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Measurements of ammonia flux to a spruce stand in Denmark
Atmospheric Environment VoL 27A, No. 2, pp. 189-202, 1993. Printed in Great Britain.
0004-6981/93 $6.00+0.00 © 1993 Pergamon Press Ltd
MEASUREMENTS OF AMMONIA FLUX TO A SPRUCE STAND IN D E N M A R K HELLE VIBEKE ANDERSEN a n d MADS F. HOVMAND National Environmental Research Institute, Frederiksborgvej 399, DK-4000 Roskilde, Denmark
and POUL HUMMELSHOJ a n d NIELS OTTO JENSEN Rise National Laboratory, DK-4000 Roskilde, Denmark (First received 26 March 1992 and in final form 20 August 1992) Abstract--This work demonstrates the existence of a linear relation between the deposition velocity of ammonia and the friction velocitymeasured above a spruce stand in the western part of Denmark. In order to estimate the ammonia deposition velocityand flux to a Norway spruce forest, concentration gradients of ammonia and several meteorological parameters were measured in a meteorology tower during two periods, 1 week in spring and 1 week in late summer 1991. The estimated deposition velocities lie in the range -0.125 to 0.201 m s-1 with a mean of 0.026 m s-1. The deposition velocity and the flux were generallylargest in the afternoon. On the basis of 24-h measurements of ammonia and routine meteorological measurements the relation between deposition velocity and friction velocity is extrapolated to an estimate of the average flux for the growing season May to September 1991. The estimate gave an average flux of 87 #g NH3-N m -2 h-1 (=0.02/zg NHa-N m -2 s-1). The average deposition velocity for the period was 0.045 m s- 1. Key word index: Ammonia, dry deposition velocity, forest, flux, micrometeorology. fertilized surfaces in the United Kingdom showed that the main limitation to deposition was atmospheric The atmospheric input of different chemical com- resistance, defined largely by site roughness and wind pounds to forests and natural resorts provides a signi- speed (Sutton, 1990). These investigations include ficant contribution of plant nutrients and acidifying a few measurements above coniferous forest in Scotcompounds to these ecosystems. The input of nitrogen land showing the same picture of rapid deposition. compounds is of special importance as nitrogen in The ammonia flux experiments reported here were most ecosystems is the limiting growth factor and carried out during two periods: 24-31 May and 29 can act as an acidifying compound at the same time August-5 September 1991. The experimental site is (Nihlg~rd, 1985; Gunderson and Rasmussen, 1988; situated in Ulborg Forest District in West Jutland, Rostn, 1988). 20kin east of the North Sea coastline (56°17'N, Only a few studies of the dry deposition of ammo- 8°26'E) and 40 m above sea level (Fig. 1). The nearest nia (NH3) to forests have been reported. Duyzer et town (30,000 inhabitants) is located 15 km northeast al. (1992) measured NHa fluxes to coniferous forest in of the site. There is no industry in the area. The forest the Netherlands. They reported a deposition rate is surrounded by agriculture activities at varying disclose to the maximum possible, only limited by the tances (Fig. 2). The trees, Norway spruce (Picea abies turbulent transport rate to the canopy. They found Karst.), have a mean height about 10 m. The site was deposition velocities in the range -0.181 to 0.104 m established in 1964 on heathland. Since 1985 a study s- 1 and estimated an average deposition velocity for of ion circulation has .been carried out, with emphasis the site for 1 year to 0.036 m s- 1 and an average flux on the budget of sulphur, nitrogen and base cations to 50kg N h a -1 yr - l (~0.16/~gN m -2 s-~). Wyers (Hovmand and Bille-Hansen, 1988). et al. (1992) also measured NH3 fluxes to coniferous For the years 1989 and 1990 the average concenforest in the Netherlands. They found highly variable trations of gases and aerosols (24-h measurements) deposition velocities with a median value of w e r e " 0.032ms-1 for the deposition velocity and /zgNm -3 /zgSm -3 0.10/~g N m -2 s -1 for the flux. Dens et al. (1988) NH 3 0.47 SO2 1.3 estimated an average ammonia deposition velocity to NH2 2.1 SO 21.5 forest to 0.039 m s- 1 from throughfall measurements. NO z 1.6 The throughfall method is generally believed to be (NO~ + HNOa) 1.0 rather uncertain due to biological interaction in the canopy. Deposition measurements to natural and un- (Hovmand, 1990, Hovmand and Grunddahl, 1991.) 1. I N T R O D U C T I O N
189
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Meteorological measurements were started in January 1991 on a routine basis. Ten-minute averages of wind speed (cup anemometers), wind direction (wind vane), solar radiation (Aanderaa), absolute temperature (Aanderaa 1289) and gradients of temperature (Aanderaa 1289A) and humidity (Rotronic MP100) are collected from a 36-m high meteorology tower. The emission density of ammonia in the area ( 1 5 x l 5 k m ) surrounding the monitoring plot is 2.7 g N m - 2 yr- 1 (Asman, 1990). The average emission density for Denmark is 2.6 g N m -2 yr-~. The total ammonia emission from Denmark is about 123,000 ton N H 3 - N yr-1, mainly from agricultural activity. As the investigation area is dominated by forest and heathland the emissions are very small within a radius of 2 km of the measuring site. From most wind directions the atmospheric ammonia is transported to the measuring site from agricultural areas more than 5 km away (Fig. 2). The nitrogen requirements from the plantation is at the current growth rate 1 . 0 g N m - 2 y r - ~ . From measurements of wet deposition and atmospheric concentrations the nitrogen input from the atmosphere is estimated to 2.0 g N m -2 y r - t . Wet deposition accounts for 1.1 g N m - 2 y r - ~ and the dry
deposition is estimated with great uncertainty to 0.9 g N m -2 y r - 1 (Hovmand and Bille-Hansen, 1988; Hovmand and Grundahl, 1991). In order to predict the nitrogen status of the forest ecosystem, more reliable dry deposition estimates of nitrogen compounds are needed. Of special interest is the deposition of ammonia since ammonia in regions with agricultural activities is expected to account for a major part of the nitrogen deposition. A regional average estimate is difficult to assess, because emissions are highly variable throughout the country. There are decreasing concentrations from emission areas into background areas and highly variable fluxes from place to place. The aim of this investigation is therefore to determine the ammonia flux to a coniferous forest in an area where yearly variation in ammonia concentrations are well documented.
2. E X P E R I M E N T A L
2.1. Meteorological measurements During the campaigns, the meteorological measurements were as follows: wind direction (wind vane) at a height of 36 m; wind speed (cup anemometers) at 36, 30, 24, 18 and
Ammonia flux measurements
191
Fig. 2. The experimental site. The radius of the circle is 2 km. Forests are indicated in dark grey, heathland in grey, and arable land in white. Only the two major farms in the area are shown.
14 m; absolute temperature (Pt 100) at 18 m; temperature difference (Pt 500) between 18 and 36 m; tree top surface temperature (Heimann KT15) at 10m; relative humidity (Rotronic MP100) at 36, 18, 8 and 4 m; fluxes of momentum, heat and water vapor (Gill Sonic and Ophir IR-2000) at 21 m. The set-up is shown in Fig. 3. This instrumentation was run parallel to the climatological measurements mentioned in the Introduction. From the measured parameters, other meteorological parameters can be derived. Among these are: surface roughness, Zo, and displacement height, d, from the logarithmic wind profile at neutral stability. From the first analysis d is found to be 8 m and Zo~ 1.4 m (at this stage we have not enough data to see if this is wind direction dependent). The friction velocity, u, is found from the sonic anemometer from u2,=-(w'u'), where ( ) indicates average values, w' is the fluctuation in the vertical wind speed and u' the fluctuation in the horizontal wind speed. Stability is calculated from the Monin-Obukhov length L = -ua, T/(Kg(w'T')), where T is the temperature in K, K the von K~rm~n constant (~ =0.4), g acceleration due to gravity, and (w'T') the kinematic heat flux, also measured by the sonic anemometer, in (m s - 1 K). The ratio of height over Monin-Obukhov length expresses the stability: (z-d)/L, where z is the height above ground level (z=21 m) and d the displacement height (d=8 m). Stability is controlled by the Monin-Obukhov length, L. On sunny days with positive heat flux upwards, L is negative and we have unstable conditions, (z-d)/LO. In between, when (z-d)/L~O, we have nearneutral stability. In this paper near-neutral is defined as -0.1 <(z-d)/L
2.2. Ammonia measurements During the two campaigns, ammonia was sampled from the meteorological tower. Samples were taken during 3-h consecutive periods, except for periods around sunrise and sunset. The samplers were placed at 18 and 36 m above ground. Occasionally, measurements were made in the top of the canopy (9 m) but these are not reported here. Each sampling consisted of three parallel measurements at each level. The sampling was done by denuders (Ferm, 1979). Glass tubes, 50-cm long and 3-ram i.d., were coated with 0.5% (w/v) oxalic acid in ethanol, leaving a 15-era uncoated inlet to establish laminar flow. The flow rate was 3 : min- 1 and each denuder was connected individually to a critical orifice with flow meter. All flow meters were connected to a vacuum pump. During exposure the denuders were heated a few degrees above ambient temperature to prevent condensation in the tubes. After exposure the tubes were extracted with 3 ml of deionized water and ammoniacal N was determined by the indophenol blue method. The detection level for a 3-h exposure was about 0.05/~g N H 3 - N m -3, occasionally lower. Sixteen per cent of the measured concentrations were lower than 0.05 #g NH3-N m -3, all of these occurred during the May campaign. 2.3. Filter pack and CO2 and O3 measurements During the two campaigns, measurements with filter packs and CO2 and 03 monitors were also performed. On a scaffold filter pack measurements were sampled at two heights, 9 and 16 m (Fig. 3). The filter packs are described in detail by Fuglsang (1986). The filter pack measurements
192
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Fig. 3. The instrumentation of the meteorology tower. Cup anemometer: 14, 18, 24, 30 and 36 m; wind vane: 36 m; temperature: 18 m; temp. diff.: 36-18 m; humidity: 4, 8, 18 and 36 m; gill sonic: 21 m; ophir humidity: 21 m; surface temp.: 10 m; NH 3 denuder: 9, 18 and 36 m; 0 3 and CO 2 measurements: 9, 18 and 36 rn; filter-pack samplings were made from the scaffold: 9 and 16 m.
Ammonia
flux measurements
were 3-6-h exposures. They provided information about the levels of particulate SO~-, NH~ and total (NO/ + HNO3) and gaseous SOs and NH3. COs and O3 gradients were measured in the meteorology tower. These measurements were determined by monitors placed in a cabin (Fig. 3). The data obtained by filter pack and CO~ and O3 measurements are not presented here. The measurements showed that no extreme concentration levels of the different species occurred during the campaigns.
3. DEPOSITION VELOCITY AND FLUX ~ A T I O N
The method used to determine the flux of NH3 to the forest is a combination of vertical mean-profile measurements of NH3 and of profile as well as eddycorrelation flux measurements of other quantities. These other quantities are temperature, water vapour and horizontal momentum. It is based on the notion that the vertical transport above the forest is totally dominated by turbulent motions of the air. Thus the exchange between two different layers of a particular constituent is only determined by the difference in concentration between these layers, multiplied by a turbulent diffusivity. This turbulent diffusivity is related to the turbulent field and not the quantity being transported. Hence the direct flux measurements of, say, momentum, FM, can be used to determine the flux of ammonia, FNH3, if the corresponding mean profiles are also measured, namely
different fluxes. The found diffusion coefficients are used to calculate the NH 3 flux. It is only an approximation to regard temperature and momentum as scalars, therefore Equation (1) is not used directly in this study. Further, Garratt (1978) found that flux profile relations are problematic when used above rou~,h surfaces, e.g. a forest. We have detected minor inconsistencies in the heat and momentum fluxes derived from the sonic anemometer used. We want to investigate these inconsistencies further before we base our flux estimates directly on this. We wish to develop a method which can also be used to calculate fluxes from more routinely available data. Hence, we will interpret Equation (1) through the surface layer similarity expressions for mean pro-
files. For the wind profile wehavethe followinglogarithmic dependenceof height: u(z)--? [In (-~-od;- ~. (-~--~; ],
u(zs)-u(zl)
(1)
where CNa~(z) is the concentration of NH3 at height z, u(z) is the wind speed ("concentration" of momentum) and the subscripts 1 and 2 refer to two different heights. One assumes that the flux measured in the air is equal to the flux which actually arrives at the forest "surface". The conditions which must be fulfilled can be found from the conservation equation for a scalar C. These are stationarity (0C/at=0, where t is time), horizontal homogeneity (aC/c~x=O, where x is horizontal distance), and no local sinks or sources for C (such as "fast" chemical reactions). Under these conditions the flux is constant with height. These assumptions are not entirely correct. If the concentration changes by a factor of two during the 3-h sampling period (aC/~t~0), then there will be an error in the apparent deposition velocity, Vd, of about 0.001 m s - 1. Therefore, in relation to the deposition velocities obtained for NH3 in this context, a change in concentration during sampling might introduce a very small error compared to measurement uncertainties. Flux measurements were carried out for momentum, heat and water vapour. The fluxes are interesting in relation to the evaporation from the forest and they provide a possibility for comparison between
(2)
where ~/M((z-d)/L) is the integrated stability correction function for momentum (Paulson, 1970). For a discussion on the influence of the displacement height, d, see Raupach (1979). The same way as for the wind profile a logarithmic profile can be derived for the air concentration of a scalar C. Thus
c( )-Co = FNa3 -- FMCNa~(Zz) -- CNU~(Zl),
193
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/
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/j
(3)
where Co is the surface concentration of the scalar. This is the main difference from the wind profile, where the "surface concentration" is zero (Uo~0). For scalars this is, in general, not the case, the exception being particles and those gases which rapidly dissolve or react with the surface. The value of gc, de, zoc and the stability function, ~Pc, is not necessarily the same as for m o m e n t u m - not even the same as for the more similar (quasiscalar) quantity heat flux. In principle they could all be different for cacti constituent. In the rest of this analysis we assume K - - g , dc=d and ~Pc=~Pa (see remarks below). It is not necessary to assume zoc =z0 here, because zoc will disappear from the equation later on. Results from a first analysis show that values for heat and water vapour is an order of magnitude less than Zo (but variable). The parameter c , in Equation (3) is similar to u , in Equation (2); c . is a measure of the concentration gradient. In the same way as the kinematic momentum flux can be written FM = - - uZ,, the relation between c, and the turbulent flux of the constituent can be written Fc = - u . c . .
(4)
The negative sign is a convention such that ff c . is positive (concentration increasing with height) the flux is downwards (negative).
194
H.V. ANDERSENet al.
Using two levels of measurements in the air and using Equation (3), c, can be isolated
c,=
The highest possible deposition velocity, Vd. . . . ~z~,is given by
x(C2-Cd
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This eliminated Co and zoc as mentioned earlier. Using Equation (5) in (4) we can now calculate the flux of C. From this the deposition velocity can be determined by division with the concentration C(z): Vd(Z)=
Fc C(z) "
(6)
The deposition velocity is by definition connected to a certain height z. The definition of Vd implies a surface concentration Co close to zero or Co,~ C(z). In this study it is assumed that Co~,C(z) due to the acidic surfaces (low pH) of the needles and top soil. The micrometeorological observations performed in this project might enable us to establish W-functions that are valid for the site in question. However, that has to await more thorough analysis, which is beyond the scope of the present paper. Even with the tower we use, which is roughly three times the canopy height, we are still in a height range where the values of W are influenced by the proximity of the tree tops. This means that we cannot rely on the usual consistency checks between profile-derived flux quantities and direct eddy fluxes (Raupach, 1979). A special analysis of this aspect will be forthcoming. In this context we have resorted to the use of stability correction functions for the "standard situation". Because of the effect described above, we have not been very particular in our choice of the large variety of the V-functions reported in the literature, not even regarding the differences between WM, Wn, etc. (for typical unstable conditions with L ~ - 1 0 0 m, the effect on va is less than 10%). For unstable conditions we have used the expressions found by Jensen et al. (1984): W = ~ - - 1,
~=(1-16(~Ld))
~t
Vd. . . .
(5)
(7)
which is based on the work of Businger et al. (1971), but have ease of calculation compared to the functional relationship given by Paulson (1970). For stable conditions we have used
This relationship is recognized only to work for 5 ( z - d ) / L < . 1. A rule of thumb, which we have not implemented, says that above this limit, the term should be kept as a constant of order unity. We have often encountered values of L so small that this is clearly violated, perhaps with the consequence that we have estimated too low deposition velocities during stable nights.
Iz)- U(Z)"
(9)
It comes from a straightforward application of Equation (6). This is an upper limit because momentum disappears by friction and pressure forces and therefore has no surface resistance. Physically, Equation (9) means that the material cannot be deposited faster at the surface than the turbulence can transport material towards the surface. The estimation of deposition velocities and fluxes are based on conserved species. Chemical transformation would affect the gradient: Harrison et al. (1989) report that chemical reactions involving the gases HNO3, HCI and NH3 are too slow to influence the concentration gradients and the surface exchange process. However, the reaction of ammonia with acid aerosols may be faster (Huntzicker et al., 1980). In this investigation, we are not able to say whether or not chemical transformation are occurring at significant rates, but we assume that it is not the case.
4. RESULTSAND DISCUSSION 4.1. Meteorology Generally the two campaigns in May and August/September 1991 can be characterized by no precipitation, normal wind speed and wind directions mainly from the northwest. Figure 4 shows relative frequencies of wind directions in wind roses from the campaigns compared with 8-year average (1971-1979) from Karup meteorological station located 45 km east of Ulborg (Troen and Petersen, 1989). It is seen that the wind directions during the two campaigns were very similar but not representative for a normal year in the area. The wind speed at 36-m height in Ulborg is comparable to standard observations at 10-m height over open terrain. This is due to the large surface roughness of the forest combined with the displacement height. The average wind speed at 36 m during the two campaigns was 4.7 and 4.9 m s - 1, respectively. The climatological mean in Denmark is 4-5 m s- 1 (Larsen and Jensen, 1983). The dry and clear weather during both of the campaigns is consistent with the fairly large amplitudes in daily variation of the observed parameters (Figs 5a and 5b). Thus the value of u, typically varies between 0.1 and 1 m s-~. The diurnal cycle is also manifested in the variations in vertical atmospheric stability which comprises both very stratified, ( z - d ) / L > 1, as well as quite convective, ( z - d)/L < - 1, periods. This means that the deposition velocity is expected to vary considerably; more than the decade as indicated by the variation in u, itself (see Section 3).
Ammonia flux measurements
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4.2. Ammonia concentrations The concentration levels during the measuring periods in August/September were generally higher than during the period in May (Figs 6a and 6b), probable due to agricultural activities and higher temperatures. Average concentrations from the periods were close to values from 1989 and 1990 (Hovmand, 1990; Hovmand and Grunddahl, 1991). High concentration events were correlated with stability in terms of ( z - d ) / L and low wind speeds (Figs 5a and 5b). The concentration seems to be inversely proportional to wind speed as shown in Fig. 7. The data have a wide scatter, mainly caused by change in source strength and distance to source with wind direction. 4.3. Concentration difference of ammonia between the two heights The difference in ammonia concentration between the two heights are shown in Figs 8a and 8b. It is indicated whether or not the difference is significantly different from zero at the 90% level. In total, 80 measuring periods are made. Thirty-four periods have
a significant difference in concentration between 36 and 18 m. In 45 measuring periods there was no significant difference in concentration between the two heights. One measuring period was left out of further analysis for experimental reasons. The grouping of concentration differences significantly or not significantly different from zero is dependent on two factors, the precision of the measurements and the atmospheric stability. During stable conditions the air mixes very slowly and the difference might become relatively large. Then the requirements of the precision are low concerning a concentration difference significantly different from zero. During unstable conditions the air mixes very well and the demands on the precision increase considerably with respect to significant concentration difference. In this context the near-neutral condition is in between the stable and unstable situation. Further, the precision is to some extent dependent on the concentration (i.e. small concentrations having poor precision), which again is connected to the atmospheric stability with respect to wind speed (Section 4.2). It is an inherent difficulty of this investigation method that well-mixed hir exhibits small concentration differences. It means that even
196
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though the deposition might be considerable, it is difficult to detect concentration differences significantly different from zero. Of the measured concentration differences, which significantly differ from zero, 24 are positive and 10 negative (Figs 8a and 8b). The 10 situations with negative concentration difference occur during nighttime and early morning. The negative differences observed here might be due to: (i) emission from the
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The other five negative concentration differences occur during stable conditions with slow, meandering winds. The averaging period is 3 h and during such conditions, with low-level turbulence, a local source only needs to be swept once during a period to influence the concentration difference. In the episodes observed here, the wind actually sweeps either northeast or west-northwest (see Fig. 2) with short distances to
agriculture areas. All these episodes have relatively high concentrations. In this investigation, we do not have sufficient information about other gases and particles in the two heights, which might influence the ammonia gradient. We know from filter-pack measurements in a nearby scaffold (shown in Fig. 3), that no extreme levels of sulphur dioxide or aerosols were occurring.
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Fig. 6. NH3 concentrations (18 m) as 3-h mean values. (a) Data lrom the campaign in May. The measurements were done in the following intervals: 2300-0200, 0500-0800, 0800-1100, 1100-1400, 1400-1700 and 2000-2300 h. (b) Data from the campaign in August/September. The measurements were done in the following intervals: 2300-0200, 0800-1100, 1100-1400, 1400-1700 and 2000-2300 h except measurements on 31 August: 0500-0800; and 2 September: 2300-0800.
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NH3 concentration, 18 m (/zg N / m 3 ) Fig. 7. Wind speed, u (18 m), vs 3-h mean values of the concentration of N H 3 from both campaigns,
Duyzer et al. (1992) report some negative gradients measured above coniferous forest in the Netherlands. Their observations are during daytime and relatively low concentrations. They explain the phenomena as evaporating NH3 from a drying surface or a compensation point for NH3. Wyers et al. (1992) report that negative gradients occasionally occur.
To estimate deposition velocities and fluxes certain criteria are set up for the data. Eleven of the 80 measuring periods are left out of further analysis due to missing meteorological measurements, concentration of NH3 less than 0.02 #g N H 3 - N m-3 or varying stability class during exposure period. Concentration measurements of ammonia are determined as triplicates and the deviation from the mean is expressed as a coefficient of variation. For both heights the coefficient of variation should be smaller than 20%. Sixteen of the remaining 69 sets of measurements does not meet this requirement. The estimated deposition velocities lie in the range -0.125 to 0.201 m s i with a mean o f 0 . 0 2 6 m s - t . These values are in agreement with values reported by Duyzer et al. (1992) and Wyers et al. (1992) from coniferous forests in the Netherlands. Figure 9 shows the estimated deposition velocity vs the time of the measurement. There is a tendency to an increased deposition velocity in the late afternoon. This is consistent with the tendency to strongest convection (unstable conditions) in the afternoon.
Ammonia flux measurements
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This would typically occur 3 h after solar zenith (Larsen and 3enscn, 1983). The period of maximum deposition, 1400-1700 h, actually corresponds to ~ 13451645 h solar time. Figure 10 shows the estimated flux. Negative values mean downward flux. The tendency towards a maximum in the afternoon is also seen here as for the deposition velocity. For the particular height combination 36 and 18 m, and a displacement height of 8 m, combination of the ~(A) 27:2-F
equations from Section 3 gives Fc ~ ~cu.AC/( 1 - A~F).
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One could neglect the negative deposition velocities which might be caused by inadequate mixing of the atmosphere. Still, the bias could be present in some of the "accepted" values. Therefore, if an average deposition velocity, with or without the negative deposition velocities/positive fluxes, were used to obtain a climatological value for the deposition flux an error would probably be made towards underestimating the input to the forest. One can make more rigorous demands on the precision of the ammonia measurements than the variation coefficient of 20% in the two heights. If the concentration measurements should meet the demand of a variation coefficient smaller than 10% in both heights, 40 of the 69 sbt of measurements are excluded. This more rigorous demand gives data showing a good relation between deposition velocity, vd(ts m), and friction velocity, u,, as shown in Fig. 11 (dosed circles). Taking all data (Fig. 11, closed circles and pluses) estimated from gradients with concentration measurements with coefficient of variation smaller than 20%, a relation is still seen, but a group of measurements with u, .~0.45-0.75 m s - ~ give a low, sometimes even negative, deposition velocity, mainly related to westerly winds. An orthogonal regression fit between u, and Vd(~a m)are made on two sets of data, classified according to the level of coefficient of variation (CV) discussed above: CV<20%: Vd=0.10u,--0.02(corr. 0.60, N = 5 2 )
(11)
C V < 10%: vd=0.11u,--0.02(corr. 0.84, N=29), (12) where u. and Vd are in m s -1 . In calculating Equation (11) one data set is left out (001X)-4)300h, 5 September, /)d(lSm)=--0.125 m s - t ) . It is a negative deposition velocity (significant at the 90% level) related to the group of significant negative concentration differences with low concentrations, discussed in Section 4.3. In Fig. 11 the two regression lines are drawn. It is seen that they are close to each other, though the correlation is higher for Equation (12). In Fig. 11 the best linear fit of the maximum possible deposition
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Fig. 11. The estimated deposition velocity, Vd(18.), from both campaigns vs the friction velocity, u..
velocity, Vd. . . . (18m) (Equation 9) is drawn. From Equations(4) and (6) it is seen that va(lsm)= -u,c./Clsm. Neglecting the intercept in Equations (11) and (12), the slope of the found relation between Vd and u . corresponds to c . / C l s m. Using Equation (5) it is seen that c , / C ~ s m is the product of the relative gradient, (C36 m - - C ~ 8 m)/C~s =, and the stability correction. When this product is constant, the relative gradient is inversely dependent on the stability. Therefore, the relative gradient, and by that the deposition velocity, is found to be controlled by the meteorology. From Equation (11) the flux can be calculated for a longer period, i.e. the approximate "growing season" 25 April-30 September 1991. From the routine
Ammonia flux measurements
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Fig. 12. For the period May-September 1991,daily averages of"(a) measured ammonia concentrations, (b) estimated ammonia deposition velocity (18 m) and (c) estimated ammonia flux. meteorological measurements, a 24-h average of the friction velocity, u., is calculated for the period. In order to correct for stability effects in the calculation of daily values of u,, we use a bulk Richardson number, z0 -- 1.4 m and d = 8 m found during the two campaigns. These averages are used in Equation (11) to calculate a 24-h average deposition velocity. For a flux estimation these deposition velocities are multiplied with the corresponding 24-h average concentration of NH3 measured by filter pack. The filter pack method is found to give good separation of gaseous NH3 and particulate NH~ (Harrison and Kitto, 1990; Wiebe et al., 1990). Figure 12 shows the daily average of (a) the measured NH3 concentrations, (b) the estimated deposition velocity and (c) the estimated flux. It is seen that the concentrations are more variable than the deposition velocity. The concentration pattern dominates the flux pattern. For the half-year period 25 April-30 September 1991, the average friction velocity is 0.69 m s - ' . The average deposition velocity for the period is 0.045 m s- '. The flux estimation for the period is 8 7 / ~ g N H 3 - N m - 2 h -1 (ffi0.02pg N H 3 - N m - 2 s - ' ) . This might be an underestimation due to inclusion of negative concentration differences which in some cases are connected to meteorological situations where the gradient method is not valid, see Section 4.3 and 4.4.
The present ammonia deposition estimate exceeds earlier estimates from the measuring site. The dry deposition of all nitrogen species is raised from 0.8-0.9 g to 1.6 g N m - 2 yr- t. This nearly doubles the nitrogen excess input estimated to the forest.
5. SUMMARY AND CONCLUSIONS
Ammonia gradients and several micrometeorological parameters have been measured above a spruce forest in the western part of Denmark. From the measurements deposition velocities and fluxes were estimated. We found the following. • The deposition velocity of NH3 was mainly determined by the turbulent transport to the canopy. • The deposition velocity and the flux were generally largest in the late afternoon, corresponding well to the tendency to strongest convection at that time. • The deposition velocities lay in the range from -0.125 to 0.201 m s -t, with a mean of 0.026 m s -1" • Some negative gradients were found. One group were probably related to inadequate mixing of air passing from local sources, rather than emission from the forest.
202
H.W. ANDERSEN et al.
• The average flux of NH3 was estimated for the a p p r o x i m a t e growing season (25 A p r i l - 3 0 September 1991) to 8 7 # g N H 3 - N m - 2 h -1. Acknowledoements---This work was supported by The National Forest and Nature Agency and EEC, Forest and Forestry Division. We wish to thank John E. Hansen and Soren W. Lund for tower climbing, and Birte Vaabengaard for preparation and analysis of denuders. We also thank Michael S. Courtney for the use of his data acquisition system to collect meteorology data, and Gunner Dalsgaard and Arent Hansen for technical assistance.
REFERENCES
Asman W. A. H. (1990) A detailed ammonia emission inventory for Denmark. DMU LUFT-A133, Emissions and Air Pollution, National Environmental Research Institute, Roskilde, Denmark. Businger J. A., Wyngaard J. C., Izumi Y. and Bradley E. F. (1971) Flux profile relationships in the atmospheric surface layer. J. atmos. Sci. 28, 181-189. Duyzer J. H., Verhagen H. L. M., Weststrate J. H. and Bosveld F. C. (1992) Measurement of the dry deposition flux of NH3 on to coniferous forest. Envir. Pollut. 75, 3-13. Ferm M. (1979) Method for determination of atmospheric ammonia. Atmospheric Environment 13, 1385-1393. Fuglsang K. (1986) A filterpack for determination of total ammonia, total nitrate, sulfur dioxide and sulfate in the atmosphere. MST LUFT-A103, Air Pollution Laboratory, National Agency of Environmental Protection, Riso National Laboratory, Roskilde, Denmark. Garratt J. R. (1978) Flux profile relations above tall vegetations. Q. Jl R. Met. Soc. 104, 199-211. GundersenP. and Rasmussen L. (1988) Nitrification, acidification and aluminium release in forest soils. In Nordic Council of Ministers; Critical loads for sulphur and nitrogen. Miljorapport 1988: 15. Harrison R. M: and Kitto A.-M. N. (1990) Field intercomparison of filter pack and denuder sampling methods for reactive gaseous and particulate pollutants. Atmospheric Environment 24A, 2633-2640. Harrison R. M., Rapsomanikis S. and Turnbull A. (1989) Land-surface exchange in a chemically reactive system; surface fluxes of HNO3, HCI and NH3. Atmospheric Environment 23, 1795-1800. H ovmand M. F. (1990) Atmosfieren. Nedfald af kv~elstofforbindelser. DMU report no. 7, Emissions and Air Pollution, National Environmental Research Institute, Roskilde, Denmark.
Hovmand M. F. and Bille-Hansen J. (1988) lonbalance i skovokosystemer, reed m/tling af atmosf~erisk stoftilforsel. II. (English summary). MST LUFT-A127, Air Pollution Laboratory, National Agency of Environmental Protection, Riso National Laboratory, Roskilde, Denmark. Hovmand M. F. and Grundahl L. (1991) Atmosf~eren. Nedfald af kvaelstofforbindelser. (Extended abstract in English.) DMU report no. 36, Emissions and Air Pollution, National Environmental Research Institute, Roskilde, Denmark. Huntzicker J. J., Cary R. A. and Ling C.-S. (1980) Neutralization of sulfuric acid aerosol by ammonia. Envir. Sci. Technol. 14, 819-824. Ivens W. P. M.F., Draaijers G. P.J., Bos M.M. and Bleuten W. (1988) Dutch forests as air pollutant sinks in agricultural areas. Report AD 1988-01, Department of Physical Geography, University of Utrecht, the Netherlands. Jensen N. O., Petersen E. L. and Troen I. (1984) Extrapolation of mean statistics with special regard to wind energy applications. WMO. report WCP-86, WMO/TD-No. 15. Larsen S. E. and Jensen N. O~ (1983) Summary and interpretation of some Danish climate statistics. Riso-R-399, Riso National Laboratory, Roskilde, Denmark. Nihlg/trd B. (1985) The ammonium hypothesis: an additional explanation to the forest dieback in Europe. Ambio 14, 2-8. Paulson C. A. (1970) The mathematical representation of wind speed and temperature profiles in the unstable atmospheric surface layer. J. appl. Met. 9, 857-861. Raupach M. R. (1979) Anomalies in flux-gradient relationships over forest. Boundary-Layer Met. 16, 467-486. Ros6n K. (1988) Effects of biomass accumulation and forestry on nitrogen in forest ecosystems. In Nordic Concil of Ministers; Critical loads for sulphur and nitrogen. Miljorapport 1988: 15. Sutton M. (1990) The surface/atmosphere exchange of ammonia. Ph.D. thesis, University of Edinburgh, U.K. Troen I. and Petersen E. L. (1989) European wind atlas. Riso National Laboratory, Roskilde, Denmark. Wiebe H.A., Anlauf K. G., Tuazon E.C., Winer A. M., Biermann H.W., Appel B.R., Solomon P. A., Cass G.R., Ellestad T.G., Knapp K. T,, Peake E., Spicer C. W. and Lawson D. R. (1990) A comparison of measurements of atmospheric ammonia by filter packs, transition-flow reactors, Simple and annular denuders and Fourier transform infrared spectroscopy. Atmospheric Environment 24A, 1019-1028. Wyers G.P., Vermeulen A.T. and Slanina J. (1992) Measurement of dry deposition of ammonia on a forest. Envir. Pollut. 75, 25-28.
0004-6981/93 $6.00+0.00 © 1993 Pergamon Press Ltd
MEASUREMENTS OF AMMONIA FLUX TO A SPRUCE STAND IN D E N M A R K HELLE VIBEKE ANDERSEN a n d MADS F. HOVMAND National Environmental Research Institute, Frederiksborgvej 399, DK-4000 Roskilde, Denmark
and POUL HUMMELSHOJ a n d NIELS OTTO JENSEN Rise National Laboratory, DK-4000 Roskilde, Denmark (First received 26 March 1992 and in final form 20 August 1992) Abstract--This work demonstrates the existence of a linear relation between the deposition velocity of ammonia and the friction velocitymeasured above a spruce stand in the western part of Denmark. In order to estimate the ammonia deposition velocityand flux to a Norway spruce forest, concentration gradients of ammonia and several meteorological parameters were measured in a meteorology tower during two periods, 1 week in spring and 1 week in late summer 1991. The estimated deposition velocities lie in the range -0.125 to 0.201 m s-1 with a mean of 0.026 m s-1. The deposition velocity and the flux were generallylargest in the afternoon. On the basis of 24-h measurements of ammonia and routine meteorological measurements the relation between deposition velocity and friction velocity is extrapolated to an estimate of the average flux for the growing season May to September 1991. The estimate gave an average flux of 87 #g NH3-N m -2 h-1 (=0.02/zg NHa-N m -2 s-1). The average deposition velocity for the period was 0.045 m s- 1. Key word index: Ammonia, dry deposition velocity, forest, flux, micrometeorology. fertilized surfaces in the United Kingdom showed that the main limitation to deposition was atmospheric The atmospheric input of different chemical com- resistance, defined largely by site roughness and wind pounds to forests and natural resorts provides a signi- speed (Sutton, 1990). These investigations include ficant contribution of plant nutrients and acidifying a few measurements above coniferous forest in Scotcompounds to these ecosystems. The input of nitrogen land showing the same picture of rapid deposition. compounds is of special importance as nitrogen in The ammonia flux experiments reported here were most ecosystems is the limiting growth factor and carried out during two periods: 24-31 May and 29 can act as an acidifying compound at the same time August-5 September 1991. The experimental site is (Nihlg~rd, 1985; Gunderson and Rasmussen, 1988; situated in Ulborg Forest District in West Jutland, Rostn, 1988). 20kin east of the North Sea coastline (56°17'N, Only a few studies of the dry deposition of ammo- 8°26'E) and 40 m above sea level (Fig. 1). The nearest nia (NH3) to forests have been reported. Duyzer et town (30,000 inhabitants) is located 15 km northeast al. (1992) measured NHa fluxes to coniferous forest in of the site. There is no industry in the area. The forest the Netherlands. They reported a deposition rate is surrounded by agriculture activities at varying disclose to the maximum possible, only limited by the tances (Fig. 2). The trees, Norway spruce (Picea abies turbulent transport rate to the canopy. They found Karst.), have a mean height about 10 m. The site was deposition velocities in the range -0.181 to 0.104 m established in 1964 on heathland. Since 1985 a study s- 1 and estimated an average deposition velocity for of ion circulation has .been carried out, with emphasis the site for 1 year to 0.036 m s- 1 and an average flux on the budget of sulphur, nitrogen and base cations to 50kg N h a -1 yr - l (~0.16/~gN m -2 s-~). Wyers (Hovmand and Bille-Hansen, 1988). et al. (1992) also measured NH3 fluxes to coniferous For the years 1989 and 1990 the average concenforest in the Netherlands. They found highly variable trations of gases and aerosols (24-h measurements) deposition velocities with a median value of w e r e " 0.032ms-1 for the deposition velocity and /zgNm -3 /zgSm -3 0.10/~g N m -2 s -1 for the flux. Dens et al. (1988) NH 3 0.47 SO2 1.3 estimated an average ammonia deposition velocity to NH2 2.1 SO 21.5 forest to 0.039 m s- 1 from throughfall measurements. NO z 1.6 The throughfall method is generally believed to be (NO~ + HNOa) 1.0 rather uncertain due to biological interaction in the canopy. Deposition measurements to natural and un- (Hovmand, 1990, Hovmand and Grunddahl, 1991.) 1. I N T R O D U C T I O N
189
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Meteorological measurements were started in January 1991 on a routine basis. Ten-minute averages of wind speed (cup anemometers), wind direction (wind vane), solar radiation (Aanderaa), absolute temperature (Aanderaa 1289) and gradients of temperature (Aanderaa 1289A) and humidity (Rotronic MP100) are collected from a 36-m high meteorology tower. The emission density of ammonia in the area ( 1 5 x l 5 k m ) surrounding the monitoring plot is 2.7 g N m - 2 yr- 1 (Asman, 1990). The average emission density for Denmark is 2.6 g N m -2 yr-~. The total ammonia emission from Denmark is about 123,000 ton N H 3 - N yr-1, mainly from agricultural activity. As the investigation area is dominated by forest and heathland the emissions are very small within a radius of 2 km of the measuring site. From most wind directions the atmospheric ammonia is transported to the measuring site from agricultural areas more than 5 km away (Fig. 2). The nitrogen requirements from the plantation is at the current growth rate 1 . 0 g N m - 2 y r - ~ . From measurements of wet deposition and atmospheric concentrations the nitrogen input from the atmosphere is estimated to 2.0 g N m -2 y r - t . Wet deposition accounts for 1.1 g N m - 2 y r - ~ and the dry
deposition is estimated with great uncertainty to 0.9 g N m -2 y r - 1 (Hovmand and Bille-Hansen, 1988; Hovmand and Grundahl, 1991). In order to predict the nitrogen status of the forest ecosystem, more reliable dry deposition estimates of nitrogen compounds are needed. Of special interest is the deposition of ammonia since ammonia in regions with agricultural activities is expected to account for a major part of the nitrogen deposition. A regional average estimate is difficult to assess, because emissions are highly variable throughout the country. There are decreasing concentrations from emission areas into background areas and highly variable fluxes from place to place. The aim of this investigation is therefore to determine the ammonia flux to a coniferous forest in an area where yearly variation in ammonia concentrations are well documented.
2. E X P E R I M E N T A L
2.1. Meteorological measurements During the campaigns, the meteorological measurements were as follows: wind direction (wind vane) at a height of 36 m; wind speed (cup anemometers) at 36, 30, 24, 18 and
Ammonia flux measurements
191
Fig. 2. The experimental site. The radius of the circle is 2 km. Forests are indicated in dark grey, heathland in grey, and arable land in white. Only the two major farms in the area are shown.
14 m; absolute temperature (Pt 100) at 18 m; temperature difference (Pt 500) between 18 and 36 m; tree top surface temperature (Heimann KT15) at 10m; relative humidity (Rotronic MP100) at 36, 18, 8 and 4 m; fluxes of momentum, heat and water vapor (Gill Sonic and Ophir IR-2000) at 21 m. The set-up is shown in Fig. 3. This instrumentation was run parallel to the climatological measurements mentioned in the Introduction. From the measured parameters, other meteorological parameters can be derived. Among these are: surface roughness, Zo, and displacement height, d, from the logarithmic wind profile at neutral stability. From the first analysis d is found to be 8 m and Zo~ 1.4 m (at this stage we have not enough data to see if this is wind direction dependent). The friction velocity, u, is found from the sonic anemometer from u2,=-(w'u'), where ( ) indicates average values, w' is the fluctuation in the vertical wind speed and u' the fluctuation in the horizontal wind speed. Stability is calculated from the Monin-Obukhov length L = -ua, T/(Kg(w'T')), where T is the temperature in K, K the von K~rm~n constant (~ =0.4), g acceleration due to gravity, and (w'T') the kinematic heat flux, also measured by the sonic anemometer, in (m s - 1 K). The ratio of height over Monin-Obukhov length expresses the stability: (z-d)/L, where z is the height above ground level (z=21 m) and d the displacement height (d=8 m). Stability is controlled by the Monin-Obukhov length, L. On sunny days with positive heat flux upwards, L is negative and we have unstable conditions, (z-d)/L
2.2. Ammonia measurements During the two campaigns, ammonia was sampled from the meteorological tower. Samples were taken during 3-h consecutive periods, except for periods around sunrise and sunset. The samplers were placed at 18 and 36 m above ground. Occasionally, measurements were made in the top of the canopy (9 m) but these are not reported here. Each sampling consisted of three parallel measurements at each level. The sampling was done by denuders (Ferm, 1979). Glass tubes, 50-cm long and 3-ram i.d., were coated with 0.5% (w/v) oxalic acid in ethanol, leaving a 15-era uncoated inlet to establish laminar flow. The flow rate was 3 : min- 1 and each denuder was connected individually to a critical orifice with flow meter. All flow meters were connected to a vacuum pump. During exposure the denuders were heated a few degrees above ambient temperature to prevent condensation in the tubes. After exposure the tubes were extracted with 3 ml of deionized water and ammoniacal N was determined by the indophenol blue method. The detection level for a 3-h exposure was about 0.05/~g N H 3 - N m -3, occasionally lower. Sixteen per cent of the measured concentrations were lower than 0.05 #g NH3-N m -3, all of these occurred during the May campaign. 2.3. Filter pack and CO2 and O3 measurements During the two campaigns, measurements with filter packs and CO2 and 03 monitors were also performed. On a scaffold filter pack measurements were sampled at two heights, 9 and 16 m (Fig. 3). The filter packs are described in detail by Fuglsang (1986). The filter pack measurements
192
H . V . ANDERSEN et al.
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Cup anemometer. Wind vane.
36m
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Fig. 3. The instrumentation of the meteorology tower. Cup anemometer: 14, 18, 24, 30 and 36 m; wind vane: 36 m; temperature: 18 m; temp. diff.: 36-18 m; humidity: 4, 8, 18 and 36 m; gill sonic: 21 m; ophir humidity: 21 m; surface temp.: 10 m; NH 3 denuder: 9, 18 and 36 m; 0 3 and CO 2 measurements: 9, 18 and 36 rn; filter-pack samplings were made from the scaffold: 9 and 16 m.
Ammonia
flux measurements
were 3-6-h exposures. They provided information about the levels of particulate SO~-, NH~ and total (NO/ + HNO3) and gaseous SOs and NH3. COs and O3 gradients were measured in the meteorology tower. These measurements were determined by monitors placed in a cabin (Fig. 3). The data obtained by filter pack and CO~ and O3 measurements are not presented here. The measurements showed that no extreme concentration levels of the different species occurred during the campaigns.
3. DEPOSITION VELOCITY AND FLUX ~ A T I O N
The method used to determine the flux of NH3 to the forest is a combination of vertical mean-profile measurements of NH3 and of profile as well as eddycorrelation flux measurements of other quantities. These other quantities are temperature, water vapour and horizontal momentum. It is based on the notion that the vertical transport above the forest is totally dominated by turbulent motions of the air. Thus the exchange between two different layers of a particular constituent is only determined by the difference in concentration between these layers, multiplied by a turbulent diffusivity. This turbulent diffusivity is related to the turbulent field and not the quantity being transported. Hence the direct flux measurements of, say, momentum, FM, can be used to determine the flux of ammonia, FNH3, if the corresponding mean profiles are also measured, namely
different fluxes. The found diffusion coefficients are used to calculate the NH 3 flux. It is only an approximation to regard temperature and momentum as scalars, therefore Equation (1) is not used directly in this study. Further, Garratt (1978) found that flux profile relations are problematic when used above rou~,h surfaces, e.g. a forest. We have detected minor inconsistencies in the heat and momentum fluxes derived from the sonic anemometer used. We want to investigate these inconsistencies further before we base our flux estimates directly on this. We wish to develop a method which can also be used to calculate fluxes from more routinely available data. Hence, we will interpret Equation (1) through the surface layer similarity expressions for mean pro-
files. For the wind profile wehavethe followinglogarithmic dependenceof height: u(z)--? [In (-~-od;- ~. (-~--~; ],
u(zs)-u(zl)
(1)
where CNa~(z) is the concentration of NH3 at height z, u(z) is the wind speed ("concentration" of momentum) and the subscripts 1 and 2 refer to two different heights. One assumes that the flux measured in the air is equal to the flux which actually arrives at the forest "surface". The conditions which must be fulfilled can be found from the conservation equation for a scalar C. These are stationarity (0C/at=0, where t is time), horizontal homogeneity (aC/c~x=O, where x is horizontal distance), and no local sinks or sources for C (such as "fast" chemical reactions). Under these conditions the flux is constant with height. These assumptions are not entirely correct. If the concentration changes by a factor of two during the 3-h sampling period (aC/~t~0), then there will be an error in the apparent deposition velocity, Vd, of about 0.001 m s - 1. Therefore, in relation to the deposition velocities obtained for NH3 in this context, a change in concentration during sampling might introduce a very small error compared to measurement uncertainties. Flux measurements were carried out for momentum, heat and water vapour. The fluxes are interesting in relation to the evaporation from the forest and they provide a possibility for comparison between
(2)
where ~/M((z-d)/L) is the integrated stability correction function for momentum (Paulson, 1970). For a discussion on the influence of the displacement height, d, see Raupach (1979). The same way as for the wind profile a logarithmic profile can be derived for the air concentration of a scalar C. Thus
c( )-Co = FNa3 -- FMCNa~(Zz) -- CNU~(Zl),
193
~Ccrln L \ oc
/
\
/j
(3)
where Co is the surface concentration of the scalar. This is the main difference from the wind profile, where the "surface concentration" is zero (Uo~0). For scalars this is, in general, not the case, the exception being particles and those gases which rapidly dissolve or react with the surface. The value of gc, de, zoc and the stability function, ~Pc, is not necessarily the same as for m o m e n t u m - not even the same as for the more similar (quasiscalar) quantity heat flux. In principle they could all be different for cacti constituent. In the rest of this analysis we assume K - - g , dc=d and ~Pc=~Pa (see remarks below). It is not necessary to assume zoc =z0 here, because zoc will disappear from the equation later on. Results from a first analysis show that values for heat and water vapour is an order of magnitude less than Zo (but variable). The parameter c , in Equation (3) is similar to u , in Equation (2); c . is a measure of the concentration gradient. In the same way as the kinematic momentum flux can be written FM = - - uZ,, the relation between c, and the turbulent flux of the constituent can be written Fc = - u . c . .
(4)
The negative sign is a convention such that ff c . is positive (concentration increasing with height) the flux is downwards (negative).
194
H.V. ANDERSENet al.
Using two levels of measurements in the air and using Equation (3), c, can be isolated
c,=
The highest possible deposition velocity, Vd. . . . ~z~,is given by
x(C2-Cd
fz2-d
F. fz2-d
+
j- c,i--K-)J
This eliminated Co and zoc as mentioned earlier. Using Equation (5) in (4) we can now calculate the flux of C. From this the deposition velocity can be determined by division with the concentration C(z): Vd(Z)=
Fc C(z) "
(6)
The deposition velocity is by definition connected to a certain height z. The definition of Vd implies a surface concentration Co close to zero or Co,~ C(z). In this study it is assumed that Co~,C(z) due to the acidic surfaces (low pH) of the needles and top soil. The micrometeorological observations performed in this project might enable us to establish W-functions that are valid for the site in question. However, that has to await more thorough analysis, which is beyond the scope of the present paper. Even with the tower we use, which is roughly three times the canopy height, we are still in a height range where the values of W are influenced by the proximity of the tree tops. This means that we cannot rely on the usual consistency checks between profile-derived flux quantities and direct eddy fluxes (Raupach, 1979). A special analysis of this aspect will be forthcoming. In this context we have resorted to the use of stability correction functions for the "standard situation". Because of the effect described above, we have not been very particular in our choice of the large variety of the V-functions reported in the literature, not even regarding the differences between WM, Wn, etc. (for typical unstable conditions with L ~ - 1 0 0 m, the effect on va is less than 10%). For unstable conditions we have used the expressions found by Jensen et al. (1984): W = ~ - - 1,
~=(1-16(~Ld))
~t
Vd. . . .
(5)
(7)
which is based on the work of Businger et al. (1971), but have ease of calculation compared to the functional relationship given by Paulson (1970). For stable conditions we have used
This relationship is recognized only to work for 5 ( z - d ) / L < . 1. A rule of thumb, which we have not implemented, says that above this limit, the term should be kept as a constant of order unity. We have often encountered values of L so small that this is clearly violated, perhaps with the consequence that we have estimated too low deposition velocities during stable nights.
Iz)- U(Z)"
(9)
It comes from a straightforward application of Equation (6). This is an upper limit because momentum disappears by friction and pressure forces and therefore has no surface resistance. Physically, Equation (9) means that the material cannot be deposited faster at the surface than the turbulence can transport material towards the surface. The estimation of deposition velocities and fluxes are based on conserved species. Chemical transformation would affect the gradient: Harrison et al. (1989) report that chemical reactions involving the gases HNO3, HCI and NH3 are too slow to influence the concentration gradients and the surface exchange process. However, the reaction of ammonia with acid aerosols may be faster (Huntzicker et al., 1980). In this investigation, we are not able to say whether or not chemical transformation are occurring at significant rates, but we assume that it is not the case.
4. RESULTSAND DISCUSSION 4.1. Meteorology Generally the two campaigns in May and August/September 1991 can be characterized by no precipitation, normal wind speed and wind directions mainly from the northwest. Figure 4 shows relative frequencies of wind directions in wind roses from the campaigns compared with 8-year average (1971-1979) from Karup meteorological station located 45 km east of Ulborg (Troen and Petersen, 1989). It is seen that the wind directions during the two campaigns were very similar but not representative for a normal year in the area. The wind speed at 36-m height in Ulborg is comparable to standard observations at 10-m height over open terrain. This is due to the large surface roughness of the forest combined with the displacement height. The average wind speed at 36 m during the two campaigns was 4.7 and 4.9 m s - 1, respectively. The climatological mean in Denmark is 4-5 m s- 1 (Larsen and Jensen, 1983). The dry and clear weather during both of the campaigns is consistent with the fairly large amplitudes in daily variation of the observed parameters (Figs 5a and 5b). Thus the value of u, typically varies between 0.1 and 1 m s-~. The diurnal cycle is also manifested in the variations in vertical atmospheric stability which comprises both very stratified, ( z - d ) / L > 1, as well as quite convective, ( z - d)/L < - 1, periods. This means that the deposition velocity is expected to vary considerably; more than the decade as indicated by the variation in u, itself (see Section 3).
Ammonia flux measurements
195
WIND ROSE
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Fig. 4. Relative frequencies of winds coming from each of 12 sectors. Comparison between the two campaigns in Ulborg and Karup meteorological station located 45 km east of Ulborg. The Karup data are 8-year average (1971-1979) from Troen et al. (1989).
4.2. Ammonia concentrations The concentration levels during the measuring periods in August/September were generally higher than during the period in May (Figs 6a and 6b), probable due to agricultural activities and higher temperatures. Average concentrations from the periods were close to values from 1989 and 1990 (Hovmand, 1990; Hovmand and Grunddahl, 1991). High concentration events were correlated with stability in terms of ( z - d ) / L and low wind speeds (Figs 5a and 5b). The concentration seems to be inversely proportional to wind speed as shown in Fig. 7. The data have a wide scatter, mainly caused by change in source strength and distance to source with wind direction. 4.3. Concentration difference of ammonia between the two heights The difference in ammonia concentration between the two heights are shown in Figs 8a and 8b. It is indicated whether or not the difference is significantly different from zero at the 90% level. In total, 80 measuring periods are made. Thirty-four periods have
a significant difference in concentration between 36 and 18 m. In 45 measuring periods there was no significant difference in concentration between the two heights. One measuring period was left out of further analysis for experimental reasons. The grouping of concentration differences significantly or not significantly different from zero is dependent on two factors, the precision of the measurements and the atmospheric stability. During stable conditions the air mixes very slowly and the difference might become relatively large. Then the requirements of the precision are low concerning a concentration difference significantly different from zero. During unstable conditions the air mixes very well and the demands on the precision increase considerably with respect to significant concentration difference. In this context the near-neutral condition is in between the stable and unstable situation. Further, the precision is to some extent dependent on the concentration (i.e. small concentrations having poor precision), which again is connected to the atmospheric stability with respect to wind speed (Section 4.2). It is an inherent difficulty of this investigation method that well-mixed hir exhibits small concentration differences. It means that even
196
H.V. ANDERSENet
though the deposition might be considerable, it is difficult to detect concentration differences significantly different from zero. Of the measured concentration differences, which significantly differ from zero, 24 are positive and 10 negative (Figs 8a and 8b). The 10 situations with negative concentration difference occur during nighttime and early morning. The negative differences observed here might be due to: (i) emission from the
~o 270 .....~..
al.
forest, (ii) stable stratified conditions with inadequate mixing from a local source and (iii) chemical transformation. Five of the episodes with negative concentration differences occur during near-neutral and unstable conditions. All episodes have very low concentrations of NH3 (0.02-0.09 gg N H 3 - N m-3) and all of them occur during periods with relative humidity above 85%.
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Fig. 5.(a) Different meteorological measurements from the campaign in May 1991. (b) Different meteorological measurements from the campaign in August/September 1991.
The other five negative concentration differences occur during stable conditions with slow, meandering winds. The averaging period is 3 h and during such conditions, with low-level turbulence, a local source only needs to be swept once during a period to influence the concentration difference. In the episodes observed here, the wind actually sweeps either northeast or west-northwest (see Fig. 2) with short distances to
agriculture areas. All these episodes have relatively high concentrations. In this investigation, we do not have sufficient information about other gases and particles in the two heights, which might influence the ammonia gradient. We know from filter-pack measurements in a nearby scaffold (shown in Fig. 3), that no extreme levels of sulphur dioxide or aerosols were occurring.
198
H.V. ANDERSENetal. 5 E z or)
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Fig. 6. NH3 concentrations (18 m) as 3-h mean values. (a) Data lrom the campaign in May. The measurements were done in the following intervals: 2300-0200, 0500-0800, 0800-1100, 1100-1400, 1400-1700 and 2000-2300 h. (b) Data from the campaign in August/September. The measurements were done in the following intervals: 2300-0200, 0800-1100, 1100-1400, 1400-1700 and 2000-2300 h except measurements on 31 August: 0500-0800; and 2 September: 2300-0800.
10-
4.4. Estimates of deposition velocities and fluxes
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Duyzer et al. (1992) report some negative gradients measured above coniferous forest in the Netherlands. Their observations are during daytime and relatively low concentrations. They explain the phenomena as evaporating NH3 from a drying surface or a compensation point for NH3. Wyers et al. (1992) report that negative gradients occasionally occur.
To estimate deposition velocities and fluxes certain criteria are set up for the data. Eleven of the 80 measuring periods are left out of further analysis due to missing meteorological measurements, concentration of NH3 less than 0.02 #g N H 3 - N m-3 or varying stability class during exposure period. Concentration measurements of ammonia are determined as triplicates and the deviation from the mean is expressed as a coefficient of variation. For both heights the coefficient of variation should be smaller than 20%. Sixteen of the remaining 69 sets of measurements does not meet this requirement. The estimated deposition velocities lie in the range -0.125 to 0.201 m s i with a mean o f 0 . 0 2 6 m s - t . These values are in agreement with values reported by Duyzer et al. (1992) and Wyers et al. (1992) from coniferous forests in the Netherlands. Figure 9 shows the estimated deposition velocity vs the time of the measurement. There is a tendency to an increased deposition velocity in the late afternoon. This is consistent with the tendency to strongest convection (unstable conditions) in the afternoon.
Ammonia flux measurements
199
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3.9.
4.9.
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Dote 1991
Fig. 8. The concentration difference of NH 3 (3-h mean) between 36 and 18 m. Closed circles (O) indicate that the concentration differenceis significantly different from zero at the 90% level.Open circles (©) indicate that the concentration difference is not significant. (a) Data from the campaign in May. (b) Data from the campaign in August/September.
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0.15~
E
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0.05
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....
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20-23
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This would typically occur 3 h after solar zenith (Larsen and 3enscn, 1983). The period of maximum deposition, 1400-1700 h, actually corresponds to ~ 13451645 h solar time. Figure 10 shows the estimated flux. Negative values mean downward flux. The tendency towards a maximum in the afternoon is also seen here as for the deposition velocity. For the particular height combination 36 and 18 m, and a displacement height of 8 m, combination of the ~(A) 27:2-F
equations from Section 3 gives Fc ~ ~cu.AC/( 1 - A~F).
(10)
During stable conditions the stability correction in the denominator becomes large• Furthermore, u, is low. Hence the flux is small, even though the concentration and/or concentration difference might be large due to a low level of turbulence.
200
H.W. ANDERSENet al.
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::i:
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20-23
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One could neglect the negative deposition velocities which might be caused by inadequate mixing of the atmosphere. Still, the bias could be present in some of the "accepted" values. Therefore, if an average deposition velocity, with or without the negative deposition velocities/positive fluxes, were used to obtain a climatological value for the deposition flux an error would probably be made towards underestimating the input to the forest. One can make more rigorous demands on the precision of the ammonia measurements than the variation coefficient of 20% in the two heights. If the concentration measurements should meet the demand of a variation coefficient smaller than 10% in both heights, 40 of the 69 sbt of measurements are excluded. This more rigorous demand gives data showing a good relation between deposition velocity, vd(ts m), and friction velocity, u,, as shown in Fig. 11 (dosed circles). Taking all data (Fig. 11, closed circles and pluses) estimated from gradients with concentration measurements with coefficient of variation smaller than 20%, a relation is still seen, but a group of measurements with u, .~0.45-0.75 m s - ~ give a low, sometimes even negative, deposition velocity, mainly related to westerly winds. An orthogonal regression fit between u, and Vd(~a m)are made on two sets of data, classified according to the level of coefficient of variation (CV) discussed above: CV<20%: Vd=0.10u,--0.02(corr. 0.60, N = 5 2 )
(11)
C V < 10%: vd=0.11u,--0.02(corr. 0.84, N=29), (12) where u. and Vd are in m s -1 . In calculating Equation (11) one data set is left out (001X)-4)300h, 5 September, /)d(lSm)=--0.125 m s - t ) . It is a negative deposition velocity (significant at the 90% level) related to the group of significant negative concentration differences with low concentrations, discussed in Section 4.3. In Fig. 11 the two regression lines are drawn. It is seen that they are close to each other, though the correlation is higher for Equation (12). In Fig. 11 the best linear fit of the maximum possible deposition
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-Best fit of Vd(,,,,=.,s,~) . . . . . . Orthogonal Regr*ssion Line, eq.(11), corr. 0.60 .......... Orthogonal Regression Line, ~1.(12), corr. 0.84
Fig. 11. The estimated deposition velocity, Vd(18.), from both campaigns vs the friction velocity, u..
velocity, Vd. . . . (18m) (Equation 9) is drawn. From Equations(4) and (6) it is seen that va(lsm)= -u,c./Clsm. Neglecting the intercept in Equations (11) and (12), the slope of the found relation between Vd and u . corresponds to c . / C l s m. Using Equation (5) it is seen that c , / C ~ s m is the product of the relative gradient, (C36 m - - C ~ 8 m)/C~s =, and the stability correction. When this product is constant, the relative gradient is inversely dependent on the stability. Therefore, the relative gradient, and by that the deposition velocity, is found to be controlled by the meteorology. From Equation (11) the flux can be calculated for a longer period, i.e. the approximate "growing season" 25 April-30 September 1991. From the routine
Ammonia flux measurements
201
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"
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,
,
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May
June
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.
.
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,+-
Fig. 12. For the period May-September 1991,daily averages of"(a) measured ammonia concentrations, (b) estimated ammonia deposition velocity (18 m) and (c) estimated ammonia flux. meteorological measurements, a 24-h average of the friction velocity, u., is calculated for the period. In order to correct for stability effects in the calculation of daily values of u,, we use a bulk Richardson number, z0 -- 1.4 m and d = 8 m found during the two campaigns. These averages are used in Equation (11) to calculate a 24-h average deposition velocity. For a flux estimation these deposition velocities are multiplied with the corresponding 24-h average concentration of NH3 measured by filter pack. The filter pack method is found to give good separation of gaseous NH3 and particulate NH~ (Harrison and Kitto, 1990; Wiebe et al., 1990). Figure 12 shows the daily average of (a) the measured NH3 concentrations, (b) the estimated deposition velocity and (c) the estimated flux. It is seen that the concentrations are more variable than the deposition velocity. The concentration pattern dominates the flux pattern. For the half-year period 25 April-30 September 1991, the average friction velocity is 0.69 m s - ' . The average deposition velocity for the period is 0.045 m s- '. The flux estimation for the period is 8 7 / ~ g N H 3 - N m - 2 h -1 (ffi0.02pg N H 3 - N m - 2 s - ' ) . This might be an underestimation due to inclusion of negative concentration differences which in some cases are connected to meteorological situations where the gradient method is not valid, see Section 4.3 and 4.4.
The present ammonia deposition estimate exceeds earlier estimates from the measuring site. The dry deposition of all nitrogen species is raised from 0.8-0.9 g to 1.6 g N m - 2 yr- t. This nearly doubles the nitrogen excess input estimated to the forest.
5. SUMMARY AND CONCLUSIONS
Ammonia gradients and several micrometeorological parameters have been measured above a spruce forest in the western part of Denmark. From the measurements deposition velocities and fluxes were estimated. We found the following. • The deposition velocity of NH3 was mainly determined by the turbulent transport to the canopy. • The deposition velocity and the flux were generally largest in the late afternoon, corresponding well to the tendency to strongest convection at that time. • The deposition velocities lay in the range from -0.125 to 0.201 m s -t, with a mean of 0.026 m s -1" • Some negative gradients were found. One group were probably related to inadequate mixing of air passing from local sources, rather than emission from the forest.
202
H.W. ANDERSEN et al.
• The average flux of NH3 was estimated for the a p p r o x i m a t e growing season (25 A p r i l - 3 0 September 1991) to 8 7 # g N H 3 - N m - 2 h -1. Acknowledoements---This work was supported by The National Forest and Nature Agency and EEC, Forest and Forestry Division. We wish to thank John E. Hansen and Soren W. Lund for tower climbing, and Birte Vaabengaard for preparation and analysis of denuders. We also thank Michael S. Courtney for the use of his data acquisition system to collect meteorology data, and Gunner Dalsgaard and Arent Hansen for technical assistance.
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