Characterizing ionic components of aerosol in rural environments: temporal variability, size distributions, and the form of particle nitrate

Abstract

Properties of the ambient aerosol were determined at several rural U.S. locations during a series of field studies. Study sites included Bondville, Illinois (February, 2003), San Gorgonio Wilderness Area, California (April and July, 2003), Grand Canyon National Park, Arizona (May, 2003), Brigantine National Wildlife Refuge, New Jersey (November, 2003) and Great Smoky Mountains National Park, Tennessee (July/August, 2004). Daily PM2.5 aerosol and trace gas samples were collected using a URG cyclone/annular denuder/filter pack sampling system. Additionally, semi-continuous measurements of PM2.5 aerosol composition were made at 15 minute intervals using a Particle Into Liquid Sampler (PILS) coupled to two ion chromatographs. A Micro Orifice Uniform Deposition Impactor (MOUDI) was used to collect 48 hr size-resolved aerosol samples in 10 particle size categories from <0.18 μm to >18 μm. Measured aerosol species included Cl-, SO42-, NO3-, Na+, NH4+, K+, Mg2+ and Ca2+. Measured trace gases included HNO3, NH3 and SO2. The aerosol measured at most locations was neutral, although acidic aerosol was observed on most days at Great Smoky Mtns NP and occasionally seen at Bondville, IL. Aerosol concentrations and composition observed at many of the study sites varied strongly in time, especially at elevated locations where mountain-valley winds frequently advected polluted air masses from lower elevations to the study site during day and returned cleaner air to the site in drainage flows at night. Ammonium and sulfate were found predominantly in submicron particles (mode peak typically 0.3 - 0.5 μm) at all sites. Nitrate was sometimes found in submicron particles and sometimes in coarse particles, with a mode peak at several micrometers. Formation of submicron ammonium nitrate was favored when temperatures were low and more than sufficient ammonia was present to neutralize fine particle sulfate, consistent with thermodynamic expectations. A predominance of ammonium nitrate was observed at Bondville, IL in winter and at San Gorgonio, CA in spring. Coarse mode nitrate was more important at Great Smoky Mtns NP in summer and at Grand Canyon in spring, similar to previous observations made at Big Bend and Yosemite National Parks in summer. During fall at Brigantine, NJ and during summer at San Gorgonio, CA, a mix of fine and coarse particle nitrate was observed. Formation of coarse particle nitrate occurs as a result of reaction of nitric acid or its precursors with sea salt or soil dust and is favored when conditions are hot and dry and/or ammonia availability is limited. The traditional 2.5 μm aerodynamic size cut used in routine monitoring networks to define a division between fine and coarse particle modes unfortunately results in frequent inclusion of the lower tail of a coarse particle nitrate mode into fine particle aerosol samples (PM2.5), confusing understanding of mechanisms controlling formation of aerosol nitrate. A division at 1 μm would provide a more natural division between these two distinct populations of particles. The relatively frequent occurrence of coarse particle nitrate observed in this study indicates a need to more closely examine common assumptions regarding the importance of ammonium nitrate at rural locations, to include pathways for coarse mode nitrate formation in regional scale air quality models, and to consider impacts of coarse particle nitrate on visibility, nitrogen deposition, and other issues.