ALBERT

Description: 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 of 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. We here 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 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 appears to have potential for improvement, but cannot be recommended for use in large scale simulations in its present state due to large uncertainties. As an interim solution for improving flux predictions, we recommend to reduce the minimum allowed Rw and the temperature response parameter in the unidirectional model and to revisit the temperature dependent Γw parameterization of the bidirectional model.

Description: Recent advances in laser spectrometry offer new opportunities to investigate ecosystem-atmosphere exchange of environmentally relevant trace gases. In this study, we demonstrate the applicability of a quantum cascade laser (QCL) absorption spectrometer to continuously measure ammonia concentrations at a high time resolution and thus to quantify the net exchange between a semi-natural peatland ecosystem and the atmosphere based on the eddy-covariance approach. Changing diurnal patterns of both ammonia concentration and fluxes were found during different periods of the campaign. We observed a clear tipping point in early spring with decreasing deposition velocities and increasingly bi-directional fluxes that occurred after the switch from dormant vegetation to CO2 uptake, but was triggered by a significant weather change. While several biophysical parameters such as temperature, radiation, and surface wetness were identified to partially regulate ammonia exchange at the site, the seasonal concentration pattern was clearly dominated by agricultural practices in the surrounding area. Comparing the results of a compensation point model with our measurement-based flux estimates showed considerable differences in some periods of the campaign due to overestimation of non-stomatal resistances caused by low acid ratios. The total cumulative campaign exchange of ammonia after nine weeks, however, differed only in a 6 % deviation with 911 and 857 g NH3-N ha−1 deposition being found by measurements and modeling, respectively. Extrapolating our findings to an entire year, ammonia deposition was lower than reported by Hurkuck et al. (2014) for the same site in previous years using denuder systems. This was likely due to a better representation of the emission component in the net signal of eddy-covariance fluxes as well as better adapted site-specific parameters in the model. Our study not only stresses the importance of high-quality measurements for studying and assessing land surface-atmosphere interactions, but also demonstrates the potential of QCL spectrometers for continuous observation of reactive nitrogen species as important additional instruments within long-term monitoring research infrastructures such as ICOS or NEON.