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

Three summer rainfall events were sampled simultaneously across a network of 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 trajectory, suggesting a different source region for the transported sulphate. Two of the air masses apparently originated in northern Canada, one near Great Slave 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‰, indistinguishable from the means of the individual events. This value is similar to that reported for long‐range transported sulphate reaching the coastal Atlantic site of Shelburne, Nova Scotia, and supports the existence of a continent‐wide, well‐mixed atmospheric reservoir of sulphur. No correlation was observed between percent sea spray and sulphur isotopic composition presumably because of the small (less than 10%) calculated sea salt contributions. Sulphate‐oxygen isotopic compositions varied very widely from −0.7 to 18‰. Two of the events apparently derived their sulphate through scavenging of aged sulphate aerosols. Sulphate‐oxygen isotopes for the other event were correlated with those of the associated rainwater, suggesting that sulphate from that event was oxidized within the transporting air mass, implying that sulphur was transported in the atmosphere in an isotopically exchangeable form.