ATMOSPHERIC METHANE AND SULFUR-COMPOUNDS AT A REMOTE CENTRAL CANADIAN LOCATION

During summer and fall 1987, at a remote location in central Ontario, Canada, aerosol sulphate (SO42-), sulphur dioxide, aerosol methane sulphonic acid (MSA, CH3SO3H) and methane concentrations were measured. In addition, sulphur isotopic composition of aerosol SO42- was determined. The objective was to compare the concentration and composition of potentially biogenic constituents in air from a boreal forest and wetland region to the north with those in air from the more polluted region to the south. In general, SO2 and SO42- concentrations were low when air arrived from the north. The influence of the Wawa, Ontario, iron sintering plant and the Noranda, Quebec, smelters on atmospheric SO2 is revealed by the presence of several outlying high concentrations. The northern sector also provides air with the lowest MSA concentrations (median 1.6 ppt(v)). If an MSA/SO42- ratio of 0.065-0.4 for the products of dimethyl sulphide (DMS) oxidation is valid, the average amount of northern sector SO4(2-) that is of DMS origin is 3-18%, respectively. Regardless of wind direction, the S-34/S-32 isotope ratio of aerosol SO42- tended to decrease from June into summer/autumn (del S-34/S-32 of 4.1 vs 3.5, respectively). The median CH4 concentration was not significantly higher in the northern unpolluted wetland sector than in the more polluted southern sector. However, the variability of its concentration was higher in the southern sector.