Long-term observations from 1980 to 1990 of aerosol methanesulfonate (MSA), sulfate, sodium, and other related chemical species as well as a shorter time series of aerosol sulfur isotope composition (Δ34S) at Alert, Northwest Territories, Canada, were used in two separate methods to determine the contributions of sea salt, biogenic sources, and pollution to aerosol SO4=. The first method, based on sulfur isotope composition, assumed a three-source model of anthropogenic, biogenic, and sea-salt sources for aerosol SO4=. Applying a Monte Carlo uncertainty propagation technique to the three-source model, the MSA/biogenic SO4= mass ratio was estimated to be between 0.2 and 0.6 in summer and much lower in October to March (<0.08). The second method, based on a multivariate statistical analysis technique, apportioned aerosol SO4= into three major components attributable to the same three sources and yielded an estimate of MSA/biogenic SO4= of 0.2 to 0.9 for the summer months, in agreement with the isotopic estimate. Based on the isotopic composition, aerosol SO4= in summer is 25 to 30% biogenic, 1 to 8% sea-salt, and the rest (62 to 74%) anthropogenic in origin. At other times of the year it is <14% biogenic, 1 to 8% sea salt, and the rest anthropogenic in origin. ¿ American Geophysical Union 1993