ECN publication
Ammonia exchange over coniferous forest
Published by: Publication date:
ECN 1996
ECN report number: Document type:
ECN-C--96-082 ECN publication
Number of pages: Full text:
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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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