Ammonia exchange over coniferous forest

Atmospheric Environment Vol. 32, No. 3, pp. 441—451, 1998( 1998 Elsevier Science Ltd

All rights reserved. Printed in Great Britain1352—2310/98 $19.00#0.00PII: S1352–2310(97)00275–6

AMMONIA EXCHANGE OVER CONIFEROUS FOREST

G. PAUL WYERS* and JAN WILLEM ERISMANs

*Netherlands Energy Research Foundation ECN, P.O. Box 1, 1755 ZG Petten, The Netherlands; andsNational Institute of Public Health and Environment RIVM, P.O. Box 1, 3720 BA Bilthoven,

The Netherlands

(First received 17 November 1995 and in final form 6 October 1996. Published February 1998)

Abstract—Surface exchange of ammonia (NH3) over a coniferous forest was measured for a period of more

than 2 yr using the aerodynamic gradient technique. The results indicate that NH3

is efficiently depositedwhen the canopy is saturated with water. At night and when the canopy is dry, the NH

3flux is generally

directed towards the canopy although emissions from the canopy are sometimes observed. The surfaceresistance (R

#) to deposition is dependent on the relative humidity which determines the thickness of

microscale water layers present on the leaf surfaces. During the day exchange of NH3

is clearly bi-directional. However, strong differences are observed in the exchange of NH

3between 1993 and 1994: in

1993 emission of NH3occurs much more frequently than in 1994. The leaf surface is observed to be a sink as

well as a source for NH3. Therefore the bi-directional nature of NH

3exchange applies to the leaf surface as

well as the stomata. Preceding fluxes to the leaf surfaces may lead to accumulation of NH3

and to anincreased resistance to deposition or even to emission, when the NH

3concentration in chemical equilib-

rium with the leaf surface exceeds the atmospheric NH3

concentration. It is hypothesized that the observeddifferences in frequency of emission between 1993 and 1994 are related to differences in the accumulation ofNH

3on the leaf surface. When the net flux is directed towards the canopy and the canopy is dry, R

#is much

lower than the stomatal resistance. It has been suggested that transpired water causes deliquescence ofparticles that are preferentially deposited in the stomatal regions. This process may then result in a partiallywetted leaf surface under otherwise dry conditions, promoting uptake of NH

3. ( 1998 Elsevier Science Ltd.

All rights reserved.

Key word index: Micrometeorology, dry deposition, ammonia fluxes, bi-directional exchange, compensa-tion point, Douglas Fir.

1. INTRODUCTION

Quantification of the surface exchange of NH3

oversemi-natural vegetation is necessary to relate the ob-served effects to the atmospheric input. DepositedNH

3disturbs the nutrient balance and contributes to

acidification after nitrification in the soil. In regionswith intensive livestock production or a high use offertilizer, NH

3is often the most important contribu-

tor of fixed nitrogen (N) and potential acidification toneighbouring nature areas. In the Netherlands 47% ofthe atmospheric input of potential acid and 69% ofthe input of N were estimated to be supplied by dryand wet deposition of NH

3and ammonium (NH`

4)

(collectively NHx) in 1993 (Heij and Schneider, 1995).

In that year the national average total deposition ofNH

xamounted to 2000 mol ha~1. The contribution

of dry deposition of NH3

to the deposition of poten-tial acid and N is estimated at, respectively, 31 and48% (Heij and Schneider, 1995). Strict measures have

been proposed in the Netherlands, some of which arealready in effect, to reduce the NH

3emissions from

agricultural activities. As these measures will havea considerable impact on the economy of the agricul-tural sector, it is clear that the atmospheric inputof NH

xto nature areas must be known accurately

and on a local scale to underpin such measures. Drydeposition of trace gases on a local scale can beestimated with an inference model (e.g. Garland,1977; Wesely and Hicks, 1977; Fowler, 1978), but forNH

3both the concentration field and the surface

exchange parameters carry large uncertainties (Eris-man, 1993; Erisman et al., 1998). The factors control-ling NH

3exchange have been investigated in micro-

meteorological and controlled-environment studies.The reader is referred to Sutton et al. (1995) for anoverview of the results of recent research intoplant—atmosphere exchange of NH

3.

Ammonia exchange over coniferous forest has beenaddressed in several micrometeorological studies

441

(Duyzer et al., 1992, 1994; Wyers et al., 1992; Andersenet al., 1993), in which it was recognized that NH

3is

efficiently deposited to coniferous forest with meandeposition velocities of 22—36 mms~1. Furthermore,it was observed in all of these studies that concentra-tion gradients of NH

3are sometimes directed away

from the canopy. Explanations given by these authorsfor reversed gradients include advection from nearbysources, chemical reactions in the air column abovethe canopy, measurement errors or emissions fromstomata and/or leaf surface, but these (apparent) emis-sions were not considered of significant influence onthe annual deposition flux. A compensation point forNH

3—the air concentration below which NH

3is

emitted and above which it is deposited—has beenobserved for some crops (Farquhar et al., 1980). Theexistence of a compensation point near or below1 kgm~3 has also been suggested for coniferous forest(Langford and Fehsenfeld, 1992; Duyzer et al., 1994;Andersen et al., 1996). In the Netherlands, however,such low concentrations are rarely achieved, and thussuch emissions would indeed be of little significance.However, Wyers et al. (1993) observed emissions froma forest canopy up to air concentrations of 8 kgm~3.

To improve insight into the air—surface exchangemechanisms of NH

3and other air pollutants over

coniferous forest, a permanent deposition monitoringstation for SO

2, NO

2and NH

3was installed at

the Speuld site in a large forested area in the centre ofthe Netherlands. The objectives of this depositionmonitoring effort are to derive parametrizations forsurface exchange of these pollutants over forest for usein national inference models, to obtain an indepen-dent determination of the deposition fluxes for a largeforested area, and to detect trends in the deposition asa result of emission reductions. At this station theNH

3flux has been measured continuously since

November 1992. Since January 1995 the station hasbeen part of a European deposition monitoring net-work within the framework of the EU-LIFE pro-gramme. The scope of the deposition monitoringstation has been expanded and now also includesacidifying aerosol, heavy metals, base cations and fog.In this paper the parameters, derived from this dataset, are discussed that determine the magnitude andthe direction of the NH

3flux.

2. EXPERIMENTAL

2.1. Measurement of the vertical concentrationgradient of ammonia

The Speuld site is an approximately 35 yr old Douglas Firstand of 2.5 ha, which is surrounded by a much larger for-ested area of approximately 50 km2, containing both decidu-ous and coniferous species (mainly oak and larch). The meantree height at the site is 20 m. The forest is dense; thestem density is 800 ha~1 and the one-sided leaf areaindex is approximately 11 (Jans et al., 1994). Micro-meteorological measurements at this site are performed ona 36 m tower.

The vertical concentration gradient of NH3

was deter-mined by measuring its concentration at heights of 24.5, 28and 34 m above the forest floor. These concentrations weremeasured by three continuous-flow denuders, which wereconnected to a single NH`

4-detector. This technique is de-

scribed in detail elsewhere (Wyers et al., 1993). The site wasvisited once a week for calibration and for refilling of thesolution containers. The relative precision of the 30 minmean gradient measurements is 1.1%. Assuming an NH

3concentration equal to the annual mean of 5 kgm~3, neutralatmospheric conditions and a friction velocity of 0.4 m s~1,this precision corresponds with a minimum detectable flux ofapproximately 20 ngm~2 s~1.

2.2. Calculation of the flux

Fluxes of momentum and sensible heat were measuredwith a sonic anemometer (Kayo Denki DAT 310 with TR61probe) mounted at a height of 36 m. For the calculation ofthe flux, modified flux-profile relations were applied, whichhave been derived for this site from a comparison of sensibleheat fluxes and ozone fluxes directly measured by eddycorrelation with fluxes calculated from vertical gradients(Duyzer and Weststrate, 1995). The modification concernsa height-dependent correction factor a, which ranges from0.75 for z"24 m to 0.9 for z"34 m. When this correction ismade, flux-profile functions can be applied to gradientmeasurements that are partly performed within the rough-ness layer over the forest. The flux is calculated from a least-squares fit in which the concentration is plotted as a functionof the logarithm of the stability-corrected height:

F"

ku*s (z)

a ln((z!d)/z0)!b[t

H((z!d)/¸)!t

H(z

0!¸)]

(1)

in which F is the flux, k the Von Karman constant, u*

thefriction velocity, v(z) the concentration at height z, z

0the

roughness length, d the displacement (15 m for this site),¸ the Monin—Obukhov length and t

H((z!d)/¸) the integ-

rated flux-profile function for heat.The parameters a andb are used to correct the flux-profile relations at heightsbelow 36 m; b"1 for stable conditions (¸'0) and b"afor unstable conditions (¸(0). A resistance layer model isused to generalize the measurements (Wesely and Hicks,1977; Erisman et al., 1994). The canopy resistance (R

c), de-

scribing the actual uptake at the receptor surface, is cal-culated from:

R#"»

$(z)~1!R

!(z)!R

"(2)

in which R!(z) and R

"are, respectively, the aerodynamic and

the boundary layer resistance. R!

is calculated followingGarland (1978); R

"is approximated using the formulation

presented by Hicks et al. (1987). »$(z) is the deposition

velocity which is calculated as the ratio of F and v(z). How-ever, for gases that experience bi-directional exchange, suchas NH

3, the deposition velocity cannot be used to calculate

the magnitude of the flux. Where bi-directional fluxes occur,other resistance model formulations may be applied whichaccount for surface concentrations, for example in relation tothe so-called compensation point concentration of NH

3for

plants (Sutton and Fowler, 1993; Sutton et al., 1995).

3. RESULTS

During the period November 1992—1994 approx-imately 9800 hourly measurements of the NH

3gradi-

ent were judged to be of good quality, representinga temporal coverage of 56%. The main causes for lossof data were instrument failure, calibration and main-tenance. During this period the average NH

3

442 G. P. WYERS and J. W. ERISMAN

Fig. 1. Ammonia flux, concentration (s), surface resistance (R#), canopy water storage (CWS) and global

radiation at the Speuld forest measurement site, 13—16 August 1994. Here and in subsequent figuresnegative fluxes indicate deposition and positive fluxes emission.

concentration at z"36 m was 5.2 kg m~3 and themedian concentration was 3.5 kg m~3.

After removal of gradients measured during condi-tions of insufficient homogeneous fetch (sector300°—60°), the exchange fluxes were calculated follow-ing the procedure outlined above. A time series ofmeasurements from August 1994 is shown in Fig. 1and illustrates how the flux and the canopy resistancemay respond to changes in global radiation and cano-py water storage (CWS). The value of CWS is cal-culated from measured rainfall and total evaporation,using a model based on measurements of the canopywater storage by microwave absorption (Bouten,1992). The canopy is considered saturated with waterwhen CWS'2 mm. At the start of this period thecanopy is saturated with water due to rainfall. Dryingof the canopy lasts 2—3 days. During the whole periodthe flux is consistently directed towards the canopy.A very clear but unusual diurnal variation is observedin the canopy resistance R

#. During the day the R

#is

essentially zero, which suggests an uptake rate forNH

3that is much faster than stomatal uptake. During

the night R#

increases to values of several hundreds m~1 and R

#becomes progressively larger as drying

of the canopy continues. This diurnal variation inR

#disappears when the canopy becomes saturated

with water. Under these conditions R#

approacheszero, during the day as well as at night.

In Fig. 2 an overview of the measurements is pre-sented for 1993, where the flux is plotted as a functionof the maximum possible flux (F

.!9), which is cal-

culated from the product of concentration and max-

imum deposition velocity (».!9

). ».!9

is defined hereas R~1

!, because measurements of water vapour fluxes

at this site (Bosveld, pers. comm., 1993) have providedindications that R

"approaches zero. A distinction is

made between day, night, wet canopy, drying canopyand dry canopy. The canopy is considered dry whenthe canopy water storage (CWS) is (0.25 mm, wetwhen CWS '2 mm and drying for intermediatevalues. When the canopy is wet, fluxes are generallydirected towards the canopy and their magnitude mayapproach F

.!9. Above a drying canopy, the fluxes are

usually smaller than F.!9

and emission of NH3

issometimes observed during the day. When the canopyis dry, there is a pronounced difference in depositionbehaviour between day and night. At night, the flux ismostly directed downward, and is generally muchsmaller than F

.!9. Emissions are observed more fre-

quently than above a wet or drying canopy. Duringthe day, emissions of NH

3are very common, leading

on average to a net upward flux under these condi-tions. A few runs in Fig. 2 show F'F

.!9, although

this may be attributed to scatter in the data.In 1994 (data not shown) the NH

3exchange pattern

above a wet or drying canopy is similar to that ob-served in 1993. However, marked differences betweenthose two years are observed when the canopy is dry.Emissions of NH

3during the night are very rarely

seen in 1994, and during the day emissions are muchless common than in 1993. This is also illustrated inFigs 3 and 4, where the average diurnal variations inF and F

.!9for the years 1993 and 1994 are compared

for different classes of canopy wetness. In 1993 the flux

Ammonia exchange over coniferous forest 443

Fig. 2. Overview of NH3flux measurements in 1993, showing the observed dry deposition flux for NH

3(F)

as a function of the maximum possible flux (F.!9

) under different conditions.

above a wet canopy is on average a factor two lowerthan F

.!9. When the canopy is drying, the mean ratio

F/F.!9

decreases to 0.38 and for a dry canopy thisratio amounts to only 0.07. In the early afternoon in1993 (at 14 : 00, see Fig. 3) the average flux evenbecomes positive, indicating a net emission at thattime of the day. In 1994 the average flux closelyresembles F

.!9for a wet canopy. The mean ratio

F/F.!9

is 0.85. When the canopy is drying or dry, theflux remains directed towards the surface throughout

the day with mean ratios of F/F.!9

of 0.63 and 0.56,much larger than the ratios for similar conditions in1993. Another difference between the two years con-cerns the magnitude of F

.!9, which in dry conditions

is larger in 1993 than in 1994. This difference can bemainly attributed to the difference in NH

3concentra-

tion, which decreases from a mean value of 7.6 kgm~3

in 1993 to 6.1 kg m~3 in 1994 (both values refer to drycanopy conditions) and to a lower mean »

.!9in 1994

(for dry canopy conditions) relative to 1993.


Fig. 3. Average diurnal variation in measured NH3fluxes (F) and maximum possible NH

3fluxes (F

.!9) for 1993.

Fig. 4. Average diurnal variation in measured NH3fluxes (F) and maximum possible NH

3fluxes (F

.!9) for 1994.

Clearly, dry deposition of NH3

was much moreefficient in 1994 than in 1993. It is now well knownthat surface exchange of NH

3is bi-directional and

that the net NH3

flux is the resultant of componentdeposition and emission fluxes (e.g. Sutton et al.,

1995). In 1993 the emission component of the flux wasapparently on average much stronger than in 1994 orthe deposition component was much weaker. Below,the parameters determining the magnitude and direc-tion of NH

3exchange over this forest are discussed in


Fig. 5. Median surface resistances (R#, 50 percentile) for different classes of relative humidity (RH), plotted

as a function of humidity-class average for a dry canopy.

relation to the observed differences in exchange be-haviour in 1993 and 1994.

4. DISCUSSION

4.1. ¼et or drying canopy

When the canopy is wet or drying, the water pres-ent on the needle surfaces will act as a strong sink forNH

3and will (partially) cover the stomata, limiting

their role in the exchange of NH3. Under these condi-

tions, NH3

exchange can be considered to occur onlywith the wet or moist leaf surface. When the canopy issaturated with water (CWS'2 mm), NH

3is efficient-

ly deposited to the leaf surface with »$

approaching»

.!9. In other words, the surface resistance to NH

3deposition is effectively zero. This is to be expected forgases such as NH

3and SO

2that are well soluble in

water and has been observed in the laboratory(Adema et al., 1986) as well as in the field (Erismanand Wyers, 1993; Sutton et al., 1993; Duyzer et al.,1994; Erisman et al., 1994). However, even when thecanopy is saturated with water (CWS'2 mm), thereare some differences between 1993 and 1994 in thesink strength of the leaf surfaces for deposition of NH

3(Figs 3 and 4). As mentioned above, the mean ratioF/F

.!9in 1993 is significantly lower (0.52) than in 1994

(0.85). This can only be attributed to a difference in thesolubility of NH

3in the water film present on the leaf

surface.When CWS is between 0.25 and 2 mm, the canopy

is still moist from previous rainfall or dew. For a

drying canopy the deposition efficiency, expressed asF/F

.!9, is less than that for a wet canopy, and in 1993

the deviation of the flux from F.!9

is again larger thanfor 1994 (see above). This suggests that the solubilityof NH

3in the water film decreases due to increased

concentrations in that film as a result of evaporationand/or that the water film no longer covers the entireleaf surface but is merely present in the form of iso-lated droplets. Emissions of NH

3, under these condi-

tions rarely observed at night but more frequentlyduring the day, could be produced by evaporatingwater films, although some contribution from stom-atal emissions during the day cannot be ruled out.

4.2. Night: dry canopy

The canopy is considered completely dry whenCWS is below 0.25 mm. At night when the stomataare (nearly) closed, R

#can be considered equal to the

leaf surface/cuticle resistance (R8). This resistance dis-

plays an exponential dependence on the relative hu-midity (RH). In Fig. 5 the median R

#is plotted as

a function of average RH for different humidityclasses. R

#ranges from +10 s m~1 at RH "94—95%

to 200 s m~1 at RH"42—43%. For RH'60%, themedian R

#is higher in 1993 than in 1994, which

suggests that in 1993 the leaf surface is a lessefficient sink than in 1994. The R

#—RH relationships

for 1993 and 1994 can be approximated by R#"

2022 exp(!0.054 RH) and R#"472 exp(!0.029RH)

respectively, with R#in s m~1 and RH in %. A similar

expression is given by Sutton et al. (1995) which was


based on field measurements as well as laboratoryobservations. This decrease in R

#at higher RH is

thought to be related to the formation of a micro-scalewater layer on the leaf surface (van Hove, 1989) whichalso serves to increase the uptake of NH

3, although

not to the same extent as macroscopic surface wetnessdue to e.g. precipitation or dew (Erisman and Wyers,1993).

4.3. Day: dry canopy

During the day the exchange of NH3

may occurwith both leaf surfaces and stomata, and under theseconditions the exchange is clearly bi-directional(Fig. 2). In Fig. 5 the median R

#is plotted against RH

for 1993 and 1994. The resistances are very low, below25 sm~1, and much lower than the stomatal resist-ances (R

4) for NH

3which reach minimum values of

100—200 sm~1 on warm, sunny days. For 1993, R#

increases with increasing RH; in 1994, however, thereis no correlation with RH. Similarly low R

#for a vis-

ually dry canopy have also been observed at this siteby Duyzer et al. (1994). These authors suggest, on thebasis of the experimental work by van Hove et al.(1990) and Taylor and Tingey (1983), that NH

3may

travel shorter distances in the substomatal cavity thanH

2O and thus R

4for NH

3will be smaller than the

R4for water vapour. They admit, however, that in the

cited experimental work it would not have been pos-sible to exclude deposition to leaf surfaces. Here wesupport another possible mechanism to explainthe observed low resistances. Burkhardt et al. (1995)have shown that particles are preferentially depositedin the stomatal regions of conifer needles. Most ofthese particles are hygroscopic. As RH in the immedi-ate vicinity of the stomata will be high due to transpi-ration, the stomatal regions may become wet due todeliquescence of deposited particles. This could leadto high humidity on leaf surfaces, even under condi-tions of low ambient humidity.

4.4. Ammonia emissions from forests

The origin of emissions of NH3

from semi-naturalvegetation is still not clear. Several processes havebeen proposed to explain these emissions. NH

3may

be emitted from the stomata (e.g. Sutton et al., 1995), itmay be emitted from (drying) leaf surfaces or theobserved emissions may be an artefact due to NH`

4-

aerosol evaporation near the canopy (e.g. Brost et al.,1988). Emissions of NH

3over forest have also been

observed in earlier studies. Of the 34 concentrationgradients measured over a spruce forest in Denmarkby Andersen et al. (1993), 10 were directed upward, i.e.the concentration near the canopy was higher thanaloft. They concluded that at least several of thesereversed gradients were due to advection from nearbysources. During six campaigns carried out at the siteSpeuld in 1988—1990, Duyzer et al. (1994) observedthat approximately 10% of the measurements in-dicated significant emission fluxes. These were con-sidered not very important on an annual basis and

thought to occur when the NH3

concentration isbelow 1 kgm~3 and the temperature is high. Van Osset al. (1998) suggest that shifts in the aerosol NH

4NO

3equilibrium with NH

3and HNO

3could explain some

of the NH3

emissions observed at the Speuld site.These coincide with downward gradients of NO~

3aerosol larger than allowed by the aerodynamic res-istance. Measurements and model calculations showthat this process could explain the emissions occur-ring at higher ambient temperatures (van Oss et al.,1998). However, since there are no significant differ-ences in average temperature or RH between 1993and 1994 (see below), changes in the NH

4NO

3equi-

librium constant or in the reaction rate cannot explainthe strongly contrasting exchange observed betweenthese years.

The present study indicates that in some periodsemissions are significant (Figs 2 and 3). Emissionsoccur mainly, but not exclusively, during the day andin the warmer seasons. Emissions have been observedto occur under conditions of sufficient fetch, at ambi-ent NH

3concentrations as high as 12 kgm~3 and at

temperatures as low as 4°C. Therefore, many of theemissions observed in this study cannot be attributedto shifts in aerosol—gas equilibria. In Fig. 6 the NH

3flux is plotted for the period 1—6 July 1993. A veryclear daily cycle is present with net deposition at nightand net emission during the day. This alternationbetween deposition and emission continues until thecanopy becomes saturated with water (CWS'2 mm)by two rain showers on 5 July. In this case, as long asthe canopy is wet, NH

3is consistently and efficiently

deposited. When the canopy is rapidly drying (in themorning of 6 July), NH

3is emitted. Also indicated in

Fig. 6 is the modelled flux, calculated following Suttonand Fowler (1993). Their model assumes bi-direc-tional exchange with stomata in parallel with irrevers-ible deposition to leaf surfaces. Net emission occurswhen the stomatal emission flux exceeds the depos-ition flux to the leaf surface. In Fig. 6 the modelledflux closely approximates the observed flux. Thisgood fit was obtained assuming an apoplastic pH of7 and an apoplastic [NH`

4] of 850 kM. R

8was cal-

culated using the expression derived from the noctur-nal R

#—RH correlation in 1993. Discrepancies are

observed when the canopy is drying (5—6 July) whenthe model predicts efficient deposition, whereas themeasurements indicate a small deposition flux (5 July)or an emission flux (6 July). This difference is probablycaused by an emission flux from the drying leaf sur-face, leading to a smaller net deposition flux on 5 Julyand a net emission flux on 6 July. When modelled andmeasured fluxes are compared for a meteorologicallysimilar period in July 1994 (Fig. 7), using the sameapoplastic pH and [NH`

4], but the expression for

R8

derived from the nocturnal R#—RH correlation in

1994, agreement is only observed at night. During theday the measurements indicate consistent and efficientnet deposition, whereas the model predicts NH

3emis-

sion. To obtain a good fit for July 1994, a value of zero


Fig. 6. Observed NH3

flux, modelled flux (Flux-par), global radiation and canopy water storage (CWS) atthe Speuld forest measurement site, 1—6 July 1993. The model parametrization is described in Section 4.4.

has to be adopted for R8

during the day. This is alsoapparent from the R

#derived from the measurements,

which show near-zero values during the day andvalues up to 70 sm~1 during the night (Fig. 7). Themodel of Sutton and Fowler (1993) can be fitted to thedata to describe the exchange of NH

3for both of these

periods with a reasonable accuracy, but this requiresdifferent values of R

8. Clearly, R

8is affected by more

complex factors than just humidity, such as depos-ition fluxes of other trace gases, particle depositionand air concentrations of NH

3. Furthermore, emis-

sions from drying leaf surfaces are sometimes ob-served which are not accounted for in that model.

4.5. ¹he influence of leaf surface chemistry

In Figs 1 and 7, R#

is observed to increase oversuccessive nights. In Fig. 1 this is mainly the result ofdrying of the canopy, but in the period shown inFig. 7 the canopy is already dry. During the day theR

#remains low, which may be due to the deliques-

cence of deposited salts by transpired water (seeabove). Also plotted in Fig. 7 is the accumulated flux.This parameter represents the sum of F in the preced-ing period since the last time that the canopy wassaturated with water (CWS'2). The increase in theaccumulated flux is accompanied by an increase in thenocturnal R

#. This ‘‘saturation effect’’ has also been

observed in a study over heathland by Erisman andWyers (1993). This suggests that the net NH

3flux to

or from the leaf surface is influenced by NHx

alreadypresent on the leaf surface. The leaf surface chemistryis determined by the amount of water present, the

material deposited in the preceding period and thematerial leached or emitted from the needles. Whenthe leaf surface contains relatively little NH

x, R

#will

be small and deposition very efficient. When the NHx

concentration on the leaf surface increases by addi-tional deposition of NH

9or by evaporation of water,

R#can be expected to increase and the dry deposition

rate to decrease. If the gaseous NH3

concentration atequilibrium with dissolved NH

xon the leaf surface

becomes higher than the actual NH3

air concentra-tion, NH

3may even be emitted from the leaf surface.

Other pollutants deposited on the leaf surface such asSO

2could also be of influence on the exchange of

NH3

(so called co-deposition, e.g. Adema et al., 1986).However, although concurrent deposition measure-ments for SO

2were available for the entire period, no

clear indications could be found for an influence ofSO

2on the surface exchange of NH

3. A model de-

scribing NH3

exchange should thus include the leafsurface composition as a parameter. The estimates ofthe volume of water present on the leaf surface and theaccumulated flux still carry very large uncertainties,however.

4.6. Ammonia exchange in 1993 and 1994

A central question is whether it is possible toexplain the observed differences in NH

3exchange

between 1993 and 1994. For wind speed, friction velo-city, RH or global radiation (cloud cover) the meanvalues for 1993 and 1994 are very similar. Differencesin mean »

.!9are also small (63 mms~1 in 1993 and

56 mms~1 in 1994). SO2

only shows a very slight


Fig. 7. Observed NH3

flux, modelled flux (Flux-par), global radiation, surface resistance (R#) and accumu-

lated flux at the Speuld forest measurement site, 16—19 July 1994. Throughout this period CWS is below0.25 mm. The model parametrization is described in Section 4.4.

decrease from 7.3 to 6.9 kgm~3. The year 1994 wassomewhat (1.1°C) colder, and the mean CWS wasslightly higher (0.65 mm in 1994 vs 0.58 in 1993).Largely as a result of emission reduction measures,the average NH

3concentration decreased from

5.6 kgm~3 in 1993 to 4.5 kg m~3 in 1994. The lowertemperature and higher CWS in 1994 may, in prin-ciple, have led to a lower emission component ora higher deposition component, through a small de-crease in the compensation point and in R

8. However,

the slight decrease in the compensation point of theorder of 0.1—0.2 kgm~3 will be more than offset by thedecrease in air concentration of 1.1 kg m~3, leading toa stronger emission component in 1994 which is incontradiction with the observations. Another possi-bility is that a higher deposition of N compoundsyields an increased apoplastic NH`

4concentration

and raises the compensation point (Sutton et al.,1995). However, from the data presented here it can beconcluded that the net deposition flux of NH

3in 1994

was higher than in 1993 (Figs 3 and 4), whereas for theother N compounds the differences in depositionfluxes are expected to have been small (Heij andSchneider, 1995). This then would have resulted ina higher compensation point and a stronger emissioncomponent in 1994, again in contradiction with themeasurements.

An important difference between 1993 and 1994 isthe mean value of the accumulated flux. The accumu-lated flux is taken here as a first-order approximationof the NH

xconcentration on the leaf surface. The

annual average accumulated flux for the year 1993was 14 mgm~2, whereas in 1994 it was only5 mgm~2. In 1994 the canopy was more frequentlywashed-off (CWS'2 mm) than in 1993, which mayhave resulted in these large differences in the accumu-lated flux. It is proposed here that the larger averageaccumulated flux in 1993 has led to a higher leafsurface resistance to deposition and to a larger andmore frequent emission component.

5. CONCLUSIONS

Surface exchange of NH3

over a coniferous foresthas been measured continuously for a period of morethan 3 yr. This has provided a unique dataset formodel development and evaluation. The measure-ments show that NH

3exchange is the result of inter-

action between physical, chemical and biologicalprocesses. The leaf surface can be both a sink anda source for NH

3, the latter through cuticular desorp-

tion of deposited NH3. Therefore, the bi-directional

nature of NH3

exchange applies to the leaf surface aswell as the stomata. When the net flux is directedtowards the canopy, the leaf surface resistance (R

8) is

often smaller than the stomatal resistance (R4), even

when the canopy is dry, which suggests that underthose conditions the leaf surface is a stronger sinkthan the stomata. The value of R

8is not a simple

function of RH, as previously suggested, but alsoinfluenced by other factors such as the accumulation


of NH3

and other pollutants on the leaf surface. Onthe basis of the canopy water storage the followingthree cases can be distinguished:

¼et canopy: When the canopy is fully saturatedwith water (CWS'2 mm), NH

3is efficiently depos-

ited to leaf surfaces, with mean F/F.!9

of 0.52 and 0.85for 1993 and 1994, respectively.

Drying canopy: For a drying canopy (0.25(CWS(2 mm) deposition is less efficient than for a wetcanopy, and the deviation of F from F

.!9is larger for

1993 than for 1994. Evaporation of water on the leafsurfaces may lead to emission of NH

3, which is some-

times observed during the day.Dry canopy: At night, when the canopy is dry

(CWS(0.25 mm) the flux is mostly directed towardsthe canopy, although emissions are sometimes ob-served. The nocturnal R

#is a function of RH, which

determines the thickness of microscale water layers onleaf surfaces. During the day emissions are very com-mon in 1993, but much less frequent in 1994. Duringthe day R

#can reach values much smaller than R

4,

which is believed to be due to high RH on the leafsurface caused by transpiration, and to deliquescenceof deposited hygroscopic particles.

Ammonia accumulating on the leaf surface in thepreceding period can have a large influence on boththe magnitude and the direction of the NH

3flux. It is

suggested that a different average leaf surface chem-istry is responsible for the observed differences in NH

3exchange between 1993 and 1994. The larger averageaccumulated flux in 1993 may have led to a higherR

8and to a larger and more frequent emission com-

ponent. Exchange of NH3

over semi-natural vegeta-tion should therefore be estimated with dynamicmodels taking into account such memory effects.

Acknowledgements—The authors wish to acknowledge thecontribution of Rene Otjes, who was responsible for themaintenance of the instrumentation for the NH

3gradient

measurements. The size and quality of the described data seton NH

3exchange are largely the result of his efforts. They

further wish to thank Mark Sutton for his thorough reviewof the manuscript and many helpful comments. This studywas part of the EUROTRAC-BIATEX programme. Fund-ing of the BIATEX-programme of ECN by the Ministry ofHousing, Spatial Planning and Environment and the Minis-try of Economic Affairs is gratefully acknowledged.

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Ammonia exchange over coniferous forest