The suggestion by Dr. Franklin S. Harris, Jr. , that these books be written arose pursuant to the editor's plaints that despite the implicitly or explicitly ack nowledged importance of both aerosols and particulate matter in innumerable domains of technology and human welfare, investigations of these subjects were generally not supported independently of the narrowest conceivable domains of their appli cations. Frank Harris, who has long been a contributor in one of the important domains of aerosol macrophysics, atmospheric optics, challenged the editor to elaborate his views. Ideally, they would have taken the form of a monograph; however, there is as yet an insufficient body of information to present a unified treatment. At the same time, substantial efforts are in progress in the component fields to hold the promise for the emergence of unifying elements which will even tually facilitate their presentation to be made with a high degree of integrity. There are numerous pertinent and systematic tie-ins between project-oriented aerosol work and basic physical investigations which are themselves quite closely akin to much classical and current work in physical science. The most significant aspect of these tie-ins is their potential for making substantial contributions to the functional needs of the applications areas while stimulating significant questions of basic physics. For this to be possible, it is necessary that the most relevant areas of physics be identified in such a manner as to make clear their re levance for aerosol-related studies and vice versa.

Aerosols, which are gas-phase dispersions of particulate matter, draw upon and con tribute to multidisciplinary work in technology and the natural sciences. As has been true throughout the history of science with other fields of interest whose un derlying disciplinary structure was either unclear or insufficiently well developed to contribute effectively to those fields, "aerosol science" has. developed its own methods and lore somewhat sequestered from the main lines of contemporary physical thought. Indeed, this independent development is the essential step in which syste matic or phenomenological descriptions are evolved with validity of sufficient gen erality to suggest the potential for development of a physically rigorous and gen eralizable body of knowledge. At the same time, the field has stimulated many ques tions which, limited to its own resources, are hopelessly beyond explanation. As Kuhn pointed out in The Structure of Scientific Revolution [2nd enlarged edition (University of Chicago Press, Chicago 1970) Chapter II and Postscript-1969) this is a very common juncture in the development of a science. In brief, the transition from this earlier stage to the mature stage of the science involves a general re cognition and agreement of what the foundations of the field consist of. By this critical step, a field settles upon a common language which is well defined rather than the ambiguous, and often undefined descriptors prevalent at the earlier stage.

Aerosols, which are gas-phase dispersions of particulate matter, draw upon and con tribute to multidisciplinary work in technology and the natural sciences. As has been true throughout the history of science with other fields of interest whose un derlying disciplinary structure was either unclear or insufficiently well developed to contribute effectively to those fields, "aerosol science" has. developed its own methods and lore somewhat sequestered from the main lines of contemporary physical thought. Indeed, this independent development is the essential step in which syste matic or phenomenological descriptions are evolved with validity of sufficient gen erality to suggest the potential for development of a physically rigorous and gen eralizable body of knowledge. At the same time, the field has stimulated many ques tions which, limited to its own resources, are hopelessly beyond explanation. As Kuhn pointed out in The Structure of Scientific Revolution [2nd enlarged edition (University of Chicago Press, Chicago 1970) Chapter II and Postscript-1969) this is a very common juncture in the development of a science. In brief, the transition from this earlier stage to the mature stage of the science involves a general re cognition and agreement of what the foundations of the field consist of. By this critical step, a field settles upon a common language which is well defined rather than the ambiguous, and often undefined descriptors prevalent at the earlier stage.

Cloud physics has achieved such a voluminous literature over the past few decades that a significant quantitative study of the entire field would prove unwieldy. This book concentrates on one major aspect: cloud microphysics, which involves the processes that lead to the formation of individual cloud and precipitation particles. Common practice has shown that one may distinguish among the following addi tional major aspects: cloud dynamics, which is concerned with the physics respon sible for the macroscopic features of clouds; cloud electricity, which deals with the electrical structure of clouds and the electrification processes of cloud and precipi tation particles; and cloud optics and radar meteorology, which describe the effects of electromagnetic waves interacting with clouds and precipitation. Another field intimately related to cloud physics is atmospheric chemistry, which involves the chemical composition ofthe atmosphere and the life cycle and characteristics of its gaseous and particulate constituents. In view of the natural interdependence of the various aspects of cloud physics, the subject of microphysics cannot be discussed very meaningfully out of context. Therefore, we have found it necessary to touch briefly upon a few simple and basic concepts of cloud dynamics and thermodynamics, and to provide an account of the major characteristics of atmospheric aerosol particles. We have also included a separate chapter on some of the effects of electric fields and charges on the precipitation-forming processes.

This project conducts theoretical and computational studies of the physical transformation processes of aerosols which underlie their atmospheric formation, interaction, transport, and removal and derives results that contribute to improved capabilities for modelling aerosol physical and chemical evolution in support of the environmental component of the National Energy Strategy. The subject of study is submicrometer aerosol particles with primary focus upon the ultrafine fraction. This report summarizes technical progress during the first two and one-half years of the project. Results of calculations of equilibrium vapor pressures over adhering pairs of 50, 100, and 200 nm particles are reported showing substantial depression of equilibrium vapor pressure relative to isolated spheres. Calculations are given of collective, long-range intermolecular energies for irregular particles to be used for growth rate calculations for realistic particles. Molecular dynamic simulations of thermal collisions of small clusters with each other and with single atoms are presented as a function of cluster size in the range from 1 to 8 atoms. Calculations of aerosol condensation in which vapor depletion and heating effects are taken into account for atmospheric cloud nucleation modelling are reported.