Ammonia exchange over coniferous forest

Surface exchange of ammonia (NH3) over a coniferous forest was measured for a period of more than 2 yr using the aerodynamic gradient technique. The results indicate that NH3 is efficiently deposited when the canopy is saturated with water. At night and when the canopy is dry, the NH3 flux is generally directed towards the canopy although emissions from the canopy are sometimes observed. The surface resistance (Rc) 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 NH3 is clearly bi-directional. However, strong differences are observed in the exchange of NH3 between 1993 and 1994: 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 leaf surface as well as the stomata. Preceding fluxes to the leaf surfaces may lead to accumulation of NH3 and to an increased resistance to deposition or even to emission, when the NH3 concentration in chemical equilibrium with the leaf surface exceeds the atmospheric NH3 concentration. It is hypothesized that the observed differences in frequency of emission between 1993 and 1994 are related to differences in the accumulation of NH3 on the leaf surface. When the net flux is directed towards the canopy and the canopy is dry, Rc 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 NH3.