Dry deposition of reactive nitrogen to European ecosystems: a comparison of inferential models across the NitroEurope network

Abstract

Inferential models have long been used to determine pollutant dry deposition to ecosystems from measurements of air concentrations and as part of national and regional atmospheric chemistry and transport models, and yet models still suffer very large uncertainties. An inferential network of 55 sites throughout Europe for atmospheric reactive nitrogen (N<sub>r</sub>) was established in 2007, providing ambient concentrations of gaseous NH<sub>3</sub>, NO<sub>2</sub>, HNO<sub>3</sub> and HONO and aerosol NH<sub>4</sub><sup>+</sup> and NO<sub>3</sub><sup>−</sup> as part of the NitroEurope Integrated Project. <br><br> Network results providing modelled inorganic N<sub>r</sub> dry deposition to the 55 monitoring sites are presented, using four existing dry deposition routines, revealing inter-model differences and providing ensemble average deposition estimates. Dry deposition is generally largest over forests in regions with large ambient NH<sub>3</sub> concentrations, exceeding 30–40 kg N ha<sup>−1</sup> yr<sup>−1</sup> over parts of the Netherlands and Belgium, while some remote forests in Scandinavia receive less than 2 kg N ha<sup>−1</sup> yr<sup>−1</sup>. Turbulent N<sub>r</sub> deposition to short vegetation ecosystems is generally smaller than to forests due to reduced turbulent exchange, but also because NH<sub>3</sub> inputs to fertilised, agricultural systems are limited by the presence of a substantial NH<sub>3</sub> source in the vegetation, leading to periods of emission as well as deposition. <br><br> Differences between models reach a factor 2–3 and are often greater than differences between monitoring sites. For soluble N<sub>r</sub> gases such as NH<sub>3</sub> and HNO<sub>3</sub>, the non-stomatal pathways are responsible for most of the annual uptake over many surfaces, especially the non-agricultural land uses, but parameterisations of the sink strength vary considerably among models. For aerosol NH<sub>4</sub><sup>+</sup> and NO<sub>3</sub><sup>−</sup> discrepancies between theoretical models and field flux measurements lead to much uncertainty in dry deposition rates for fine particles (0.1–0.5 μm). The validation of inferential models at the ecosystem scale is best achieved by comparison with direct long-term micrometeorological N<sub>r</sub> flux measurements, but too few such datasets are available, especially for HNO<sub>3</sub> and aerosol NH<sub>4</sub><sup>+</sup> and NO<sub>3</sub><sup>−</sup>

TRANC – a novel fast-response converter to measure total reactive atmospheric nitrogen

“…Unlike wet deposition, which is widely monitored in regional networks of wet-only or bulk precipitation collectors, measurements of overall dry (turbulent) N r exchange fluxes have largely remained experimental and were limited to selected research sites and to measurement campaigns of typically a few days to a few months due to technical complexity and to the large equipment and operational costs involved (Flechard et al, 2011) except for multi-year NO y eddy flux measurements at Harvard forest presented in Munger et al (1996Munger et al ( , 1998 and in Horii et al (2006). This is due to the fact that dry N r exchange constitutes a variety of compounds with different exchange patterns and that in general, concentration and flux measurements of N r species are challenging from a metrological point of view (e.g.…”

“…Hesterberg et al, 1996;Flechard et al, 2011). The conversion of NH 3 , NO 2 and a mixture of both gases was tested by parallel sampling of calibration gases by the TRANC-CLD system and specific NH 3 and NO 2 analysers.…”

“…The dry deposition of N r typically contributes between one third and two thirds to the total atmospheric N deposition (Simpson et al, 2006). The environmental effects of excess atmospheric N r deposition to ecosystems may include soil acidification, eutrophication of water bodies, nutrient imbalances, leaching of base cation and nitrate, loss of biodiversity, direct toxicity to plants, increased N 2 O emissions, and inhibition of soil methane (CH 4 ) oxidation (Galloway et al, 2003;Erisman et al, 2007;Flechard et al, 2011).…”

Observations of reactive nitrogen oxide fluxes by eddy covariance above two midlatitude North American mixed hardwood forests

“…Estimates of nitrogen deposition therefore depend on models with accurate surface and micrometeorological parameters, and sufficient observations (or adequate modelling) of all the relevant species. However, due to a lack of direct observations, dry deposition model parameterizations are based on limited data sets and unconfirmed assumptions (Wesely and Hicks, 2000;Flechard et al, 2011). Moreover, mixing ratio observations are sparse and are rarely available for all the necessary species, requiring spatial interpolation and assumptions about the unknown contributions (Holland et al, 2005).…”

Evaluating the calculated dry deposition velocities of reactive nitrogen oxides and ozone from two community models over a temperate deciduous forest

“…Therefore, large uncertainties still exist in modeling V d . A recent study (Flechard et al, 2010) modeled the V d of inorganic reactive nitrogen species (i.e. NH 3 , NO 2 , HNO 3 , and HONO and aerosol NH 4 þ and NO 3 À ) over 55 monitoring sites throughout Europe, using four existing dry deposition models.…”