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Title:
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Ammonia exchange over coniferous forest
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Author(s):
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Published by:
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Publication date:
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ECN
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1996
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ECN report number:
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Document type:
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ECN-C--96-082
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ECN publication
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Number of pages:
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Full text:
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29
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Download PDF
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Abstract:
Surface exchange of ammonia over a coniferous forest was measured for aperiod of more than two years using the aerodynamic gradient technique. The
results indicate that ammonia is efficiently deposited when the canopy is
saturated with water. At night and when the canopy is dry, the ammonia flux
is generally directed towards the canopy although emissions from the canopy
are sometimes observed. The surface resistance to deposition is dependent on
the relative humidity which determines the thickness of microscale water
layers present on the leaf surfaces. During the day exchange of ammonia is
clearly bidirectional. However, strong differences were observed in the
exchange of ammonia between 1993 and 1994: in 1993 emission of ammonia occurs
much more frequently than in 1994. The leaf surface is observed to be a sink
as well as a source for ammonia. Therefore the bidirectional nature of
ammonia exchange applies to the leaf surface as well as the stomata.
Preceding fluxes to the leaf surfaces may lead to accumulation of ammonia and
to an increased resistance to deposition or even to emission, when the
ambient ammonia concentration in chemical equilibrium with the leaf surface
exceeds the atmospheric ammonia concentration. It is hypothesized that the
observed differences in frequency of emission between 1993 and 1994 are
related to differences in the accumulation of ammonia on the leaf surface.
When the net flux is directed towards the canopy and the canopy is dry, the
surface resistance to uptake is much lower than the stomatal resistance. It
has been suggested that transpired water causes deliquescence of particles
that are preferentially deposited in the stomatal regions. This process may
then result in a partially wetted leaf surface under otherwise dry
conditions, promoting uptake of ammonia. 8 refs., 1 tab., 29 refs.
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