Aero.solScience, 197t, Vol. 2, pp. 305 to 314. Pergamon Press,Printed in Great Britair~. NEW RESULTS IN BACKGROUND AEROSOLS STUDIES FROM THE ATLANTIC EXPEDITION OF THE R.V. METEOR, SPRING 1969" C. JUNGE and R. JAENICKE Max Planck-Institut ftir Chemie, Mainz, Germany (Received 26 Aoril 1971 Abstract--Long-term variations in background aerosols can change cloud formation and the global radiation budget. To improve our knowledge of these aerosols, measurements of the aerosol size distribution have been perfomled on the Atlantic Expedition of the R.V. Meteor. To cover the size range of the background aerosols, several instruments -- designed for low particle concentrations -- were used. The total number concentration showed an average around 600 cm -3, thus twice as high as expected for background aerosol. The mean aerosol distribution indicates a concentration maximum around 0.001 t~ radius, supporting the idea of a dynamic equilibrium in the size distribution. Besides a relative maximum around 0"3 ~z, the distribution shows a smooth shape between 0"3 a and 100 ~. Air masses from the Sahara desert observed in mid-Atlantic showed a concentration increase between 0.1 and 10 tz. INTRODUCTION: BACKGROUND AEROSOLS IN THE TROPOSPHERE IN TIMES when we are increasingly concerned with problems of global changes of our environment, studies of the natural background conditions are of basic importance. One crucial area of research is the atmosphere. Although the atmosphere is an essential part of man's environment we know very little about the composition of ' clean air '. Studies of ' clean air' have become urgent because man is modifying the atmosphere at a rate and to an extent such that the natural conditions may soon be destroyed; it will then become impossible for future generations to determine the preindustrial conditions and to estimate the inflicted changes. In this paper we will discuss only atmospheric background aerosols and present some new aerosol data obtained in the Spring of 1969 during the Atlantic Expedition of the R.V. Meteor. Background aerosols are defined as the aerosols which are affected only slightly by anthropogenic activities and are far away from local natural sources. Recent studies on electrical conductivity (COBB and WELLS, 1970) indicate that the total con- centration of aerosols over most parts of the North Atlantic has increased by a factor of about two since the beginning of this century, an increase probably due to pollution from North America. Aerosols in this region, therefore, can no longer be considered background aerosols. Because our results inciude measurements from other parts of the ocean, we feel that our observations and conclusions are still applicable. Data collected on local particles sources over continent (dry dust) and sea (sea-salt), indicate that background aerosols *A preliminary version of this paper was presented at the Colloquium on the Physics and Chemistry of Aerosols, Fontenay-aux-Roses, 1970; Journal of Aerosol Science Vol. 2, No. 2 1971. 305