Dry deposition and internal circulation of nitrogen, sulphur and base cations to a coniferous forest

Atmospheric Environment 33 (1999) 4421}4430

Dry deposition and internal circulation of nitrogen, sulphur and base cations to a coniferous forest Martin Ferm*, Hans Hultberg Swedish Environmental Research Institute P.O. Box 47086, SE-402 58 Gothenburg, Sweden Received 21 June 1998; received in revised form 25 March 1999; accepted 19 April 1999

Abstract The dry deposition of sulphur, nitrogen and base cations to a spruce stand was estimated during a "ve year period using a surrogate surface resembling needles, throughfall and bulk deposition measurements. The deposition was calculated from the ratio between the deposition of an ion and sodium on the surrogate surface and the net throughfall of sodium to the forest. The dry deposition represented a large fraction of the total atmospheric input of base cations. For Na>, Mg>, Ca>, and K> they were 66, 67, 53 and 42%, respectively. The internal circulation was 95% of non-marine net throughfall fro K> and 76% for Ca>. The dry deposition of SO to the canopies regulates the internal circulation of  Ca>. The dry deposition of SO to the canopies regulates the internal circulation of Ca>. The estimated dry  depositions of ammonium and nitrate are close to the net throughfall of Kjeldahl-N and nitrate, respectively. The obtained deposition velocities are comparable to other studies. The calculated dry deposition of ammonium was compared to the net throughfall of ammonium at three nearby forest stands receiving di!erent ammonium amounts on the soils. No correlation to nitrogen level was found, but most ammonium was lost and converted to organic nitrogen in the canopies of the wettest forest stand.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Throughfall; Surrogate surface; Ion leakage; Organic nitrogen; Sulphate; Nitrate; Ammonium; Calcium and potassium deposition

1. Introduction For a sustainable development it is necessary to reduce emission of air pollutants so that the wet and dry depositions do not exceed the critical load. Dry deposition to complex terrains like forests is substantial at the same time as these ecosystems are sensitive and the dry deposition is di$cult to estimate. Throughfall is measured at hundreds of sites in Europe and elsewhere. The technique cannot be used to estimate the dry deposition of base cations because the ion leakage (internal circulation) a!ects the throughfall, especially for K> and Ca>. Base cations are lost from soils during acidi"cation. The atmospheric input of base cations has a positive e!ect, but has decreased during the latest decades (Hedin et al., 1994). The loss of base cations from the soil and the decreased

* Corresponding author.

deposition of base cations reduce the critical load for acidifying compounds, which increases the need for correct estimates of true wet and dry deposition. Di!erent ways of estimating the critical load for acidity on the forest soils at Ga rdsjoK n have given values in the range 0.1}0.44 keq ha\ yr\, (Andersson et al., 1992). Several ways of estimating the dry deposition of base cations have previously been used. Ulrich (1983) calculated the dry deposition of H>, Mg> and Ca> from the assumption that a "xed relation exists between the dry deposition to the forest and the wet deposition and used the depositions of Na> to calculated this relation. Beier et al. (1992) calculated the dry deposition of a compound to the forest from the net throughfall of the compound at the forest edge multiplied with the ratio of the net throughfall of sodium to the forest and the forest edge. Saxena et al. (1997) used polypropylene trays to quantify dry deposition of particles. The trays were leached with de-ionised water. The deposition was

1352-2310/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 2 1 1 - 3

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obtained by dividing the amounts of ions in the leachate with the area of the tray. Draaijers et al. (1997) used an inferential modelling technique to estimate the dry deposition of base cations to Europe. Dambrine et al. (1998) used plastic Christmas trees to estimate the dry deposition of base cations. It is well known that low depositions of ammonium to forests cannot be estimated from net throughfall of ammonium, which is often negative during the vegetation period. Inorganic nitrogen can e.g. be taken up by microorganisms living on the tree and sooner or later appear in throughfall as organic nitrogen (Ferm, 1993). The dry deposition of NH and particulate NH> have previously   been measured using eddy correlation or the gradient technique. The aim of this project was to apply a simple and cost-e$cient way to estimate the dry deposition to a forest. A fog sampler protected from precipitation by a roof cover was exposed continuously and washed monthly in connection with throughfall and bulk collection. The collector consists of a lot of Te#on strings that resembles the needles of conifers. A special way of interpreting the results was used. It is, however, di$cult to validate the #uxes obtained, but the results looked so reasonable and consistent that it was thought useful to publish the results.

2. Experimental The experiments were carried out at the Ga rdsjoK n research area (58304N, 12301E) ca. 15 km from the open Swedish west coast. The measurements presented here started in May 1992 and was completed in April 1997. The forest is hilly and the investigated stand consists mainly of Norway spruce. This project use results from three micro-catchments, one is ca. 0.5 ha and part of the NITREX experiment (Hultberg et al., 1994; Wright and Rasmussen, 1998), the 0.6 ha ROOF experiment (Hultberg and Ske$ngton, 1998) and the 3.7 ha control catchment (Ferm and Hultberg, 1998) within the Integrated Monitoring project (IM within UN ECE LRTAP programme), see Fig. 1. 2.1. Dry deposition estimates A surrogate surface consisting of Te#on strings has previously been used to estimate the dry deposition of particles to the forest (Ferm and Hultberg, 1995 and Hultberg and Ferm, 1995). The surrogate surface consist of 70 m Te#on string wounded around a Te#on frame and protected from rain by a 1.25;1.55 m roof. The dry deposition of an ion x (DD ) is obtained by multiplying V the ratio between the ion and the sodium deposition to the stings by the sodium deposition to the forest (net

Fig. 1. Topographic map showing the location of the three micro-catchments and the three surrogate surfaces.

throughfall). [X] 11 NTF Na>. DD " V [Na>] 11

(1)

Internal circulation or ion leakage (IC) or uptake (UP) is calculated from Eq. (2): TF#SF"WD#DD#IC!UP.

(2)

TF denotes throughfall, SF stem#ow and WD wet deposition. The net throughfall (NTF) is calculated from TF#SF!WD, i.e. it is equal to the dry deposition if internal circulation or uptake does not take place, which is the case for Na>. The net throughfall was not corrected for stem#ow here. Measurement at Ga rdsjoK n have shown that the stem#ow to throughfall ratio is low for Norway spruce and varies between 1}3% except for Ca> for which it was 4.4% (Moldan et al., 1998). Construction of a new low-cost surrogate surface. A new design of surrogate sampler that is substantially cheaper to produce was made, see Fig. 2. It is constructed from a polypropylene bottle (1 l) with a wide screw cap. A large hole was made in the bottom of the bottle and three rectangles were cut out from its side. The material between the rectangles (20 mm wide) served as a support. 15 small holes were drilled above as well as under each rectangle. A Te#on string (or tubing) having an external diameter of 0.75 mm was wound between the holes. Altogether ca. 13 m of Te#on string was used. A funnel was attached to the bottom of the bottle using Nylon screws and nuts. The exit of the funnel was connected to the collection bottle below. The whole sampler was protected from rain and snow by a roof (+1.2; 1.5m) made of water resistant plywood and placed on four legs about 2 m above the ground. The screw cap was attached under the centre of the roof and the sampler was screwed into it. At stormy places the collecting bottle can be secured by strings attached to the legs. This sampler is better protected from rain because it is shorter than the other type. This funnel is situated only 18 cm below the roof compared to 31 cm for the "rst type used (Ferm and

M. Ferm, H. Hultberg / Atmospheric Environment 33 (1999) 4421}4430

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Fig. 2. The surrogate surface that is simple to produce mounted under a roof.

Hultberg, 1995). This equipment was placed at a point of land by the lake. 2.2. Throughfall and wet deposition Throughfall was sampled using 28 randomly placed samplers in the forest. During May}November the sampler consisted of 150 mm diameter funnel with a "lter connected to a 5 l polyethylene bottle protected from light by an aluminium foil. During the winter 200 mm buckets with disposable polyethylene bags inside were used. Wet deposition was sampled using a 200 mm bulk collector. 2.3. Air concentrations The total (gaseous#particulate) ammonium and nitrate concentrations were measured using a "lter pack (EMEP, 1995). The "lter pack was intermittently exposed during 2 min every hour for one month (Ferm and Hultberg, 1998). Di!usive samplers (Ferm and Rodhe, 1997; Ferm and Svanberg, 1998) were used for SO , NO , and   NH . Both the "lter pack and the di!usive sampling  started April 1994. Concentration in air before April 1994 were taken from a coastal station 70 km south of Ga rdsjoK n (RoK rvik 59325N).

sodium ratios before and after the move was compared. In another test carried out during 1997 the low-cost sampler was placed at a windy point of land by the lake. An old type of sampler was placed inside the forest where there is very little wind, see Fig. 1. Results from these two samplers were compared to the old type of sampler placed on top of the lab. The calculated depositions were divided by the air concentrations in order to obtain deposition velocities. The results are compared to other investigations. 2.5. Loss of inorganic nitrogen in throughfall To see if the loss of inorganic nitrogen in throughfall is a function of the nitrogen input to the forest, three di!erent forest stands with di!erent nitrogen inputs were compared. One stand was the earlier mentioned NITREX area that was fertilised with 146 mmol N m\ yr\ using labelled ammonium nitrate (Hultberg et al., 1994) (the one used during the 5 yr). Another was the untreated control catchment that has been studied since 1979. The last was a covered ROOF catchment. This stand is protected from rain and throughfall by a 6300 m roof and receives only de-ionised water containing some base cations (Hultberg and Ske$ngton, 1998).

2.4. Investigation of the method 3. Results In order to see if the calculated depositions or internal circulations were a!ected by the location of the surrogate surface, some tests were made. The surrogate surface was moved to a less windy place in October 1993. The absolute deposition to the surrogate surface and the ion to

3.1. Investigation of the method Several pollutants are present in both gas and particle phases. Only pollutants in particulate phase are

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Fig. 3. Deposition ammonium, nitrate and non-marine sulphate to the string sampler divided by air concentration (left scale in l min\) and deposition of sodium to the string sampler divided by its net throughfall to the forest (right scale in m). The results are average of 5 yr.

deposited on the Te#on strings. An exception may be nitric acid which can be deposited on almost any material, even on Te#on (Appel et al., 1988). The amount that can be adsorbed is, however, limited. The deposition rate to the surrogate surface was calculated by dividing the amount deposited on the strings per month by the air concentration during the same period. This average ratio expressed in l min\ is shown for ammonium, nitrate and sulphate in Fig. 3 as a function of month. Only the non-marine fraction of the sulphate deposition to the surrogate surface was used. The sodium concentration was not measured on the particle "lter in the "lter pack. The non-marine air concentration of sulphate could therefore not be calculated. The particulate ammonium concentration in air was calculated from the total ammonium concentration, measured with the "lter pack and the gaseous ammonia concentration measured using di!usive samplers. The gaseous fraction was only 34% of the total concentration. Fig. 3 shows that the ratios are very similar for ammonium and sulphate. This is also the case for all individual months, with a few outliers. These ratios indicate a deposition of ammonium sulphate to the surrogate surface. These ratios are lower during the summer and higher during the winter. A possible explanation to the high deposition of ammonium and non-marine sulphate might be that snow or very "ne rain drops, which have a very low fall speed and high concentration of these ions is deposited to the strings. Sodium and nitrate will also be deposited through snow on the string, but since their depositions are normally so high on the strings, the e!ect of snow deposition is not noticeable. This hypothesis is further supported by an increased acidity on the strings in winter. During the snow free seasons, the deposited material is almost neutral while the precipitation is always acidic. The problem has now been solved in the low-cost surrogate surface, which is shorter.

The deposition to concentration ratio for nitrate is probably partly caused by high deposition rates for large particles, but can also be caused by the fact that the "lter pack underestimates the concentration of nitrates. Pakkanen et al., 1999 found that the HNO fraction in  the "lter pack was 16% lower compared to a denuder and the particulate nitrate fraction 24% lower than a virtual impactor. The deposited nitrate amount is divided by the total (gas#particulate) nitrate concentrations. The ratio should therefore be lower if nitric acid was not deposited on the strings. No seasonal trend can be seen in the nitrate data. It is very di$cult to measure the atmospheric concentration of sea spray. A ratio between the deposited sodium amount to the strings and the net throughfall deposition of sodium to the forest, expressed in m, was therefore introduced in Fig. 3. No seasonal trend can be seen in this noisy curve. The corresponding ratio for Cl\ and Mg> follow this curve closely, even individual data. Nitrate can also be presented as a ratio between the deposition to the surrogate surface and the net throughfall. This ratio has a large range. If the outliers are removed there is still a high noise and no seasonal trend. The deposition to NTF ratios for the nitrates are very similar to the same ratio for Na>, Cl\ and Mg>. The deposition to the surrogate surface were much higher during the "rst year compared to the following years probably because the surrogate surface was placed at a more windy place the "rst year. The di!erent ions to sodium ratio (ss X/Na>) were, however, rather similar before and after the move. This is shown in Table 1. January was excluded from the averages because there was a sea spray storm in January '93. In order to prove that the calculated deposition is not a!ected to any greater extent by the location of the surrogate surface, simultaneous exposure at di!erent locations has to be carried out. Two identical surrogate surfaces was compared. One was placed at a moderately exposed site at our lab (the place used 1993 to 1997 in this study) and one was placed at a calm place far into the forest. The calculated deposition and internal circulations of di!erent ions on the forest stand using ion to sodium ratios from the three di!erent surrogate surfaces are shown in Table 2. In the last column the sodium deposition to the di!erent surrogate surfaces is shown. The strings at the lab have the highest sodium deposition, but contains more Te#on string (70 m) than the low-cost type (13 m). The sampler inside the forest also contains 70 m string, but has received ten fold less amount of sodium. Despite these di!erences the calculated depositions are similar within ca. 20}30%. There is only one exception the potassium deposition calculated from the surrogate surface in the forest. The sampler at the lab was also analysed with respect to organic-N during 1997. Most of the organic nitrogen was found in the summer. The

M. Ferm, H. Hultberg / Atmospheric Environment 33 (1999) 4421}4430

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Table 1 Dry deposition to the surrogate surface (ss) in lmol month\, NTF in mmol m\ month\ and x/Na> ratios on the surrogate surface (molar bases). Two years are compared (July}July, with the exception of Jaunary) SO\ nm  ss ss NTF NTF ss X/Na> ss X/Na>

92/93 94/95 92/93 94/95 92/93 94/95

28 11 0.98 0.83 0.037 0.034

NH>  45 31 !0.08 !0.47 0.059 0.093

NO\ 

Cl\

Na>

Mg>

K> nm

Ca> nm

145 68 1.67 0.80 0.19 0.02

856 347 11.0 8.9 1.12 1.03

767 335 9.2 7.3 1 1

94 39 1.43 1.11 0.12 0.11

7 5 3.22 1.92 0.009 0.014

13 5 0.70 0.67 0.017 0.016

Table 2 Calculated dry deposition and internal circulations during 1997 using ion to sodium ratins from three di!erent surrogate surfaces (mmol m\ yr\)

Lab Lake Forest

DD pNH> 

DD gNH 

DD SO\ 

DD SO 

DD NO\ 

DD K>, nm

DD Ca>, nm

DD Mg>

IC K>

IC Ca>

ss Na>

16.0 9.7 14.3

4.5 5.7 3.8

10.9 9.9 11.5

3.7 4.7 3.1

25.3 19.9 24.9

1.4 0.9 3.3

2.1 1.4 2.1

12.7 11.1 11.0

20.4 20.9 18.5

4.2 4.9 4.3

5.7 1.8 0.5

p, incidates particulates ammonium and g, gaseous ammoniua. The deposition of gaseous NH was estimated from the assumption that  its depostion velocity was twice that for SO . The last column shows the total sodium depostion to the strings (m mol yr\). 

average calculated dry deposition of organic nitrogen was 2.8 mmol m\ yr\, which is 23% of the calculated ammonium deposition to the surrogate surface. A very small fraction of the particles that travel in a trajectory toward a string is later trapped in the sample. From the measurement of particulate sulphate in the air it can be calculated that ca. 1% of the non-marine sulphate particles were trapped by the surrogate surface at the lake and ca. 1 by the surrogate surface in the forest. Wiman and As gren (1985) have published a model for the deposition velocity of particles to a forest due to di!usion, impaction and sedimentation. the deposition velocity is calculated as a function of wind speed, aerodynamic particle diameter and needle diameter. This model was applied for an accumulation mode particle of 0.5 lm (ammonium sulphate or nitrate) and a coarse particle of 3 lm (sodium chloride or base cation). The ratio between their deposition velocities varies signi"cant with wind speed. The reason for this is that the relative importance of the three deposition processes changes with wind speed. If only one process is considered at a time, the ratio between the deposition velocities for the two particle sizes is practically independent on wind speed. The needle diameter is, however, not as important as the wind speed for this ratio. The marine fraction of the sulphate deposition to the NITREX area during 1997 was 6.9 mmol m\ yr\ implying that the non-marine fractions (accumulation mode particles) were 4.0, 3.0 and

4.6 mmol m\ yr\ for the laboratory, lake and forest, respectively (cf. Table 2). The most windy place (the lake) gave the lowest ion to sodium ratio and thus the lowest calculates deposition. This was not only the case for the yearly average, but also for most of the monthly averages. The reason for this is that the deposition due to di!usion becomes important at places with lower wind speeds. This can also be seen in the particulate ammonium deposition and the non-marine fractions of potassium and calcium. A possible explanation to the fact that the ion to sodium ratios did not vary more than ca. 50% for the accumulation mode particles at the di!erent wind exposed places is that the deposition due to impaction was the dominant process. Most of the ions deposited on the stings have then been deposited during windy periods. Topochemical reactions (gases reacting with deposited particles, the string or the needle) can also occur and alter the observed ion to sodium ratios. These reactions have been assumed to have a similar e!ect on the particles on the needles as on the particles on the strings and therefore have been neglected. Most of the deposition occurs on the wind exposed tops of the trees. In order to minimise the e!ects of lower wind speed around the surrogate surface compared to the top of the canopy, it is recommended to place the surrogate surface at an open and wind exposed place. This has been the approach in a recent study of base cation deposition at 11 sites in Sweden.

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3.2. Five-year measurement period The seasonal variation in weather parameters during the period is presented in Fig. 4. As can be seen there is no seasonal variation in wind speed. In Table 3 the ion to sodium ratios on the strings, in precipitation and in net throughfall are presented. As can be seen the ion to sodium ratios are much higher in precipitation for H>, NO\, SO\ nm and NH> than on the strings. Wet    deposition on the Te#on strings can therefore overestimate the deposition of these species. The ammonium deposition on the strings are about twice the non-marine sulphate deposition at all seasons, indicating that the deposition consist of (NH ) SO . The NTF of am  monium is negative due to uptake or conversion. The ion to sodium ratios of NO\, Cl\ and Mg> are very similar  on the strings compared to NTF indicating that the interaction of these species with the canopy is small.

Fig. 4. Weather data at Ga rdsjoK n for the period. Wind speed is expressed in m s\, precipitation in 0.1 mm month\, temperature in 3C and global radiation in 0.1 W m\.

3.3. Dry deposition of sulphur and nitrogen compounds In Fig. 5 are the calculated dry deposition of SO and  its concentration in air plotted as a function of month. The two curves resemble each other but the calculated dry deposition of SO is often negative in the summer  depending on very low or negative net throughfall depositions of sulphate. Cadle et al. (1991) found that HNO could be ab sorbed by three di!erent routs on conifers: surface, trans-cuticular and stomatal deposition. The surface-deposited HNO can be recovered by washing the tree  while the trans-cuticular and stomatal-deposited HNO  are mainly assimilated by the plant. In rural areas most of HNO will be surface-deposited and thus found in  throughfall. Ferm et al. (1984) found that most of the atmospheric nitrate at the Swedish west-coast is particulate. Particulate nitrate is in coastal areas usually present in two modes (Wall et al., 1988): accumulation mode particles consisting of NH NO and coarse nitrate par  ticles formed by reaction between HNO and sea spray  (Pio and Lopes, 1998) or alkaline coarse particles. The vapour pressure product of NH and HNO concentra  tions over pure NH NO is, however seldom exceeded   on the Swedish west-coast (Ferm, 1998,1992). Ga rdsjoK n is

Fig. 5. Calculated dry deposition of SO and SO concentration   in air.

not far from the coast and it is therefore likely that most of the nitrate particles belong to the coarse mode. The nitrate deposition calculated from Eq. (1) should therefore be close to the real dry deposition of nitrate to the forest. In Fig. 6 the calculated deposition of nitrates (sum of HNO and particulate NO\) are plotted together with   the net throughfall and atmospheric concentration. The NTF and calculated DD are very similar indicating that the canopy interactions are small and that the concept with the surrogate surface function reasonably well. The average #uxes are shown in Table 4. The high correlation between the calculated DD and the total atmospheric

Table 3 Ion to sodium ratios of the intergrated depostions during 5 yr (molar bases) on the surrogate surface, in precipitation and in net throughtfall

ss WD NTF

H>

SO\ nm 

NH> 

NO\ 

Cl\

Mg>

K> nm

Ca> nm

0.02 0.67 0.08

0.03 0.33 0.12

0.06 0.61 !0.01

0.16 0.67 0.14

1.15 1.07 1.34

0.12 0.12 0.16

0.008 0.07 0.25

0.015 0.05 0.08

Index nm denotes the non-marine fraction.

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Table 5 Dry depositions (mmol m\ yr\), air concentrations (nmol m\) and deposition velocities (mm s\)

SO  SO\ nm  Total-NO\  Total-NH>  NH 

Fig. 6. NTF of NO\ (open squares), calculated dry deposition  of nitrates (sum of HNO and particulate NO\) ("lled squares),   and its concentration in air ("lled diamonds).

DD

Air

Vd

7.5 4.5 19.5 15.9 11.6

27.4 28.5 34.2 62 21.3

8.7 5.0 18.1 8.2 (17.4)

All are averages during 5 yr except the NH concentrations  which only represent 3 yr.

Table 4 Wet deposition, calculated dry deposition, NTF, internal circultion minus uptake (IC-UP), all in mol m\ yr\ WD SO  SO nm  SO\ m  NO\  NH>  org-N K> nm K> m Ca> nm Ca> m Mg> Na> Cl\

19.4 3.2 35.0 32.2 3.8 3.6 1.1 2.7 1.2 6.5 54 58

DD 7.5 4.5 6.3 19.5 21.7 1.2 2.2 2.0 2.3 12.9 105 129

NTF

12.0 6.3 14.1 0.1 15.3 25.2 2.2 8.5 2.3 16.1 105 139

IC-UP

!5.4 !21.6 24.1 6.5

Fig. 7. Calculated dry deposition of NH and NH> ("lled   squares), net throughfall of NH> (open triangles) and Kjeldahl N (open squares). The total NH> concentration in air is present ed as the dotted line (right axes).

(3.2) (10.0)

Index nm denotes the non-marine fraction and m the marine.

concentration suggest a rather constant deposition velocity throughout the year. The calculated deposition also show a similar annual variation as the total nitrate concentration in air. Bytnerowicz et al. (1991) compared natural and Te#on-coated branches of pine trees. After exposure in ambient air the branches were rinsed with water. The nitrate #uxes (amount per total surface area and time) were very similar on natural and Te#on coated branches. Hofschreuder et al. (1997) found similar nitrate depositions on branches and arti"cial branches of polyethylene. Several ways of estimating the dry deposition of NH V were tried. The dry deposition of particulate ammonium was calculated using Eq. (1). Estimation of the dry deposition of NH is, however, more di$cult. It is well known  that NH can be emitted from stomata due to the equilib rium between NH and NH> present in leaf tissues. This   is called the compensation point for NH . When NH is  

deposited to nitrogen-limited ecosystems, such as forests, it indicates that the compensation point is low (see e.g. Sutton et al., 1995). In an earlier estimate of ammonium deposition to another forest stand, an approach based on relations with the sulphur deposition was used. A close agreement with the net throughfall of organic nitrogen was then obtained (Ferm, 1993). Since the calculated SO  depositions were often negative during the summer an analogy with individual sulphur dioxide #uxes cloud not be used here. Duyzer et al. (1994), Wyers and Erisman (1992) estimate the dry deposition velocity for NH to  forests as 20}30 and 32 mm s\, respectively. It was here assumed that the dry deposition velocity of NH was  constant through the year and equal to twice the average deposition velocity for SO which was 8.7 mm/s, see  Table 5. The NH concentrations were measured using  di!usive samplers and the average concentration was 21 nmol m\, i.e. 34% of the total NH concentration. V The calculated dry deposition of NH #particulate  NH> is also rather close to the NTF of organic nitrogen  (21.7 compared to 15.3 mmol m\ yr\, see Table 4). In Fig. 7 the calculated dry deposition of NH and the NTF V of NH> and Kjeldahl-N are plotted as a function of 

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month for the whole period. The deposition of KjeldahlN is almost equal to the organic-N deposition since NTF of NH> is so small. As can be seen from the "gure the  calculated dry deposition of NH is of the same magniV tude as the NTF of Kjeldahl-N and has no seasonal trend. The air concentration of NH which is also shown V in the "gure does not have any trend either. The #uxes are shown in Table 3. The 5 yr average became 10.1 mmol m\ yr\ for particulate NH> and  11.6 mmol m\ yr\ for NH .  3.4. Internal circulation of base cations A close correlation between the calculated dry deposition of SO and internal circulation of Ca> has earlier  been observed (Hultberg and Ferm, 1995). This was also observed here during he whole 5 yr period, see Fig. 8. At the peaks of SO deposition the leakage of Ca>  also shows a peak, but the #ux never exceeds 2 mmol m\ month\. Turunen et al. (1995) found crystals of CaSO outside stomata of needles.  The dry deposition of base cations represented a large fraction of total atmospheric input. For Na>, Mg>, Ca>, and K> they were 66, 67, 53 and 42%, respectively. The internal circulation was 95% of non-marine NTF for K> and 76% for Ca> In Table 4 the wet deposition, calculated dry deposition and NTF, internal circulation minus uptake is given. The di!erence between estimated DD and NTF for Cl\ is very small and within experimental error. 3.5. Deposition velocities In Table 5 the deposition velocities have been calculated for SO and particulate sulphate. The particulate  sulphate concentrations are not corrected for marine contribution since sodium concentrations were not measured. The deposition velocities for nitrates were calculated by dividing the calculated dry deposition of nitrate (assuming that also HNO was deposited to the  surrogate surface) and dividing it with the total nitrate concentrations in air. The deposition velocity 18 mm s\ therefore also includes HNO . The total nitrate concen trations may, however, be too low because very large particles do not enter the inlet. The deposition of NO  was not assumed to a!ect the NTF of nitrate. The average NO concentration in air was 94 nmol m\. The  estimated deposition velocity for SO and particulate  SO\ of 8.7 and 5 mm s\, respectively, correspond well  with earlier estimates by Hultberg and Grennfelt (1992) of 7 to 8 mm s\ of the sum of SO and particulate SO\   to the Norway Spruce forest at Ga rdsjoK n. Wyers and Duyzer (1997) estimated the dry deposition velocity for particulate nitrate and sulphate particles to a coniferous forest using the gradient technique. For sulphates they found deposition velocity from 0 to

Fig. 8. Dry deposition of SO and internal circulation of Ca>  as a function of time.

40 mm s\ that had a near linear dependence on the friction velocity, with an average of 7 mm s\. For nitrates they found deposition velocities that were higher than theoretical possible from atmospheric resistances point of view (12 mm s\). They were also temperature dependent indicating that equilibrium displacements in particulate NH NO had occurred.   3.6. Ewect of fertilisation on the loss of inorganic nitrogen in throughfall Three forested micro-catchments were studied. The reference area received 38 mmol NH> m\ yr\ as wet  deposition in 1997. The NITREX area received 146 extra through sprinkling, i.e. 184 mmol NH> m\ yr\. The  covered roof area did not receive any wet deposition of NH> at all. The corresponding "gures for nitrate are: 39,  185 and 0 mmol NO\ m\ yr\. The net throughfall  and calculated dry depositions for 1997 using the lowcost surrogate surface is shown in Table 6. The net throughfall of nitrate was always smaller than the calculated dry deposition, especially for the reference area. In the reference area the net throughfall of ammonium was negative. This was partly compensated by the highest net throughfall of organic nitrogen (from conversion of ammonium). Compared to the calculated dry deposition of particulate and gaseous ammonium there is almost balance with the net throughfall of reduced nitrogen, except at the reference area. If the dry deposition of organic nitrogen is also taken into account the throughfall measurements are smaller than the calculated deposition. The organic nitrogen amount was 23% of the NH>  amount on the surrogate surface and no algae could be observed on the threads. A possible explanation may be that there are no carbohydrates on the Te#on as there are on the needles. It has earlier been observed that a higher throughfall amount of organic nitrogen is obtained if the samplers are preserved with iodine prior to sampling (Ferm and Hultberg, 1998), probably because

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Table 6 Net throughfall, calculated dry deposition and internal circulation at three di!erent forest stands (mmol m\ yr\)

Control NITREX ROOF

NTF Na>

NTF NO\ 

DD NO\ 

NTF NH> 

NTF org-N

DD pNH> 

DD gNH 

DD org-N

DD SO 

IC Ca>

IC K>

162 114 137

10.4 9.5 19.4

29.5 19.9 25.0

!13.8 3.4 2.2

16.7 10.9 10.9

14.2 9.7 12.7

7.6 5.7 6.4

3.1 2.0 3.4

6.2 4.7 5.3

9.3 4.9 6.5

26 21 16

no material is attached to the wall in this case. The throughfall collectors were, however, not preserved here. The NITREX and covered roof areas are situated side by side and facing south, while the reference area is facing north and is therefore more wet. The higher wetness in the reference area favours green algae and lichens which are believed to be responsible for the losses of inorganic nitrogen. It is reasonable to assume that some microorganisms that live on the needles can be found in litterfall and that balance should not be expected. The dry deposition of SO is also favoured by wetter surfaces and the  SO deposition was also higher in the reference area.  4. Conclusions The technique used to estimate the dry deposition of particles to forests and internal circulation is very coste$cient, does not need electricity and is therefore suitable for estimating the annual country wide depositions to forest. With the used way of calculating the deposition #uxes, the location of the surrogate surface is not very important. The technique is di$cult to check, but gives reasonable estimates of the deposition velocities for SO  and particulate SO\. It also gives a reasonable estimate  of the dry deposition of NO\ (slightly higher than the  NTF) and NH> (slightly higher than NTF of Kjeldahl-N).  The calculated internal circulation of Ca> resembles the calculated dry deposition of SO . The internal circulation  of Mg> is very small and that of K> is substantial. The calculated #uxes of dry deposition and internal circulation can be used to correct the estimate of critical loads for acidi"cation of forest ecosystems. Acknowledgements This work was a part of the BIATEX, LIFE and the Integrated Monitoring Programme within UN ECE and has been founded by EU and the Swedish Environmental Protection Board. References Andersson, B.I., Ske$ngton, R.A., Hultberg, H., Sverdrup, H.U., 1992. Assessment of critical loads for coniferous forests and

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