Geert Draaijers

Deposition to forests in Europe: most important factors influencing dry deposition and models used for generalisation

Dry deposition of gases and particles to forests is influenced by factors influencing the turbulent transport, such as wind speed, tree height, canopy closure, LAI, etc. as well as by factors influencing surface condition, such as precipitation, relative humidity, global radiation, etc. In this paper, an overview of these factors is given and it is shown which are the most important determining temporal and spatial variation of dry deposition of sodium and sulphur. Furthermore, it is evaluated how well current deposition models are able to describe the temporal and spatial variation in dry deposition. It is concluded that the temporal variation is not modelled well enough, because of limited surface-wetness exchange parameterisations. The influence of forest characteristics are modelled reasonably well, provided enough data describing the forests and the spatial variation in concentration is available. For Europe these data are not available. The means to decrease the atmospheric deposition through forest management is discussed.

Particle deposition to forests : Summary of results and application

Abstract Particle deposition to forest was studied at Speulder forest using experimental and modelling results. In this paper a short overview of the main results is given and they are applied in generalisation of deposition in The Netherlands. The results of the Aerosol project show that the deposition of particles to forests has been underestimated until now. Particle deposition makes a reasonable contribution to the total deposition of acidifying components and base cations to forests. Parametrisations of the deposition processes were derived from the experiments and model developed for particle deposition to Speulder forest. The parametrisations were used together with concentrations, meteorological observations and land-use data to determine the deposition on a 5 x 5 km basis in The Netherlands. Results of the canopy exchange experiments at Speulder forest and of the comparison of atmospheric deposition estimates and throughfall estimates at Speuld and other locations in The Netherlands show that throughfall fluxes and atmospheric deposition estimates are reasonably in agreement for all components, except nitrate. There is still large uncertainty in canopy exchange processes of oxidised nitrogen and in deposition estimates of the different gases contributing to the total oxidised nitrogen flux. The contribution of aerosol deposition to the total deposition in The Netherlands is 7% for SO42−, 11% for NH4+ and 9% for NO3−; for deciduous forests these numbers are 14,15 and 17%, and for coniferous forests 10, 17 and 20%, respectively. Dry deposition is estimated to amount to about 21% of total deposition for Na+, 20% for K+, 27% for Mg2+ and 25% for Ca2+.

The contribution of ammonia emissions from agriculture to the deposition of acidifying and eutrophying compounds onto forests

In the vicinity of a large ammonia emission area, dry and wet deposition of acidifying and eutrophying compounds onto Douglas Fir forests was studied by sampling throughfall, stemflow and bulk precipitation. Deposition amounts of NH(4)(+) and SO(4)(2-) were recognised to be among the highest of Central Europe, resulting in extremely high inputs of (potential) acid to the forest soils (13.1 kEq ha(-1) year(-1)). The contribution of NH(3) emissions from agriculture to the total acid deposition to the forests was 52%. The total nitrogen deposition amounted to 115.0 kg ha(-1) year(-1), 83% originating from NH(3) emissions and 17% from NO(x) emissions. Calculated mean dry deposition velocities of NH(3) and SO(2) were much larger than reported in the literature. A synergistic effect between NH(3) and SO(2) in the process of dry deposition is suggested and evidence for this effect is discussed. When deposition models do not take this interaction into account, they will underestimate NH(3) and SO(2) deposition amounts in areas with intensive animal husbandry.

Field intercomparison of throughfall measurements performed within the framework of the Pan European intensive monitoring program of EU/ICP Forest.

A 6-month field intercomparison study on throughfall measurements was performed at Speulder forest near the west coast in The Netherlands. Twenty throughfall sampling systems were evaluated on accuracy, sampling strategy and performance under field conditions. Throughfall fluxes of NO3-, K+ and Kjeldahl-N generally could be determined with a larger accuracy than fluxes of SO4(2-), NH4+, Na+, Cl-, Mg2+, Ca2+, and alkalinity. Throughfall fluxes of H+ generally had the lowest accuracy. Only 20% of the sampling systems differed more than 20% from the best estimate, whereas 45% of the systems stayed within a 10% range from the best estimate. The difference from the best estimate was mainly caused by aspects related to sampling strategy, like, for example, collecting area, sampler placement. The inaccuracy induced by the sampling system appeared to be much larger than that resulting from the analysis of the samples by different laboratory as determined by ring-tests. The field intercomparison described in this article gave a good insight in the different aspects contributing to the overall accuracy of the measurements. However, performing a future field intercomparison is recommended for throughfall and stemflow in order to also take in account other aspects that might influence the performance of the different measurement systems (e.g. tree species, climate zone, summer/winter period).A 6-month field intercomparison study on precipitation measurements was performed at Schagerbrug near the west coast in the Netherlands. Twenty bulk sampling systems and two wet-only samplers were evaluated on accuracy, sampling strategy and performance under field conditions. Bulk precipitation fluxes of NO3-, NH4+, H+ and Kjeldahl-N generally could be determined with a greater accuracy than bulk precipitation fluxes of SO4(2-), Na+, Cl-, Mg2+, Ca2+, Alkalinity and H+. Bulk precipitation fluxes of K+ generally had the lowest accuracy. Only 20% of the sampling systems differed less than 10% from the best estimate, whereas most systems (60%) differed more than 20% from the best estimate. The inaccuracy induced by the sampling system appeared to be much larger than that resulting from the analysis of the samples by different laboratories as determined by ring-tests.

CONCENTRATION VARIATIONS IN RAIN AND CANOPY THROUGHFALL COLLECTED SEQUENTIALLY DURING INDIVIDUAL RAIN EVENTS

Abstract Canopy throughfall was collected sequentially during individual rain events in a Norway spruce stand at Klosterhede in Denmark (1989) and a Douglas-fir stand at Speuld in the Netherlands (1993). Simultaneously, precipitation was sampled sequentially. Each fraction was analyzed for pH, Na + , K + , Ca 2+ , Mg 2+ , NH 4 + , Cl − , NO 3 , and SO 4 2− . Analysis of successive precipitation collected at both sites generally showed the highest concentrations of Na + , Cl − , Mg 2+ , Ca 2+ , NO 3 − , and SO 4 2− in the initial fraction of most rain events followed by decreasing ion concentrations in subsequent samples. For K + , and especially for NH 4 + , several concentration maxima were observed throughout the events. At Speuld, the sequential throughfall showed a slight increase in concentration in the beginning, followed by a continuous decrease. At the end of the events, concentrations of throughfall increased again. At Klosterhede, decreasing ion concentrations of all ions were observed throughout some events, while constant and even increasing ion concentrations were seen throughout others. The ion concentrations in throughfall were inversely connected to the intensity of the throughfall. At both sites, the concentrations in throughfall in the final part of the event never reached a constant value and were always higher than in the simultaneously collected rain water. This indicates that the sources for the ions in throughfall were never emptied. The gradual and steadily changing concentration curves suggest that the processes involved in dry deposition and canopy exchange are slow. It was further found that the concentrations of NH 4 + and SO 4 2− in the initial throughfall at Speuld sometimes exceeded concentrations which could damage plant tissues.

Mapping wet deposition of acidifying components and base cations over Europe using measurements

Precipitation concentrations of various acidifying components and base cations in 1989 on a European scale were obtained from national organisations responsible for wet deposition monitoring in their countries and from the EMEP database. In total, results were gathered from 824 monitoring locations scattered over Europe. Annual mean concentrations were corrected for the contribution of dry deposition onto the funnels of bulk samplers. Spatial analysis based on regionalised variable theory revealed autocorrelation in all ion concentrations and reasonable bounded models were fitted to the experimental variograms. Maps of concentrations of acidifying components and base cations were compiled using the block-kriging interpolation technique. To obtain fluxes, concentrations were multiplied by long-term mean precipitation amounts from the EPA database. Wet deposition fluxes over Europe were mapped on a 50 x 50 km scale for nonmarine sulphate, nitrate, ammonium, hydrogen, chloride, sodium, magnesium, potassium and calcium. An extensive uncertainty analysis was performed to assess the quality of the maps.

Atmospheric sulphur deposition to forest stands: Throughfall estimates compared to estimates from inference

Abstract Wet and dry deposition of sulphur was estimated for 30 forest stands in the Netherlands using a throughfall method and an inferential method. Dry deposition estimates of the throughfall method were significantly higher compared to estimates from inference. The major sources of uncertainty of the throughfall dry deposition estimates were associated with non-representative throughfall sampling, wet deposition estimates, canopy exchange processes, deposition of neutral salts, dry deposition directly onto the throughfall collectors, and with the omission of stemflow fluxes and dry deposition directly to the undergrowth vegetation and forest floor. These uncertainties were found to act both ways to approximately the same extent and were not able to explain the observed gap between the two dry deposition estimates. For the inferential method, major sources of uncertainty in the dry deposition estimates arose in calculation of the dry deposition velocity of SO 2 and the omission of occult deposition of SO 4 2− . In this study, uncertainties associated with the spatial averaging of air concentrations of SO 2 and SO 4 aerosol and the calculation of the deposition velocity of SO 4 aerosol were found to be relatively small. Modifying the R c parametrization of SO 2 , based on recent dry deposition measurements made over heather in the Netherlands, resulted in fairly good agreement between both dry deposition estimates. Occult deposition of SO 4 2− was found to contribute significantly to the total sulphur deposition to the forest stands. Both the modified R c parametrization and the incorporation of occult deposition led to systematically higher sulphur deposition estimates by the inferential method compared to originally inferred deposition. This implies that in the Netherlands, sulphur deposition to forest ecosystems might have been underestimated heretofore.

Research on the impact of forest stand structure on atmospheric deposition

Dry and wet deposition onto thirty forest stands in relation to stand structure is studied by sampling throughfall and bulk precipitation. Nine measurement sites are situated in Pseudotsuga menziesii stands, ten in Pinus sylvestris and eleven in Quercus robur stands. All stands are situated within a radius of 1.2 km to assure a more or less equal air pollution load. In each stand, detailed forest structure inventories are made to determine aerodynamic roughness, collecting efficiency and surface area parameters. Measurements to data cover a four month period (April-July 1990). First results show relatively high throughfall deposition in Pseudotsuga menziesii stands. Lowest throughfall fluxes are recorded for Quercus robur and intermediate values for Pinus sylvestris stands. There are indications of a relatively strong canopy exchange in Quercus robur stands during the measurement period. Many results from forest stand structure inventories are not available yet. However, a strong relation is observed between throughfall deposition in Pseudotsuga menziesii stands and total crown volume.

Base cation deposition in Europe - Part II. Acid neutralization capacity and contribution to forest nutrition

An assessment was made of the capacity of base cations to neutralize acid deposition and of the contribution of base cation deposition to forest nutrition in Europe. In large parts of southern Europe more than 50% of the potential acid deposition was found counteracted by deposition of non-sea salt Mg2+ + Ca2+ + K+. In central and northwestern Europe, base cation deposition usually amounted to less than 25% of the acid Input. Smallest base cation deposition relative to potential acid deposition was found in southern Scandinavia, Denmark, northern Germany and The Netherlands. A similar spatial pattern was found for the neutralization of acid anions in precipitation. Whereas in Scandinavia weathering is the dominant supplier of base cation to forest soils, in eastern and southern Europe, forests mainly rely on atmospheric deposition for the supply of base cations. Using error propagation, the random and systematic error in acid neutralization capacity for an average grid cell of 10 x 20 km was estimated to equal 45-55% and 50-55%, respectively.

Base cation deposition in Europe - Part I. Model description, results and uncertainties

Deposition of base cations (Na+, Mg2+, Ca2+, K+) in Europe was mapped for 1989 with a spatial resolution of 10 x 20 km using the so-called inferential modeling technique. Deposition fields resembled the geographic variability of sources, land-use and climate. Dry deposition constituted on average 45% of the total base cation deposition in Europe. Modeled deposition estimates compared reasonably well with deposition estimates derived from throughfall and bulk-precipitation measurements made at 174 sites scattered over Europe. Using error propagation, the random and systematic error in total deposition for an average grid cell of 10 x 20 km was estimated to equal 35-50% and 25-40%, respectively. Within individual grids a relatively large variability in deposition is expected.