Organic forms of nitrogen (N) are an important component of atmospheric deposition but are not routinely measured. As summarized by Jickells et al. (2013), global datasets of precipitation chemistry indicate that water soluble organic nitrogen (WSON) contributes ~25% of the total N in wet deposition, on average. In the U.S, various studies show that annual averages of WSON range from <5% to ~30%. Generally, measurements of WSON in precipitation and aerosol are limited in North America, precluding development of a complete picture of the spatial and temporal patterns of the contribution of WSON to total nitrogen in wet and dry deposition at regional to continental scales.

The Clean Air Status and Trends Network (CASTNET) is EPA's long-term, rural monitoring network of approximately 90 stations that measure changes in air quality and assesses atmospheric deposition over broad geographic regions of the U.S. (https://www.epa.gov/castnet). CASTNET provides weekly concentrations of inorganic nitrogen species, but to this point has lacked the ability to report the total organic fraction.

A pilot study was conducted in 2020 to investigate the feasibility of quantifying water soluble total nitrogen and WSON on CASTNET Teflon filters, including assessment of potential effects from routine shipping, storage and handling of the filters. Bulk WSON in PM is calculated by measuring the concentration of WSTN and then subtracting the concentrations of the measured inorganic components (NH₄⁺, NO₂^-, and NO₃^-). A 27 site one-year study was conducted from November 2022 to November 2023 to assess seasonal and spatial patterns as well as to estimate the contribution and correlation of WSON to the total nitrogen deposition budget. Preliminary interpretation of data has been performed, and seasonality as well as impacts from extreme events such as forest fires can be identified. Averaged across all the sites, preliminary data show that WSON is about 15% of total N, with increases in late-spring and summer months, although results can differ at individual sites. Leveraging the existing sites and sampling media to capture the WSON fraction could offer a more complete assessment of shifts in aerosol chemistry and source impacts on air quality.

¹ WSP USA Environment & Infrastructure, Inc., Gainesville, FL

² U.S. Environmental Protection Agency, Durham, NC

³ U.S. Environmental Protection Agency, Washington DC

⁴ WSP USA Environment & Infrastructure, Inc., Jacksonville, FL

* Corresponding Author: nathaniel.topie@wsp.com