Elevated NH3 and NO2 air concentrations and nitrogen deposition rates in the vicinity of a highway in Southern Bavaria

ARTICLE IN PRESS

Atmospheric Environment 39 (2005) 4531–4542 www.elsevier.com/locate/atmosenv

Elevated NH3 and NO2 air concentrations and nitrogen deposition rates in the vicinity of a highway in Southern Bavaria Manfred Kirchnera,, Gert Jakobia, Ernst Feichta, Markus Bernhardtb, Anton Fischerb a

GSF—National Research Centre for Environment and Health, Institute of Ecological Chemistry, Ingolstaedter Landstrasse 1, D-85764 Neuherberg, Germany b Department of Ecology, Centre of Live and Food Sciences of Technische Universita¨t Mu¨nchen, Am Hochanger 13, D-85354 Freising, Germany Received 4 August 2004; received in revised form 4 August 2004; accepted 29 March 2005

Abstract A transect study consisting of air concentration and deposition measurements of nitrogen compounds was performed to estimate the potential influence of car emissions on the nitrogen input to ecosystems. Therefore, two transects each consisting of 4 plots, the first in a coniferous forest and the second one in an extensively farmed grassland, were installed perpendicular to a highway south of Munich (Bavaria). Both profiles were influenced mainly by car emissions and showed only small local influences caused by agricultural activities. In the framework of a pilot study based upon denuder measurements we found a strong temporal dependency of both nitrogen dioxide (NO2) and ammonia (NH3) concentrations on traffic density. In the main study air concentrations of NO2 and NH3 were measured by passive samplers; they used as the basis for the estimation of dry deposition. These estimations have been compared with the results of analyses from simultaneously conducted canopy throughfall deposition and open air bulk measurements of + nitrate (NO
3 ) and ammonium (NH4 ). Additionally, within the forest transect the variety of different soil vegetation species was recorded and quantified. We obtained a strong gradient of gas concentrations along both profiles. Whereas the bulk deposition remained quite constant along the non-forested transect, the nitrogen throughfall deposition rate diminished substantially with the distance from the highway. The deposition rate at the forest edge was twice of that inside. The nitrogen load estimated for the examined forest in the vicinity of the highway was comparable to other forest ecosystems situated near diffuse emission sources from agriculture. It could be shown that changes in soil composition and soil vegetation along the forest transect are caused by decreasing nitrogen deposition with distance from the highway. The application of road salt in winter leads to further impacts. r 2005 Published by Elsevier Ltd. Keywords: Road traffic; Emissions; Transect; Forest edge; Ground vegetation

1. Introduction Corresponding author. Fax: +49 89 3187 3371.

E-mail address: [email protected] (M. Kirchner). 1352-2310/$ - see front matter r 2005 Published by Elsevier Ltd. doi:10.1016/j.atmosenv.2005.03.052

Fuel combustion represents the major source of manmade nitrogen oxides (NOx). Although in the last 10 y

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emissions by gasoline powered cars could be reduced significantly following the introduction of the three-way catalytic converters (Kean et al., 2000), NOx emissions remain a problem for the environment because of the increasing road traffic in general and the increasing portion of diesel cars and heavy duty vehicles. The emission factors of NOx range in 5707400 for gasoline powered cars and in 376073000 mg km
1 for diesel cars depending on driving speed; for heavy duty vehicles they may be up to 10 times higher (Lenaers, 1996; NOREM, 1998; Ramamurthy and Clark, 1999). The emitted nitric oxide (NO) is rapidly oxidized to form nitrogen dioxide (NO2), which can be removed from the atmosphere via dry, wet and occult deposition resulting nitrates. Caused by the low deposition velocity (vd) of 0.1–0.24 cm s
1 for meadows (Hesterberg et al., 1996) and 0.2–1.0 cm s
1 for forests (Sutton et al., 1993; Volpe-Horii, 2002) lifetime of NO2 can amount to some days and deposition occurs far from the sources. Furthermore wet deposition of nitrate (NO
3 ) will dominate the overall NOy deposition; the dry deposition of nitrate may be of minor extent. While agriculture, particularly livestock farming, represents the main source of ammonia (NH3), traffic might be the most important factor influencing ammonia concentrations at urban locations and near roads since the introduction of three-way-catalysts, as recent inventories of non-agricultural sources show (Sutton et al., 2000; Kean et al., 2000; Lo¨flund et al., 2002). Emission measurements performed in petrolengine vehicles equipped with catalytic converters show NH3 emission factors of 50740 mg km
1 (NOREM, 1998; Kean et al., 2000; Baum et al., 2001); in the case of diesel cars the range of NH3 emission factors is 0,4–10,9 mg km
1 (Fraser and Cass, 1998). In some regions the vehicle NH3 emissions could grow dramatically in the future being approximately equivalent to the magnitudes of livestock emissions when road traffic increases and particularly fuel sulphur contents further decrease (Fraser and Cass, 1998). NH3 is the most abundant basic gas in the atmosphere and is important as a neutralizer for acidic species and forms ammonium sulphate, nitrate or chloride and may be deposited by dry and wet deposition processes (Aneja et al., 2001). Characterized by a relatively high deposition velocity of 0.5–2.2 cm s
1 for meadows (Hesterberg et al., 1996; Rihm, 1996) and 0.8–4.5 cm s
1 for forests (Duyzer et al., 1994) and a short lifetime in the atmosphere of 30 min–5 d (Warneck, 1988; Walker et al., 2000) NH3 will deposit to a great extent near its source. However, ammonium (NH+ 4 ) aerosols tend to deposit at larger distances because of their lower deposition velocity and resultant higher atmospheric lifetime of 1–15 d (So¨derlund and Svensson, 1976; Aneja et al., 2001).

Given a certain atmospheric concentration of the singular nitrogen compounds different earth surfaces corresponding to their aerodynamic properties are characterized by different load of deposited materials. Vegetation with a higher leaf area index acts as a greater sink for atmospheric gases and particles than meadows. In forests dry deposited materials thus increase the wet deposition, which differs less between different surfaces. Generally this is valid for all kinds of natural and manmade substances; however, there are differences between the singular components due to canopy retention and leaching effects (Draaijers et al., 1988). Especially at forest edges high atmospheric input caused by higher amount of transported air and greater leaf area index compared to the interior of the forest is found (Beier and Gundersen, 1989; Spangenberg, 2002). It has to be distinguished between more or less intensive and diffuse or localized sources located upwind from the forest edge. Several studies provide results from measurements at forest edges which are influenced by agricultural emissions originating from liquid manure spreading on grassland or industrial animal farming (Hasselrot and Grennfelt, 1987; Lindberg and Owens, 1993; Spangenberg, 2002; Kirchner et al., 2002), while fewer measurements have been performed to quantify the nitrogen input to grassland and forests caused by road traffic. The excessive deposition of reduced (NHy) and oxidized (NOy) forms of nitrogen observed in Central Europe during the last decades (Melzer et al., 1992; Spangenberg, 2002) may severely influence the nitrogen status of natural ecosystems (Hesterberg et al., 1996; Krupa, 2003). The uptake of N in plants is performed by shoots and roots (Krupa, 2003). While acute effects on plants could be observed only in the vicinity of animal farms (Hofmann et al., 1990), chronic consequences become more and more evident in many regions. Accumulation and saturation of inorganic N may lead to soil acidification and hence NO
3 leaching into the groundwater, when the forest soil is N-saturated (Smidt, 2002); enhanced NO
3 avaibility in the soil will also increase both denitrification and the production of NO and N2O in the process (Rennenberg and Gessler, 1999). It is a contributory factor behind soil acidification which in turn leads to the leaching of nutrients and the mobilization of heavy metals (Fabian, 1987). As a result of changes in soil and water environments excessive nitrogen deposition may cause modifications in the species composition of flora and fauna and influence competitive ability of species resulting in ecosystem changes and possibly in a decrease of biodiversity (Ellenberg, 1985; Melzer et al., 1992; Prietzel et al., 1997; Pitcairn et al., 1998). The main purpose of the present study is to quantify the extent of the enhancement of nitrogen input along two transects perpendicular to a highly frequented road.

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2. Experimental design 2.1. Location and experimental sites The investigation area is located south of Munich in the Bavarian district of Upper Bavaria along the highway A 95 from Munich to Garmisch-Partenkirchen; a map is shown in Fig. 1. This pre-alpine region is characterized by prevailing winds from SW to W, abundant precipitations, as moist air masses from NW and N are forced to rise over terrain of increasing altitude. Such meteorological conditions were observed during the first year of observation (2002) in contrast to the second year (2003), which was characterized by extremely dry weather. Perpendicular to highway two measuring transects have been installed. An open field (‘grassland’) transect was located 40 km south of Munich in an extensively managed litter meadow with some bushes and trees (j ¼ 471460 , l ¼ 111200 , z ¼ 615 m) 5 km far from a small village (Antdorf, province of Weilheim/Schongau). This transect was oriented eastwards from the highway, where approximately 22,600 vehicles (about 60% gasoline powered cars) pass by per day. At four measuring points (A I–A IV) in a distance of 40, 170, 300 and 410 m from the highway bulk and passive samplers were installed and changed once every two or four weeks. Additionally a meteorological station recorded air temperature and humidity, wind speed and direction, solar radiation and precipitation data. South of the grassland transect there is a bog (Ho¨llfilz), adjacent to the litter meadow in the north and east there are forest stands and small agricultural areas with only sporadic spreading of liquid manure; the averaged number of cattle in the surrounding areas is approximately 1.2 ha
1. The forest transect was established 5 km south of Munich in the Forstenrieder Park, a 90–110 y old

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homogenous Norway spruce forest (Picea abies L. Karst) (j ¼ 481020 , l ¼ 111260 , z ¼ 610 m), at the eastern side of the highway which intersects the forest in a 90 m wide aisle and with a traffic density of approximately 62,400 vehicles per day. The investigated forest edge is exposed to the prevailing winds from SW and W. The stand is characterized mainly by the influence of the vehicular emissions. Any other sources located nearer than 2 km can be excluded. Canopy troughfall measurements were performed at a distances 50, 130, 260 and 520 m from the highway (F I–F IV) , 20, 100, 230 and 490 m behind the front trees, respectively. At each of these locations throughfall was sampled on a 2- or 4weekly basis by 12 funnels, arranged in an area of about 20  20 m, in order to minimize the non-random variability in the stand (Beier and Gundersen, 1989). Additionally open field bulk precipitation (FF) was collected in a clearing of 90  190 m at approximately 270 m from the highway. All sites were instrumented with passive samplers, too. While in the clearing (FF) the passive sampler measurements served in order to have comparable data base for open field conditions without direct influence of traffic emissions, another passive sampler (site F 0) was placed at a distance of 20 m from the driveway directly at the forest edge, in order to quantify the concentrations in the vicinity of the vehicular sources. In November 2002 soil samples were taken along the forest transect. Each point was characterized by three samples, consisting of three sub-samples. Each soil sample was divided into three layers (litter, mineral soil of 0–5 cm depth and mineral soil of 5–10 cm). To investigate changes in vegetation composition with increasing distance from the highway, vegetation sampling was carried out in June 2003. At each point of the transect five plots (5m  5 m) were sampled. The traffic-related concept of the investigations was based upon the results obtained from a prior pilot study (March 2001) in which active NH3 concentration measurements had been performed on the eastern side of the same highway in order to detect dependencies and diurnal trends. The measurements had been performed 2 km from the forest transect site (Forstenrieder Park) at the highway police station Oberdill, where electricity power was available. Measurements had been conducted at a distance of 30 m from the eastern highway road board. 2.2. Measurement technique and analytical procedures

Fig. 1. Location of the two investigation areas in Southern Bavaria: forest (spruce) and open-field (grassland) transect.

In the pilot phase we performed continuous NH3 concentration measurements using the wet annular denuder system AMOR, which has been developed by the Netherlands Energy Research Foundation (ECN) for ambient air (Wyers et al., 1993) and which served as reference system during a field intercomparison of

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diffusive samplers for measuring NH3 (Kirchner et al., 1999). The method is very selective and quite independent meteorological parameters. With a time resolution of 2 min the measuring range is between 0.05 and 1000 mg m
3. Along the two transects situated in areas where no electricity power was available passive samplers were used in order to obtain 4-weekly average concentrations of atmospheric gases. Because of the concentration values which are thought to decrease with growing distance from the highway, battery powered ventilated samplers which have an average lower detection limit of 0.05 mg m
3 for monthly concentrations were used. The samplers contain two different glass fibre filter halves (diameter 5 mm); the first is impregnated with triethanolamine (TEA) in combination with eugenol and potassium carbonate for NO2, the second with citric acid for NH3 measurements. After the exposure the first filter half was analysed by ion chromatography and additionally by photometry as control method (VDI 2453, Blatt 1, 1990) and the second one by photometry using the Berthelot reaction for quantitative assessment (Kirchner et al., 1999). The NH3 sampler type had been tested in a field intercomparison together with other devices showing a good agreement with the continuously working denuder (Kirchner et al., 1999). Similar comparisons had been performed for NO2 passive samplers (Kirchner, 1997). Passive sampler results serve as basis for estimations of NO2–N and NH3–N (dry) deposition. Precipitation was collected in open field (A I–A IV, FF) and as throughfall (F I–F IV) by using bulk samplers with a frequency of 4 weeks. Bulk samplers are continuously open collectors which sample material that enters during both wet and dry periods. The use of bulk samplers was necessary because electricity power was not available. Previous studies showed that the differences between bulk and wet samplers, which are open only when it is raining or snowing, are small in relation to the major ion concentrations (Della Lucia et al., 1996; Umweltbundesamt, 1996, Kirchner et al., 2001); reasonable adjustment factors for Bavaria are 0.89 for NH+ 4 (NH4–N deposition), 0.83 for NO
3 (NO3–N deposition), but lower than 0.70 for K, Ca and Mg (Della Lucia et al., 1996; Umweltbundesamt, 1996, Kirchner et al., 2001). Changes in chemical composition of the precipitation water after sampling were minimized by immediate storage in refrigerator; this is necessary because particularly NH+ 4 concentration can decrease significantly during storage (Karlsson et al., 2000). The concentra
2
2

tions of NH+ 4 , NO3 , SO4 , PO4 and Cl were analysed either by segmented continuous flow analyser or ion chromatography with conductivity detection, while the concentrations of cations (Ca2+, Mg2+, K+ and Na+) were determined using the Inductively Coupled Plasma

Atomic Emission Spectrometry (ICP-AES). Rain samples from site F I and A I were analysed by another four laboratories in order to test the reliability of analyses. The results from these intercomparisons were very satisfactory. Soil analysis was carried out using the methods of the AG Boden (1996). Ground vegetation was sampled and classified according to the method established by BraunBlanquet (1964). The ecological–sociological investigations were performed along the forest transect (F I–F IV) and until 1 km from the highway. In order to estimate the maximum range of the influence of traffic emissions cluster analysis was adopted (Mather, 1976). 2.3. Critical levels and loads The critical level/critical load concept for environmental protection is widely accepted as an appropriate tool to provide estimates of targets for decision-makers (Krupa, 2003). It encompasses not only direct injuries by high concentrations of NH3 and NO2, but also secondary and chronic effects such as increased susceptibility to stress or altered competitive ability, which can be the consequence of long-lasting deposition of nitrogen compounds. Both critical levels and loads serve to interpret the results from concentration and deposition measurements. WHO (2000) established 30 mg m
3 (annual mean) and 75 mg m
3 (24-h mean) as critical levels for NO2 regarding all vegetation types. Critical levels for NH3, which have been derived from eco-toxicological models (Van der Eerden et al., 1998), are 8 mg m
3 (annual mean) and 270 mg m
3 (24-hour-mean). The critical loads have been established at 5–10 kg N ha
1 y
1 for ombrotrophic bogs, at 10–15 kg N ha
1 y
1 for unmanaged grassland and at 15–20 kg N ha
1 y
1 for forests corresponding to their nutritional status and species composition (Krupa, 2003; Umweltbundesamt, 1996; Hesterberg et al., 1996; Gauger et al., 2000; Kirchner et al., 2001). So the sensivity of forests on silicate soil to excess of nitrogen input may be higher than the susceptibility of those on calcareous soils.

3. Results and discussion 3.1. Ammonia denuder measurements The results obtained from the NH3 concentration denuder measurements, which were performed in a short pilot phase of two weeks in March 2001, are shown in Fig. 2. Although actual 2 min data were monitored, the results are reported as hourly values by separating workdays and weekend. Additionally, the hourly frequency of vehicles on highway is reported. Both curves show a similar daily cycle which reveals a

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Fig. 2. Hourly mean ammonia concentrations and traffic densities for workdays and weekend at the Munich–Garmisch–Partenkirchen highway near Oberdill (Forstenrieder Park) during March 2001.

relationship between NH3 air concentrations and traffic density. The concentrations are characterized by a very clear daily variation with a minimum in nights and higher values during days. It must be distinguished between working days with a bimodal and work-free days with a unimodal distribution of both curves. Between Monday and Friday huge numbers of commuters get to the city of Munich in the early morning and leave it in the late afternoon. On Saturday and Sunday there is a more time-independent weekend excursion traffic from Munich to the Upper Bavaria Lake region and to the Alps with maximum vehicle movements between late morning and late afternoon. Whereas the traffic intensity (with hourly peak values between 4000 and 5500 vehicles) is similar during the two weeks, the absolute NH3 concentrations reflect the current weather conditions. Wind direction and air humidity are the main influencing factors. Decrease in concentrations during night hours is enhanced due to the frequently occurring dew formation on vegetation, which is an important sink for soluble gaseous species such as NH3. Generally, when combined with water, NH3 is rapidly converted to NH+ 4 ions (Krupa, 2003). Analogous measurements at one of the most polluted crossroads of Munich with approximately 1,20,000 vehicles per day showed similar diurnal variations of NH3, but with a higher concentration level (Kirchner et al., 2001). At these two places concentration peaks between 20 and 70 mg m
3 could be observed. Similar hourly NH3 measurements in urban sites of Rome

ranged from 3.8 to 45.6 mg m
3 (Perrino et al., 2002). A comparison between NH3 concentrations and traffic density at some traffic junctions in Munich, Bavaria and Salzburg, Austria, gave a linear correlation with r2 ¼ 0:96 (Lo¨flund et al., 2002). The close link between NH3 concentrations and dispersion from traffic sources could be confirmed by parallel CO and NO2 measurements conducted at some crossroads in Munich which showed the same diurnal cycle (Kirchner, 2002). Referring to short measurements performed in Rome Perrino et al. (2002) came to the same suggestion that NH3 and CO might have traffic emissions as source. 3.2. Concentration measurements along the transects The results of NH3 air concentration measurements conducted along the transect Antdorf during 2002 are given in Fig. 3. The transect shows a decrease of NH3 concentrations with increasing distance from source caused by mixing and dry deposition processes. The yearly means of NH3 concentration decreased along the Antdorf transect from 3.5 (A I) to 2.0 mg m
3 (A IV) in 2002, where westerly winds dominated easterly winds with a frequency relation for winds 40.2 m s
1 by 1.24, and respectively from 4.9 to 4.2 in 2003, where the frequencies of both wind direction sections were similar (1.04). The concentrations are influenced mainly by traffic and secondarily by agricultural emissions. The concentration levels reflect the presence of a scrub strip between road board and measuring devices. In areas which are unaffected by NH3 sources mean air

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Fig. 3. Seasonal distribution of ammonia concentrations at the grassland transect (Antdorf) near the Mu¨nchen–Garmisch–Partenkirchen highway in 2002. The results are given for 4 plots with increasing distance from the highway.

concentrations amount to 1–1.5 mg m
3, in contrast NH3-loaded rural areas show concentrations of 10–15 mg m
3. In the vicinity of animal stables concentrations can exceed these values by a factor 5 to 10 (Fischer-Riedmann, 1995; Kirchner et al., 2001). NO2 at this transect shows a similar dependency on the distance from the source: The mean concentration of NO2 dropped from 7.4 (A I) to 3.9 mg m
3(A IV) in 2002 and 10.2 (A I) to 7.0 mg m
3 (A IV) in 2003. The corresponding mean annual concentrations for NH3 along the forest transect decreased from 3.0 near the highway (F 0) to 2.1 mg m
3 (F I), 1.3 (F II), 1.0 (F III) and 1.0 mg m
3 (F IV) within the forest during the wet year of 2002, whereas mean concentrations of 4.3 (F 0), 3.4 (F I), 2.5 (F II), 2.1 (F III) and 1.7 mg m
3 (F IV) were measured in the dry year 2003. The increase from the interior of the forest (F IV) to the forest edge (F 0) was about a factor of 3.1 in 2002 and 2.5 in 2003. The NH3 concentrations at the clearing site FF (1.1 respectively 2.1) were similar to those of F III during both periods. The comparison of the results obtained during the two years makes evident that the dry conditions in 2003, which are accompanied by drier surfaces and lower solubility of NH3 upon the needles, were responsible for the higher values. NO2 showed values of 13.8 (F 0), 9.6 (F I), 8,0 (F II), 6,2 (F III) and 4.6 mg m
3 (F IV) in 2002 and 17.2 (F 0),

14.0 (F I), 11,6 (F II), 8,9 (F III) and 7.1 mg m
3 (F IV) in 2003. The elevated concentration level of NO2 far from the highway is caused by urban emissions from combustion processes as regards domestic fuel and traffic. Unaffected areas at background stations in the Bavarian Alps are characterized by NO2 concentration levels below 2 mg m
3 (Kirchner, 1997). In order to distinguish enhanced concentrations caused by edge effects from those due to direct car emissions along the forest aisle we compared the NH3 and NO2 air concentrations measured at the forest plot F III and in the clearing site FF, both situated at the same distance (300 m) from the highway. Since the differences between the two sites were scarcely ascertainable, we follow that the enhanced concentrations in air and bulk trials are caused predominantly by the car emissions from the highway. Pure forest edges effects which arise from the special barrier and filter conditions would be more important if the highway would not intersect the forest in a narrow aisle but would be located at the edge of the forest. The difference of NO2 levels between the two transects reflects the different traffic densities, too. In contrast, the NH3 concentration along the grassland transect was higher than in the forest. This can be explained by the higher background concentrations attributable to agricultural activities and the less effective dry deposition

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rate caused by the lower leaf area index for grass than for forest. Regarding the situation at the sites A IV and F IV, which both are less influenced by traffic, there is a certain indication of an annual variation with elevated NH3 values between early spring and late autumn as described in literature (Fischer-Riedmann, 1995) and during periods with a smaller number of days with precipitation. 3.3. Bulk and throughfall deposition measurements along the transects For the grassland transect we observed in the case of NH+ 4 –N only a slightly enhanced bulk deposition near the highway compared to areas some 100 m away because of the fact that the overwhelming portion of bulk deposition is governed by rain and snow precipitation, which is scarcely influenced by adjacent sources. The yearly NH4–N input increases from 6.2 (A IV) to 6.7 kg ha
1 (A I) in 2002 and from 4.0 (A IV) to 4.9 kg ha
1 (A I) in 2003; the corresponding precipitation amounts were 143679 mm in 2002, approximately 15% higher than the 1951–2000 average, and 85875 mm in 2003, approximately 40% lower than long-time average. In the extreme dry year 2003 the deviation between the NH+ 4 deposition rates at A I and A IV were greater (22%) than during the wet year 2002 (8%). Because of the relatively low frequency of fog events in the area of Antdorf, which are mostly restricted to the early morning hours (2002: 18 d; 2003: 22 d), disregarding of occult deposition measurements is leading to less discrepancies from total input than in mountainous regions. Contaminations such as bird dropping may exert a marked influence particularly upon NH+ 4 content of precipitation water. However, due to the fact that the sampling sites along the two transects had been chosen carefully, contaminations could be minimized; in the majority of cases PO
4 concentrations were below detection limit. Additionally we experienced in a short pilot study that the selected length of sampling interval of 4 weeks appears to have little influence on determined concentrations for major anions and cations in the prealpine region, particularly if the precipitation is abundant and the groundwater level is high in order to exert a cooling effect to the bulk sampler water; Madsen (1982) and Granat (1974) found similar effects. For drier regions a maximum exposure interval of 2 weeks is reasonable (Slanina et al., 1987). Meteorological situations which facilitate rapid conversion of NH3 to NH+ 4 are restricted to periods with fog and drizzle. Little differences between A I and A IV may occur in the case of higher frequency of wind from the sectors northeast to southeast; in 2003 the frequency quotient of E winds in comparison to W winds were higher (0.96) than in 2002 (0.80).

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Comparing the quotient of deposition between A I and A IV in order to estimate the gradient along the transect different results have been obtained for the singular ions; no gradient (0.9–1.1) could be detected in + 2+ the case of NO
and Mg2+, low gradients 3 , SO4 , K (1.1–1.5) for Ca2+ and NH+ and strong gradients 4 (42.5) in the case of Na+ and Cl
. The results obtained for the forest transect show stronger gradients. The chemistry of precipitation that reaches the forest floor is influenced not only by the chemical and hydrological characteristics of incident precipitation but also by wash-off of dry-deposited materials from canopy surfaces, release and uptake by trees. Therefore throughfall water is generally enriched in most chemical elements compared to bulk precipitation. Along the transect all sites received similar precipitation below crown (2002: 827733 mm; 2003: 491717 mm), but substantial chemical differences were found in the canopy throughfall fluxes; at the open field rain gauge in the clearing the precipitation amounts in 2002 were 1291 mm and in 2003 year 831 mm.While in 2002 the NH4–N deposition decreased from 12.1 (F I) to 7.7 (F II), 7.6 (F III) and 5.8 kg ha
1 y
1 (F IV), the NO3–N deposition behind the edge was 8.7 (F I) in comparison to 6.5 (F II), 7.2 (F III) and 6.7 kg ha
1 y
1 (F IV) within the forest. In 2003 the NH4–N throughfall deposition rates were 10.9 (F I), 6.5 (F II), 7.7 (F III) and 5.6 kg ha
1 y
1 (F IV), the NO3–N decreased from 7.3 (F I) to 4.9 (F II), 6.2 (F III) and 4.9 kg ha
1 y
1 (F IV). The high NH+ 4 deposition at F I may be caused by NH3 car emissions in combination with the forest edge effect. The important role of salt use along the highways in wintertime is reflected by 45.6 (29.7) and 22.8 (13.4) kg Na+ ha
1 y
1 measured in 2002 and 2003) at the same plot. The deposition ratio between the inputs of F I and F IV reveals no gradient (0.9–1.1) for K+, low gradients (1.1–1.5) for Ca2+, Mg2+, NO
3 and + SO2+ 4 , strong gradients (1.5–2.5) for NH4 and very strong gradients (42.5) for Na+ and Cl
. Comparing these data with the open field conditions of site FF, all ions show low deposition rates during the two years (NH+ 4.3 vs. 3.6 kg ha
1; NO
4.3 vs. 4 –N: 3 –N:
1 3.9 kg ha ; Cl
: 1.4 vs. 2.4 kg ha
1; Na+: 1.2 vs. 1.0 kg ha
1). The composition of nitrogen throughfall and bulk deposition far from the traffic sources showed a
NH+ 4 –N/NO3 –N ratio of 1:1. The plots close to the highway are characterized by a clearly increased share of NH+ 4 –N (approximately 60%), which is caused by the rapid conversion of NH3 to NH+ 4 . The results are comparable to those obtained from transects near animal farms (Spangenberg, 2002). In background areas NH3 dry deposition is quite balanced with wet deposition of reduced forms of nitrogen during normally wet years. In dry years and near sources dry deposition seems to be much more important. In the case of

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oxidized forms wet deposition overwhelms the dry portion in bulk gauges in open field areas and in forests with the exception of forest edges. 3.4. Estimation of total deposition The total nitrogen deposition can be estimated by adding dry and wet deposition. The wet deposition can be calculated from the bulk deposition using the previously mentioned correction factors for NO3 (0.83) and NH+ 4 (0.89). A commonly used approach in describing dry fluxes is the resistance analogy, which assumes that the flux is proportional to the concentration. The proportionality coefficient is generally known as the deposition velocity which can be expressed by the reciprocal value of a series of resistances. In any case a quantification of all nitrogen deposition processes without flux measurements is difficult, because each kind of land cover will exhibit a unique receptiveness to the different nitrogen components and receptiveness will change with environmental conditions. Regarding the two horizontal transects dry deposition was estimated from an interferential approach (Sutton et al., 1994), where the measured concentration of NH3 and NO2 are combined with published surface-specific deposition velocities vd The concentrations may be multiplied by the typical deposition velocity for these gases derived from measuring campaigns which had been performed in Central Europe (Wyers et al., 1992; Erisman et al., 1993; Sutton et al., 1994; Ferm and Hultberg, 1999). Based upon these data we chose 1.4 and 2.2 cm s
1 as the basis for the estimation of dry deposition velocity vd of NH3 above extensively farmed meadows respectively in spruce forests, respectively; the corresponding literature-based mean values for NO2 are 0.17 and 0.35 cm s
1 for the two vegetation types. Along the grassland transect the total nitrogen deposition increased from 16.8 (A IV) to 23.0 kg ha
1 (A I) in 2002. The corresponding nitrogen input in 2003 increased from 22.0 in rural background conditions to 25.6 kg ha
1 near the highway. Referring to the background conditions at A IV wet deposition (56%) dominates the total deposition during the wet year 2002, whereas in 2003 dry deposition (69%) overwhelms wet deposition (31%). Near the highway (A I) the dry portion is greater than the wet portion in both years (2002: 56%; 2003: 70%). In all cases the reduced nitrogen (NH3, NH+ 4 ) is the prevailing chemical form (72–82%). Along the forest transect we obtained a strong gradient of total nitrogen deposition (Fig. 4). Directly at the forest edge at the first row of spruce trees (F 0) total input is 29.7 in 2002 and 37.9 kg ha
1 in 2003. The total deposition amounts to 14.5 and 18.7 kg ha
1 490 m behind the front trees (F IV) and is reduced by

approximately 50% compared to F 0. Whereas the relation between dry and wet deposition in the centre of the forest is balanced during the wet year 2002, the dry portion is greater in 2003 (65%). Near the highway (F 0) total deposition consists of dry deposition to an amount of 74% in 2002 and 81% in 2003. The reduced nitrogen predominates over the oxidized species (NO2, NO3). Comparing the throughfall deposition, which is enriched by canopy exchanges as well as dry deposition of aerosols and gases and the theoretically estimated total deposition the results were similar along the forested transect. Whereas the deposition amounts differ as consequence of the incomplete consideration of dry portions by the throughfall concept, the decreases of input from F I to F IV were comparable. The comparison shows that the assumption of an ammonia deposition velocity (vd) of 2.2 cm s
1 as a representative mean of different measuring campaigns obtained from literature review reflects very well the real conditions during the normally wet year 2002. Under the very dry climatic conditions of 2003 this value may overestimate the dry deposition. These assumptions may be confirmed by investigations of Erisman et al. (1993), which indicate a reduction of vd under dry conditions. In the framework of this transect study a parameterization of wetness of plant surfaces possibly based upon precipitation, humidity and radiation data seems to be extremely speculative. Furthermore the purpose of this project was to derive discrepancies of input on similar surfaces. 3.5. Impacts on soil and forest plant communities Along the forest transect F I–F IV a gradient of nutrient soil content can be determined. With increasing distance from the highway the storage of nitrogen in upper soil (0–10 cm) decreases from 1149 at F I to 733 kg ha
1 at F IV. A similar gradient has been shown for the pools of K, Ca and Mg. This decrease is ascribable to the raise of dust by cars and the always relevant edge effect. Due to the application of road salt in winter, the storage of Na and Cl in soil increases from 2.0, respectively 2.7 kg ha
1 in the centre of the forest (F IV), to 24.8,respectively 8.7 kg ha
1 behind the forest edge (F I). Caused by the input of the different substances, pH diminishes from 4.9 at F I to 3.8 at F IV. Correspondingly, the base saturation of the soil decreases from 75% to 14%. As consequence of the high variance of the results for the five subplots at each site statistically significant differences between the sites F I–F IV could be found for base saturation, nitrogen, Ca2+, Mg2+ and Na+. Furthermore, the C/N relation decreases with distance from highway. High nitrate export in seepage was observed near the highway. During 2003 the average NO3 concentration of seepage water in the deeper root zone (40 cm depth) reached approximately 200 mg l
1 at F I compared to an

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Fig. 4. Annual deposition of nitrogen compounds at Forstenrieder Park transect (FI: 50 m from highway; FIV: 520 m from highway) in 2002 and 2003 by comparing total deposition and throughfall. The total deposition consists of dry deposition fluxes calculated on the basis of passive sampler measurements of NH3 and NO2 and measured open-field wet deposition.

average value of 15 mg l
1 in the interior of the forest. Monthly mean values ranged between 65 and 441 mg l
1, the highest concentrations were reached in autumn 2003 after seepage water had run dry during August and September. Whereas the contamination is restricted to the area adjacent to the highway and the NO3 concentration in seepage water diminishes with depth, the infiltration may not be negligible considering the threshold value of 50 mg l
1 for drinking water (European Commission, 2002). Parallel to the changing soil conditions along the transect, which reaches from the border to the centre of the forest, plant abundances of Rubus fructicosus (blackberry) and Mycelis muralis (wall lettuce) decrease. Plant covers of Vaccinium myrtillus (blueberry) and Hylocomium splendens (stair-step moss) show a reverse development. Plant cover increases with increasing distance from the highway. While plant cover of V. myrtillus and H. splendens are positively correlated to the soil contents of N, Ca2+, Mg2+ and Na+, R. fructicosus and M. muralis are negatively correlated to them. Deeper in the forest no more changes in ground vegetation have been observed respectively statistically proved. This may indicate that the immediate impact by traffic upon forest ecosystems is only extended approximately 0.5 km from the highway.

4. Conclusion The study based on air concentration and deposition measurements along two transects both perpendicular to a highway was conducted in 2002 and 2003, two years with meteorologically different conditions. All plots near the highway showed an enhancement of NH3 and NO2 air concentrations by up to 300% compared to rather background sites situated at approximately 500 m from the traffic source. By using an interferential approach we estimated dry deposition rates, which showed corresponding gradients of NH3 and NO2. Additionally we performed bulk deposition measurements to estimate the wet deposition rates. In the case of grassland transect we only obtained a marked gradient for NH+ deposition. Referring to the open field 4 conditions of the grassland transect the total deposition as sum of dry and wet deposition amounts to 23.0–25.6 kg ha
1 y
1 in the vicinity of the highway and 16.8–22.0 kg ha
1 y
1 500 m from the source. Along the forest transect the total deposition ranged from 22.9 to 31.1 kg ha
1 y
1 near the highway, and from 14.5 to 18.7 kg ha
1 y
1 in the centre of the forest in 2002 and 2003. Throughfall shows a similar decrease along the transect as total deposition. The calculated total input may overestimate the real deposition and the relation

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between reduced and oxidized portions of N particularly in the extremely dry year of 2003. The discrepancy between the two concepts may also arise from plant uptake of nitrogen (Saxena et al., 1989). Whereas the concentrations of NH3 and NO2 are significantly below critical levels at both transects, partly as a consequence of the presence of a scrub strip, the nitrogen input exceeds significantly the critical loads for bogs (5–10 kg ha
1 y
1) and spruce forests (15–20 kg ha
1 y
1), both very frequent in Southern Bavaria. The fact that no exceedance of critical levels occurred is confirmed by the absence of visible injuries on forest vegetation near the highway. However, parallel measurements of ion content in soil and statistical investigations about the distribution of ground vegetation along the transect show changed abundances in some nitrophile species as a result of the enhanced storage of some ions in soil and the changed C/N relation. Near the highway we observed a marked codeposition of Na+ and Cl
as consequence of the application of road salt during winter. All average monthly NO3 concentrations in the seepage water below spruce reached values higher than the drinking water limit of 50 mg l
1. The groundwater protection function of forests near a highway may be endangered; the situation is comparable to forest edges exposed to agricultural emissions (Spangenberg, 2002). Considering the great road density and the huge number of vehicles in Central Europe the importance of edge effects may not be underestimated. More attention must be given to traffic-related emissions which in the case of NH3 have been underestimated in the past. Petrol vehicles fitted with catalytic converters provide larger NH3 emissions than estimated previously (Fraser and Cass, 1998; Sutton et al., 2000), even if NH3 emission factors from literature vary from study to study between 0.01 and 0.2 g km
1 (Kean et al., 2000; Shores et al., 2002). The inconsistent data may actually be the result of different driving conditions and different vehicle fleets. In comparison NOx emission factors are found to be in a range from 0.4 to 13 g km
1 (Steinemann and Zumsteg, 2003). Based on the proven nitrogen load the silvicultural management should reinforce, that as much nitrogen as possible should be immobilized in the internal circulation. The at least the short-term advantage of deciduous forests becomes evident. In order to minimize the input of nitrogen and other traffic-related pollutants into ecosystems dense scrub strips may be planted along the highways. Priority should be given to NH3 emission reduction in the vicinity of sensitive ecosystems (Kluizenaar and Farrell, 2000); a distance of some hundreds of metre would be desirable. In any case a general reduction of nitrogen emissions still remains indispensable. Emissions of NH3 and NO2 can be reduced by the employment of environment-friendly propulsion tech-

nologies for vehicles; the further development and adoption of compressed natural gas (CNG), liquefied petroleum gas (LPG) and hydrogen powered cars in combination with hybrid techniques would be appropriate measures.

Acknowledgements The authors wish to thank the Bavarian State Ministry for Environment, Health and Consumer Protection for supporting the study and Bavarian State Institute for Forestry for the valuable input to the work. The provision of the precipitation data by Deutscher Wetterdienst is greatly acknowledged.

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