van der Graaf, SC; Kranenburg, R; Segers, AJ; Schaap, M; Erisman, JW (2020). Satellite-derived leaf area index and roughness length information for surface-atmosphere exchange modelling: a case study for reactive nitrogen deposition in north-western Europe using LOTOS-EUROS v2.0. GEOSCIENTIFIC MODEL DEVELOPMENT, 13(5), 2451-2474.

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 (N-r) 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 N-r flows. In these models, the parameterization of the atmosphere-biosphere exchange of N-r 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 (z(0)) input maps. As LAI, we use the Moderate Resolution Imaging Spectroradiometer (MODIS) MCD15A2H product. The monthly z(0) 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 z(0) values are updated, too, and linked to a population density map. The approach to derive these dynamic z(0) estimates can be linked to any land use map and is as such transferable to other models. We evaluated the sensitivity of the modelled N-r deposition fields in LOng Term Ozone Simulation - EURopean Operational Smog (LOTOS-EUROS) v2.0 to the abovementioned changes in LAI and z(0) inputs, focusing on Germany, the Netherlands and Belgium. We computed z(0) values from FLUXNET sites and compared these to the default and updated z(0) 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 z(0)) to 0.60 (updated z(0)). The implementation of these updated LAI and z(0) input maps led to local changes in the total N-r deposition of up to similar 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.