The interest in air pollution modelling has shown substantial growth over the last five years. This was particularly evident by the increasing number of participants attending the NATO/CCMS International Technical Meetings on Air Pollution modelling and its Application. At the last meeting 118 papers and posters were selected from an abundance of submitted abstracts divided over five modelling topics: (i) model assessment and verification, including policy applications, (ii) air pollution modelling in coastal areas with emphasis on the mediterranean region, (iii) accidental atmospheric releases, including warning systems and regulations, (iv) modelling of global and long-range transport and (v) new developments in turbulent diffusion. A round-table discussion chaired by John Irwin (USA) and Jan Kretzschmar (Belgium) on the harmonization of air pollution models was attended by more than 50 scientists and is reported in these proceedings. The opening paper addressed the main issue of this conference: modelling over complex terrain. Of particular interest were coastal areas where the surface inhomogeneities introduce small-scale circulation and varying atmospheric stability, often combined with a complex topography. As the conference was located on the beautiful island of Crete, problems faced by the host nation, particularly Athens and its environs were obvious examples for consideration. These together with other regions with similar geographical features were addressed. Heavily populated and industrialized as they often are, air quality is generally poor there and emission regulations are desired. Obviously, a major task of air pollution dispersion modelling is to assist policy makers in formulating sensible regulations.

In 1949, when the North Atlantic Treaty was ratified, one of its articles explicitly noted '1hat member countries should contribute towards the further development of peaceful and friendly international relations." Specific problems related to the human environment were addressed by the Committee of Challenges of Modern Society (CCMS) of NATO, established in 1969. This provided a framework within which a series of International Technical Meetings (ITMs) on Air Pollution Modelling has been held. This volume documents the proceedings of the 18th meeting in this series. Science, like the arts and sports, provides an ideal vehicle for "developing peaceful and friendly international relations". National boundaries have never been barriers to the movement of air pollution, and fortunately this has also proved true of scientists studying the transport of air pollution. It is thus satisfying to record that since the mid-seventies it has been commonplace to find Eastern European scientists among attendees at the ITMs which have (in a very modest way) participated in a precursor to the process which has led to historical changes in Europe and which will undoubtedly lead to a tremendous increase in personal and intellectual exchange on a worldwide basis.

Environmental Chemistry is a relatively young science. Interest in this subject, however, is growing very rapidly and, although no agreement has been reached as yet about the exact content and limits of this interdisciplinary discipline, there appears to be increasing interest in seeing environmental topics which are based on chemistry embodied in this subject. One of the first objectives of Environmental Chemistry must be the study of the environment and of natural chemical processes which occur in the environment. A major purpose of this series on Environmental Chemistry, therefore, is to present a reasonably uniform view of various aspects of the chemistry of the environment and chemical reactions occurring in the environment. The industrial activities of man have given a new dimension to Environmental Chemistry. We have now synthesized and described over five million chemical compounds and chemical industry produces about hundred and fifty million tons of synthetic chemicals annually. We ship billions of tons of oil per year and through mining operations and other geophysical modifications, large quantities of inorganic and organic materials are released from their natural deposits. Cities and metropolitan areas of up to 15 million inhabitants produce large quantities of waste in relatively small and confined areas. Much of the chemical products and waste products of modern society are released into the environment either during production, storage, transport, use or ultimate disposal. These released materials participate in natural cycles and reactions and frequently lead to interference and disturbance of natural systems.

In 1949, when the North Atlantic Treaty was ratified, one of its articles explicitly noted '1hat member countries should contribute towards the further development of peaceful and friendly international relations." Specific problems related to the human environment were addressed by the Committee of Challenges of Modern Society (CCMS) of NATO, established in 1969. This provided a framework within which a series of International Technical Meetings (ITMs) on Air Pollution Modelling has been held. This volume documents the proceedings of the 18th meeting in this series. Science, like the arts and sports, provides an ideal vehicle for "developing peaceful and friendly international relations". National boundaries have never been barriers to the movement of air pollution, and fortunately this has also proved true of scientists studying the transport of air pollution. It is thus satisfying to record that since the mid-seventies it has been commonplace to find Eastern European scientists among attendees at the ITMs which have (in a very modest way) participated in a precursor to the process which has led to historical changes in Europe and which will undoubtedly lead to a tremendous increase in personal and intellectual exchange on a worldwide basis.

The interest in air pollution modelling has shown substantial growth over the last five years. This was particularly evident by the increasing number of participants attending the NATO/CCMS International Technical Meetings on Air Pollution modelling and its Application. At the last meeting 118 papers and posters were selected from an abundance of submitted abstracts divided over five modelling topics: (i) model assessment and verification, including policy applications, (ii) air pollution modelling in coastal areas with emphasis on the mediterranean region, (iii) accidental atmospheric releases, including warning systems and regulations, (iv) modelling of global and long-range transport and (v) new developments in turbulent diffusion. A round-table discussion chaired by John Irwin (USA) and Jan Kretzschmar (Belgium) on the harmonization of air pollution models was attended by more than 50 scientists and is reported in these proceedings. The opening paper addressed the main issue of this conference: modelling over complex terrain. Of particular interest were coastal areas where the surface inhomogeneities introduce small-scale circulation and varying atmospheric stability, often combined with a complex topography. As the conference was located on the beautiful island of Crete, problems faced by the host nation, particularly Athens and its environs were obvious examples for consideration. These together with other regions with similar geographical features were addressed. Heavily populated and industrialized as they often are, air quality is generally poor there and emission regulations are desired. Obviously, a major task of air pollution dispersion modelling is to assist policy makers in formulating sensible regulations.

1. Contents of these proceedings. These proceedings contain most of the papers which were presented at the NATO ARW (Advanced Research Workshop) on "Large Scale Computations in Air Pollution Modelling." The workshop was held, from June 6 to June to, 1998, in Residence Bistritza, a beautiful site near Sofia, the capital of Bulgaria, and at the foot of the mountain Vitosha. 2. Participants in the NATO ARW. Scientists from 23 countries in Europe, North America and Asia attended the meeting and participated actively in the discussions. The total number of participants was 57. The main topic of the discussions was the role of the large mathematical models in resolving difficult problems connected with the protection of our environment. 3. Major topics discussed at the workshop. The protection of our environment is one of the most important problems facing modern society. The importance of this problem has steadily increased during the last two-three decades, and environment protection will become even more important in the next century. Reliable and robust control strategies for keeping the pollution caused by harmful chemical compounds under certain safe levels have to be developed and used in a routine way. Large mathematical models, in which all important physical and chemical processes are adequately described, can successfully be used to solve this task.

The study of turbulence in the atmosphere has seen considerable progress in the last decade. To put it briefly: boundary-layer meteorology, the branch of atmospheric science that concentrates on turbulence in the lower atmosphere, has moved from the surface layer into the boundary layer itself. The progress has been made on all fronts: theoretical, numerical and observational. On the other hand, air pollution modeling has not seen such a rapid evolution. It has not benefited as much as it should have from the increasing knowledge in the field of atmospheric turbulence. Air pollution modeling is still in many ways based on observations and theories of the surface layer only. This book aims to bring the reader up to date on recent advances in boundary-layer meteorology and to pave the path for applications in air pollution dispersion problems. The text originates from the material presented during a short course on Atmospheric Turbulence and Air Pollution Modeling held in The Hague during September 1981. This course was sponsored and organized by the Royal Netherlands Meteorological Institute, xi xii PREFACE to which both editors are affiliated. The Netherlands Government Ministry of Health and Environmental Protection and the Council of Europe also gave support.