PNNL

Abstract

To improve understanding of the nature and variability of the ionic fraction of atmospheric fine aerosol particles in non-urban environments, one to two month measurement campaigns were conducted at several rural locations in the United States. Study sites included Yosemite National Park (NP) (July – September 2002), Bondville, Illinois (February 2003), San Gorgonio Wilderness Area, California (April and July 2003), Grand Canyon NP, Arizona (May 2003), Brigantine National Wildlife Refuge (NWR), New Jersey (November 2003), and Great Smoky Mountains NP, Tennessee (July/August 2004). PM2.5 ion composition was measured at 15-minute intervals using a Particle Into Liquid Sampler (PILS) coupled to two ion chromatographs. Comparison of PILS measurements with parallel traditional 24-hr denuder/filter-pack measurements reveal generally good agreement between the two techniques, although PILS measurements of PM2.5 NH4+ appear to be biased slightly low. High-time resolution aerosol concentration measurements provide much better estimates of the range of aerosol concentration levels experienced at these rural locations. Ratios of peak 15-min to 24-hr nitrate concentrations, for example, ranged from 1.7 at Brigantine NWR to 7.0 at the Great Smoky Mountains NP. A strong influence of diurnal upslope/downslope transport patterns was observed on aerosol concentrations at several locations, including Yosemite NP, San Gorgonio Wilderness Area, and Great Smoky Mountains NP, with peak concentrations typically occurring during afternoon upslope transport. High time resolution aerosol composition measurements also provide new insight into relationships between individual aerosol species and the influence of environmental conditions on aerosol composition. Observations at several locations revealed important information about mechanisms of particle nitrate formation. At Yosemite and Grand Canyon NPs, for example, evidence was observed for reaction of nitric acid or its precursors with sea salt or soil dust. Observations from several sites also revealed the importance of aerosol acidity (Great Smoky Mountains NP, Bondville) and temperature/humidity (San Gorgonio, Bondville) on fine particle ammonium nitrate formation.