Environmental Pollution 102, S1 (1998) 327–335 reformated layout similar to printed version

Spatial variation in annual nitrogen deposition in a rural region in

a b a a a Werner Eugster ∗, Silvan Perego , Heinz Wanner , Alex Leuenberger , Matthias Liechti , Markus Reinhardt c , Peter Geissbühler d, Marion Gempeler a , and Jürg Schenk a

a University of Bern, Institute of Geography, Hallerstrasse 12, CH-3012 Bern, Switzerland bSwiss Federal Institute of Technology, Air and Soil Pollution Laboratory, CH-1015 Lausanne, Switzerland c Swiss Meteorological Office, Krähenbühlstrasse 58, CH-8004 Zürich, Switzerland dPaul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

Received 27 March 1998; accepted 9 September 1998

Abstract We studied the spatial variation in annual nitrogen deposition to the region of Switzerland by combining (1) mesoscale atmospheric modeling of gaseous dry deposition, (2) wet deposition data measured within the study area as well as published data, and (3) estimates of dry deposition of aerosol particles based on concentration measurements and sedimentation samples from a near-by region. Gaseous 1 1 dry deposition shows greatest regional variation. Total nitrogen input to this region is in the range of 22–51 kg N ha− yr− (13–42 kg N gaseous dry deposition; 7–8 kg N wet deposition; 2 kg N dry deposition of aerosol particles). Gaseous dry deposition is dominated ≈ 1 1 by reduced nitrogen components from agricultural sources. The highest nitrogen inputs were found over lakes (51 kg N ha− yr− ). Our 1 1 results indicate that current nitrogen loads exceed the nutrient critical loads by several kg N ha− yr− in all nitrogen-sensitive ecosystem types occurring in our 70 50 km2 study area. ×

Keywords: Nitrogen input; dry deposition; pollutant dispersion modeling; critical loads; Switzerland

Introduction loads for various ecosystem types in our study area.

Nitrogen is essential for biomass production in undisturbed natural ecosystems as well as in intensely managed agro- Study area ecosystems. The nitrogen cycle is therefore regarded as one of the major ecological cycles, together with the carbon and Selection of a study area water cycles (Larcher, 1995), and has received great atten- In a previous study (Hesterberg et al., 1996; Eugster and tion in scientific research since the very beginning of eco- Hesterberg, 1996; Neftel et al., 1994; Eugster, 1994) con- logical studies. ducted in a non-urban, densely populated rural area of In this paper we focus on the atmospheric inputs of ni- Switzerland in the lower Reuss valley north of Lucerne trogen in three forms, (1) gaseous dry deposition, (2) wet and west of Zürich, it was concluded that reduced nitro- deposition (input by precipitation), and (3) dry deposition gen compounds, primarily gaseous ammonia (NH , 45.9%) of aerosol particles (sedimentation and impaction/diffusion 3 and wet deposited ammonium (NH+, 19.8%), were more of dry and wettened aerosol particles, but excluding those 4 abundant than oxidized nitrogen compounds in the total an- incorporated in precipitation), to obtain the current nitrogen nual average deposition of 28.3 kg N ha 1 year 1. Be- loads in a densely populated rural part of Switzerland. The − − cause of the local importance of pig production, high am- current loads are then compared with the nutrient critical monia losses from stables and slurry applications raised the question whether the results obtained in that area were also ∗Corresponding author. Tel.: +41 31 631-8551; fax: +41-31-631-8511; e-mail: [email protected] representative of other rural parts of Switzerland. There-

327 328 W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335 fore we selected a study area that had (1) lower cattle den- Methods and data sity, (2) more diverse agricultural land use, (3) less influ- ence from large urban centers (like Zürich), and (4) in- To obtain annual nitrogen deposition amounts on the re- corporated both source regions and potential receptor areas gional scale, we used various scale- and process-specific with semi-natural and natural undisturbed ecosystems that concepts and methods for the three forms of nitrogen de- are expected to be most vulnerable, i.e. have lowest critical position listed in the introduction. Because of the high de- loads of nitrogen. mand for input data and the heterogeneous quality, avail- ability and temporal coverage of such data, we tried to de- termine an annual average of the time period 1990–1996. The Seeland region For such a 7-year period, we assumed that land use and land cover changes can be neglected whenever information was These criteria were all met in the Seeland region, where only available for a single year. there is an abrupt change from intensive farming in the rural plains of the Seeland in the Southeast to the steep south slope of the in the Northwest. The Dry deposition of trace gases Jura south slope contains extensive mesic to xeric grass- The dynamic mesoscale Metphomod (Meteorology and lands at lower elevations intermixed with deciduous and Photochemistry Model) numerical model (version 1.1; mixed forests on calcareous soil with thin organic soil hori- Perego, 1996) was used to model gaseous dry deposition. zons, small soil nitrogen pools, and negligible influence of Metphomod is a three-dimensional Eulerian model based on groundwater nitrogen supplies due to the porous and well- the primitive equations of fluid dynamics. The atmospheric drained calcareous geology of the Jura mountains. In ad- chemistry module used 39 substances with 82 chemical re- dition, the three lakes of Neuchâtel, Biel and Murten in actions, plus NH3 that does not participate in the photo- this region add a non-terrestrial ecosystem component to the chemical cycle but is essential to obtain credible amounts of study area that was not previousely recognized as receiving gaseous nitrogen deposition in our study area. The chem- considerable inputs of nitrogen from the atmosphere (Rihm, istry follows the RADM mechansim described in Stockwell 1997). (1986) which does not consider aerosol formation. The model domain size was 70 50 km2, with 22 ver- tical layers. Horizontal grid resolution× was 1 1 km2 and Climate vertical spacing was 100 m between 450 and 2650× m a.s.l. The Seeland region has a temperate climate with a bi-modal Topography was described by the RIMINI digital elevation precipitation distribution. Annual precipitation measured at model (Bundesamt für Landestopographie, 1992), which has a horizontal resolution of 250 250 m2 with 1 m ver- the Swiss Meteorological Office (SMO) weather station in × Biel (434 m a.s.l.; Fig. 1a) totals 1090 mm (30-year av- tical resolution. erage 1966–1995) with a primary maximum in December We defined 17 turbulent transport classes and modeled a (120.9 mm) and a secondary maximum in August (115.3 representative day for each class selected from the period 1990–93 (see below). The frequency of occurrence was mm). Annual average air temperature is 9.23◦C with an ab- used as a weighting factor to obtain annual deposition val- solute maximum and minimum of 30.70◦C and –7.90◦C, respectively. ues. For each representative day we worked out emission in- ventories of all trace gas species of interest, and all available meteorological input data. Each model run started at noon Land use of the previous day and was stopped at midnight of the se- lected day (36-hour runs). The first 12 hours were used for The land use classification is based on a multispectral- initialization of the model with the current meteorological multitemporal analysis of four Landsat 5 TM scenes conditions and obtaining the appropriate spatial distribution (Liechti, 1996) on 30 April, 17 June, 3 July and 4 August of locally emitted and advected trace gases within the model 1994. It revealed the following land use pattern in the cen- domain. tral section of our study area: 45.2% arable lands; 32.7% forests and woodlands; 12.4% lakes and rivers; 6.9% ur- Deposition module ban areas and infrastructure; 2.8% unclassified land. The arable land consists of 62.2% meadows and pastures; 15.9% The deposition module of Metphomod is a three-resistor cereals; 11.4% maize; 4.5% sugar beets; 2.5% orchards; chain with an aerodynamic resistance Ra, a laminar 1.3% vineyards; 1.1% potatoes; and 1.1% other agricultural boundary-layer resistance Rb, and a canopy resistance Rc crops (especially vegetables that were not distinguishable (Erisman et al., 1994; Eugster and Hesterberg, 1996). For on Landsat TM images). Rb the approximation by Wesely and Hicks (1977) was

W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335 329

a d Basel

Zürich 4 Biel 3 Lucerne

1 Bern 2

5

Geneva

b e

c f

Figure 1: (a) Location of the model domain in Switzerland (red box), with the weather stations (triangles) used in the classification of tur- bulent transport classes (see text for details). The size of the red box corresponds to the model domain shown in panels b–f. (b) Total annual nitrogen deposition of gaseous dry deposition, wet deposition, and aerosol particle deposition (sedimentation and impaction/diffusion) in 1 1 1 1 kg N ha− year− . (c) Total annual dry deposition of oxidized gaseous nitrogen (NOy) in kg N ha− year− . (d) Ratio of dry deposition of oxidized nitrogen (NOy-N) vs. dry deposition of reduced nitrogen (NH3-N), annual average. (e) Ratio of emission vs. deposition of total nitrogen in the study area, annual average. Thin lines in panels (a)–(e) indicate the 1:1 ratio; broken lines are topographic contour lines; and bold lines are rivers and lake shore lines. (f) Regions defined for regional averaging. Yellow indicates Seeland rural plains; red indicates the Jura south slope; deep blue indicates lakes; pink indicates urban area including Bern; green indicates forested hills (, Schaltenrain, Bargenholz); gray shading shows the topography. 330 W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335

proaches for the application they were designed for, none of them gave satisfactory results as a basis for mesoscale trace gas flux modeling. The problem is a scale issue: weather classifications tend to be at continental scale rather than at a specific regional scale. Therefore, Leuenberger (1996) de- veloped an empirical classification (Fig. 2) based on eas- ily available data in our region that include (1) prevailing wind direction, (2) thermal stratification of the atmospheric boundary layer (ABL), and (3) typical wind speed (with or without a diurnal cycle). Within our model domain, or in its vicinity, he selected five SMO weather stations (Fig. 1a): (1) Payerne (491 m a.s.l.), (2) Plaffeien (1042 m), (3) Napf (1406 m, mountain top station), (4) (1599 m, mountain top station), (5) Moléson (1972 m, mountain top station). This classification uses the 10-minute data from Figure 2: Turbulent transport classes and their frequency of oc- these stations from the years 1990–1993 and takes account currence in the study area. The wind direction profile specifica- of the channeling effect of the and the Jura mountains tions are: (a) no changes in wind direction; (b–d) SW winds above 1000, 1500, or 1600 m a.s.l, respectively; (e) layer with SW winds (Furger, 1990) by looking at the wind direction profile (Fig. around 1600 m a.s.l.; (f) variable wind direction above 1300 m 2). Classes that did not differ from a class with random a.s.l.; (g) layer with NE winds around 1400 m a.s.l.. Class names wind direction at all stations at the p <0.05 significance correspond to Leuenberger (1996). level (Bonferroni statistics; Sachs, 1978) were discarded, and classes with >10% frequency of occurrence were re- fined using threshold windspeeds given in Fig. 2. This clas- used and for Rc the seasons defined by Wesely (1989) were sification covers 93% of all observed cases with 5% and 2% used to parametrise maximum and minimum resistances for unclassified northeasterly and southwesterly flow patterns, SO2 for the various surface types. These Rc values were respectively. then mapped to corresponding values for other chemical species, e.g., for NO and NO2 the same Rc as for SO2 was 1 Emission inventories used over land, but a constant value of 500 s m− was used over water surfaces because of the low solubility in water We used three types of emission inventories: (1) the large- 1 (for comparison: Rc for O3 was set to 2000 s m− over scale EMEP inventory at 50 50 km2 resolution; (2) a lo- × water surfaces). For NH3 over terrestrial ecosystems an Rc cal inventory with 1 1 km2 spatial and 1 h temporal res- × value was used that corresponds to 20% of the Rc value for olution for oxidized nitrogen compounds and most VOCs, SO2, while Rc = 0 was set for water surfaces. For HNO3 and 5 5 km2/1 h resolution for some photochemically im- × and HONO Rc = 0 was used both for land and water sur- portant but difficult to determine compounds (Kunz et al., faces, assuming that dry deposition of these species is only 1995 and later updates where the time span was extended to limited by turbulent conditions in the atmosphere (Ra+Rb), a full year, and emissions from residential heating sources not by the surface type. Typical daytime Rc values for SO2 were added); (3) an ammonia emission inventory produced 1 used in Metphomod are 50–500 s m− over agricultural by Reinhardt (1995) that covers our model domain. The 1 1 land, 60–1000 s m− and 150–1000 s m− over deciduous EMEP inventory was primarily used to prescribe advective 1 and coniferous forests, respectively, 100–500 s m− over inputs into our model domain from distant areas that are not 1 pasture and 200–1000 s m− over urban areas. The higher covered by the updated Kunz et al. (1995) inventory. values are typically used for fall conditions if the vegetation was harvested, or for fall and winter conditions in the case Wet deposition of forests and natural vegetation types. Additional details can be found in Perego (1996). We sampled wet deposition with wet-only deposition sam- plers during two field campaigns of 16 weeks in 1995 and Turbulent transport classes 12 weeks in 1996 (26 July–28 November and 10 April–10 June, respectively) at five sites distributed over the central Leuenberger (1996) tested the performance of various ex- part of our study area. The sampling buckets were made isting weather classification schemes (Hess and Brezowsky, of white polyethylene and the outer wall of the container 1969; Schuepp, 1968; Wanner and Kunz, 1977; Perret, in which the buckets were placed was insulated with a thick 1987; Rickli, 1988; Wanner and Furger, 1990; Furger, 1990; layer of aluminum-covered styrofoam to minimize evapora- Künzle and Neu, 1994). Despite the usefulness of these ap- tion of collected precipitation water. A detailed description W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335 331

1 1 2 Table 1: Summary of regionalized median nitrogen deposition fluxes in kg N ha− year− ; total area (km ) and elevation range (m a.s.l.). The 95% confidence interval of the nonparametric Hodges-Lehmann estimate of median difference is shown in brackets if different from the median 0.2. See Fig. 1f for definition of regions. ± Variable Seeland Jura Lakes Bern Forested hills Entire domain rural plains south slope urban area

n (km2) 113 43 124 60 25 3381 Elevation range (m a.s.l.) 429–602 429–1033 429–437 489–728 440–571 413–1579 Gaseous dry deposition 12.7 12.6 41.7 35.9 17.4 13.2 (12.5, 13.0) (11.6, 13.4) (40.7, 42.8) (32.3, 39.5) (14.7, 24.2) (12.9, 13.4) Reduced N 70.1% 65.6% 83.5% 51.7% 77.6% 72.2% Wet deposition 7.0 7.4 7.0 7.1 7.0 7.7 Reduced N 64.3% 65.0% 65.0% 65.0% 65.0% 65.0% Aerosol dry deposition 2.0 2.3 2.0 2.0 2.0 2.2 Reduced N 57.2% 56.5% 57.1% 56.9% 56.9% 56.6% Total deposition 21.7 22.1 50.7 45.0 26.4 23.5 (21.4, 21.9) (21.1, 23.2) (49.6, 51.8) (41.4, 48.5) (23.6, 33.1) (23.2, 23.7) Reduced N 67.4% 64.4% 79.9% 54.0% 73.0% 68.7% of the wet-only sampler can be found in Gempeler (1997). Dry deposition of aerosol particles

The only data set that gives information about the depen- The samples were stored at a temperature below 4◦C, dence of particulate dry deposition as a function of altitude then filtered with a syringe through a 45 µm filter and stored in this part of Switzerland was obtained by Gälli Purghart in the refrigerator until processing was possible on a Dionex (1989) between 16 July 1985 and 16 September 1986. We ion chromatograph for nitrate, sulfate and chlorine, or a used her sedimentation data (dry-only sampling buckets) photometrical determination of ammonium could be made. and size-fractionated aerosol concentration data sampled According to Müller et al. (1982) we expected our mea- with a Berner cascade impactor with 9 stages. Three of surements to experience an 18% loss in nitrogen concen- the four sites used by Gälli Purghart were identical to the tration between sampling and analysis. Accounting for the sites with the long-term wet deposition measurements pre- imperfect sampling efficiency of any wet-only samplers, we viousely mentioned, and the fourth site was located at 1550 assumed an overall collection efficiency of 80% of true ni- m elevation, giving a vertical range that matches our study trogen wet deposition. area (400–1606 m a.s.l.). Because sedimentation measurements do not include im- To scale up our samples to annual totals, we referenced paction and diffusion of small aerosol particles, we es- our measurements against long-term (8–12 year) data ob- timated the impaction/diffusion deposition by using mea- tained in a height profile at some distance southwest of our sured aerosol concentration at seven size fractions <8 µm, study area (unpublished data courtesy of Jürg Fuhrer). The multiplied by an estimated deposition velocity vd and a ni- corresponding weeks for all available years showed that trogen content for each size fraction, and added this value 65% of annual precipitation and 66% of annual nitrogen to the sedimentation measurements. The deposition veloc- wet deposition occurs during these 28 weeks. The July– ities used here were 0.2, 0.1, 0.1, 0.2, 0.3, 2, and 10 mm 1 November period had precipitation similar to the April– s− for the size fractions 0.06–0.125, 0.125–0.25, 0.25–0.5, June period (31% vs. 34%) but nitrogen deposition was 0.5–1, 1–2, 2–4, and 4–8 µm. Average relative nitrogen clearly higher during the July–November period (40% vs. content of the aerosol particles was assumed to be 0.062, 26%). Based on this reference data we multiplied our mea- 0.079, 0.092, 0.102, 0.071, 0.027, 0.016, and 0.009 of to- + surements by 1.52 to scale up to a full year, and then by tal mass for NH4 -N, and 0.031, 0.048, 0.056, 0.060, 0.034, 1.25 to adjust this value according to our estimated overall 0.020, 0.012, and 0.016 for NO3−-N, respectively. These sampling efficiency. This procedure basically enlarges the fractions are the average of the published values of Schu- measured data from our 28 weeks by 90%. For validation of mann (1989) and the unpublished values of Richard Heim- our approach, we used independent data (Krieg, 1997) from gartner (EAWAG, Switzerland). The ammonium and nitrate Payerne located in the western half of our study area (Fig. content of the sedimentation data was directly determined 1a, location 1). (Gälli Purghart, 1989). Again, a validation data set (Krieg, 332 W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335

1997) for low elevations was available from Payerne from dominance of oxidized nitrogen deposition, but rather a ra- the years 1993 and 1995. tio slightly lower than 1.0. This underlines the rural charac- ter of our study area. Results The ratio of locally emitted nitrogen to the correspond- ing local deposition (Fig. 1e) confirms the importance of Median total nitrogen deposition (Fig. 1b, Table 1) for the traffic axes as net sources of nitrogen, but it also reveals 1 1 the whole model domain is 23.5 kg N ha− year− with that the vast agricultural areas in the Seeland are net sources peak inputs over lakes and the urban area of Switzerland’s of nitrogen. Only lakes and forests and the extensively man- capital, Bern. In contrast to these extended areas with aged slopes of the Jura mountains (top third in Fig. 1e) are high nitrogen loads, the local influence of minor cities net receptor areas. (Biel, Neuchâtel, Fribourg, Solothurn) and larger towns (Burgdorf, Murten, and ) is restricted to a Wet deposition small area of a few km2 in the close vicinity of these ur- ban sources. At distances of typically less than 5–10 km, The altitudinal gradient of wet deposited nitrogen we mea- the nitrogen deposition reaches the level of the rural parts sured in our study area shows almost no variation among of our study area. the three sites at lowest elevation between 435 and 562 m First, we present the results for the three forms of ni- a.s.l. (data not shown). The adjusted annual total is esti- 1 1 trogen deposition considered here, followed by the spatial mated to be 7.0–7.1 kg N ha− year− at the lowest eleva- 1 1 variation among regions. tions, slightly increasing to 7.4 kg N ha− year− at 620 m a.s.l. Our data suggest a steep gradient between 620 m and 875 m a.s.l., with an annual wet deposition total of 9.6 kg N Gaseous dry deposition 1 1 ha− year− at the highest elevation of our transect. But be- The spatial variation in Fig. 1b is mostly determined by cause of the uncertainty of the value at the highest elevation, gaseous dry deposition which revealed to be the dominant we decided not to extend this steep gradient beyond an al- form of nitrogen deposition and contributes 60–82% to the titude of 900 m, although precipitation measurements from total nitrogen deposition in Fig. 1b. It is worth noting climate stations show a further increase of annual precipi- that in our model runs the dry deposition over lakes was tation with height. With this conservative modification we of the same order of magnitude as the deposition over the parametrised an empirical equation for spatial extrapolation 1 1 urban area of Bern. However, the chemical form of the that assigns a value of 7.0 and 9.9 kg N ha− year− for al- nitrogen deposited over urban areas differs from that over titudes z<620 m and z>900 m, respectively, with a gradient 1 1 1 the lakes: only the urban area of Bern shows a signifi- of 0.0102 kg N ha− year− m− between 620 and 900 m. cant peak of oxidized nitrogen deposition (Fig. 1c), while Based on this parametrization we obtained an annual nitro- 1 1 over the rest of the domain the pattern seen in Fig. 1c gen wet deposition of 7.7 kg N ha− year− for the entire is determined by the primary traffic axes (Fribourg–Bern– model domain (Table 1). Burgdorf–Solothurn and Solothurn–Biel–Neuchâtel), sec- ondary traffic axes (from Biel passing east of Aarberg and Dry deposition of aerosol particles continuing to the east of Bern, and — less important — the Aarberg–Murten–Payerne axis), and the corresponding Wet (liquid) aerosols and aerosol particles incorporated in population density along these axes. Most parts of the Jura cloud water are considered to be included in our wet depo- mountains in our domain receive comparably low inputs of sition samples and are excluded in this section. For the days 1 1 oxidized nitrogen (generally below 5 kg N ha− year− ). without precipitation events we estimated an additional de- 1 1 NO2 and NO account for 65% and 32% of oxidized nitrogen position of 2.2 kg N ha− year− for the entire model do- deposition in the Seeland region, and 57% and 38% along main (Table 1). This value is based on the altitudinal gradi- the Jura south slope, respectively. The remaining fraction is ent found in Gälli Purghart’s (1989) data set. mostly in the form of HNO3. Aerosol sedimentation is 3–4 times more important than Fig. 1d shows the ratio of oxidized versus reduced dry impaction and diffusion, and should be included in the an- deposition of nitrogen. The only area where oxidized forms nual total, despite the uncertainty of available data and the of nitrogen exceeded the inputs of reduced nitrogen (ratio lack of recent data from within our study area. As with wet >1.0 in Fig. 1d) were the urban area of Bern, with the ad- deposition, we had to assume horizontally homogeneous jacient part of the traffic axis to Solothurn (passing NW of distribution of aerosol inputs, although there might be im- Burgdorf), and the industrialized area between the lakes of portant local variations around point sources (chimneys). Neuchâtel and Biel, and some minor areas along the north- If we assume that buckets give only a low estimate of western lake shores where the motorway is located. The aerosol deposition over forests, the contribution of aerosol urban centers of Biel and Neuchâtel do not reveal a clear deposition is still less than wet deposition, even when we W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335 333 -1

yr LEGEND

-1 55 gaseous dry deposition 50 NH-N 45 critical loads x

40 NO-N y 35 mesotrophic fens wet deposition 30 calcareous species-rich grassland (Mesobromion) 25 } NH-N calcareous forests 4 20 acidic forests } } NO-N 15 montane-subalpine 3 particulate aerosol deposition 10 grassland 5 nitrogen deposition, kg N kg ha deposition, nitrogen 0 bogs and shallow NH-N Seeland Jura urban forested totalsoft-water bodies 4 rural south lakes area of study hills NO-N plains slope Bern area 3

Figure 3: Average annual loads of the regions defined in Fig. 1f and the total model domain, and critical loads for selected ecosystem types in the study area (taken from Rihm 1997). The shading of the bars indicates the contributions of reduced and oxidized forms of nitrogen in gaseous, wet, and particulate aerosol deposition. Error bars show the 95% confidence interval for the annual loads.

1 1 double our estimate to 4.0 kg N ha− year− , assuming Validation of wet and particulate aerosol deposition that forests receive twice the amount we estimated based on Gälli Purghart’s (1989) bucket samples. The gaseous A comparison of our wet deposition estimate with the vali- dry deposition discussed previousely remains the dominant dation data set of Krieg (1997) from Payerne (491 m a.s.l.) 1 1 fraction of total nitrogen deposition. shows an underestimate of 1.2 kg N ha− year− (or 15%) for the time period of our own measurements. This differ- ence may be due to (a) differences in sampling technique (wet-only vs. bulk deposition samplers), (b) differences in Regional variation in total deposition nitrogen loss between sampling and time of analysis, or (c) spatial variation that we were not able to resolve with our The annual total nitrogen deposition (Table 1, Fig. 3) in measuring design. our study area shows a factor 2 regional variation for the For dry deposition of aerosol particles, we can only com- regions that we defined in Fig. 1f. Independent of region, pare concentration measurements with Krieg’s (1997) vali- we found that gaseous dry deposition dominates (60–82% dation set from Payerne, because deposition was not mea- of total nitrogen deposition), and that reduced nitrogen is sured. Krieg’s (1997) data show a 37% lower concentra- the prevailing chemical form (54–80% of total nitrogen de- tion of aerosol particles in 1993/95 than what Gälli Purghart posited; 52–84% of gaseous dry deposition). Ammonia dry (1989) measured at the lowest site of her height profile dur- deposition is most important over lakes and contributes 69% ing 1985/86. If this difference in concentrations is also lin- (or 34.8 kg N ha 1 year 1) to the annual total (Table 1, Fig. − − early related to a difference in deposition, then our estimate 3). On the other hand, oxidized nitrogen is most impor- of aerosol dry deposition rather overestimates inputs from tant in the urban area of Bern, with a contribution of 46% this source. However, aerosol deposition is only in the or- (or 20.7 kg N ha 1 year 1) to the annual total. Because − − der of a few kg N ha 1 year 1, and therefore even a 37% we defined the urban area of Bern as a rectangle that also − − overestimation results only in an uncertainty of 1 kg N includes city forests and agricultural land at the border of ha 1 year 1. ± the built-up area, the fraction of oxidized nitrogen is below − − 50%, although the city center and central areas reveal a clear dominance of oxidized nitrogen deposition (Fig. 1d). Ammonia surface resistance over water

The high ammonia inputs to the lakes result from the as- sumption that ammonia does not experience a surface resis- Discussion tance due to its solubility in water (Erisman et al., 1994, for pH < 7–8), such that the transfer resistance that determines Validation of our estimates of wet and particulate aerosol the uptake rate of ammonia reduces to Ra+Rb. While such deposition is discussed first, followed by a discussion on an assumption is confirmed by experimental data over e.g. the assumption that Rc = 0 for NH3 over water surfaces moorland for winter conditions (Fowler et al., 1998), the 1 which may be questioned in light of recent publications. Fi- same study finds Rc typically in the range 100–120 s m− nally, total nitrogen inputs are compared with nutrient crit- during August. And Lee et al. (1998) find that Rc varies 1 ical loads that were defined by Rihm (1997) for the most between 0 and 360 s m− over sea water with a pH of 8.3 susceptible ecosystem types found in the study area. in their chamber experiment. If we therefore assume that an 334 W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335

1 Rc of 120 s m− would be more appropriate for the lakes tributes 13.8–33.5%, and dry deposition of aerosol particles in our study area than Rc = 0, then the deposition estimate 3.9–10.4%, to annual total nitrogen deposition. reported for lakes in Table 1 and Fig. 3 would reduce by 27– For the modeling of gaseous dry deposition we see the 46% depending on turbulence conditions. However, Lee et greatest potential for improvements in (a) implementing a al. (1998) conclude that their high Rc values are highly un- mosaic approach for each model grid cell that would al- certain. Therefore we assume that our deposition values for low inclusion of more detailed land cover information for lakes are equally uncertain, while the confidence in our Rc deposition modeling, (b) using a deposition module that is values for terrestrial ecosystems is considerably higher. based more on plant physiology, allowing us to also simu- late drought conditions and showing the behavior of stom- atal resistance more realistically, and (c) having emission Comparison of current loads with nutrient critical loads inventories that would allow us to model specific days, not Median total nitrogen deposition significantly exceeds the just representative days with average conditions. Addition- nutrient critical loads (Fig. 3) of bogs and shallow soft- ally, experimental measurements over the lakes are needed water bodies, montane-subalpine grasslands, acidic and cal- to validate our model results that rely strongly on the as- careous forests, and calcareous species-rich grasslands with sumption that Rc 0 for NH3 over a water surface. ≈ 95% confidence in all regions (Table 1). The range of crit- There is a 95% confidence that current nitrogen loads ical loads for mesotrophic fens (Fig. 3) is reached in the exceed the nutrient critical loads of sensitive ecosystems forested hills region and exceeded in lakes and the urban in our study area (bogs, shallow soft-water bodies, mon- area of Bern, but is not significantly exceeded in the agricul- tane and subalpine grassland, calcareous and acidic forests tural plains of the Seeland and along the Jura south slope. growing on nutrient-poor soils, and calcareous species-rich The lowest elevations of the Jura south slope receive an grassland). The clear dominance of gaseous dry deposi- annual load of nitrogen that is only insignificantly lower tion in our study area strongly suggests that local measures than that of the rural plains of the Seeland (Table 1, Fig. are very likely to be effective in decreasing atmospheric ni- 3), despite the fact that local sources of nitrogen emissions trogen inputs to sensitive ecosystems because of the short are almost absent in this area. The exceedance of the criti- (<10 km) transport distances of the most important trace cal loads for calcareous forests and calcareous species-rich gas, ammonia (Asman and van Jaarsveld, 1990). 1 1 grassland by >6.1 and >1.1 kg N ha− year− (p <0.05), Ammonia dry deposition is more important than depo- respectively, is remarkable because the Jura region is the sition of oxidized forms of nitrogeneous trace gases in the most diverse in natural and semi-natural ecosystems, with rural Seeland. Therefore we expect that decreasing ammo- the highest species diversity found in the study area. nia losses from agricultural sources has a better potential for Besides the montane and subalpine forests in the Jura reducing nitrogen inputs than reducing only the emission of mountains, there are no extended forested areas at lower el- sources that emit NOx. evations in the Seeland. Forests, primarily beech forests and mixed forests intermixed with spruce plantations, are found Acknowledgements only at locations with medium or low soil fertility where agricultural management is of less interest. Such conditions This study was funded by the Swiss Agency for the En- prevail on sandstone hills (some are drumlins from the last vironment, Forests and Landscape and supported by the glaciation), and therefore the region we named “forested Swiss National Science Foundation (grant 21-42050.94). hills” in Fig. 1f shows increased nitrogen deposition, not We wish to thank Brigitte Gälli Purghart and Jürg Fuhrer only because of the difference in surface roughness, but also (Bern, Switzerland) for supplying us with unpublished data due to differences in elevation (Table 1), and thus in prevail- on aerosol deposition and wet deposition, Ted Wachs (Uni- ing concentrations of nitrogeneous trace gases and aerosol versity of Bern) for lingustic assistance, and Michel Piot particles in the air. For forested hills, we obtained an an- (University of Bern) for statistical advice and support. nual nitrogen load that is 22% above the comparable value for the rural plains in the Seeland and exceeds the nutrient critical loads for both acidic and calcareous forest types by References 1 1 6.6 kg N ha− year− or more (p <0.05). Asman, W. H. A. and van Jaarsveld, H. A. (1990) A Variable-Resolution Statistical Transport Model applied for Ammonia and Ammonium. Conclusions Technical Report 228471007, National Institute of Public Health and Environmental Protection, Bilthoven. In this study most effort was put into modeling gaseous dry Bundesamt für Landestopographie (1992) RIMINI — Das digitale Höhen- modell der Schweiz mit einer Maschenweite von 250 Metern. deposition because atmospheric input of nitrogen is strongly Technical product information, Swiss Agency for the Topography, governed by trace gas deposition. Wet deposition con- Wabern, Switzerland. W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335 335

Erisman, J. W., van Pul, A., and Wyers, P. (1994) Parametrization of Sur- Liechti, M. (1996) Landnutzungskartierung im Gebiet des Seelandes und face Resistance for the Quantification of Atmospheric Deposition of des angrenzenden Juras - Rechnergestützte Erfassung der kleinräu- Acidifying Pollutants and Ozone. Atmospheric Environment, 28, migen Landnutzung mit multitemporalen Landsat-5 Thematic Map- 2595–2607. per Daten unter Einbezug von Arealstatistik und Pixelkarte. Mas- ter’s thesis, University of Bern, Institute of Geography, 111 pp. Eugster, W. (1994) Mikrometeorologische Bestimmung des NO2-Flusses an der Grenzfläche Boden/Luft. Geographica Bernensia, G37, 164 Müller, K. P., Aheimer, G., and Gravenhorst, G. (1982) The Influence of pp. Immediate Freezing on the Chemical Composition of Rain-samples. In Deposition of Atmospheric Pollutants, eds Georgii and Pankrath, Eugster, W. and Hesterberg, R. (1996) Transfer Resistances of NO De- 2 pp. 125–132. D. Reidel Publ. Comp., Dordrecht, Holland. termined From Eddy Correlation Flux Measurements Over a Litter Meadow at a Rural Site on the . Atmospheric Environ- Neftel, A., Wanner, H., Blatter, A., Eugster, W., Fahrni, M., Hester- ment, 30, (8), 1247–1254. berg, R., and Rosset, M. (1994) Stickstoffeintrag aus der Luft in ein Naturschutzgebiet. Vol. 28 of Umwelt-Materialien Luft. Swiss Fowler, D., Flechard, C. R., Sutton, M. A., and Storeton-West, R.L. (1998) Agency for the Environment, Forests and Landscape, Bern, 135 pp. Long Term Measurements of the Land-Atmosphere Exchange of Ammonia over Moorland. Atmospheric Environment, 32, (3), 453– Perego, S. (1996) Ein numerisches Modell zur Simulation des Som- 459. mersmogs. Geographica Bernensia, G47, 202 pp. Furger, M. (1990) Die Radiosondierungen von Payerne: Dynamisch- Perret, R. (1987). Une classification des situations météorologiques à klimatologische Untersuchungen zur Vertikalstruktur des Wind- l’usage de la prévision. Arbeitsbericht der Schweiz. Meteorol. feldes. Lenticularis, Opfikon (Switzerland), 191 pp. Anstalt, 46, 127 pp. Gälli Purghart, B. C. (1989) Schwermetalle auf grössenfraktioniertem Reinhardt, M. (1995) Abschätzung der Emission und Deposition von Am- Aerosol und in der Deposition: Untersuchungen an einem Höhen- moniak während verschiedener Jahreszeiten im Gebiet des See- profil im Kanton Bern. ADAG, Zürich, 126 pp. landes und des angrenzenden Juras. Master’s thesis, University of Bern, Institute of Geography, 80 pp. Gempeler, M. (1997) Bestimmung der Nassdeposition von Stickstof- fverbindungen im Gebiet des bernischen Seelandes. Master’s thesis, Rickli, R. (1988). Untersuchungen zum Ausbreitungsklima der Region University of Bern, Institute of Geography, 113 pp. Biel. Geographica Bernensia, G32, 120 pp. Hess, P. and Brezowsky, H. (1969). Katalog der Grosswetterlagen Eu- Rihm, B. (1997) Critical Loads of Nitrogen and their Exceedances: Eutro- ropas. Berichte des Deutschen Wetterdienst, 113. phying Atmospheric Deposition. Vol. 275 of Environmental Series. Swiss Agency for the Environment, Forests and Landscape, Bern, Hesterberg, R., Blatter, A., Fahrni, M., Rosset, M., Neftel, A., Eugster, W., and Wanner, H. (1996) Deposition of Nitrogen-Containing Com- 82 pp. pounds to an Extensively Managed Grassland in Central Switzer- Sachs, L. (1978) Angewandte Statistik. Springer Verlag, Berlin, 5th edi- land. Environmental Pollution, 91, (1), 21–34. tion, 552 pp. Krieg, F. (1997) Stickstoffhaltige Gas- und Aerosolkomponenten in Pay- Schuepp, M. (1968). Regionale Klimabeschreibungen, 1. Teil. Beiheft z. erne 1995. Vol. 76 of Umwelt-Materialien. Swiss Agency for the d. Ann. Schweiz. Meteorol. Zentralanstalt, 2, 245 pp. Environment, Forests and Landscape, Bern, 84 pp. Schumann, T. (1989) Precipitation Scavenging of Aerosol Particles: A Kunz, S., Künzle, T., and Rihm, B. (1995) TRACT Emissionsmod- Winter Time Field Study. PhD thesis, ETH Zürich Nr. 11382, ell Schweiz. Ein raum-zeitlich hochaufgelöster Emissionskataster Zürich, Switzerland, 295 pp. für Forschungszwecke im Bereich des atmosphärischen Schadstoff- Stockwell, W. R. (1986) A Homogeneous Gas Phase Mechanism for Use transports. Technical report, Meteotest, Fabrikstrasse 29, 3012 Bern, in a Regional Acid Deposition Model. Atmospheric Environment, Switzerland, 63 pp. 20, 1615–1632. Künzle, T. and Neu, U. (1994). Experimentelle Studien zur räumlichen Wanner, H. and Furger, M. (1990). The Bise - Climatology of a Regional Struktur und Dynamik des Sommersmogs über dem Schweizer Mit- Wind North of the Alps. Meteorol. Atmos. Phys., 43, 105–115. telland. Geographica Bernensia, G7, 211 p. Wanner, H. and Kunz, S. (1977). Die Lokalwettertypen der Region Bern. Larcher, W. (1995) Physiological Plant Ecology. Springer, Berlin, 3rd edi- Beitr. zum Klima der Region Bern, 9, 96 pp. tion, 506 pp. Wesely, M. L. (1989) Parametrization of Surface Resistances to Gaseous Lee, D. S., Halliwell, C., Garland, J. A., Dollard, G. J., and Kingdon, R. D. Dry Deposition in Regional-Scale Numerical Models. Atmospheric (1998) Exchange of Ammonia at the Sea Surface U˝ A Preliminary Environment, 23, 1293–1304. Study. Atmospheric Environment, 32, (3), 431–439. 506 p. Wesely, M. L. and Hicks, B. B. (1977) Some Factors That Affect the De- Leuenberger, A. (1996) Berechnung der gasförmigen Deposition von oxi- position Rate of Sulfur Dioxide and Similar Gases on Vegetation. J. dierten Stickstoffverbindungen im Gebiet des bernischen Seelandes. Air Pollution, 27, 1110–1116. Master’s thesis, University of Bern, Institute of Geography, 167 pp.