Sources of aerosol sulphate at Alert: Apportionment using stable isotopes

From July 1993 to September 1994, seasonal variations in the sources of SO4 2- aerosols in the Arctic lower atmosphere at Alert, Canada, (82 degrees 30 ' N, 62 degrees 20'W) were investigated using the sulphur isotope abundance of as little as 10 mu g of sulphur analyzed by combustion-flow isotope-rat io mass spectrometry. In conjunction with air mass trajectories and in para llel with measurements of aerosol composition, the sulphur isotope composit ion was used to discern sources of aerosol SO42-. Total SO42- is composed o f seasalt SO42-, marine biogenic, and nonmarine SO42-. From June through Se ptember the fraction of biogenic SO42- in the non-sea-salt (nss) component ranged from 0.09 to 0.40 with an average of 0.31 +/- 0.11. Summertime nonma rine SO42- is likely anthropogenic in origin since it is isotopically indis tinguishable from SO42- in the polluted winter/spring period of arctic haze (delta(34)S = +5 parts per thousand). In summer there was no significant d ifference in isotope composition of aerosol sulphate between air which rece ntly traversed Eurasia and the Arctic Ocean and air arriving from North Ame rica. In contrast to summer and late winter/spring, delta(34)S values for n onmarine SO42- in fall and early winter were often less than +5 parts per t housand. These isotopically light samples were divisible into two groups: ( 1) those associated with air mass trajectories potentially affected by Nort h American soils and/or smelters and (2) three weekly samples between Octob er and December which could be attributed to fractionated sea-salt aerosol formed on refrozen Arctic Ocean leads. For the latter the ratio of SO42-/Na was estimated to be a factor of 3.6 lower than in bulk seawater. From Nove mber to May, nonmarine aerosol SO42- was apportioned into 10 aerosol compon ents using positive matrix factor analysis of 18 aerosol ions and trace ele ments [Sirois ann Barrie, this issue]. In turn, a multiple linear regressio n of delta(34)S values against the scores of the components was used to pre dict the isotope composition of six components. It was concluded that the m ain mass of anthropogenic SO42- had a delta(34)S value near +5 parts per th ousand and that biogenic SO42- had a delta(34)S Of +16 +/- 3.9 parts per th ousand. Reasonable agreement between model results and sulphur isotope meas urements at Alert show that SO42- apportionment using positive matrix facto r analysis is a reasonable approach which gives realistic results.