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Sulfate is a major component of bulk atmospheric deposition ( including dust, aerosol, fog, and rain). We analyzed sulfur and oxygen isotopic compositions of water-soluble sulfate from 40 sites where year-round dust traps collect bulk atmospheric deposition in the southwestern United States. Average sulfur and oxygen isotopic compositions (delta(34)S and delta(18)O) are 5.8 +/- 1.4 (CDT) and 11.2 +/- 1.9 (SMOW) (n = 47), respectively. Samples have an oxygen 17 anomaly (Delta(17)O), with an average value of 1.0 +/- 0.6%. Except for a weak positive correlation between Delta(18)O and Delta(17)O values (r(2) approximate to 0.4), no correlation exists for delta(18)O versus delta(34)S, Delta(17)O versus delta(34)S, or any of the three isotopic compositions versus elevation of the sample site. Exceptional positive Delta(17)O values ( up to 4.23%) are found in samples from sites in the vicinity of large cities or major highways, and near-zero Delta(17)O values are found in samples close to dry lakes. Comparison of isotopic values of dust trap sulfate and desert varnish sulfate from the region reveals that varnish sulfate has average isotopic values that are similar to4.8% lower for delta(18)O, similar to2.1% higher for delta(34)S, and similar to0.3% lower for Delta(17)O than those of the present-day bulk deposition sulfate. Although other factors could cause the disparity, this observation suggests a possibility that varnish sulfate may have recorded a long-term atmospheric sulfate deposition during the Holocene or Pleistocene, as well as the differences between sulfur and oxygen isotopic compositions of the preindustrial bulk deposition sulfate and those of the industrial era.