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|Title:||Geochemical Characterization of Coastal Precipitation: Natural versus Anthropogenic Sources|
|Authors:||Wadleigh, Anne Marie|
|Abstract:||<p>Chemical, isotopic and air mass back trajectory analyses were combined with statistics on 39 coastal rain events at Shelburne, Nova Scotia to obtain specific information on the sources of sulphate in precipitation at remote sites.</p> <p>δ ³⁴S (SO₄) was found to be highly correlated with the measured sea spray component. δ ¹⁶O (SO₄) did not correlate with sea spray and only related to δ ¹⁶O (H₂O) on a seasonally weighted basis.</p> <p>The chemistry of a particular rain event was related to the provenance of its corresponding air mass as determined from the back trajectory; particularly the marine versus continental character. In some cases the air masses were vertically homogeneous and the sulphate well-mixed. In other cases, considerable vertical shearing of the air mass was observed and the sulphate appeared to have been derived from a specifIc pressure level.</p> <p>Seasonal variations were observed in much of the data. Percent sea spray values were higher in the winter than in the summer as were δ ³⁴S (SO₄). Air mass back trajectories supported a different origin for summer versus winter excess sulphate. δ ¹⁶O and δ ³⁴S (SO₄)* (where * indicates excess sulphate, that is, due to sources other than sea spray) were lower in the winter than in the summer as was δ ¹⁶O (H₂O).</p> <p>Most variables were separated into "marine" and "continental" indicators using factor analysis. Only the oxygen isotope data could not be included in either of these groups due to a mixture of controlling mechanisms.</p> <p>In general, the Shelburne rain samples are described by a two endmember mixing model consisting of the continent and the ocean. The continental endmember has a sulphur isotopic composition of approximately 4 ‰ and represents a steady-state value for the reservoir of continental atmospheric sulphate influenced by both anthropogenic and natural contributions.</p> <p>Under certain circumstances, more specific sources can be identified. Two subsets of the data are of special importance in this regard. The first is a group of samples whose air masses recently centred over the Bay of Fundy. The σ ³⁴S (SO₄)* values averaged 3 ‰ and the σ ¹⁶O (SO₄)* are correlated to their corresponding δ ¹⁶O (H₂O) values. These represent locally formed sulphate and are probably derived from oxidation of reduced sulphur gases commonly released from tidal areas. The other subset is a group of seven samples which contain a sulphate component comprising between 16 and 36% of the total sulphate which has a suIphur isotopic composition near 20 ‰ and an oxygen composition ranging from -2 to 5 ‰. It is suggested that this component represents a contribution by dimethyl sulphide (DMS) to the sulphate of Shelburne rainwater derived from the zone of high biological productivity nearby.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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