining the most reliable choice for regional scale applications.

A modelling method for estimating transboundary air pollution in southeastern Europe

Abstract A new methodology is presented for the model estimation of an import–export pollution balance across country borders based on the classification of the synoptic scale atmospheric circulation. As an application, the transboundary transport of SO 2 and NO x across the Greek borders is investigated for the year 1995. The annual total inflow–outflow fluxes were derived by weighting the model results with the annual relative frequency of the corresponding synoptic type appearance. The results of the new modelling method are similar to those following the day-by-day calculation of the inflow–outflow fluxes for the whole year 1995 using the same models. The advantage of the new methodology is that it is definitely less time-consuming than the day-by-day calculations. Hence, the proposed methodology can be employed as a practical tool for several applications requiring the implementation of complex models over longer time periods.

Integrated Assessment of Air Pollution Abatement Strategies in Urban Areas: Application to the Greater Athens Area

This paper presents an integrated system for the assessmentof technical and non-technical measures that are putforward in order to reduce air pollution levels in urbanareas. In contrast to the majority of the currentlyemployed assessment tools, this system allows for theevaluation of any proposed air pollution control measure interms of its combined impact on air quality and socialwelfare, by correlating the environmental and economicaspects of alternative air pollution abatement solutions.Based on the multi-pollutant, multi-effect concept, thesystem presented aims in providing policy-makers with areliable tool for the objective assessment of the mostcost-effective packages of measures, the latter beingallocated according to the particular features and needs ofthe areas examined.

Assessing ozone abatement strategies in terms of their effectiveness on the regional and urban scales

Abstract Integrated ozone abatement strategies should take into account that an emission intervention which is effective on the regional scale may not necessarily be effective for a city and its surroundings. In the context of a study performed for the European Commission a methodology was developed for assessing ozone abatement strategies in terms of their effectiveness on both the regional and the urban scale. Towards this aim, the assumptions made for three regional emission reduction scenarios were assumed to be valid also for the emission situation in the urban areas of Athens and Stuttgart and the corresponding emission inventories were compiled. The EMEP MSC-W ozone model was used to calculate the regional scale ozone distribution over a 6 month summer period applying the 1990 meteorology. Local scale transport and chemical transformation processes were analysed with the ozone fine structure (OFIS) model. Both the regional and the local scale simulations were performed for the base case (1990 situation) and the three emission reduction scenarios. The significance of regional scale emission reductions was demonstrated by the aid of a second series of simulations assuming that the emission interventions were implemented only at local scale. The results of the simulations reveal that ozone exposure in conglomerations as the ones considered in this study depends on both urban and regional scale influences. Urban VOC control is found to be effective in reducing ozone primarily on the local or urban scale, whereas urban NOx control may cause an increase of urban peak ozone while contributing to an effective reduction of regional ozone.

Development and Validation of the Multilayer Model MUSE - The Impact of the Chemical Reaction Mechanism on Air Quality Predictions

As a new constituent of the European Zooming Model (EZM) system, the multilayer model MUSE is designed to serve as an efficient tool for simulating transport and transformation of air pollutants in the urban scale and thereby in supporting local scale air quality management in the most cost effective way.