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Institute of Energy and Climate Research

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Aims of the project

Forests are complex sources of biogenic volatile organic compounds (VOC) in the planetary boundary layer. Previous studies estimate that global emissions of biogenic VOC are in the range of 490 Tg C/year to 1150 Tg C/year and thus a factor of 5 to 10 higher than anthropogenic emissions. Therefore, biogenic VOC contribute significantly to the formation of photo-oxidants in the troposphere. Due to fast vertical transport processes, they may even have an impact on the chemistry of the upper troposphere. Recent airborne measurements show surprisingly high mixing ratios of reactive compounds such as acetone, formaldehyde, methanol, hydrogen peroxide and organic hydroperoxides. These compounds are believed to originate from primary biogenic emissions or to be oxidation products of biogenic VOC.

All together, the impact of biogenic VOC on tropospheric photochemistry, air quality, and the formation of secondary products affecting our climate on a regional and global scale are far from being understood. A considerable lack of knowledge exists concerning a typical forest stand as a net source of reactive trace compounds, the amount of primary emitted VOC which are transported directly into the PBL, and the amount of VOC which are chemically processed within the canopy, the products of which are transported into the PBL. The gas phase chemistry above the canopy which is driven by high concentrations of reactive precursor compounds and high UV radiation is also not yet understood. The goal of the proposed project is to investigate these questions to improve our understanding of biosphere-atmosphere interactions and effects on the planetary boundary layer. This will be achieved by the investigation of the following themes:

  1. Sources of reactive trace gases in a mixed forest stand: Emission rates of VOC from the trees will be investigated as well as the role of the soil as a source and sink of reactive trace compounds.
  2. The forest stand as a chemical reactor: Chemical processes and formation of oxidation products of VOC (secondary VOC, photo-oxidants, organic aerosols) inside the forest stand will be studied.
  3. The forest stand as a source of biogenic VOC and their oxidation products: The emission of trace compounds (primary emitted VOC, oxidation products, photo-oxidants, aerosols) from the canopy to the planetary boundary layer will be investigated. The net fuxes of these compounds will be quantified.
  4. Atmospheric chemistry above the canopy: Chemical processes above the canopy, the influence of the emission of reactive compounds on these processes, and the formation of oxidation products will be investigated. Of primary interest is the influence of the vertical distribution of different trace gases on the chemical processing and the formation of photo-oxidants.