Narragansett Bay, the largest estuary in the northeastern U.S., is a heavily urbanized watershed impacted by deposition and runoff. Nutrient budgets and local policy rely on deposition data from a 1990 study that did not include any direct observations of dry deposition of gaseous ammonia (NH3) and particulate ammonium (NH4+) due to uncertainties in their flux direction and measurement difficulty. Recent precipitation measurements show that wet deposition of NH4+ to the Bay has increased by a factor of 6 over the past three decades, leading to a 2.5-fold increase in wet nitrogen (N) deposition. The documented increase in wet deposition of NH4+ concurrent with managed nutrient reductions in urbanized estuaries has potentially increased the impacts of atmospheric N deposition, including the dry deposition of NH3 and NH4+. However, a scarcity of measurements hinders our interpretation of this important N source. For the first time over Narragansett Bay, and to our knowledge over open water, dry (particulate and gas phase) deposition of total NHx (NHx = NH3 + NH4+) and bidirectional NH3 flux was quantified using a relaxed eddy accumulation sampling technique. We find that total N entering the Bay from the atmosphere has doubled since 1990. Dry deposition of NHx comprises 9.6% of total N deposition. During the fall season, the dominant flux direction for NH3 is upward, which also has implications for urban air quality. We estimate that NH3 emitted from the Bay to the atmosphere makes up to 10% of the local NH3 emission budget.

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.