Measurements reported by Eldred and Cahill <1994> provide an extended record of fine-particle sulfur concentrations at Shenandoah National Park. Begun in 1982, the measurements were upgraded in 1986 to improve time resolution and again in 1988 to improve sensitivity and particle size discrimination. Standard hypothesis rejection tests indicate a statistically significant upward trend in measured concentrations from 1982 to 1992. However, similar tests also indicate statistically significant differences between measurements made by the different methods, even when the apparent trend is accounted for. Does the trend in measured concentrations reflect increasing atmospheric concentrations, or is it an artifact of evolving measurement methods? This paper frames the question in rudimentary Bayesian terms and shows that one's interpretation of the trend is sensitive to one's prior confidence in the measurements.¿ 1997 American Geophysical Union