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Vertical fluxes of air constituents and energy between phytosphere and atmosphere

Vertical fluxes of air constituents and energy between phytosphere and atmosphere

Eddy-Kovarianz-SystemClassical air pollution problems caused by very high concentrations of sulphur dioxide (SO2) and London-type smog have decreased to acceptable levels in most parts of Europe. Nevertheless, there are still a number of potential ecological threats such as acidification and eutrophication of terrestrial and aquatic ecosystems, increased tropospheric ozone (O3) concentrations and stratospheric ozone depletion, as well as greenhouse effects and human health problems caused by aerosols. Reactive atmospheric nitrogen species contribute to all these phenomena.

During the 1970s it was recognised that transboundary air pollution has ecological as well as economic consequences e.g. for the forest and fish industries. As a consequence, the countries of the UNECE (UN Economic Commission for Europe) developed a legal, organisational and scientific framework to deal with these problems. In 1979 the UNECE Convention on Long-Range Transboundary Air Pollution (LRTAP) was signed; it entered into force in 1983 (see www.unece.org/env/lrtap). In this context, the so called multi-effects or Gothenburg protocol requires the quantification - or at least estimation - of fluxes of atmospheric reactive nitrogen and sulphur species as well as of ozone between the ecosystems under consideration and the atmosphere near the ground.

Ideally, fluxes should be measured continuously and in an area-covering manner. Of course, this is not feasible. Another problem is that for some air constituents the toxicologically relevant flux is only a part of the total flux. Therefore modelling of fluxes has become a useful tool. Measurement and modelling techniques separate into two main categories, according to the type of species under consideration and their deposition properties: gases and fine particles (0.002 µm < dp < 2.5 µm, with dp the aerodynamic diameter of particles) on the one hand and coarse particles (dp ³ 2.5 µm) on the other hand. 'Particles' in this context may be solid or liquid (including rain and cloud drops). In general, fluxes of inert gases or fine particles are governed by turbulent diffusion in the atmosphere, by molecular diffusion within the (quasi-laminar) boundary layer adjacent to plant and soil surfaces, and by chemical reactions at the surfaces. In case of reactive gases or fine particles, also chemical reactions in the air have to be taken into account. Fluxes of very large particles (dp > 100 µm) are predominantly controlled by gravitational forces whereas fluxes of smaller particles (dp < 100 µm) are a result of diffusive, gravitational and inertial effects (interception, including impaction and turbulent inertial effects), depending on particle size and density. The following figure shows a separation of air constituents with respect to their deposition properties (particle size and mass, state).

atmospheric constituents

conceptual_separation_of_air_constituentsConceptual separation of atmospheric constituents with respect to their deposition properties (Grünhage et al. 1993)

 

Bulk deposition is the sum of wet-only deposition and of sedimenting dry particles.

Vertical fluxes of air constituents can be assessed by e.g. the following approaches (cf. Dämmgen et al., Environmental Pollution 134, 535-548, 2005):

  • Flow rate approach
    The mass flow density is determined as the product of the concentration of the air constituent and air flow through unit area.
    This approach is realized in the eddy covariance method, which is applied to assess vertical fluxes of trace gases and aerosols.
  • Potential-gradient approach
    The mass flow density is determined from a driving force gradient and the respective conductance per unit area (columnar conductance) by analogy with Ohm's, Darcy's and Fick's laws.
    This approach is the basis of the aerodynamic gradient method which is also applied to trace gases and aerosols, and serves as the basis for inferential modelling.
  • Surrogate surface approach
    The mass flow density into the receptor system under consideration equals the mass flow density into a surrogate system. The surrogate approach presupposes that the sink properties of the surrogate system are representative over time for those of the receptor system. In practice, this can be only achieved for particles whose deposition is totally independent of the receptor and surrogate system. This is only true for sedimenting particles, i.e. particles whose vertical movement is (in practice) only governed by gravitational forces.
    This approach is the basis of the methods to measure wet-only and bulk deposition.

For an overview see:

Grünhage, L., Haenel, H.-D. & Jäger, H.-J. (2000): The exchange of ozone between vegetation and atmosphere: micrometeorological measurement techniques and models. Environmental Pollution 109, 373-392.

Dämmgen, U., Grünhage, L. & Jäger, H.-J. (1997): Description, assessment and meaning of vertical fluxes of matter within ecotopes: a systematic consideration. Environmental Pollution 96, 249-260.