This book presents the Proceedings of the Fourth International Conference ENVIROSOFT 92. Since the beginning of this conference cycle in 1986, EN VIROSOFT has provided a forum for discussion and debate for scientists, engineers and decision makers involved in the development and application of computer techniques to environmental sciences. Our main goal is to pro vide an opportunity for cross-fertilization among the several groups involved in the study of the environment. A couple of years ago a reviewer of ENVIROSOFT 90 made a very in teresting and appropriate comment. He congratulated us for the effort of providing a forum for discussion among environmental scientists but criti cized us for maintaining, in the proceedings and in the conference schedule, a strict division between 'atmospheric' papers, 'water' papers, etc. I agree with the reviewer and, in this conference, I made an effort to separate the articles not on the basis of their physical medium (air, water, groundwa ter, etc.) but on the basis of the methods and approaches adopted by the authors. Again, I hope that this change will improve the dialogue among scientists and, in particular, the interactions between 'air' and 'water' re searchers.

This book comprises of the edited proceedings of a seminar on environmental modelling. It provides a comprehensive overview of current environmental problems and the progress achieved in their numerical simulation.

Finishing this book is giving me a mixture of relief, satisfaction and frus tration. Relief, for the completion of a project that has taken too many of my evenings and weekends and that, in the last several months, has become almost an obsession. Satisfaction, for the optimistic feeling that this book, in spite of its many shortcomings and imbalances, will be of some help to the air pollution scientific community. Frustration, for the impossibility of incorporating newly available material that would require another major review of several key chap ters - an effort that is currently beyond my energies but not beyond my desires. The first canovaccio of this book came out in 1980 when I was invited by Computational Mechanics in the United Kingdom to give my first Air Pollution Modeling course. The course material, in the form of transparencies, expanded, year after year, thus providing a growing working basis. In 1985, the ECC Joint Research Center in Ispra, Italy, asked me to prepare a critical survey of mathe matical models of atmospheric pollution, transport and deposition. This support gave me the opportunity to prepare a sort of "first draft" of the book, which I expanded in the following years."