1 Executive Summary.- 2 Environmental Problems, Policies and EUROTRAC.- 2.1 The Atmosphere as a Recipient and Transport Medium for Pollutants.- 2.2 Environmental Problems.- 2.2.1 Environmental problems covered by EUROTRAC.- 2.2.2 Photochemical oxidant effects.- 2.2.3 Acidification and eutrophication.- 2.2.4 Links to global effects.- 2.3 Policies.- 2.3.1 Policies used to combat regional air pollution.- 2.3.2 Policies used to combat global problems.- 2.4 EUROTRAC and its Application Project.- 2.4.1 EUROTRAC: the successful scientific programme.- 2.4.2 The Application Project (AP).- 2.4.3 The organisation of the Application Project report.- 2.5 Acknowledgements.- 2.6 References.- 3 Photo-oxidants.- 3.1 Environmental Policy Issues Related to Photo-oxidants.- 3.2 Scientific Findings of Importance for Environmental Policy.- 3.2.1 Trends in photo-oxidant concentrations.- 3.2.2 Emissions.- 3.2.3 Processes influencing the concentration of photo-oxidants.- 3.2.4 Local and urban photo-oxidants.- 3.2.5 Regional or continental photo-oxidants.- 3.2.6 Photo-oxidants on a global scale.- 3.3 Applications and Importance for Environmental Policy, Potential for Applications.- 3.3.1 Local and urban photo-oxidants.- 3.3.2 Regional or continental photo-oxidants.- 3.3.3 Photo-oxidants on the global scale.- 3.4 References.- 4 Acidification and Deposition of Nutrients.- 4.1 Environmental policy issues related to acidic deposition and deposition of nutrients.- 4.2 Scientific Assessment - Issues and Highlights.- 4.2.1 Emissions.- 4.2.2 Atmospheric processes.- 4.2.3 Non-linearities in source-receptor relationships.- 4.2.4 Deposition.- 4.3 Applications and Implications for Policies.- 4.3.1 Quantification of emissions.- 4.3.2 Cloud chemistry.- 4.3.3 Atmospheric deposition.- 4.3.4 The development and application of source-receptor models for policy.- 4.3.5 The influence of sulphate aerosols on the radiation balance.- 4.4 Conclusions. Future Potential for Applications.- 4.4.1 Gaps in knowledge - uncertainties.- 4.4.2 Needs for improvements of application models etc.- 4.5 References.- 5 Contribution of EUROTRAC to the Development of Tools for the Study of Environmentally Relevant Trace Constituents.- 5.1 Introduction.- 5.1.1 Instruments.- 5.1.2 Laboratory studies.- 5.1.3 Models.- 5.1.4 Further tools.- 5.2 Instruments.- 5.2.1 Instrument development within EUROTRAC.- 5.2.2 Instrument subprojects within EUROTRAC.- 5.2.3 Instrument development in the experimental subprojects.- 5.2.4 Some reflections on instrument development within EUROTRAC.- 5.2.5 Conclusions and recommendations.- 5.3 Laboratory Studies.- 5.3.1 Laboratory studies and their scope.- 5.3.2 Some highlights of the scientific advances 1 concerned with photo-oxidants.- 5.3.3 Some highlights of the scientific advances concerned with heterogeneous chemistry.- 5.3.4 Incorporation of laboratory results into simulation model.- 5.3.5 Development of laboratory techniques within the programmes.- 5.3.6 Needs for the future.- 5.4 Models.- 5.4.1 Introduction.- 5.4.2 Regional simulation models.- 5.4.3 Urban simulation models.- 5.4.4 Simulation models on a global scale.- 5.4.5 Validation of simulation models.- 5.4.6 Process-oriented models.- 5.4.7 Conclusions and recommendations for future applications.- 5.5 Other Tools.- 5.5.1 Databases from field experiments.- 5.5.2 Emission and land-use databases.- 5.5.3 Monitoring networks.- 5.5.4 Hidden tools.- 5.5.5 A network of scientists.- 5.6 References.- Appendices.- A. The Application Project Description.- B. The EUROTRAC subprojects with their aims.