Articles | Volume 16, issue 21
Research article
31 Oct 2016
Research article |  | 31 Oct 2016

Non-stomatal exchange in ammonia dry deposition models: comparison of two state-of-the-art approaches

Frederik Schrader, Christian Brümmer, Chris R. Flechard, Roy J. Wichink Kruit, Margreet C. van Zanten, Undine Zöll, Arjan Hensen, and Jan Willem Erisman

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 fluxes in the past, but their computational expense and challenging implementation into existing air quality models call for steady-state simplifications. 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 non-stomatal compensation-point (χw) model after Wichink Kruit et al. (2010), with NH3 flux measurements from five 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 (Γw) for bidirectional non-stomatal exchange models. Results indicate that the Rw-only model has a tendency to underestimate fluxes, while the χw model mainly overestimates fluxes, although systematic underestimations can occur under certain conditions, depending on temperature and ambient NH3 concentrations at the site. The proposed Γw 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 flux predictions, we recommend reducing the minimum allowed Rw and the temperature response parameter in the unidirectional model and revisiting the temperature-dependent Γw parameterization of the bidirectional model.

Short summary
We found a systematic mismatch of modeled and measured NH3 fluxes using two state-of-the-art dry deposition models and data from five sites in Europe. Results of our analysis indicate a too large minimum non-stomatal resistance and too strong temperature response in the parameterization of a unidirectional surface–atmosphere exchange scheme, as well as room for improvement on the emission potential parameterization of a bidirectional model, both directly impacting predicted NH3 exchange fluxes.
Final-revised paper