My background is the study of micrometeorological exchange processes, e.g., heat and water vapor exchange and its extension to trace gas exchange processes. The first experience concerning the representation of these processes, especially the dry deposition process, in global scale chemistry and climate studies some years ago, was a surprising one. The common applied approach at that time was to define constant removal rates over land and the oceans whereas my own experience was that large differences exist between different surface cover types and the seasons. Hence it seemed quite obvious to improve the representation of the dry deposition process in these models by implementation of a "state of the art" dry deposition parameterization for trace gases such as ozone and nitric acid. The impact in terms of relative changes in the concentrations and burden compared to the old approach turned out to be large and model validation showed reasonable agreement with measurements, which was quite a stimulating result. The gained knowledge with the development of the dry deposition scheme also yielded some new challenges that are still being explored, e.g., aerosol dry deposition and the bi-directional atmosphere-biosphere exchange of trace gases such as the oxidized nitrogen species (NOx). For these gases alternative formulations that explicitly consider the interactions between the emission and dry deposition fluxes through chemistry and turbulence explicitly are currently being developed and implemented in global scale models.  Extension of the surface exchange formulations to other trace gases and aerosols is rather limited due the small selection of trace gases (and aerosols) for which surface flux measurements are available. Moreover, the observations also seem to be biased towards vegetated surfaces in Europe and the USA, which makes the evaluation of the role of surface fluxes over non-vegetated surfaces and other regions sometimes rather speculative.

Within this presentation I will try to give a flavor of the related topics of the surface trace gas exchange  with a specific emphasis on the dry deposition process. The topics that I will discuss within the rather short timeframe are; the available measurements; the mechanism that controls the dry deposition process; bi-directional exchange fluxes; available dry deposition models for different scales and finally a rather crucial subject; the available sources of surface cover properties to constrain these models. In addition to these presented topics I will propose some points of discussion that address some of the uncertainties involved in the surface trace gas exchange process.  Hopefully, this overview and the discussion will reveal some of the controlling mechanisms of the dry deposition process; indicate the significance of this process for the budget of different trace gases and aerosols, and offer some guidelines for selecting a modelling approach to describe the atmosphere-biosphere exchange within a regional chemistry and climate model.