Continuous measurement of PM_2.5 ion concentrations at IMPROVE sites

Several recent studies have demonstrated the utility of continuous measurements of aerosol composition. In order to better understand the nature and variability of the ionic fraction of PM_2.5 contributing to regional haze, several continuous measurement campaigns have been undertaken at IMPROVE monitoring sites over the past two years. Ion composition was measured at 15 minute intervals using a Particle Into Liquid Sampler (PILS) coupled to two Dionex ion chromatographs. Study sites included Yosemite National Park (July-Sept 2002), Bondville, Illinois (February 2003), San Gorgonio Wilderness Area, California (April and July 2003), Grand Canyon N.P. (May 2003), and Brigantine National Wildlife Refuge, New Jersey (November 2003). An additional campaign is scheduled for Great Smoky Mountains N. P. in summer 2004. PILS data are revealing interesting correlations between ion species and significant temporal variations in ion concentrations. For example, PILS data from Yosemite clearly show an association between nitrate and sodium due to the displacement of chloride from sea salt by reaction with nitric acid or its precursors. High time resolution measurements of potassium ion also help identify periods of fluctuating influence by local and regional fires. Observations from Bondville reveal rapid transitions between nitrate and sulfate dominated aerosols. San Gorgonio data reveal a strong diurnal change in the concentration of ammonium nitrate. Concentrations measured by PILS show reasonable agreement with concentrations measured by more traditional time-integrated filter pack techniques.