The one-dimensional multilayer canopy-chemistry model model CACHE calulates the vertical transport of heat, water vapor, and chemical trace species in the atmosphere between the soil surface and about 3 km and heat and water transport in the soil. Chemical transformation rates are described with the RACM gas phase chemistry mechanism (Stockwell et al., 1997, JGR 102). The RACM mechanism includes oxidation schemes for isoprene, α-pinene, and d-limonene.
Leaf surface temperatures and heat and water vapor flux between the leaves or needles and the surrounding air are calculated for each model layer by solving the energy balance for each canopy layer. The short wave and long wave radiative flux within the canopy are computed according to Norman (Norman, 1979, American Soc. of Agricult. Eng., Monograph No. 2). The required photolysis frequencies above the canopy are modelled according to Madronich (1987, JGR 92). Inside the canopy the photolysis frequencies are parameterized on the basis of the computed decrease of the photosynthetic active radiation. The stomatal resistances are described according to Jarvis (1976, Phil. Trans. R. Soc. B273).
Emissions of isoprene, monoterpenes, and other VOC by the leaves are modelled in the standard version of CACHE according to Guenther et al. (1995, JGR 100) and Steinbrecher et al. (1999, European Communities, EUR 18910 EN, ISBN 92-828-6990-3), taking into account the predicted leaf temperatures and PAR fluxes in the canopy. Alternatively, the process based isoprene emission module SIM-BIM (Zimmer et al., 2003, Atmos. Environ. 37) can be applied. Emissions of NO from the soil surface are taken as temperature and soil water dependent. The deposition of trace gases at the soil and leaf surfaces is described according to Wesely (1989, Atmos. Environ. 23).
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| Difference between simulated monoterpene flux from vertically integrated emissions on branch basis (potentiel flux) and the fluxes fluxes at different heights indicates the effect of in-canopy processes on effective emission fluxes | Ratio between potential fluxes and fluxes at canopy top for monoterpenes with one double bond (API) and monterpenes with two double bonds (LIM) for a pine forest |
The model permits the interpretation of the observed diurnal course of ozone and VOC by investigating the role of turbulent exchange, chemical formation and degradation, emission, and deposition during the course of the day. The simulation results show that the fluxes of isoprene and α-pinene into the atmosphere are 10 - 15 % lower than the potential emission fluxes on branch basis due to chemical BVOC degradation within the canopy. The flux of limonen can be reduced by up to 30 % as compared to the potential flux. BVOC degradation by the NO3 radical was found to occur in the lower part of the canopy also during daytime.
More results (Poster)
Reference
Forkel, R., Klemm, O. Graus, M., Rappenglück, B., Stockwell, W.R., Grabmer, W., Held, A., Hansel, A, Steinbrecher, R. (2006) Trace gas exchange and gas phase chemistry in a Norway spruce forest: A study with a coupled 1-dimensional canopy atmospheric chemistry emission model, Atmos. Environ., 40, S1, S28-S42.
Contact: Renate Forkel