As gas phase atmospheric ammonia reacts with acidic aerosol particles
it affects the chemical, physical, and optical properties of the parti
cles. A knowledge of the source strengths of NH3 is useful in determin
ing the effect of NH3 on aerosol properties on a regional basis. Here,
an attempt is made to determine the direction and magnitude of the ai
r/sea flux of ammonia for the North Atlantic Basin from both measured
and modeled seawater and atmospheric ammonia concentrations. Previousl
y reported measured seawater concentrations range from less than 30 to
4600 nM with the highest concentrations reported for the Caribbean Se
a, the North Sea, and the Belgium coast. Measured atmospheric ammonia
concentrations range from 2 to 500 nmol m(-3) with the largest values
occurring over the Sargasso Sea, the Caribbean Sea, and the North Sea.
For comparison to the measurements, seawater ammonia concentrations w
ere calculated by the Hamburg Model of the Ocean Carbon Cycle (HAMOCC3
). HAMOCC3 open ocean values agree well with the limited number of rep
orted measured concentrations. Calculated coastal values are lower tha
n those measured, however, due to the coarse resolution of the model.
Atmospheric ammonia concentrations were calculated by the Acid Deposit
ion Model of the Meteorological Synthesizing Center (MSC-W) and by the
global 3-dimensional model Moguntia. The two models predict similar a
nnually averaged values but are about an order of magnitude lower than
the measured concentrations. Over the North Sea and the NE Atlantic,
the direction and magnitude of the air/sea ammonia flux calculated fro
m MSC-W and Moguntia agree within the uncertainty of the calculations.
Flux estimates derived from measured data are larger in both the posi
tive and negative direction than the model derived values. The discrep
ancies between the measured and modeled concentrations and fluxes may
be a result of sampling artifacts, inadequate chemistry and transport
schemes in the models, or the difficulty in comparing point measuremen
ts to time-averaged model values. Sensitivity tests were performed whi
ch indicate that, over the range of values expected for the North Atla
ntic, the accuracy of the calculated flux depends strongly on seawater
and atmospheric ammonia concentrations. Clearly, simultaneous and acc
urate measurements of seawater and atmospheric ammonia concentrations
are needed to reduce the uncertainty of the Bur calculations, validate
the model results, and characterize the role of oceanic ammonia emiss
ions in aerosol processing and nitrogen cycling for the North Atlantic
.