PREFACE This report is one of a series of monographs on ambient air quality prepared by distinguished authorities in the field of environmental science under grants from the American Petroleum Institute. The first monographs to be issued will be concerned with what are now regarded as the major atmospheric pollutants. Additional papers will be published as they are completed. It is planned to extend this series to cover many of the minor air contaminants as well. Copies of the monographs may be purchased from the Institute at $1.00 per copy. Requests from non-profit institutions for complimentary copies will be considered.