Reductions in oxidized nitrogen and sulfur emissions have resulted in a shift in atmospheric chemistry and the need to reassess the primary species contributing to PM formation and total deposition. Reduced nitrogen (NHx) has replaced oxidized nitrogen as the dominate nitrogen compound in the atmosphere, contributing more than 50% in most areas and the contribution is expected to continue increasing. EPA is considering NHx in the secondary NAAQS review for the ecological effects of NOx/SOx/PM. Currently, monitoring networks across the U.S. provide ambient measurements of oxidized and reduced nitrogen species using different measurement methods and varying time-scales, but NHx (NH₃ + NH₄⁺) is not measured routinely in any of the national networks. The Chemical Speciation Network (CSN) measures ambient NH₄⁺ concentrations on a nylon filter following a MgO-coated denuder at more than 160 urban sites throughout the U.S using a MetOne SASS/Super SASS. The Interagency Monitoring of Protected Visual Environments network (IMPROVE) compliments the CSN, providing comparable PM_2.5 speciation data collected using the PM2.5 sampler with 4 channels at more than 100 sites located in rural areas including National Parks and Class I areas. IMPROVE does not currently measure NHx. The Clean Air Status and Trends Network (CASTNET) measures weekly concentrations of NH₄⁺ using a Teflon filter and most CASTNET sites are co-located with NADP’s Ammonia Monitoring Network (AMoN).

In this study EPA, in partnership with the National Park Service, deployed acid-impregnated filters in the Super SASS and IMPROVE PM sampler in Gainesville, FL and Duke Forest, NC for 6-months in 2017 to assess whether this modification could be implemented across the networks to measure NHx at more than 200 locations throughout the US. The Super SASS and PM samplers were co-located with CASTNET and AMoN, as well as URG annular denuder systems (ADS) as the reference method. Bias, calculated as the median difference between the ADS versus the Super SASS and ADS versus the PM sampler, was -0.03 and 0.09, respectively, at Duke Forest and -0.17 and -0.14, respectively, at Gainesville. NHx concentrations at both sites were below 2 µg m^-3 with low variability throughout the sample period.

Results from the 2017 study showed unexpected breakthrough on the ADS with high capture of NH₃ on the backup denuder. Further analysis of the filters was performed to determine the cation/anion ratio and additional testing of the URG ADS system was performed at the AIRS site on the EPA RTP, NC campus.