Features of the atmospheric cycle of aerosol trace elements and sulphur dioxide revealed by baseline observations in Canada

Selected applications of baseline aerosol, SO₂ and deposition chemistry observations in Canada are reviewed to illustrate how new insight can be gained into features of the atmospheric pathway of trace substances such as sources, transformation and removal. A strong annual variation in Arctic aerosol concentration is a manifestation of particle residence times that are much longer in winter than in summer. Differences in the variation of SO₄ ⁼ and V concentrations in the Arctic winter are due to SO₂ oxidation. The mean rate of oxidation between November and April ranges from 0.04 to 0.25%/h and is a minimum in December, January and February. Br measured on filters in the Arctic peaks in concentration later (March and April) than anthropogenic particulate matter suggesting photochemical production. Acidity in Arctic aerosol and in glacial ice are correlated. The relationship yields a best estimate of acidity in the absence of anthropogenic influences of 5.8 mole/l. Coincident air and precipitation measurements of sulphur oxides indicate that on average in eastern Canada 60% of SO₄ ⁼ in rain originates from SO₂ oxidation in the storm. Trends in Arctic ice core acidity and SO₂ emissions in Europe are similar, that is, little variation in the first half of the century and a marked increase since the mid 1950's. This is consistent with meteorological and chemical evidence linking Arctic air pollution with Eurasian sources.