Background
Turbulent Transport
The troposphere and more particularly the planetary boundary layer are in general turbulent. Turbulence in the boundary layer is responsible for most of the vertical mixing which takes place in this layer and so is crucial for the vertical transport of species (which may be gases, aerosol, or heat) to and from the earth’s surface. Without turbulent transport, species emitted at the surface would remain close to the surface and would not become well mixed in the atmosphere, or be transported for long distances. Turbulence in the boundary layer is mostly caused by surface roughness, and so is a function of roughness length and wind speed. In general the atmosphere is more turbulent closer to the surface, with turbulent structure being present on very small scales. As height increases so does the size of the turbulent eddies as the air becomes better mixed and the energy in the smaller eddies is dissipated. Turbulent transport of species leads to a concentration gradient close to the surface, and also concentration differences within the turbulent eddies between air moving upwards and air moving downwards. The quantity of a species being emitted from or deposited to a surface is referred to as a flux. Emission and deposition budgets may be calculated from averaged fluxes. The area of the earth’s surface which contributes to the concentration of a species at a particular point in the atmosphere is referred to as the fetch. The size of the fetch increases with increasing height and is more difficult to determine for complex terrains such as cities and forests.