L.L. Sørensen

Atmospheric input of nitrogen into the North Sea: ANICE project overview.

The aim of the atmospheric nitrogen inputs into the coastal ecosystem (ANICE) project is to improve transport-chemistry models that estimate nitrogen deposition to the sea. To achieve this, experimental and modelling work is being conducted which aims to improve understanding of the processes involved in the chemical transformation, transport and deposition of atmospheric nitrogen compounds. Of particular emphasis within ANICE is the influence of coastal zone processes. Both short episodes with high deposition and chronic nitrogen inputs are considered in the project. The improved transport-chemistry models will be used to assess the atmospheric inputs of nitrogen compounds into the European regional seas (the North Sea is studied as a prototype) and evaluate the impact of various emission reduction strategies on the atmospheric nitrogen loads. Assessment of the impact of atmospheric nitrogen on coastal ecosystems will be based on comparisons of phytoplankton nitrogen requirements, other external nitrogen inputs to the ANICE area of interest and the direct nitrogen fluxes provided by ANICE. Selected results from both the experimental and modelling components are presented here. The experimental results show the large spatial and temporal variability in the concentrations of gaseous nitrogen compounds, and their influences on fluxes. Model calculations show the strong variation of both concentrations and gradients of nitric acid at fetches of up to 25km. Aerosol concentrations also show high temporal variability and experimental evidence for the reaction between nitric acid and sea salt aerosol is provided by size-segregated aerosol composition measured at both sides of the North Sea. In several occasions throughout the experimental period, air mass back trajectory analysis showed connected flow between the two sampling sites (the Weybourne Atmospheric Observatory on the North Norfolk coast of the UK and Meetpost Noordwijk, a research tower at 9km off the Dutch coast). Results from the METRAS/SEMA mesoscale chemistry transport model system for one of these cases are presented. Measurements of aerosol and rain chemical composition, using equipment mounted on a commercial ferry, show variations in composition across the North Sea. These measurements have been compared to results obtained with the transport-chemistry model ACDEP which calculates the atmospheric inputs into the whole North Sea area. Finally, the results will be made available for the assessment of the impact of atmospheric nitrogen on coastal ecosystems.

Deposition of nitrogen into the North Sea

The flux of nitrogen species from the atmosphere into the ocean, with emphasis on coastal waters, was addressed during the ANICE project (Atmospheric Nitrogen Inputs into the Coastal Ecosystem). ANICE focused on quantifying the deposition of atmospheric inputs of inorganic nitrogen compounds (HNO3, NO3-, NH3 and NH4+) into the North Sea and the processes governing this deposition. The Southern North Sea was studied as a prototype. Because the physical and chemical processes are described, as opposed to empirical relations, the results can potentially be transferred to other regional seas like the Mediterranean, the North Atlantic continental shelf area and the Baltic. Two intensive field experiments were undertaken, centred around the offshore tower Meetpost Noordwijk and the Weybourne Atmospheric Observatory in East Anglia (UK). Long-term measurements were made on a ferry sailing between Hamburg and Harwich/Newcastle. These measurements provided data for sensitivity studies of a variety of problems associated with the coastal region that are not easily evaluated with larger scale models, to constrain models and to test model results. Concentrations of nitrogen compounds over the North Sea and the resulting deposition presented in this paper were obtained with the Lagrangian transport-chemistry model ACDEP. The average annual deposition in 1999 was 906kg Nkm-2. The results are compared with experimental data from the ferry. Effects of temporal and spatial variations are evaluated based on experimental results and small-scale model studies. In particular, effects of the aerosol size distribution on the nitrogen deposition are discussed.

Upward fluxes of particles over forests: when, where, why?

Of the 60% of particle number fluxes over two forests that exceed the associated uncertainty bounds, approximately one-third are upward. These ‘apparent emission’ fluxes are not solely observed during periods when other micrometeorological fluxes are ill-defined, which implies they derive from a/multiple physical cause/s. Upward fluxes are slightly more frequent at night over the Danish beech forest but do not depend on wind direction or speed. Data from the pine forest in Finland indicate no diurnal cycle in the frequency with which upward fluxes are observed, although as in data from the beech forest the magnitude of upward fluxes is higher during the day. At the pine forest local emissions may account for some of the upward fluxes but other mechanisms appear also to play a role. Entrainment of particle depleted air from above the boundary layer, analysed via use of quadrant analysis and scalar correlations, appears to be important in the occurrence of upward fluxes at both sites. The rate of upward fluxes scales with prevailing geometric mean diameter (GMD) and consistent with the hypothesis of entrainment of relatively particle-depleted air upward fluxes appear to be associated with particle ensembles characterized by larger prevailing GMD.

Atmospheric nitrogen inputs into the coastal ecosystem (ANICE): the southern North Sea as a study area

The Atmospheric Nitrogen Inputs into the Costal Ecosystem (ANICE) project was initiated to improve the accuracy and performance of model tools used to estimate the atmospheric nitrogen deposition to the sea. The experimental work includes the use of a ferry for long-term measurements of gaseous species, aerosols and rain water during one year, and two intensive field campaigns. During the first ANICE intensity experiment, situations were encountered in which the air flow connected the different stations. Differences in nitrate and ammonium concentrations between the various stations were analyzed in terms of the processes determining the concentrations of these species and to quantify their variations with fetch.

Atmospheric Pollution Research on Greenland

Danish studies in the Arctic atmosphere were almost all carried out on Greenland. There were not any specific programmes within IPY but activities were performed in the framework of Arctic Monitoring and Assessment Programme (AMAP) Financed from Denmark by means from DANCEA. A long series of studies in other spheres than the atmosphere have also been carried out but they will not be described here as they are beyond the scope of this chapter.

Simulating the atmospheric CO 2 concentration across the heterogeneous landscape of Denmark using a coupled atmosphere-biosphere mesoscale model system

Although coastal regions only amount to 7 % of the global oceans, their contribution to the global oceanic air–sea CO2 exchange is proportionally larger, with fluxes in some estuaries being similar in magnitude to terrestrial surface fluxes of CO2. Across a heterogeneous surface consisting of a coastal marginal sea with estuarine properties and varied land mosaics, the surface fluxes of CO2 from both marine areas and terrestrial surfaces were investigated in this study together with their impact in atmospheric CO2 concentrations by the usage of a high-resolution modelling framework. The simulated terrestrial fluxes across the study region of Denmark experienced an east–west gradient corresponding to the distribution of the land cover classification, their biological activity and the urbanised areas. Annually, the Danish terrestrial surface had an uptake of approximately −7000 GgC yr−1. While the marine fluxes from the North Sea and the Danish inner waters were smaller annually, with about −1800 and 1300 GgC yr−1, their sizes are comparable to annual terrestrial fluxes from individual land cover classifications in the study region and hence are not negligible. The contribution of terrestrial surfaces fluxes was easily detectable in both simulated and measured concentrations of atmospheric CO2 at the only tall tower site in the study region. Although, the tower is positioned next to Roskilde Fjord, the local marine impact was not distinguishable in the simulated concentrations. But the regional impact from the Danish inner waters and the Baltic Sea increased the atmospheric concentration by up to 0.5 ppm during the winter months.

Ultrafine particle number fluxes over and in a deciduous forest

Ultrafine particles (UFP, particles with diameters (Dp) < 100 nm) play a key role in climate forcing, thus there is interest in improved understanding of atmosphere-surface exchange of these particles. Long-term flux measurements from a deciduous forest in the Midwestern USA (taken during December 2012 to May 2014) show that, although a substantial fraction of the data period indicates upward fluxes of UFP, on average the forest is a net sink for UFP during both leaf-active and leaf-off periods. The overall mean above-canopy UFP number flux computed from this large data set is -4.90 × 106 m-2 s-1 which re-emphasizes the importance of these ecosystems to UFP removal from the atmosphere. Although there remain major challenges to accurate estimation of the UFP number flux and in drawing inferences regarding the actual surface exchange from measurements taken above the canopy, the above the canopy mean flux is shown to be downwards throughout the day (except at 23.00) with largest magnitude fluxes during the middle of the day. On average, nearly three-quarters of the total UFP capture by this ecosystem occurs at the canopy. This fraction increases to 78% during the leaf-active period, but the over-storey remains dominant over the sub-canopy even during the leaf-off period.

Deposition of Nitrogen Compounds to the Danish Coastal Waters

Methods, parameterisations and models have been developed, which give insight in processes that are important for deposition of nitrogen compounds to the sea. This knowledge has been applied to the Kattegat Strait for which a nitrogen deposition of 960 kg N km-2 yr-1 was calculated with an atmospheric transport model.