Seasonal ambient ammonia and ammonium concentrations in a pilot IMPROVE NHx monitoring network in the western United States

Abstract

Ammonia and ammonium are important atmospheric trace constituents that affect particulate matter concentrations and contribute to reactive nitrogen deposition. We refer to and measure the sum of ammonia and ammonium as NHx. To better understand concentrations of NHx in remote areas, the Interagency Monitoring of Protected Visual Environments (IMPROVE) fine particulate matter (<2.5 μm) sampler was modified to measure NHx at a subset of locations in the routine IMPROVE network. To sample NHx, an additional IMPROVE PM2.5 sampler was installed. Samples were collected on phosphorous acid impregnated cellulose filters held in polypropylene filter holders. While the standard IMPROVE filter holder is made of Delrin (polyoxymethylene, POM), reactions between collected NHx and formaldehyde released by phosphorous acid degradation of the POM holder produced substantial amounts of artifact methylamine, especially during warm sampling periods. This artifact did not occur with the new polypropylene holder design, and no methylamine was measured above the method detection limit of 0.003 μg/m3. Samples collected using the new IMPROVE NHx sampling system were evaluated against samples collected with a collocated URG annular denuder/filter-pack module for 6 weeks; the observed bias was −7%. The NHx monitors were deployed at a total of nine sites in the U.S. Rocky Mountain and Great Plains regions and to the east, and at Bondville, Illinois, from April 2011 to August 2012. Collocated samplers at Rocky Mountain National Park, Colorado, and Bondville, Illinois, demonstrated excellent measurement precision. The data revealed a pattern of increasing NHx concentrations in late spring/early summer (June) and a decrease in winter, starting in September for most of the sites. This pattern is consistent with expected seasonal patterns in agricultural emissions of ammonia. Sites closer to agricultural sources at Bondville and Cedar Bluff (Kansas), however, still exhibit quite abundant winter NHx, which may reflect continued local agricultural emissions trapped within a shallower winter boundary layer. A probable impact of wildfires on NHx concentrations was observed for Bandelier NM, Chiricahua NM, and Yellowstone NP during summer/fall 2011.