Areal distribution of sulphur and oxygen isotopes in sulphate of rain overeastern North America

Three summer rainfall events were sampled simultaneously across a network o f 24 stations extending from the Mississippi River to the Atlantic seaboard and from the Gulf of Mexico to subarctic Canada. The chemical and isotopic compositions of the rain and its dissolved sulphate were measured on each sample. Each event was characterized by a different air mass back trajector y, suggesting a different source region for the transported sulphate. Two o f the air masses apparently originated in northern Canada, one near Great S lave Lake and the other in arctic Canada. The third air mass originated in the Gulf of Mexico. In spite of this, sulphur isotopic composition was very homogeneous over most of the network, averaging +3.41 +/- 0.95 parts per t housand, indistinguishable from the means of the individual events. This va lue is similar to that reported for long-range transported sulphate reachin g the coastal Atlantic site of Shelburne, Nova Scotia, and supports the exi stence of a continent-wide, well-mixed atmospheric reservoir of sulphur. No correlation was observed between percent sea spray and sulphur isotopic co mposition presumably because of the small (less than 10%) calculated sea sa lt contributions. Sulphate-oxygen isotopic compositions varied very widely from -0.7 to 18 parts per thousand. Two of the events apparently derived th eir sulphate through scavenging of aged sulphate aerosols. Sulphate-oxygen isotopes for the other event were correlated with those of the associated r ainwater, suggesting that sulphate from that event was oxidized within the transporting air mass, implying that sulphur was transported in the atmosph ere in an isotopically exchangeable form.