SULFATE SAMPLING ARTIFACT FROM SO2 AND ALKALINE SOIL

A significant sulfate artifact may be produced by the interaction of S O2(g) with basic particles on filters when the SO2 is not first remove d by a denuder. Such an artifact could invalidate most historical sulf ate measurements in regions with alkaline soil. To test this hypothesi s, we compare measurements from Teflon filters without a denuder to me asurements from nylon filters following a carbonate denuder from the I nteragency Monitoring of Protected Visual Environments (IMPROVE) netwo rk. The data at 11 remote sites in the Desert Southwest show no eviden ce of a sulfate artifact. Using a normal error distribution, we show t hat any artifact is much less than 20 ng/m(3). In addition, data from 16 collocated samplers at Meadview, AZ, show that removing the SO2 by denuders does not change the measured sulfate. The mean difference bet ween no-denuder and denuder is -4 +/- 10 ng/m(3) for Teflon filters an d 3 +/- 17 ng/m(3) for nylon filters. Brigham Young University obtaine d large differences between their denuder and filter pack samplers at Meadview. Our analysis shows that these differences are explained bett er by a bias in their denuder sampler than by an SO2 artifact. Our ana lysis also shows that their measurements are too imprecise to determin e an artifact less than about 500 ng/m(3).