An estimate of the importance of dry deposition as a pathway of acidic substances from the atmosphere to the biosphere in eastern Canada

The relative importance of dry and wet deposition of sulphate and nitrate over southeastern Canada is examined using daily air and precipitation chemistry observations made at 6 rural stations of the Air and Precipitation Monitoring Network (APN) during the period 1979 to 1982. Dry deposition was calculated from these air concentrations using deposition velocity information on land-use types, on atmospheric wind speed and stability and on dry deposition rates from over 80 published studies. Despite the large uncertainty in dry deposition estimates which is associated with the complexity of the process, valuable conclusions emerge. It is estimated that total annual deposition varies between 10 to 86 mmole m^-2 and 13 to 62 mmole m^-2 for SO⁼₄ and NO^-₃, respectively. To eastern Canada 22% and 30% of the total SO⁼₄ and NO^-₃ deposition is dry deposited, respectively. Depending on location, the contribution of SO₂ to the dry deposition of sulphur ranges from 37% to 78%. It tends to decrease with increasing distance from major sources. In eastern Canada, the best estimate of the fraction of the total dry deposition of nitrogen contributed by NO₂ and HNO₃ is in the range 50% to 60% and 31 % to 45%, respectively, depending on location. The uncertainty in the NO₂ estimate is large, approaching a factor of 3.5. Even if PAN dry deposition velocities are equal to that of NO₂ (they are expected to be lower), the contribution of PAN to total nitrate deposition is negligibly small. Both dry and wet deposition are episodic in nature. It is estimated that the top-20% deposition events yield 47% to 70% of the total deposition.