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Titel:
Non-stomatal exchange in ammonia dry deposition models: comparison of two state-of-the-art approaches
 
Auteur(s):
Hensen, A.; Schräder, F.; Brümmer, C.; Flechard, C.R. |; Wichink Kruit, R.J.; Zanten, M.C. van; Zöll, U.; Erisman, J.W.
 
Gepubliceerd door: Publicatie datum:
ECN Environment & Energy Engineering 31-10-2016
 
ECN publicatienummer: Publicatie type:
ECN-W--16-046 Artikel wetenschap tijdschrift
 
Aantal pagina's:
17  

Samenvatting:
Abstract. The accurate representation of bidirectional ammonia (NH3/ biosphere–atmosphere exchange is an important part of modern air quality models. However, the cuticular(or external leaf surface) pathway, as well as other non-stomatal ecosystem surfaces, still pose a major challenge to translating our knowledge into models. Dynamic mechanistic models including complex leaf surface Chemistry have been able to accurately reproduce measured bidirectional ?uxes in the past, but their computational expense and challenging implementation into existing air quality models call for steady-state simpli?cations. Here we qualitatively compare two semi-empirical state-of-the-art parameterizations of a unidirectional non-stomatal resistance (Rw) model after Massad et al. (2010), and a quasi-bidirectional nonstomatal compensation-point (w) model after Wichink Kruit et al. (2010), with NH3 ?ux measurements from ?ve European sites. In addition, we tested the feasibility of using backward-looking moving averages of air NH3 concentrations as a proxy for prior NH3 uptake and as a driver of an alternative parameterization of non-stomatal emission potentials (0w) for bidirectional non-stomatal exchange models. Results indicate that the Rw-only model has a tendency to underestimate ?uxes, while the w model mainly overestimates ?uxes, although systematic underestimations can occur under certain conditions, depending on temperature and ambient NH3 concentrations at the site. The proposed 0w parameterization revealed a clear functional relationship between backward-looking moving averages of air NH3 concentrations and non-stomatal emission potentials, but further reduction of uncertainty is needed for it to be useful across different sites. As an interim solution for improving ?ux predictions, we recommend reducing the minimum allowed Rw and the temperature response parameter in the unidirectional model and revisiting the temperature-dependent 0w parameterization of the bidirectional model.

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