BACKGROUND POLLUTION IN THE ARCTIC AIR MASS AND ITS RELEVANCE TO NORTH AMERICAN ACID RAIN STUDIES L.A. Barrie Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario, Canada, M3H 5T4 ABSTRACT. The Arctic air mass is a unique meteorological feature of the northern hemispheric atmosphere. Possessing well-defined meteorological characteristics, it occupies not only the polar region but also a large fraction of the Canadian and Eurasian land masses during the period November to April. Poor pollutant removal by precipitation and dry deposition within the air mass and a strong transport pathway between Eurasian mid-latitudinal sources and the north, result in elevated levels of acidic anthropogenic aerosols and gases in the air mass during winter. In summer, weak north/south transport and strong pollutant removal between the Arctic and mid-latitudes and within the Arctic, results in lower airborne concentrations of acidic pollutants. Due to the presence of the relatively polluted Arctic air mass, 'background' air concentrations of S0~, S02 and total N0~ are elevated in western Canada during winter. Typical mean monthly concentrations from December to March are 0.8 to 2.1, 1.O to 2.4 and O.1 - 0.6 ~g m-3, respectively. In the absence of the neutralizing influence of alkaline soil dust, the acidity of snow forming in western Canada during winter is expected to range from 5 to 20 ~eq ~-l. i. INTRODUCTION The North American airshed east of the Rockies between 30 and 60*N is a convergence zone for three air masses that originate in the Arctic; the Pacific and the Carribean (graphically illustrated in Figure 1). From the point of view of the acid deposition problem, these air masses provide the 'background' to man-made pollution on the continent. In meteorological terminology, an air mass is a large body of air which is approximately horizontally homogeneous in its moisture content and vertical distribution of temperature. The meteorological characteristics are acquired primarily from the source region (in this case the Arctic) and seco~ffarily, from the modifying influence to which the air mass is subjected after leaving its source region. The ~e~cemtration of trac~ constituents in this air is uniquely determined in t~e regio~ of origin by a balance between natural and man-made Water, Air, and Soil Pollution 30 (1986) 765-777. © 1986 by D. Reidel Publishing Company.