Incidents of elevated nitrate in southern Ontario

Abstract

A study of nitrate in suspended particulate in the lower Great Lakes region of southern Ontario, Canada, during the period 1976–1977 has indicated that elevated levels (in excess of 9.9 μg m⁻³) occur simultaneously across the region on certain days. These days most frequently fall within the cold-season months of October to March. The most common weather situation on these days is the presence of an anticyclone over or to the east of southern Ontario.

Case studies of three periods in early 1977 during which widespread elevated nitrates occurred showed nitrate levels to be dependent upon the previous history of the air mass. When back trajectories showed that the air mass had previously moved across the lower Great Lakes, nitrate levels were high. Low nitrate levels were most common when the air mass had moved across the upper Great Lakes. The low variability of nitrate concentration among the monitoring network and similar history of air mass movement indicates that long range transport is, in part, responsible for elevated ambient nitrate levels.