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Rapidly increasing interest in the problems of air pollution and source-receptor relationships has led to a significant expansion of knowledge in the field of atmospheric chemistry. In general the chemistry of atmospheric trace constituents is governed by the oxygen content of the atmosphere. Upon entering the atmosphere in a more or less reduced state, trace substances are oxidized via various pathways and the generated products are often precursors of acidic compounds. Beside oxidation processes occurring in the gas phase, gaseous compounds are often converted into solid aerosol particles. The various steps within gas-to-particle conversion are constantly interacting with condensation processes, which are caused by the tropospheric water content. Thus in addition to the gaseous state, a liquid and solid state exists within the troposphere. The solid phase consists of atmospheric conversion products or fly ash and mineral dust. The liquid phase consists of water, conversion products and soluble compounds. The chemistry occurring within this system is often referred to as hydrogeneous chemistry. The chemist interprets this term, however, more strictly as reactions which occur only at an interphase between phases. This, however, is not always what happens in the atmosphere. There are indeed heterogeneous processes such as reactions occurring on the surface of dry aerosol particles. But apart from these, we must focus as well on reactions in the homogeneous phase, which are single steps of consecutive reactions running through various phases.

The Acid Rain 2000 Conference in Tsukuba, Japan, held 10-16 December 2000, was the sixth such conference in the series, starting with Columbus, Ohio, USA, in 1975, and including Sandefjord, Norway, in 1980, Muskoka, Canada, in 1985, Glasgow, UK, in 1990, and Göteborg, Sweden, in 1995. This series of International Conferences on the acid rain problem has made a very important contribution to the process of summarising the state of current understanding and making this information available. In the 6th Conference, approximately 600 papers were presented, including talks and posters. About 300 peer-reviewed papers from the presentation appear in this volume, and will provide readers with a comprehensive review of the history and scientific aspects of the acid rain problem. The papers appear in three volumes: the first containing the plenary and keynote papers and the other two the remaining scientific papers. (Volume 1: ISBN 0-7923-7132-1; Volume 2: ISBN 0-7923-7133-X; Volume 3: ISBN 0-7923-7134-8). The Conference was arranged under the joint auspices of The Science Council of Japan, The Japanese Society of Limnology (representative academic society), Japan Association of Aerosol Science and Technology, The Japan Society for Analytical Chemistry, Japan Society for Atmospheric Environment, Chemical Society of Japan, The Ecological Society of Japan, The Japanese Society of Environmental Education, Society of Environmental Science, Japan, The Japanese Forestry Society, Japanese Society of Snow and Ice, Japanese Society of Soil Science and Plant Nutrition, and Japan Society on Water Environment, with the cooperation of Ibaraki Prefecture and Japan Environment Agency.