Atmospheric decay distances and times for sulphur and nitrogen oxides estimated from air and precipitation monitoring in eastern Canada

Previous estimates of residence times for sulphur species have been made mainly from the overall box budget approach by considering the various deposition and conversion terms in a “static” reservoir. This paper will use a “dynamic” aproach by considering daily filter-pack and precipitation observations taken at several eastern Canadian monitoring sites over a period of several years. Some of these sites are lined up from southwest to northeast along the most frequent wind direction for pollutant transport episodes. Concentrations can thus be examined in the Lagrangian sense starting at the edge of the major North American emission region and ending at a remote site in Newfoundland over 2000 km away. Plots of concentration of the monitored species in air and precipitation against distance from the emissions region, show a good fit to an exponential curve. The decay distances, defined as the distances for the initial concentration to fall to 1/e of its original value, can thus be determined. These are in air: 480 km for SO₂, 1280 km for SO⁼₄, 720 km for total sulphur (SO₂ + SO⁼₄) and 540 km for total nitrate (HNO₃ + particulate nitrate). In precipitation the decay distances are 1700 km for SO⁼₄ and 1100 km for NO^-₃. Using mean velocities for southwesterly trajectories across the region, the decay distances can be converted to decay times. These are estimated for air concentrations to be: 14 h for SO₂; 58 h for SO⁼₄; 24 h for total sulphur and 16 h for total nitrate. In the case of SO₂ and total sulphur, the decay times are equivalent to residence times.