Comparison of selenium and sulfur at remote sites throughout the United States

The Interagency Monitoring of Protected Visual Environments (IMPROVE) particulate monitoring network has been collecting aerosols for visibility apportionment at remote sites in the United States since 1988. The measurements include the major PM(2.5) components, such as sulfur, carbon, and nitrate, and trace elements, such as selenium. This paper will examine the relationship between the sulfurs and selenium concentrations at 61 sites for samples collected in the seasonal year 1993. Maps of mean sulfur and selenium measurements in summer 1993 and winter 1993-1994 indicate that there are well-defined regional patterns for both elements, with concentrations in the Appalachian region that are ten times those in areas of the Pacific Northwest. The S/Se ratios of means are relatively uniform across the United States, at around 2000 in summer and 1000 in winter, indicating a strong sulfurselenium relationship. The role of conversion of SO, to sulfate can also be deduced from the means. For individual samples taken during summer 1993, there is a high correlation between the two variables in the East, especially at sites in the Northeast, where the correlation coefficients (r(2)) are around 0.9. In the West, the correlation is much lower. This is attributed to fewer sources and differing emission factors.