What controls the discrepancy between biogenic emission/uptake and above-canopy fluxes of NO and NO2?
Christof Ammann1, Michael Kortner2, Axel Thielmann2, Udo Rummel2, Meixner Franz X.2
1 Research Station ART
2 Max Planck Institute for Chemistry
2 Max Planck Institute for Chemistry
O 4.5 in Forest biogeochemistry of reactive trace gases
07.10.2009, 11:40-12:05, Kutschenhaus
It is commonly acknowledged that the main source and sink processes for NOx (= NO+NO2) in vegetated ecosystems are (a) the soil emission of NO originating from nitrification and denitrification and (b) the deposition of NO2 via plant stomates and, to a minor degree, to outer plant and soil surfaces. Consequently, these processes are usually included in atmospheric chemistry models as simple surface parameterizations. However, the exchange of reactive trace gases at the atmosphere-biosphere interface is often influenced by several interacting processes with similar time scales including (photo-)chemical reactions. This is especially the case in forest canopies where transport mechanisms control to what extent emitted and deposited species are affected by these processes during transfer to or from the atmosphere. In order to investigate this problem, we performed surface exchange measurements on the NO-NO2-O3 triad in two contrasting forest ecosystems, a primary rainforest in Rondônia/Brazil (LBA-EUSTACH) and a mixed temperate forest in Jülich/Germany (AFO2000-ECHO). At both sites, the measurements addressed processes on various scales: (a) dynamic chambers for soil emission and deposition, (b) vertical concentration profiles throughout the canopy, and (c) net turbulent fluxes above (and within) the canopy. In addition, profiles of radiation, thermal stratification, and turbulence intensity were observed.In both canopies, soil emitted NO was rapidly oxidized by O3 to NO2. Consequently, only a fraction of NO emitted from soils reached the atmospheric boundary layer as either NO or NO2. During daytime over the temperate forest, a deposition flux of NO was observed despite the strong soil emission. And in contrast to the detected NO2 uptake by the foliage, a net NO2 emission was found above the canopy. This effect may be explained by the lack of NO2 photolysis within the dark canopy.With the experimental datasets, it was tested, to what extent the interacting canopy processes for NO and NO2 could be described by simple model approaches (that may be suitable for atmospheric chemistry models). Furthermore, the influence of various controlling parameters, especially the canopy residence time, on the discrepancy between biogenic sources/sinks and net canopy fluxes was analyzed.
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