Atmosphere-biosphere trace gas exchange is often calculated in models without considering the intermittent character of the turbulent exchange. The turbulent exchange determines the residence time of emitted species, e.g. nitrogen oxides and hydrocarbons, within the canopy and consequently the exposure of these trace gases to a different chemical regime compared to that of the surface layer and higher up in the Planetary Boundary Layer (PBL). Moreover, the chemically produced species, e.g., peroxyacetylnitrate, and nitrogen dioxide, are removed by dry deposition, which is largely controlled by the biological activity. The intermittent character of the turbulent exchange leads to segregation of reacting trace gases, e.g., nitrogen oxide within the canopy and ozone in the PBL, during the quiescent periods. These quiescent periods are followed by bursts into the surface layer of trace gases, which have accumulated within the canopy during the quiescent period, as a result of gusts ventilating the canopy. The depth of penetration of the gusts into the canopy, the frequency at which these disturbances occur and the depth of mixing of these emitted trace gases into the PBL depends on the turbulent characteristics of the PBL. It is expected that the temporal and spatial average trace gas concentrations and fluxes considering the intermittent turbulent exchange are different compared to that for conditions where there is continuous mixing between the atmosphere and the biosphere, due to the non-linearity of some of the chemical interactions. The typical timescale of these atmosphere-biosphere interactions is relatively small compared to the timestep of the large scale models, e.g., 30 minutes for a typical global chemistry and climate model. A comprehensive integrated atmosphere-biosphere model of the exchange of trace gases, considering the canopy structure, hydrocarbon chemistry, extinction of radiation and turbulence, the biogenic emission of hydrocarbons and nitrogen oxides, and dry deposition, has been developed and implemented in a single column version of the Regional Atmospheric Climate Model (RACMO). The model is used to study specific features of atmosphere-biosphere trace gas exchange such as the impact of the canopy structure and the intermittent character of the turbulent exchange on the surface fluxes and concentration profiles. Parameterizations which account for these atmosphere-biosphere interactions are developed and implemented in a global and a regional chemistry and climate model. The latter will be applied to study surface trace gas exchanges in the Amazonian basin within the context of the Large scale Biosphere Atmosphere (LBA) project.
Symposium on Interdisciplinary Issues in Atmospheric Chemistry