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 1969 series covered what are regarded as the major atmospheric pollutants. The 1970 series deals with minor pollutants that are of particular interest at this time. It is planned to publish additional papers from time to time as public interest warrants. Copies of the monographs may be purchased from the Institute at $1.00 per copy. Requests from nonprofit institutions for complimentary copies will be considered.