Atmospheric ammonia measurements over a coastal salt marsh ecosystem along the Mid-Atlantic U.S.

Terrestrial-aquatic interfaces such as salt marshes, mangroves, and similar wetlands provide an optimum natural environment for the sequestration and long-term storage of carbon (C) from the atmosphere, commonly known as coastal blue carbon. There are over 4 million acres of salt marsh in the US and over half of these are along the east coast of the US. Due to anthropogenic activities, this area presents the greatest nitrogen (N) pollution problem in coastal ecosystems in the U.S. as part of atmospheric N deposition, runoff, and riverine export. Ammonia (NH₃) is the most abundant alkaline gas in the atmosphere. Agricultural intensification is the primary anthropogenic source of NH₃ leading to a doubling of reactive nitrogen (Nr) entering the biosphere. Despite this, there are limited atmospheric measurements of NH₃ concentrations in coastal areas along the east coast. The objective of this study is to advance our process-level understanding of NH₃ air-surface exchange over a tidal salt marsh at the Saint Jones Reserve (DE), which is part of the National Estuarine Research Reserve System (NERRs).

Continuous and high temporal resolution measurements of atmospheric NH₃ concentrations were measured using a cavity ring-down spectrometer, reporting 30 min concentration averages. These high temporal resolution measurements allowed the estimation of the average diurnal cycle of NH₃ fluxes using a new analytical methodology. Micrometeorological measurements were also measured using the eddy covariance system operated concurrently above the tidal marsh at the research site, which is part of the AmeriFlux network (US-StJ).

This pilot study represents one of the few atmospheric measurements of NH₃ over a tidal salt marsh in the eastern U.S. Such measurements are important to characterize the processes that influence the exchanges of NH₃ between the atmosphere and the aquatic surface and provide baseline data to form more reliable parameterizations to simulate NH₃ deposition and emissions in tidal salt marshes using surface-atmosphere transfer models.