Deposition of atmospheric acid from local and distant sources at a rural site in Nova Scotia

Abstract

An analysis of local surface wind direction during the sampling of individual precipitation events at St. Margaret's Bay, a rural site in Nova Scotia, suggests that, during a one-year sampling period, approximately 50% of the wet deposition of hydrogen and sulphate ion was due to SO 2 emissions in Halifax, a city of 260,000 located 25 km to the east. Similar analyses at other sampling sites in Nova Scotia support this finding. However, the effect of Halifax appeared to be small at distances greater than 100km. The data also support a removal coefficient in rain of 2 × 10 −4 s −1 for sulphur emissions from Halifax. The estimated deposition of 2.9gSO 4 −2y −1 and 33 meqH + m −2y −1 coming in 1979 from directions other than Halifax are consistent with regional background values attributed to distant source regions. Back trajectory analyses on 850 mb charts indicated that about 65% of the deposition from distant source regions came from the eastern United States south of the Great Lakes, 31% from the Lower Great Lakes-St. Lawrence Valley region, and 4% from the regions of Canada outside the preceding area. The dry deposition of aerosol sulphate was only 0,035gSm −2y −1, or only 1% of the total estimated deposition; wet deposition was estimated to be the dominant mechanism. The relative contributions of each source region to wet and dry deposition appeared to depend upon a complicated interplay of source strength, distance to the receptor and meteorological factors such as frequency of transport flow and precipitation amount. It is very likely that strategies for the reduction of acid precipitation in Atlantic Canada must include not only a reduction of emissions in distant source regions, but, equally importantly, a reduction within the region itself.