Fine particle sulfate was measured continuously for 90 consecutive days, from August through October 1999, at Big Bend National Park. Measurements were made with a prototype integrated collection and vaporization cell, whereby particles are humidified and collected by impaction onto a metal strip and analyzed in place by flash-vaporization and pulsed fluorescence detection of the evolved sulfur dioxide. The time resolution was 12min. Sulfate values are compared to 24-h integrated filter measurements. Time variation in observed concentrations are compared to hourly measurements of dry particle size distributions and to nephelometry at ambient humidity. For most of the study period the sulfate was the major constituent of the accumulation mode aerosol. The sulfate scattering cross section as a function of relative humidity is inferred by comparisons among these measurements using a two-component fine particle model wherein the nonsulfate accumulation mode volume is attributed to nonhygroscopic components.