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Atmospheric Nitrogen and Sulphur Dry Deposition Into Forest Stands

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Atmospheric Nitrogen and Sulphur Dry Deposition Into Forest Stands ATMOSPHERIC NITROGEN AND SULPHUR DRY DEPOSITION INTO FOREST STANDS C. MARQUES CORREIA1, J. CONSTANTIN1, A. IBROM1, G. GRAVENHORST2, 1 Department of Biological and Environmental Engineering, University of Trás-os-Montes e Alto Douro, Portugal 2 Faculty of Forestry and Forest Ecology, University of Göttingen, Germany Abstract In this study the dry input of atmospheric particles into forest stands is quantified. The chosen method, a wash- off-method using the natural leaf surfaces as collectors of the dry deposition, was standardized on spruce (Picea abies (L.) Karst.) and experimentally applied to other tree species, forest stands, and climatic regions. The field measurements were achieved predominantly in a spruce stand at Solling (Germany) and in a pine and eucalyptus stand at Herdade da Espirra (Portugal). To judge the trend of air pollution unaffected by normal meteorological deviations, continuing measurements were taken for several years at the main research site at - +, 2- Solling. At this site, the concentration developments of NO3 , NH4 and SO4 show an average annual pattern that varies in distinction according to the components. A comparison between the research sites in Germany and Portugal shows that the ion-balance of the aerosol at Solling is strongly dominated in the fine fraction by ammonium and sulphate, while at Herdade da Espirra the composition of ions is dominated by sea salt particles. Dry particle deposition constitutes an important part of the total matter input into forest ecosystems. Just the nitrogen input into Solling only by dry deposition (from particle, mist, and gas deposition) with about 30 kg N ha-1 a-1 already exceeds the critical load of 20 kg N ha-1 a-1 by far. Only a quick and drastic reduction of sulphur and nitrogen emissions could stop the further increase of the nutrient imbalance and the progressing acidification of this ecosystem. 1. INTRODUCTION Evaluation of the ambient concentrations of substances in the lower atmosphere is important, but those values alone do not allow deeper conclusions on the damaging impacts of these compounds on a forest stand. More significant is the amount of deposition an ecosystem is exposed to. Forest ecosystems are particularly important for the removal of atmospheric aerosols. The forest canopy structure and geometry, together with the surface roughness properties favour that trees act as an important sink for particles from the atmosphere. In order to evaluate the maximum tolerable deposition rates with critical loads, below which there will be no damage to a forest ecosystem, it is necessary to quantify the input of atmospheric pollutants on stand level. Until now, a weakness of all investigations on the total deposition rate into ecosystems has been that the portion of the nutrients and harmful substances dry deposition could only be assessed inadequately and hardly separated from the wet deposition. Thus far, within dry deposition, that of gaseous acid compounds has received far more attention in research despite the importance of the dry deposition of particles. The complexity of deposition processes of particles implies the difficulty to find appropriate methods for field measurements. Together with wet deposition, particle dry deposition is responsible for delivering 2- - + atmospheric loads of such compounds as SO4 , NO3 and NH4 to ecosystems. High deposition loads of sulphur and nitrogen compounds can have strong effects on the structure and function of the ecosystem (among others e.g. [1]). This paper summarizes data specific to sulphur and nitrogen air concentrations and particles dry deposition measured at three different forest stands. 169 2. MATERIALS AND METHODS The field measurements of this study were carried out predominantly in a Norway spruce stand (Picea abies (L.) Karst.) at Solling (51°46’ N, 9°35’ E), a north German low mountain range using a 50 m high research tower. In Portugal, the field measurements were achieved in a pine (Pinus pinea L.) and eucalyptus (Eucalyptus globulus Labill.) stand at Herdade da Espirra (38°38' N, 8°36' W). A wash-off-method for the direct on-site-measurement of the dry deposited trace material on living branches was chosen. The ion exchange processes occurring on natural branches with a wash-off-method were quantified through immediate sequential washings. At the beginning of a measuring period randomly selected, branches from the sun crown were washed on the tree in a Teflon bag by steady manual shaking. At the end of each sampling period the measuring twigs were then washed four times immediately sequential by four minutes, respectively, with demineralised water in Teflon bags. For the conifers, the element specific portion of leaching in the first washing can be extrapolated by simplex-logarithmic transformation of the element concentrations from the sequential washings two to four; for eucalyptus, the arithmetic mean of the extractions two to four was calculated [2]. Additionally at Solling, during each sampling period in 1993 and 1994, ten of the measuring twigs were washed with an acidified washing solution from demineralised water that was acidified by 0.05 M hydrogen chloride (HCl) to a pH-value of 4.2. Leaf surface areas were determined by an automatic image analysing system DIAS (Digital Image Analysis System, Fa. Delta-T Devices Ltd, Cambridge, England) [3] and, afterwards, the leaves were dried to determine the dry weight for each measuring period. The atmospheric concentrations of particles in Solling were measured continuously above the canopy by a six stage cascade impactor (Sierra-Model) and by a Teflon filter pack system. In each stand, the atmospheric aerosol concentration was measured with an impactor in close proximity to the selected branches during each sampling period. All samples were analysed for pH and electrical conductivity. The samples were analysed for the water soluble parts of Ca2+, Mg2+, and Mn2+ with an Inductive-Coupled Plasma Atomic Emission Spectroscope; the heavy metals ions Cd2+, Cu2+ and Pb2+ were determined by + + + - Atomic Absorption Spectrometry. The ascertainment of the soluble ions Na , K , NH4 , Cl , 2- NO3, and SO4 was carried out with an ion chromatograph. - + 2- In order to calculate also the gas dry deposition of NO3 , NH4 , and SO4 ,a resistance model was used [2]. Evaluation, statistical analysis, and graphical representation were performed with SAS (Statistical Analysis System). 3. RESULTS AND DISCUSSION 3.1. Atmospheric concentrations - + 2- The concentrations of trace substances (NO3 , NH4 and SO4 ) at Solling have not changed significantly during the last five years [2]. On this site, the concentration developments of nitrate, ammonium, and sulphate show an average annual pattern that varies in distinction according to the components. 170 The annual variation of the nitrate concentrations indicates a pronounced maximum in early spring from February to April and a second one slighter in late autumn (November). The minima are found during the summer months May to October and in December/January. The lowest concentrations are measured in December. In the aerosol at Solling, nitrate is found in considerable fractions in both the coarse and fine mode (bimodal distribution). The seasonal variation is characteristic for nitrate fine particles that are mainly balanced by ammonium. A strong positive correlation between these two trace elements is also indicated by a Pearson correlation coefficient of r = 0.87 within a confidence belt of 99.9%. Therefore a decrease in - the concentrations of NO3 during the warmer months can be observed as a result of a rise in the volatilisation of NH4NO3 in the secondary aerosol. However, with coarse particles similar processes do not occur because nitrate is mainly associated with sodium, forming a more stable compound [4,5]. The annual trend of the ammonium concentrations indicates a maximum in March/April. A more or less pronounced secondary maximum was observed in autumn. Ammonium itself is not emitted; the whole ammonium is just converted in the atmosphere out of ammonia. The high ammonium concentrations during spring and autumn indicate an increased conversion from NH3-N to NH4-N in the air. The ammonium concentration in the particles is strongly correlated with the sulphate concentration (r = 0.80 within a confidence belt of 99.9%). But the molar ratio from ammonium to sulphate exceeds in numerous measurements a factor of 2. + 2- In fact the excess of ammonium at both measuring sites (in Germany: NH4 /SO4 = 1.18 + 2- eq/eq; in Portugal: NH4 /SO4 = 1.24 eq/eq) indicates the presence of nitric acid (HNO3) in the aerosol, which also associates with ammonium. FIG. 1. Seasonal variation of air concentrations of nitrate (sampling with impactor for 67 measuring periods) and ammonium (with filter pack system for 84 measuring periods) at Solling, shown by monthly averages. Boxes represent the 25th and 75th percentile. 171 TABLE I. MEAN AIR CONCENTRATIONS OF SEVERAL TRACE ELEMENTS IN THE AEROSOL OF THE F1 STAND AT SOLLING (GERMANY) AND AT HERDADE DA ESPIRRA (PORTUGAL) Measuring site Trace element - + 2- NO3 NH4 SO4 (µg N m-3) (µg N m-3) (µg S m-3) Solling 1.12 3.90 3.80 Herdade da Espirra 0.51 0.85 0.78 He electrolytic composition at Solling is thus dominated by ammonium and sulphate. The other ions are significantly lower concentrated. Only 26% of the equivalents account for other ions. A share of 4% for the cation equivalents is contributed to hydrogen ions. This small share of acids refers to neutralization and buffer capacities of the aerosol. However, at Herdade da Espirra, the ion composition is dominated by sea salt particles. Already 57% of the sum of the equivalents account for sodium and chloride and just 24% for ammonium and sulphate. This may be due to the short distance to the near Atlantic (30 km inland, while for Solling the shortest distance to the coast is 250 km).
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