AMMONIA EXCHANGE OVER CONIFEROUS FOREST

Surface exchange of ammonia (NH3) over a coniferous forest was measure d for a period of more than 2 yr using the aerodynamic gradient techni que. The results indicate that NH3 is efficiently deposited when the c anopy is saturated with water. At night and when the canopy is dry, th e NH3 flux is generally directed towards the canopy although emissions From the canopy are sometimes observed. The surface resistance (R-c) to deposition is dependent on the relative humidity which determines t he thickness of microscale water layers present on the leaf surfaces. During the day exchange of NH3 is clearly bidirectional. However, stro ng differences are observed in the exchange of NH3 between 1993 and 19 94: in 1993 emission of NH3 occurs much more frequently than in 1994. The leaf surface is observed to be a sink as well as a source for NH3. Therefore the bi-directional nature of NH3 exchange applies to the le af surface as well as the stomata. Preceding fluxes to the leaf surfac es may lead to accumulation of NH3 and to an increased resistance to d eposition or even to emission, when the NH3 concentration in chemical equilibrium with the leaf surface exceeds the atmospheric NH3 concentr ation. It is hypothesized that the observed differences in frequency o f emission between 1993 and 1994 are related to differences in the acc umulation of NH3 on the leaf surface. When the net flux is directed to wards the canopy and the canopy is dry, R-c is much lower than the sto matal resistance. It has been suggested that transpired water causes d eliquescence of particles that are preferentially deposited in the sto matal regions. This process may then result in a partially wetted leaf surface under otherwise dry conditions, promoting uptake of NH3. (C) 1998 Elsevier Science Ltd. All rights reserved.