Characteristics of aerosol nitrate at several IMPROVE monitoring sites

Nitrate comprises an important part of aerosol mass in non-urban locations during some times of the year. Properties of aerosol nitrate were characterized at several IMPROVE monitoring sites during a series of field studies. Study sites included Big Bend National Park, Texas (July-Oct, 1999), Yosemite National Park, California (July-Sept 2002), Bondville, Illinois (February 2003), San Gorgonio Wilderness Area, California (April and July 2003), Grand Canyon N.P., Arizona (May 2003), and Brigantine National Wildlife Refuge, New Jersey (November 2003). 24 hour average PM_2.5 nitrate concentrations and gaseous nitric acid concentrations were determined by annular denuder/filter-pack measurements. Nitrate size distributions were measured using a Micro-Orifice Uniform Deposit Impactor (MOUDI). Concentrations of PM _2.5 nitrate and other ions were also measured at 15 minute intervals using a Particle Into Liquid Sampler (PILS) coupled to two Dionex ion chromatographs. Nitrate was found predominantly in submicron ammonium nitrate particles during the Bondville, San Gorgonio (April), and Brigantine campaigns. Coarse mode nitrate particles, resulting from reactions of nitric acid or its precursors with sea salt or soil dust, were more important at Big Bend, Yosemite and Grand Canyon. PILS data from Yosemite clearly show an association between nitrate and sodium reflecting displacement of chloride from sea salt. PILS data also reveal other interesting correlations between ion species and significant temporal variations in ion concentrations. 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. An overview of findings from these studies will be presented.