Abstract
Abstract. The nitrogen cycle has been continuously disrupted by human activity over the past century, resulting in almost a tripling of the total reactive nitrogen fixation in Europe. Consequently, excessive amounts of reactive nitrogen (Nr) have manifested in the environment, leading to a cascade of adverse effects, such as acidification and eutrophication of terrestrial and aquatic ecosystems, and particulate matter formation. Chemistry transport models (CTMs) are frequently used as tools to simulate the complex chain of processes that determine atmospheric Nr flows. In these models, the parameterization of the atmosphere–biosphere exchange of Nr is largely based on few surface exchange measurement and is therefore known to be highly uncertain. In addition to this, the input parameters that are used here are often fixed values, only linked to specific land use classes. In an attempt to improve this, a combination of multiple satellite products is used to derive updated, time-variant leaf area index (LAI) and roughness length (z0) input maps. As LAI, we use the Moderate Resolution Imaging Spectroradiometer (MODIS) MCD15A2H product. The monthly z0 input maps presented in this paper are a function of satellite-derived normalized difference vegetation index (NDVI) values (MYD13A3 product) for short vegetation types (such as grass and arable land) and a combination of satellite-derived forest canopy height and LAI for forests. The use of these growth-dependent satellite products allows us to represent the growing season more realistically. For urban areas, the z0 values are updated, too, and linked to a population density map. The approach to derive these dynamic z0 estimates can be linked to any land use map and is as such transferable to other models. We evaluated the sensitivity of the modelled Nr deposition fields in LOng Term Ozone Simulation – EURopean Operational Smog (LOTOS-EUROS) v2.0 to the abovementioned changes in LAI and z0 inputs, focusing on Germany, the Netherlands and Belgium. We computed z0 values from FLUXNET sites and compared these to the default and updated z0 values in LOTOS-EUROS. The root mean square difference (RMSD) for both short vegetation and forest sites improved. Comparing all sites, the RMSD decreased from 0.76 (default z0) to 0.60 (updated z0). The implementation of these updated LAI and z0 input maps led to local changes in the total Nr deposition of up to ∼30 % and a general shift from wet to dry deposition. The most distinct changes are observed in land-use-specific deposition fluxes. These fluxes may show relatively large deviations, locally affecting estimated critical load exceedances for specific natural ecosystems.
Highlights
The nitrogen (N) cycle has been continuously disrupted by human activity over the past century (Fowler et al, 2015; Galloway et al, 2004, 2008), resulting in a doubling of the total reactive nitrogen (Nr) fixation globally and even a tripling in Europe
We distinguish three different main approaches: (1) z0 values that depend on forest canopy height, (2) z0 values that depend on the normalized difference vegetation index (NDVI) and (3) new z0 values for urban areas that depend on the population density map
We examined the direct effect of the updated leaf area index (LAI) and z0 values on the modelled dry Nr deposition, as well as the related indirect effect in modelled wet Nr deposition
Summary
The nitrogen (N) cycle has been continuously disrupted by human activity over the past century (Fowler et al, 2015; Galloway et al, 2004, 2008), resulting in a doubling of the total reactive nitrogen (Nr) fixation globally and even a tripling in Europe. NOx and NH3 affect air quality through their significant role in the formation of particulate matter, impacting human health and life expectancy (Lelieveld et al, 2015; Bauer et al, 2016; Erisman and Schaap, 2004). As Nr forms are linked through chemical and biological conversion in one another within the environmental compartments, one atom of N may even take part in a cascade of Nr forms and effects (Galloway et al, 2003) To minimize these adverse effects, effective nitrogen management and policy development require consideration of all Nr forms simultaneously
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