The problern of "acid precipitation" has been recognized with growing concern in many industrialized countries. The incorporation of pollutants into cloud and rain elements and their transfer to the ground by "wet deposition" are dominant mechanisms leading to a self~cleansing of the tro- posphere but, on the other hand, to hazards to the soil, vegetation and forests. The influence of orographic and meteorological parameters and of the regional distribution of precipitation on the deposition of pollutants are insuffi- ciently known factors. During previous years, several projects and analyses have been initiated to improve our knowledge on the dry and wet deposition of pollutants and on the mechanisms of transport of gaseaus and particulate components from the atmosphere to the ground. Research activities have been supported in different fields and it appeared not only useful but necessary to bring the different research-groups tagether to endorse the communication and cooperation between scientists in the related fields. A symposiumwas arranged in Oberurself Taunus in November 1981 to discuss the results of experimen- tal and theoretical work in the field of deposition and to gain a better understanding of each other's methods, experience and Observations. The proceedings presented in this volume permit a fair sur- vey of the present-day knowledge and will be a useful tool for all working in this field. The meeting would not have been possible without the finan- cial support of the German Environmental Agency {Umwelt- bundesamt) .

In November 1988 the "'Third Oberursel Symposium" devoted to the problems of input of pollutions into forest-ecosystems and their effects on plants or soil convened. After several years of intensive research on the effects of pollutions on forest ecosystems it is obvious that not a single specific pollutant can be made responsible but a mixture of several components act together or interact with each other. The contributions of the workshop _ reflect to a large extend the results of research projects which were started at the beginning of the eighties. They review our improved knowledge on the patterns of concentration, of the mechanism of wet and dry deposition and fog interception, modelling studies and the effect of the processes on plant receptors and surfaces. Since the 1985 symposium the pathways of pollutants leading to biological damage have been examined and are more clearly recognised. The book reflects the common interest and the continuous effort of scientists from many different disciplines to better understand the physical and chemical processes which finally lead to the observed damage of forest-trees. Comparing the conclusions of the contributions of this book _ with the results of the first Oberursel symposium in 1981, our knowledge on the relevance of the different mechanisms leading to forest-decay has been considerably improved. The book indicates also in which directions future work should be concentrated. Again, I have to thank the authors for their cooperation by submitting their recent research-results."

The content of this book are lectures and research papers presented at the NATO Advanced Study Institute on -Chemistry of the Unpolluted and Polluted Troposphere-held from September 28 to October 10, 1981, on the Island of Corfu, Greece. The realization of the scientific event was made possible by the sponsorship of the NATO Scientific and Environmental Affairs Division. We must express our gratitude first to this Institution for the important assistance and cooperation we received. For additional assistance we are much obliged to our distinguished co sponsors: Carl Zeiss Werk, Oberkochen, FRGj Biotronik GmbH, Frankfurt, FRGj Bodenseewerk Perkin Elmer u. Co GmbH, Ueberlingen, FRGj and TSI Deutschland Inc., Aachen, FRG. To Mr. Dimitris Bouas, the Director of the Grand Hotel Glyfada, we would like to express our thanks and appreciation for the excellent services. The Center of Environmental Protection of the University of Frankfurt, FRG, conducted the NATO Advanced Study Institute because of the rapidly increased interest in the problems of air chemistry and air pollution. This development has contributed in recent years to a significant expansion of knowledge in the field of atmospheric chemistry, where several of the classic disciplines like meteorology, physics and chemistry have joined in combined research. The Advanced Study Institute offered an ideal opportunity to give a comprehensive survey of the present knowledge in this relatively new field of atmospheric science.

In November 1981 a first symposium with the topics of "Acid Deposition of Atmospheric Pollutants" was organised in OberursellTaunus to introduce the problems and first results of research-activities on wet and dry deposition of pollutants and on acid precipitation. In the meantime the hazard to forest and vegetation became more dramatic and research-projects to investigate the input of pollutants to forest-ecosystems have been initiated by several interdisciplinary groups. The rapidly increasing interest in the problems of forest decay and the many open questions with respect to the diagnosis of the forest-damage were the background for the organisation of a second symposium which was held in November 1985 at the same location in Oberursel/Taunus. It was mainly concerned with new techniques of sampling and analyzing pollutants in forest areas. Besides deposition, one important pathway of pollutants in orographic terrain is the interception of fog-droplets by vegetation. Special emphasis was laid on the chemical composition of fog. The symposium successfully assembled scientists from the field of atmospheric research with those studying the effect of pollutants on trees and vegetation in order to reduce the many open questions in connection with forest desease. The proceedings presented in this volume are a substantial contribution to the understanding of deposition and interception of pollutants in forest-areas. Thanks to the authors the volume contains a lot of new research .results and presents therefore a true picture of our present knowledge."

The content of this book are lectures and research papers presented at the NATO Advanced Study Institute on -Chemistry of the Unpolluted and Polluted Troposphere-held from September 28 to October 10, 1981, on the Island of Corfu, Greece. The realization of the scientific event was made possible by the sponsorship of the NATO Scientific and Environmental Affairs Division. We must express our gratitude first to this Institution for the important assistance and cooperation we received. For additional assistance we are much obliged to our distinguished co sponsors: Carl Zeiss Werk, Oberkochen, FRGj Biotronik GmbH, Frankfurt, FRGj Bodenseewerk Perkin Elmer u. Co GmbH, Ueberlingen, FRGj and TSI Deutschland Inc., Aachen, FRG. To Mr. Dimitris Bouas, the Director of the Grand Hotel Glyfada, we would like to express our thanks and appreciation for the excellent services. The Center of Environmental Protection of the University of Frankfurt, FRG, conducted the NATO Advanced Study Institute because of the rapidly increased interest in the problems of air chemistry and air pollution. This development has contributed in recent years to a significant expansion of knowledge in the field of atmospheric chemistry, where several of the classic disciplines like meteorology, physics and chemistry have joined in combined research. The Advanced Study Institute offered an ideal opportunity to give a comprehensive survey of the present knowledge in this relatively new field of atmospheric science.

In November 1988 the "'Third Oberursel Symposium" devoted to the problems of input of pollutions into forest-ecosystems and their effects on plants or soil convened. After several years of intensive research on the effects of pollutions on forest­ ecosystems it is obvious that not a single specific pollutant can be made responsible but a mixture of several components act together or interact with each other. The contributions of the workshop _ reflect to a large extend the results of research­ projects which were started at the beginning of the eighties. They review our improved knowledge on the patterns of concentration, of the mechanism of wet and dry deposition and fog interception, modelling studies and the effect of the processes on plant­ receptors and surfaces. Since the 1985 symposium the pathways of pollutants leading to biological damage have been examined and are more clearly recognised. The book reflects the common interest and the continuous effort of scientists from many different disciplines to better understand the physical and chemical processes which finally lead to the observed damage of forest-trees. Comparing the conclusions of the contributions of this book _ with the results of the first Oberursel symposium in 1981, our knowledge on the relevance of the different mechanisms leading to forest-decay has been considerably improved. The book indicates also in which directions future work should be concentrated. Again, I have to thank the authors for their cooperation by submitting their recent research-results.