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Particles collected in the desert Southwest on Teflon filters using an annular denuder sampling system absorb SO2(g) in a subsequential exposure to SO2(g) in the laboratory if the concentration of fine particulate sulfate measured with a filter pack sampling system is greater than the concentration of sulfate measured with the annular denuder sampling system. If the concentrations of sulfate determined using the two sampling systems agreed, no absorption of SO2(g) by the denuder-collected particles occurs. These results show that SO2(g) may be collected by Southwest desert fine particles during sampling, resulting in measured concentrations of fine particulate sulfate that are higher than the correct concentrations. The magnitude of this filter pack sampling artifact, 0.1-0.4 mu g SO42-/m(3), is small. However, the artifact is important relative to the concentration of particulate sulfate present in the desert Southwest, typically 0.5-2 mu g SO42-/m(3). The magnitude of the filter pack sulfate sampling artifact is not related to either the atmospheric particulate sulfate or SO2(g) concentrations.