Spatial scale variability of NH3 and impacts to interpolated concentration grids

Melissa Puchalski1, Donna Schwede2, Kristen Foley3, Gregory Beachley4, John T. Walker5 and Sybil Anderson6

Over the past decade, reduced nitrogen (NH3, NH4) has become an important component of atmospheric nitrogen deposition due to increases in agricultural activities and reductions in oxidized sulfur and nitrogen emissions from the power sector and mobile sources. Reduced nitrogen is known to cause negative impacts on air quality (i.e. PM2.5 formation) and ecosystems (i.e. eutrophication, soil acidification), yet the fate and transport of reduced nitrogen from emissions sources is not well characterized. Ammonia is believed to be deposited locally around large emissions sources, but it may be deposited and re-emitted or react with acidic gases to form NH4+ particulates, allowing for transport and deposition much further downwind. NH3/NH4 may also alternate between particulate and gas phases with differing atmospheric and meteorological conditions greatly affecting the fraction which is transported or deposited.

To better characterize NH3 concentrations on a national scale, NADP’s AMoN began measuring bi-weekly NH3 concentrations in 2012 and has grown to nearly 100 monitoring sites. Many of the sites that began measuring in 2007 have shown an increasing trend in NH3 concentrations, but little is known about how concentrations vary between site locations.

A field experiment was conducted where ambient NH3 was measured using the AMoN passive samplers at 17 sites located around the Fort Collins, CO AMoN site (CO13) and 9 sites around the University of Illinois Bondville monitoring station (IL11). Samplers were deployed for 1-year resulting in ~10 samples per location. The two different locations were selected based on their difference in terms of land-use, source type, and the proximity of sampling locations to those sources. The Fort Collins area is heavily influenced by large CAFOs to the east in Weld county and complex terrain to the west (Rocky Mountain National Park). Bondville is surrounded mostly by crop fields and fertilization.

A spatial model was used to evaluate the bi-weekly variability between the samples in each region. The results are used to determine a reasonable radius of influence around an AMoN site. The calculated variability may be used to modify the radius of influence used in the total deposition hybrid maps developed by NADP’s TDEP committee. The current method for calculating a seasonal radius of influence for NH3 was derived from gridded CMAQ estimates of NH3 concentrations. Further work needs to be done to determine how local source impacts may be used to determine the radius of influence from AMoN in the TDEP interpolation method.