Flux of NO_x between Atmosphere and Vegetation — Parameterisation of Surface Resistance/Emission

Summary

Fluxes of gases including NO, NO₂, O₃ CO₂ and water vapour were studied with the enclosure technique in three different coniferous forest ecosystems with different air pollution loads and in the laboratory on seedlings, also with different exposures of air pollutants. The results have been expressed in the form of resistances (conductances) which will serve as a basis for a more general estimate to any given forest ecosystem in northern Europe.

The flux of NO at the forest floor (both up and down) is insignificant and can be neglected in Sweden and probably also at similar northern locations. The flux of NO to or from forest canopy is also small and can be neglected in comparison with the flux of NO₂. The flux of NO₂ to the forest canopy passes only through stomata and is, in southern Sweden, determined by stomatal resistance no additional internal resistance. This is valid for the whole concentration range investigated, from about 0.2 to more than 10 ppb. At a northern location this is true only at NO₂ concentrations above about 5 ppb. At this location and at lower NO₂ concentrations, an additional (internal) resistance is found. In model calculations, this additional resistance can be expressed as a compensation point, usually between 0.3 and 0.7 ppb, although the real mechanism can also be an internal resistance with no emission. Since the ambient summertime concentration of NO₂ in the region is about 1 ppb, the rate of uptake by the northern canopy is quite small. It is not yet well understood if the higher resistance is a geographical feature related to the lower pollution load, although there is some indirect evidence that this is the case. Deposition of NO₂ to the forest floor is important. Compared to the deposition to the canopy it is lower in southern Sweden but substantially higher at the northern site. The flux of O₃ to the canopy of Scots pine and Norway spruce can be described by stomatal conductance with no internal resistance, plus an external deposition.

The conductance for the external deposition is linearly related to the PAR flux with a coefficient of 0.0015 (mm s⁻¹) / (μE m⁻² s⁻¹), valid for projected needle area and was determined up to 2000 μE m⁻² s⁻¹ in laboratory experiments.

The flux of HNO₃ and NO₂ (p) to a coniferous forest was measured with a foliar extraction method using branches distributed throughout the canopy. The deposition to individual branches was fund to be highly variable depending on their locations in the canopy. Comparison with simultaneously measured air concentrations will give an estimate of the deposition velocity.