In order to provide meaningful information for evaluating environmental consequences of alternative control strategies of air pollutant emissions, scientists have recently initiated numerous studies aiming at collecting and making use of regional data. New kinds of mathematical models have been developed that no longer are calibrated just for individual stands or catchments but can be applied on a large regional scale. Such regional models and data were reviewed at a meeting" Models to describe the geographic extent and time evolution of acidification and air pollution damage", organized by the Finnjsh Research Project on Acidification (HAPRO), and held in Aulanko, Hiimeenlinna, Finland, July 5-8, 1988. This volume is based on the outcome of that meeting. The chapters of the volume are written by leading scientists in the field. The main objective of this volume is to discuss the potential of regional models to analyze the geographic extent of acidification as well as the environmental damage and the time evolution of:the responses of the environment. First, in Part I of the volume, sensitivity distributions of ecosystems, forest soils and surface waters are described. The focus of interest is therefore extended from the most sensitive ecosystems to sensitive and moderately sensitive parts of the environment. When formulating national and interna tional targets, it is of crucial importance to know the whole distribution of critical loads within the area, and not only the level the most sensitive ecosystem can tolerate.

Sensitive receptors such as soil and fresh bodies of water are at the end of a long chain of events in the process of regional acidification. This chain begins thousands of kilometers upwind at the emitters of acidifying pollutants. The topics covered in this book are important in the study of regional acidification for two reasons. First, it is important to assess the sensitivity of terrestrial and aquatic ecosystems to the deposition of acidifying pollutants. If the sensitivity of an ecosystem is known, then international control strategies can be developed to reduce deposition in the receptor areas of greatest importance. This is an impor tant factor in designing the most effective strategies because of the very high costs of reducing emissions of acidifying pollutants. Second, it is important to be able to predict changes in ecosystems for decades into the future, whether it be an improvement owing to decreases in aci difying emissions or, alas, a further deterioration because control strategies are nonexistent or inadequate. In either event, it is important to be able to judge the results of our actions. Decision makers tend to be mistrustful of models unless they can judge their reliability. The application and testing of the models in Part III of this book cover, therefore, an important facet of model building."

Sensitive receptors such as soil and fresh bodies of water are at the end of a long chain of events in the process of regional acidification. This chain begins thousands of kilometers upwind at the emitters of acidifying pollutants. The topics covered in this book are important in the study of regional acidification for two reasons. First, it is important to assess the sensitivity of terrestrial and aquatic ecosystems to the deposition of acidifying pollutants. If the sensitivity of an ecosystem is known, then international control strategies can be developed to reduce deposition in the receptor areas of greatest importance. This is an impor­ tant factor in designing the most effective strategies because of the very high costs of reducing emissions of acidifying pollutants. Second, it is important to be able to predict changes in ecosystems for decades into the future, whether it be an improvement owing to decreases in aci­ difying emissions or, alas, a further deterioration because control strategies are nonexistent or inadequate. In either event, it is important to be able to judge the results of our actions. Decision makers tend to be mistrustful of models unless they can judge their reliability. The application and testing of the models in Part III of this book cover, therefore, an important facet of model building.