Investigation of Hourly Concentration Ratios of Ammonia Gas and Particulate Ammonium Using an Online Ion Chromatography Analyzer

The chemical composition of atmospheric nitrogen pollution has shifted from predominantly oxidized nitrogenous species (particulate NO₃^- and gaseous NOx) to reduced inorganic nitrogen (particulate NH₄⁺ and gaseous NH₃) (Li et al., 2016, Du et al., 2014). Understanding the particle formation chemistry and the transport of nonpoint source (i.e. fertilizer application, animal operations) emissions is key in assessing NH₃ bidirectional air-surface exchange, deposition, and subsequent ecological impacts. Currently, national air monitoring networks are situated for measuring integrated reduced nitrogen samples often collected for periods of a week or more. These measurements are typically filter-based and can suffer from volatilization losses especially for ammonium nitrate.

The EPA’s Clean Air Status and Trends Network (CASTNET) has operated co-located Monitor for Aerosols and Gases (MARGA) online analyzers at Beltsville, MD for a long-term period (spanning from July 2013 to July 2015 and including the Fall of 2016) in order to assess the precision of the analyzers as well as the efficacy of co-located weekly integrated filter pack measurements. The MARGAs hourly samples include both NH₄⁺ and NH₃ in addition to nitrate (NO₃^-), sulfate (SO₄^-2) and their acid gas precursors (HNO₃ and SO₂) and base cations (Ca⁺², Na⁺, Mg⁺², K⁺). The MARGAs sampled approximately 13-months of valid data, capturing all seasons at a site located in crop fields managed by USDA’s Agricultural Research Service. There is a need to better understand hourly NH₃:NH₄⁺ chemistry and a need to better characterize long-term measurements of these compounds that are deployed in national networks (CASTNET, NADP’s AMoN, IMPROVE). This communication will explore daily and seasonal trends along with analysis of episodic events of NH₃ and NH₄⁺ using the entire suite of MARGA measurements and co-located meteorological measurements.