Particulate sulfate in air was monitored daily from 15 June 1978 to 31 December 1979 at Whiteface Mountain, New York. The daily SO42- concentrations were related to surface-air trajectory ensembles to assess the relative contribution from the U.S. and from Canada. During the study period the site was influenced approximately equally by continental polar and maritime tropical air masses. However the maritime air masses from the U.S. were the principal conveyors of high SO42- concentrations at this site and transported about 4 to 5 times more SO42- than did the continental polar air masses from Canada.