7. Discussion and conclusions

7. Discussion and conclusions

7.1 The Ozone Directive and 5EAP

This report deals with the status of pollution by tropospheric ozone in the European Union in the perspective of the EU Council Directive on air pollution by ozone (92/72/EEC) and the Fifth Environment Action Programme (5EAP). The report was announced under Article 8 of the above Directive which states that "the Commission shall submit [...] a report on the information collected and on the evaluation of photochemical pollution in the Community". The discussion and conclusions in this chapter will therefore mainly address these two issues.

7.2 Information collected

The Ozone Directive defined concentration thresholds for ozone, it established a harmonised procedure for monitoring and exchanging data and it also arranged to inform and warn the population if certain thresholds were exceeded. Member States were encouraged to report earlier information. In addition the Directive required measurements of NO_x at particular stations, whereas it only recommended VOC measurements at these sites.

In addition to the above arrangements the Directive aimed:

1. to assess the individual risk of exposure of the population to values in excess of the health protection thresholds of ozone;
2. to assess the exposure of vegetation to the vegetation related thresholds of ozone;
3. to optimise the action needed to reduce ozone formation;
4. to obtain exceedance time series to assist the progress of assessment of trends;
5. to obtain information on concentrations of VOCs and NO_x.

In order to facilitate collection of data to support production of these assessments the Directive put forward station siting criteria: i.e. ozone monitoring stations located at geographically and climatologically representative sites where the risk of approaching or exceeding threshold values is highest and where it is likely that either the population or vegetation is exposed.

In relation to 1), 2), we conclude from the information reported so far that the risk and exposure assessment objectives can be fulfilled only partially. This should not mask the fact that the information made available under the Directive is a major leap forward in comparison to the pre-Directive period. However, the spatial coverage of the ozone monitoring network still requires improvement in many countries. Gaps in geographical coverage can be noted in both urban and rural monitoring. The current subset of rural/background stations is estimated to cover a maximum of 20 - 40% of forests and 30 - 50% of crops, both depending on the chosen radius of representativeness of the observations. The subset of urban/street sites covers no more than 12% of all EU15 residents and approximately 25% of EU15 city dwellers (city defined as having more than 50,000 inhabitants). This conclusion does not necessarily imply a call for 100% coverage of all people, crops and ecosystems by very dense monitoring networks in the EU15. It does, however, emphasise that knowledge on the representativeness of current stations at the European level needs to be improved. Although the Directive states that: "Member States shall provide [...] a description of the area covered by the stations, and the site-selection criteria [...]" this has not resulted in an adequate level of information provided in this matter. In the future assessments of risk and exposure may be improved if better documentation on the spatial representativeness of individual sites is made available by the Member States. This could be supplemented by information on the number and extent of locations where a similar situation and air quality occurs.

In relation to 4), for compliance with legal requirements of the Directive and also for the scientific community there is a need for good quality data in order to address the question of trends in ozone and ozone episodes. Current trends in concentrations are uneven: there appears to be increases in some rural zones, stability or reductions in others. The question rises whether reductions in concentrations can be directly linked to decreases in emissions of ozone precursors. The 1994 - 1996 time series reported in the framework of the Directive are too short to assess any trend. However, information covering a longer time period has been acquired from seven Member States. Four countries reported information covering 1989 - 1996. This is very valuable data despite the fact that the composition of networks has changed in these countries and monitoring practices may also have been altered.

The issue of linking decreases in ozone to abatement of emissions of precursors is hampered because coincident data on concentrations of NO_x are not available. Due to the rapid reaction between ozone and NO a change in the ozone concentration can easily be caused by a change in NO. The quantity of O_x (= O₃ + NO₂) is often a convenient parameter to assess the influence of fresh emissions of NO through the years. We therefore recommend co-located measurement of ozone and NO_x, in particular at stations closely situated to sources of NO_x.

In relation to 5), statistics on VOCs were reported by only a few countries. Although this limited amount of VOC data helps in the description of the formation and transport of ozone and its source species, it is nevertheless insufficient to conclude on the situation in all Member States. NO_x and NO₂ statistics were transferred by about half of the Member States. This data archive provides information relevant to health effects, but helps only marginally in understanding the climatology of ozone.

The experiences with public information required by the Directive are rather positive in many countries. The Directive defined air quality information and warning thresholds, obliging Member States to disseminate information to the public as soon as any exceedance was observed. The information that was issued received considerable attention from the media, and the increased public awareness has brought the ozone problem higher on the list of political priorities.

The question whether the population information threshold is of any real benefit to the public is difficult to answer. We addressed this question by comparing how often the health protection threshold was exceeded without the population information threshold being reached. (Note that the exceedances of these thresholds are not necessarily exceeded at the same sites during the same period). The data indicates that the ratio between the number of exceedances of the two thresholds has been higher than 3 in all EU15 countries and higher than 10 in Sweden, Denmark and Finland.

The quality and quantity of data reporting has improved in the period 1994 - 1996. The improved formats for reporting have resulted in good harmonisation of the data reports although deviations from the format still occur. Unfortunately, it still happens that particular countries completely fail to transmit their data within the time schedule set in the Directive.

7.3 Evaluation of photochemical pollution and emissions of precursors

From the measured data it is concluded that both the EU and WHO thresholds set for the protection of human health are exceeded substantially and in all Member States. We estimate that more than 90% of EU citizens were exposed to an exceedance of the threshold at least once in 1995. More than 80% of these people experienced more than 25 exceedances.

The current data set derived from monitoring locations in the urban environment is representative of only a limited fraction of the European population; we estimate 41 million people.

The number of additional hospital admissions attributable to ozone exposure in concentrations exceeding the threshold for protection of human health is estimated to be over 80 cases per year. If this pattern is extrapolated to full country populations, and to all 15 EU countries then we assess that about 700 hospital admissions could have been attributed to exceedances of this ozone threshold (110 mg.m^-3 8-h mean). However, the real impact of the pollution is greater because many effects arise below the current health protection threshold. A conservative estimate, suggests that these impacts may have led to over 3000 additional admissions in the full population of 15 EU countries.

The threshold value for providing information to the public has been exceeded in almost all Member States during a limited number of days, but rarely or never at Finnish, Danish or Irish stations. In 1995 this concerned about 31 million Europeans, which approximates 45% of the urban population living in cities with operational monitoring in that year. The warning threshold is reached occasionally, in particular in the southern Member States.

The EU15 threshold value for the protection of vegetation was exceeded substantially, by up to a factor of 3, widely and frequently. The full EU15 area of coniferous forest and arable land experienced exceedances of this threshold. In less than 1% of the area of broad-leaved forest were exceedances not observed. Recent work, however, indicates that the 65 mg.m^-3 (24-h average) threshold may be of little relevance in assessing the potential for ozone effects on vegetation. Exceedances of this current threshold occurred even at the end of the 19th century. It was exceeded during close to 1% of the time between 1876 and 1911 near Paris.

The WHO guideline for protection of crops is exceeded in all Member States except Finland. On average over all EU15 countries only a small fraction (about 6%) of arable land is not exposed to exceedances of this critical level. The magnitude of exceedance of this critical level differs from year to year. Even in the low ozone years the area of exceedance covers almost the entire EU15 surface.

In the case of forests the situation is slightly better. In Scandinavia, Ireland and the United Kingdom forests seldom experience exceedances. On average over Europe about 35% of coniferous forest is estimated to experience exceedances. This is in contrast to the broad-leaved forest, from which we estimate that approximately 70% is not protected.

The 5EAP set emission reduction targets for the precursors of ozone The 5EAP includes emission abatement targets of the ozone source species. The VOC target requires a 30% reduction in the year 2000 from the 1990 emission levels. The NO_x target also uses 1990 as its reference year, and 2000 as the year to achieve a reduction of 30%. It also aims at a stabilisation in 1994 at the 1990 levels.

Emissions of both VOC and NO_x increased until the late 1980s but declined in the 1990s. Between 1990 and 1994 VOC emissions from the EU15 countries decreased by approximately 9%. Similar emission reductions occured for NO_x, which showed a 8% drop between 1990 and 1994. However, given the current reduction rate, it is most unlikely that the remaining reductions of more than 20% will be met until 2000.

In 1990 as an average over EU15, the transport sector accounted for about 45% of total anthropogenic VOC emissions. Similarly, for NO_x the largest fraction originated from transport with an almost stable share of 64% between 1990 and 1994. The second largest source sector is industry (approximately 35%) for VOC, whereas it is the energy sector contributing about 19% for NO_x. Increased traffic is an important obstacle to attaining emission reductions necessary to meet air quality objectives of ozone.

7.4 Conclusions from the meteorological perspective

All wide-spread ozone episodes observed in North and Central Europe from 1994 occurred in spring and summer with dry, sunny weather conditions in stagnant air, in which ozone precursors such as nitrogen oxides, carbon monoxide and volatile organic compounds accumulate. Ozone episodes are shown to be systematically associated with high pressure systems and elevated daily maximum temperatures. On the first/second days of the episodes, the exceedances can be mostly attributed to local/regional photochemical formation, whereas, on the following days, long-range transport plays a major role in the cross-border redistribution of ozone concentrations.

The different situation with the occurrence of exceedances observed in different Member States are to a large part explained by the direction of the predominant wind in situations of anticyclonic circulation and, are related in a large part to the dependence of ozone levels on the meeting of the air masses with areas of high emissions of precursors. The lower occurrence of exceedances in Portugal and Ireland can be mostly attributed to the influence of clean oceanic air masses.

Meso-scale meteorology and interaction between European and local scale weather situations are of major influence on air pollution in South Europe. However, only more dense ozone monitoring networks would allow the assessment of the special meteorological scenarios, such as the land-sea-breeze-circulations which induce particularly high ozone levels in coastal urban/industrial Mediterranean areas.

Downward transport of ozone from the upper troposphere/lower stratosphere may also contribute to observed surface ozone concentrations during spring and summer.

References

Alcamo, J., Shaw, R. and Hordijk, L. (1990) The RAINS model of acidification. International Institute for Applied Systems Analysis IIASA. Kluwer, Academic Publishers, Dordrecht, the Netherlands.

Amann, M., Bertok, I., Cofala, J., Gyarfas, F., Heyes, C., Klimont, Z., Schöpp, W., Simpson, D., Hettelingh, J.-P., and Posch, M. (1996) Cost effective control of acidification and ground-level ozone. Second interim report to the European Commission, DGXI. IIASA, Laxenburg, Austria.

Amann, M., Bertok, I., Cofala, J., Gyarfas, F., Heyes, C., Klimont, Z., Makowski, M., Shibayev, S., and Schöpp, W. (1997) Cost effective control of acidification and ground-level ozone. Third interim report to the European Commission, DGXI. IIASA, Laxenburg, Austria.

Amann, M., Bertok, I., Cofala, J., Gyarfas, F., Heyes, C., Klimont, Z., Makowski, M., Schöpp, W., and Syri, S. (1998) Cost effective control of acidification and ground-level ozone. Fourth interim report to the European Commission, DGXI. IIASA, Laxenburg, Austria.

Anderson, H. R., Spix C., Medina S., et al. (1998) Air pollution and daily admissions for chronic obstructive pulmonary disease in 6 European cities: results from the APHEA project. Eur Respir J.10:1064-71.

Anderson, H.R., Ponce de Leon, A., Bland, J.M., Bower, J.S., Strachan, D.P. (1996) Air pollution and daily mortality in London: 1987-92. British Medical Journal 312; pp 665-669.

Andersson-Sköld, Y., Grennfelt, P., Pleijel, K. (1992) Photochemical ozone creation potentials. A study of different concepts. Journal of the Air and Waste Management Association, 42, pp 1152-1158.

Angeletti, G., and Restelli, G. (ed.) (1994): Physico-chemical behaviour of atmospheric pollutants, (2 vols.) Proc. 6th European Symposium, Varese, 18-22 October, 1993. European Community Report, EUR 15609/1, Commission of the European Communities, DG XII/D-1, Brussels, Belgium.

Beck, J.P. and Grennfelt, P. (1994) Estimate of ozone production and destruction over northwestern Europe. Atmospheric Environment, 28, pp 129-140.

Beck, J.P., Roemer, M.G.M. Vosbeek, M.E.J.P. and Builtjes, P.J.H. (1996) Tropospheric ozone Research: Monitoring and Modelling of photo-oxidants over Europe. RIVM report no 722201007, Bilthoven, the Netherlands.

Borrego, C., Countinho, M., and Barros, N. (1994) Atmospheric pollution in the Lisbon airshed. in Power, H., Moussiopoulos, N. and Brebbia, C.A. (eds.) Urban Air Pollution, Computational Mechanics Publications, Southampton, UK.

Borrell, P., Builtjes, P., Grennfelt, P., Hov, Æ., van Aalst, R., Fowler, D., Megie, G., Moussiopoulos, N., Warneck, P., Volz-Thomas, A. end Wayne, R. (1995) Photo-oxidants, Acidification and Tools; Policy Applications of EUROTRAC Results, EUROTRAC ISS, Garmisch-Partenkirchen, Germany.

Bouscaren , R., (1991) The problems related with the photochemical pollution in the Southern E.C. member states. Final Report, Contract No 6611-31-89.

Bower, J.S., Stevenson, K.J., Broughton, G.F.J., Lampert, J.E., Sweeney, B.P., Wilken, J. et al. (1991) Ozone in the UK: A review of 1989/90 data from monitoring sites operated by Warren Spring Laboratory. Stevenage, UK.

Bower, J.S., Stevenson, K.J., Broughton, G.F.J., Vallance-Plews, J., Lampert, J.E., Sweeney, B.P., Eaton, S.W., Clark, A.G., Willis, P.G., Stacey, B.R.W., Driver, G.S., Laight, S.E., Berwick, R. and Jackson, M.S. (1994) Air Pollution in the UK: 1992/93. Warren Spring Laboratory, Stevenage, UK.

Brönnimann, S. and Neu, U. (1997) Weekend-weekday differences of near-surface ozone concentrations in Switzerland for different meteorological conditions. Atm. Env., 31, pp 1127-1135.

Cartalis, C. and Varotsos, C. (1994) Surface ozone in Athens, Greece, at the beginning and the end of the twentieth century. Atm. Env. 28, 3-8.

Ciccioli, P., Brancaleoni, E., Di Paolo, C., Brachetti, A., Cecinato, A. (1987) Daily trends of photochemical oxidants and their precursors in a suburban forested area. A useful approach for evaluating the relative contributions of natural and anthropogenic hydrocarbons to the photochemical smog formation in rural areas of Italy, in Physico-chemical behavior of atmospheric pollutants, Proc. 4th European Symposium, Angeletti, G., and Restelli, G., (ed.). D. Reidel Publishing Co., Dordrect, the Netherlands.

Cox, R.A., Eggleton, A.J.E., Derwent, R.G., Lovelock, J.E., Pack, D.H. (1975) Long range transport of photo-chemical ozone in north-western Europe. Nature, 255, 118-121.

Cox, R.A. (1997) Overview of results of tropospheric chemistry projects supported within the third framework programme. Air Pollution Research Report 62, EUR 17769 EN, European Commission, DG XII/D, Brussels, Belgium.

Davies, T.D., Kelly, P.M., Low, P.S., and Pierce, C.E. (1992) Surface ozone concentration in Europe: Links with the regional-scale atmospheric circulation, J. Geophys. Res., vol. 97, 9819-9832.

Davies, T. D., and Schuepbach, E. (1994) Episodes of high ozone concentrations at the Earth's surface resulting from transport down from the upper troposphere/lower stratosphere: a review and case studies, Atm. Env., Vol. 28, N° 1, pp. 53-68.

De Cort M., Seynaeve, R. and Vadé, S. (1997) Environmental Radioactivity in the European Community, European Commission, EUR - JRC report 17265, 82 pp., ISBN 92-827-9739-2.

Derwent R.G. and Davies T.J. (1994) Modelling the impact of NOx or hydrocarbon control on photochemical ozone in Europe. Atm. Env., 28, 2039-2052.

Derwent, R.G. and Jenkin, M.E. (1991) Hydrocarbons and the long-range transport of ozone and PAN across Europe. Atmospheric Environment 25A, 1661-1678.

DCMR (1989) Jaarverslag 1988, Schiedam, the Netherlands.

Dumont, G. (1996) Effects of short term measures to reduce ambient ozone concentrations in Brussels and in Belgium. Paper presented at the Ministerial Conference on Tropospheric Ozone in Northwest Europe, London, UK, May 1996.

EC (1992a) Council Directive on Air Pollution by Ozone 92/72/EEC. OJ Nr L 297/1-7.

EC (1992b) Towards Sustainability; a European community programme of policy and action in relation to the environment and sustainable development. Office for official publications of the European Communities, 1993, 162 pp, ISBN 92-826-5157-6.

EC (1996) Council Directive on Ambient Air Quality Assessment and Management 96/62/EC. OJ No L 296/55-63.

EC (1996b) Proposal for a Directive of the European Parliament on a Community Strategy to Combat Acidification.

Feister, U. and Warmbt, W., 1987. Long-term measurements of surface ozone in the German Democratic Republic, J. Atmos. Chem., 5, 1-21.

Feister, U. and Pedersen, U. (1989) Ozone measurements January 1985-December 1985. Report no 1. Potsdam / Lillestrøm, Meteorological Service of the GDR/ Norwegian Institute for Air Research. EMEP/CCC-Report 3/89, Lillestrøm, Norway.

Flocke, F., Volz-Thomas, A. and Kley, D. (1994) The use of alkyl nitrate measurements for the characterization of the ozone balance at TOR-station no. 11, Schauinsland, A contribution to subproject TOR. In: Transport and transformation of pollutants in the troposphere, Proceedings of EUROTRAC Symposium '94, P. Borrel, P.M. Borrell and W. Seiler (eds.). SPB Academic publishing, The Hague, the Netherlands.

Fodor, J (1881) Hygienische Untersuchungen über Luft, Boden und Wasser. Friedrich Vieweg und Sohn, Braunschweig, Germany.

Fortezza, F., V. Strocchi, G. Giovanelli, P. Bonasoni, and T. Georgiadis (1993) Transport of photochemical oxidants along the northwestern Adriatic coast. Atm.. Env., 27A 2393-2402.

Georgiadis, T., G. Giovanelli, and F. Fortezza (1994) Vertical layering of photochemical ozone during land-sea breeze transport. Il Nuovo Cimento, 17, 371-375.

Grennfelt, P., and Schjoldager, J. (1984) Photochemical oxidants in the troposphere: A mounting menace. Ambio, 13, 61-67.

Grennfelt, P., Saltbones, J. and Schjoldager, J. (1987) Oxidant data collection in OECD-Europe 1985-87 (OXIDATE). April-September 1985. NILU OR 22/87, NILU, Lillestrøm, Norway.

Grennfelt, P., Saltbones, J. and Schjoldager, J., (1988) Oxidant data collection in OECD-Europe 1985-87 (OXIDATE). Report on ozone, nitrogen dioxide and peroxyacetyl nitrate October 1985-March 1986 and April-September 1986. NILU OR 31/88, NILU, Lillestrøm, Norway.

Grösslinger, E., Radunsky, K. and Ritter, M. (1996) CORINAIR 1990 Summary Report 1, EEA Topic Report 7/1996. European Environment Agency, Copenhagen, Denmark.

Guicherit, R. and van Dop, H. (1977) Photochemical production of ozone in Western-Europe (1971-1975) and its relation to meteorology. Atm. Env., 11, 145-155.

Guicherit, R. (1988) Ozone on an urban and regional scale- with special reference to the situation in the Netherlands, in Tropospheric Ozone, edited by Isaksen, I.S.A., D.Reidel Publ., pp 49-62.

Guillot, P., G. Bonometti, H. Hasenjaeger, A. van der Meulen, P. Hamilton, R.Haulet, J. Laurent, S. Sandroni, C. Cerutti, G. Giovanelli, T. Tirabassi, O. Vittori, and P. Piccinini (1979) First European Community campaign for remote sensing of air pollution, Lacq (France) 7-11 July 1975. Atm.. Env. 13, 895-917.

Guillot, P. (1985) Joint European campaigns on remote sensing of atmospheric pollution. Proc. 78th Annual Meeting Air Pollution Control Association. Paper 85-18.1., APCA. Pittsburgh, PA., USA.

Hess, P., and Brezowsky, H. (1969) Bericht des Deutschen Wetterdienstes, Nr. 113: Katalog des Grosswetterlagen Europas, Selbstverlag des Deutschen Wetterdienstes, Offenbach a. M.

Heyes, C., Schöpp, W., Amann, M., Bertok, I., Cofala, J., Gyarfas, F., Klimont, Z., Makowski, M. and Shibayev, S. (1996) A model for optimizing strategies for controlling ground-level ozone in Europe. IIASA, Laxenburg, Austria.

Hjellbrekke, A.-G., Schaug, J. and Skelmoen, J.E. (1996) data Report 1994. EMEP/CCC- Report 5/96. NILU, Kjeller, Norway.

Hjellbrekke, A.-G. (1997) Data Report 1995. EMEP/CCC-Report 3/97 NILU. Kjeller.

Imbard, M., Joly, A. and Vachat, R. (1986) Le modèle de prévision numérique PERIDOT: formulation dynamique et modes de fonctionnement. Technical report 161, EERM. Available from CNRM, 31057 Toulouse, France.

IPCC (1995) Climate change 1995; The science of climate change, Houghton, J.T. et al (eds.), Cambridge University Press, Cambridge, UK.

Kärenlampi, L. and Skärby, L. (eds.) (1996) Critical levels for ozone in Europe: Testing and finalizing the concepts. UN-ECE workshop report. University of Kuopio, Department of ecology and environmental science, Kuopio, Finland.

Kallos, G. (ed.) (1997) Transport and transformation of air pollutants from Europe to the East Mediterranean region. Final Report Contract: AVI-CT92-0005, European Commission, DG XII, Brussels.

Kallos, G., V. Kotroni, K. Lagouvardos, and A. Papadopoulos (1998) On the long-range transport of air pollutants from Europe to Africa. Geophys. Res. Lett., 25, 5, 619-622.

Kleinman, L.I. and Benkovitz, C.A. (1987) Source attribution for hydrocarbons and ozone. In The Scientific and technical issues facing post 1987 ozone control strategies, Transactions of an APCA International Speciality Conference. Air & Waste Management Association.

Koffi B., Koffi, N.E. and De Saeger, E. (1998) Ozone episodes and Meteorology in the European Union, EUR - JRC report, in preparation.

Koffi N. E. (1997) Trajectory study of advection processes involved during episodes of high ozone concentrations observed over Europe from 1994 to 1996, European Commission EUR-JRC contract no. 12894-97-05 F1ED ISP I, pp. 38.

Kramp, F., Buers, H.J., Flocke, F., Klemp, D., Kley, D., Pätz, H.W., Schmitz, T., and Volz-Thomas, A. (1994) Determination of OH-concentrations from the decay of C5-C8 hydrocarbons between Freiburg and Schauinsland: Implications on the budgets of olefins, a contribution to subproject TOR. In: Transport and transformation of pollutants in the troposphere, Proceedings of EUROTRAC Symposium '94, P. Borrel, P.M. Borrell and W. Seiler (eds.). SPB Academic publishing, The Hague, the Netherlands.

Le Bras, G. (1988) European Community research on air pollution: Atmospheric processes, measurement and transport. European Community Report, EUR 11590, Commission of the European Communities, DG XII/E-1, Brussels, Belgium.

Le Bras, G. (ed.) (1993) Air quality: Analysis, sources, transport, transformation and deposition of pollutants. European Community Report, EUR 15016, Commission of the European Communities, DG XII/D-1, Brussels.

Le Bras, G. and G. Angeletti (eds.) (1995) Tropospheric processes and air quality, overview of research and results within the fifth environmental R&D; programme (1989-92) STEP. European Community Report, Commission of the European Communities, DG XII/D-1, Brussels, Belgium.

de Leeuw, F.A.A.M., Sluyter, R.J.C.F., van Zantvoort, E.D.G. and Larssen, S. (1995) Exceedance of ozone threshold values in the European Community in 1994. ETC/AQ, Bilthoven, the Netherlands.

de Leeuw, F.A.A.M and van Zantvoort, E.D.G. (1996) Exceedance of ozone threshold values in the European Community in 1995. EEA Topic Report 29/1996. European Environment Agency, Copenhagen, Denmark.

de Leeuw, F.A.A.M and van Zantvoort, E.D.G. (1997) Exceedance of ozone threshold values in the European Community in 1996. EEA Topic Report 7/1997. European Environment Agency, Copenhagen, Denmark.

Lin, X., Roussel, P.B., Melo, O.T and Selorio, P.M (1995) The role of Toronto urban emissions in regional ozone episodes. Atmospheric Environment, 29, pp 565-577.

Lindsay, R.W., and Chameides, W.L. (1988) High ozone events in Atlanta, Georgia, in 1883 and 1984. Environmental Science and Technology 22, 426-431.

Lisac, I. and Grubisic, V., (1991) An analysis of surface ozone measurements at the end of the 19th century in Zagreb, Yugoslavia. Atm. Env. 25, 481-486.

Mahieu, E., R. Zander, L. Delbouille, P. Demoulin, G. Roland, C. Servais, (1997). Trends in total vertical column abundance of atmospheric gases deduced from infrared solar spectra recorded at the Jungfraujoch Station. Accepted by Journal of Atmospheric Chemistry.

Millán, M.M., B. Artíñano, L. Alonso, M. Navazo, and M. Castro (1991) The effect of meso-scale flows on regional and long-range atmospheric transport in the western Mediterranean area. Atm. Env. 25A, 949-963.

Millán, M.M., B. Artíñano, L. Alonso, M. Castro, R. Fernandez-Patier, and J. Goberna (1992) Meso-meteorological cycles of air pollution in the Iberian Peninsula, (MECAPIP). Air Pollution Research Report 44, European Commission DG XII/E-1, Brussels, Belgium.

Millán, M.M., (1993) Photo-oxidation in the Mediterranean Region: Relevant Atmospheric Processes. in Borrell, P.M. (eds.) The Proceedings of EUROTRAC symposium '92, SPB Academic Publishing, The Hague, The Netherlands.

Millan M., Salvador, M.R., Mantilla, E. and Artinano, B. (1996) Meteorology and Photochemical Air Pollution in Southern Europe: Experimental results from EC research projects, Atm. Env., Vol. 30, n° 12, pp. 1909-1924.

Millán, M.M., R. Salvador, E. Mantilla, and G. Kallos (1997) Photo-oxidant dynamics in the Western Mediterranean in summer: Results from European research projects. J. Geophys. Res., 102, D7, 8811-8823.

Moussiopoulos, N. (1994) Air pollution in Athens. in Power, H., Moussiopoulos, N. and Brebbia, C.A. (eds.) Urban Air Pollution, Computational Mechanics Publications, Southampton, UK.

Mylona, S. (1996) Emissions: The collocation and nature of the emission data. In Barrett, K. and Berge, E. (eds.) Transboundary air pollution in Europe. EMEP MSC-W Status Report 1/1996. DNMI, Oslo, Norway.

National Research Council, (1992) Rethinking the ozone problem in urban and regional air pollution, National Academy Press, ISBN 0-309-04631-9, 500 pp, Washington DC.

Olendrzynski, K. (1997) Emissions. In Berge. E (ed.) Transboundary Air Pollution in Europe. EMEP/CCC-Report 1/97, DNMI, Oslo, Norway.

Orciari, R., T. Georgiadis, F. Fortezza, L. Alberti, G. Leoncini, L. Venieri, V. Gnani, T. Montanari, and E. Rambelli (1998) Vertical evolution of photochemical ozone over greater Ravenna. Annali di Chimica, 88, (In press).

Ponce de Leon A, Anderson HR, Bland JM, Strachan DP, Bower J. Effects of air pollution on daily hospital admissions for respiratory disease in London between 1987-88 and 1991-92. J Epidemiol Comm Health 1996;50(Suppl 1):S63-S70.

PORG; United Kingdom Photochemical Oxidants Review Group (1987) Ozone in the United Kingdom, London, UK.

Ritter, M. (1997) CORINAIR 1994 Summary Report, EEA Topic Report, European Environment Agency, Copenhagen, Denmark.

Roemer, M.G.M. (1996) Trends of tropospheric ozone over Europe. PhD thesis, University of Utrecht, the Netherlands.

Roemer M.G.M. (1997). Trend analysis of ground level ozone concentrations in Europe. EMEP/CCC-Note 1/97. NILU, Kjeller, Norway.

Roemer, M.G.M., Builtjes, P.J.H., Esser, P., Guicherit, R. and Thijsse, T. (1998) C2-C5 hydrocarbon measurements in the Netherlands 1981-1991. Accepted by Atmospheric Environment.

Rombout PJA, Schwarze P. (1997) Quantitative exposure-response relationships for ozone. In: United Nations Economic Commission for Europe, World Health Organization European Centre for Environment and Health. Health effects of ozone and nitrogen oxides in an integrated assessment of air pollution. Proceedings of an International Workshop, Eastbourne, 10-12 June 1996. Leicester: MRC Institute for Environment and Health: pp 20-28.

Sandroni, S., and M. De Groot (1980) Intercomparison of remote sensors of sulphur dioxide at the 1979 European Community Campaign at Turbigo. Atm. Env., 14, 1331-1333.

Sator, F., Demuth, C., Snacken, R., Walckiers, D. (1997) Mortality in the elderly and ambient ozone concentration during the hot summer, 1994, in Belgium. Environmental Research 72, 109-117.

Scheel, H.E., H. Areskoug, H. Geiß, B. Gomiscek, K. Granby, L. Haszpra, L. Klasinc, D. Kley, T. Laurila, A. Lindskog, M. Roemer, R. Schmitt, P. Simmonds, S. Solberg, G. Toupance (1997) On the Spatial Distribution and Seasonal Variation of Lower-Troposphere Ozone over Europe. Accepted by Journal of Atmospheric Chemistry.

Schneider, J. (1998) An assessment of Austrian Ozone data based on different indicators. Umweltbundesamt, Bericht BE-106, Vienna, Austria

Seufert, G. (ed.) (1997) BEMA: A European Commission project on Biogenic Emissions in the Mediterranean Area. Atm. Env. 31 (Special Issue), SI 1-SI 255.

Simmonds, P.G. (1993) Tropospheric Ozone Research and Global Atmospheric Gases Experiment, Mace Head, Ireland. Annual report on the EUROTRAC-TOR project, ISS, Garmisch-Partenkirchen, Germany

Simpson, D. (1992) Long period modelling of photochemical oxidants in Europe B) on the linearity of country-to-country ozone calculations in Europe. EMEP MSC-W Note 1/92, DNMI, Oslo, Norway.

Simpson, D. (1995) Hydrocarbon reactivity and ozone formation in Europe. J. Atm. Chem., 20. 163-177.

Simpson, D. (1995) Biogenic emissions in Europe (2) Implications for ozone control strategies. J. Geophys. Res., Vol. 100, pp 22,891-22,906.

Simpson, D. and Malik. S., (1996) Photochemical oxidant modelling, and source receptor relationships for ozone. In Barrett, K. and Berge, E. (eds.) Transboundary air pollution in Europe. EMEP MSC-W Status Report 1/1996. DNMI, Oslo, Norway.

Simpson, D., Olendrzynski, K., Semb, A., Storen, E. and Unger, S. (1997) Photochemical oxidant modelling in Europe: multi-annual modelling and source-receptor relationships. EMEP/CCC-Report 3/97, DNMI, Oslo, Norway.

Sluyter, R.C.J.F. (ed.) (1995) Air quality in major European cities. RIVM/NILU, Bilthoven, the Netherlands.

Solberg, S., Stordal, F., Schmidbauer, N. and Hov, Ø. (1994) Non-methane hydrocarbons (NMHC) at Birkenes in South Norway, 1988-1993. NILU Report 47/93.

Staehelin, J., Thudium, J., Buehler, R., Volz-Thomas, A. and Graber, W. (1994) Trend in surface ozone concentrations at Arosa (Switzerland). Atm. Env., 28, pp 75-87.

Stedman, J.R. and Williams, M.L. (1992) A trajectory model of the relationship between ozone and precursor emissions. Atm. Env., 26A, pp 1609-1634.

Stohl, A., Williams, E., Wotawa, G. and Kromp-Kolb, H. (1996) A European inventory of soil nitric oxide emissions and the effect of these emissions on the photochemical formation of ozone. Atm. Env., 30, pp 3741-3755.

Sunyer, J., Castellsague, J., Saez, M., Tobias, A., Anto, J.M. (1996) Air pollution and mortality in Barcelona.
J. Epidemiology and Community Health, 50 (Suppl.1 ) S76-S80.

Suppan, P., Fabian, P., Vyras, L. Gryning, S.E., (1998) The behaviour of ozone and peroxyacetyl nitrate concentrations for different wind regimes during the MEDCAPHOT-Trace campaign in the greater area of Athens, Greece, Atm. Env.., in press.

UN-ECE (1996) Kärenlampi, L. and Skärby, L. (eds.) Critical levels for ozone in Europe: testing and finalising the concepts. UNECE workshop report. University of Kuopio,Finland.

Volz, A. and Kley, D., 1988, Evaluation of the Montsouris series of ozone measurements made in the nineteenth century. Nature, 332, 240-242.

Volz-Thomas, A. (1997) Personal communication on the seasonality in the Montsouris ozone data series.

WHO (1987). Air Quality Guidelines for Europe. Copenhagen: World Health Organization: 1987.

WHO (1995). Update and revision of the Air Quality Guidelines for Europe. Meeting of the Working Group "Classical" Air Pollutants. Copenhagen: World Health Organization: 1995.

WHO (1996a) Update and revision of the WHO air quality guidelines for Europe. Classical air pollutants ; ozone and other photochemical oxidants. European centre for environment and health, Bilthoven, the Netherlands.

WHO (1996b) Update and revision of the WHO air quality guidelines for Europe. Ecotoxic effects, ozone effects on vegetation. European centre for environment and health, Bilthoven, the Netherlands.

WHO (1997) Air Quality Guidelines for Europe, second edition. Copenhagen: World Health Organization: in press.

Zander, R., Demoulin, Ph., Mahieu, E., Roland, G., Delbouille, L. and Servais, C. (1997) in Tropospheric Ozone Research, ed. Hov, O, Springer-Verlag, Berlin, Germany.

van Zantvoort, E.D.G. and Sluyter, R.J.C.F. (1996) Annex to the report "Exceedance of ozone threshold values in the European Community in 1994": data from Germany. ETC/AQ, Bilthoven, The Netherlands.

Ziomas, I.C., Gryning, S.-E., and Borstein, R.D. (eds.) (1998) The Mediterranean campaign of photochemical tracers-transport and chemical evolution (MEDCAPHOT-TRACE): Athens, Greece 1994-1995. Atm. Env. 32, 12, 2043-2326.

Ziomas I.C., (1998) The Mediterranean Campaign of Photochemical Tracers - Transport and Chemical Evolution (MEDCAPHOT-TRACE). An outline. Atm. Env., in press.