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