Measurements and model results of a two-year dataset of ammonia exchange over a coniferous forest in the Netherlands

Abstract

In this study we present and analyse a two-year dataset of NH${}_3$ exchange over a temperate Douglas fir forest in the Netherlands. The atmospheric NH${}_3$ concentration ([NH${}_3$]) was measured at multiple heights above the canopy in 2009 and 2010. We applied the aerodynamic gradient method combined with four different methods for roughness sublayer correction to calculate fluxes. The results with and without this correction were on average similar, but instantaneous differences can be up to 30%. We evaluated a 1-D inferential model (DEPAC). The reference run tended to overestimate deposition and did not predict emission. The observed stomatal emission potential (${\Gamma }_s$) agrees well with values from literature and the modelled relation in DEPAC. The model performance strongly improved after implementation of a temperature dependent scaling factor in the external leaf pathway. We estimated the annual deposition load by combining observed and modelled fluxes and subsequent extrapolation of the mean (median) flux to be $11.8\pm 3.5$ ($8.5\pm 2.6$) kg N ha⁻¹ in 2009 and $11.4\pm 3.4$ ($8.7\pm 2.6$) kg N ha⁻¹ in 2010. Compared to historical measurements in the nineties at the same site, the [NH${}_3$] has stayed approximately constant and the deposition has decreased. Further research has to be done to better quantify these trends and to assess how the newly proposed external leaf pathway in DEPAC behaves in large scale transport models.