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Factors influencing local dry deposition of gases with special reference to ammonia
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Atmospheric Environment Vol. 32, No. 3, pp. 415— 421, 1998 ( 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain S1352–2310(97)00166–0 1352—2310/98 $19.00#0.00
FACTORS INFLUENCING LOCAL DRY DEPOSITION OF GASES WITH SPECIAL REFERENCE TO AMMONIA WILLEM A. H. ASMAN National Environmental Research Institute (NERI), Frederiksborgvej 399, 4000 Roskilde, Denmark (First received 15 November 1995 and in final form 17 January 1997. Published February 1998) Abstract—A model is developed that is used to compute the dry deposition of gases as a function of the downwind distance from a source. The model is applied to examine the fraction of the ammonia emission (Fr) from a point source that is deposited within different distances from the source in relation to factors affecting dispersion and deposition. The results show that Fr at 2000 m distance from the source may be as large as 60% for a 3 m high source when ammonia deposits to mature forest at rates limited only by atmospheric transfer. It is shown how the dry deposition of ammonia depends on source height, wind speed, atmospheric stability, surface resistance, surface roughness length and compensation points. The model’s application to area sources is illustrated with urine patches. Differences between emissions measured in the field and in the laboratory are attributed to dry deposition in between the urine patches. ( 1998 Elsevier Science Ltd. All rights reserved. Key word index: Dry deposition, model, NH , source height, wind speed, atmospheric stability, surface 3 resistance, surface roughness, compensation point, urine patch.
1. INTRODUCTION
A certain fraction of the emission of gases from a source of pollution is dry deposited near the source. It is important to know this fraction for two reasons: (a) If the emission rate of this source is relatively high and the release height is low and the dry deposition velocity of the component is high, it may determine the total deposition near the source to a large extent; (b) The dry deposition fraction determines how much gas is left for long-range transport. The last reason is especially important in international negotiations on the effects of air pollutants that are transported over long distances. Van der Hoven (1968) and Horst (1977) studied dry deposition as a function of the distance to sources of varying heights and under differing atmospheric conditions. Ho¨gstrom (1979) and Janssen and Asman (1988) described dry deposition close to the source in order to derive correction factors that can be used in atmospheric transport models that assume instantaneous mixing of the emitted component over the entire mixing layer. All these publications refer either to neutral atmospheric conditions, to a few source heights or to average conditions, or use a description of the dry deposition velocity in which there is no relation between the dry deposition velocity and the wind speed and atmospheric stability. Asman and van Jaarsveld (1992) developed a variable-resolution transport model for NH and NH` (ammonium) in which there 3 4
was a relation between the dry deposition velocity and the meterological conditions. However, only average results for one source height and a few results for meteorological situations with a different atmospheric stability were published (Asman and van Jaarsveld, 1990). There are two types of plume dispersion models that can take into account depletion due to dry deposition: source depletion models and surface depletion models. In the former model the material deficit, caused by dry deposition at the earth’s surface, is spread instantaneously over the whole plume. This is done by reducing the source strength by the amount deposited. Such a model (Van der Hoven, 1968) gives reasonable results for components with a relatively low dry deposition velocity, but yields depositions near the source that are too high for components with a relatively high dry deposition velocity, at least for neutral and stable atmospheric conditions (Horst, 1977). This is caused by the higher rate of removal of material from the plume at the earth’s surface than the rate at which material can be supplied from the plume. This means that the implicit assumption of instantaneous spreading in source depletion models is no longer justified. For this reason a surface depletion model was developed, based upon the work of Maas and Asman (Asman and van Jaarsveld, 1990). It is a K-model named DEPO1. The special features of the model are that the dry deposition is described in a consequent way with the same parameters as the diffusion, so that they are not treated independently
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as e.g. in Horst (1977). Moreover, dry deposition/ emission can be described in terms of differences between the airborne concentrations and the concentrations in the surface and a first order reaction rate can be taken into account. This paper will focus on the fraction that is dry deposited close to the source as a function of the source height, wind speed, atmospheric stability, surface resistance and surface concentration (compensation point) of the depositing component with NH as an example. 3 2. SETUP OF THE MODEL
The DEPO1-model is a steady-state K-model with two dimensions: the downwind distance (x-direction) and the height (z-direction). This means that only crosswind integrated concentrations/depositions can be calculated with the model. The model consists of a variable number of layers in the vertical direction. The diffusion in the x-direction is neglected, as well as the vertical wind velocity. The diffusion in the z-direction is described with the eddy diffusivity. As the model is used specifically to study dry deposition, wet deposition is not taken into account. Removal by precipitation occurs only 5—10% of the time, but can be quite effective for highly soluble gases like NH (Asman, 1995). The model 3 is based on the following equation: u(z)
C
D
Ls (x, z) L Ls (x, z) ! " K (z) ! #Q(x, z)!S(x, z) (1) H Lx Lz Lz
where u(z) is the wind speed at height z (m s~1), s (x, z) the ! crosswind integrated concentration (units m~2), K (z) the H eddy diffusivity (m2 s~1), Q(x, z) the concentration change due to sources of the component (emission, reaction) (units m~2 s~1), S(x, z) the concentration change due to sinks of the component (dry deposition, reaction) (units m~2 s~1). In this case units could be mol or kg. Dry deposition can take place only from the lowest layer; no transport is possible across the top of the upper layer (height of the top of this layer is equal to the mixing height). The wind speed is a function of height and is therefore different for each layer (Arya, 1988):
CA B
AB
A BD
z z z u 0. !( #( (2) u(z)" * ln M ¸ M ¸ z k 0. where z is the surface roughness length for momentum (m), 0. u the friction velocity (m s~1), k the von Karman’s constant * (dimensionless) and ¸ the Monin—Obukhov length (m), which defines the atmospheric stability. The values of the stability function ( follow equation (11.14) of Arya (1988). M The eddy diffusivity for heat, which is also taken for airborne material, is described by ku z K " * (3) H / H where the values of / follow equation (11.11) of Arya (1988). H The exchange flux between the surface and the atmosphere for both dry deposition and emission is described by a ‘‘big leaf’’ model, where exchange with the surface is assumed to take place through single bulk resistances. The actual direction of the flux depends on the concentration difference between the air concentration which would be in equilibrium with the concentration in the surface (s , also called 463&!#% ‘‘compensation point’’) and the concentration in the air (s (z )) at the centre of the lowest layer (which is z , the ! 3 3 reference height in the calculations). For NH s can 3 463&!#% have a value larger than 0, because NH can be present in 3 vegetation or in applied manure or chemical fertilizer. For
most other components s will be 0. The flux is defined 463&!#% by F(x)"!» (x, z ) (s (x, z )!s (x)) (4) % 3 ! 3 463&!#% where » (z )"exchange velocity (m s~1). The flux in % 3 this equation can be split up into a deposition flux F (x)"!» (x, z ) s (x, z ) and an emission flux F (x)" $ % 3 ! 3 % » (x, z ) s (x). The exchange velocity is given by % 3 463&!#% 1 » (x, z )" (5) % 3 R (z )#R #R@ (x) ! 3 " # where R is the the aerodynamic resistance at a certain ! reference height given by: (ln (z /z )!( (z /¸)#( (z /¸)) 3 0. H 3 H 0. R (z )" (6) ! 3 ku * and the formulation of ( follows equation (11.14) of Arya H (1988). The laminar boundary layer resistance for gases R (s m~1) can be approximated by (Hicks et al., 1987): " 2 Sc 2@3 R" (7) " k u Pr * where Sc is the Schmidt number, defined by Sc"l/D with ' l being the kinematic viscosity of air (1.44]10~5 m2 s~1 at 10°C) and D the diffusivity of the gas (m2 s~1). Pr is the ' Prandtl number (0.72). It should be noted that although both l and D are temperature dependent, their ratio is not, in that ' they vary in the same way with temperature. As a result R is " not a function of temperature. The resistance R@ (s m~1) is not the usual surface resist# ance R , but is a surface resistance without the presence of # a concentration of the gas in the surface (called ‘‘concentration corrected surface resistance’’ hereafter). R@ depends on # the properties of the gas and the surface. Its meaning can be illustrated best by comparing » and R@ on the one hand, % # with the deposition velocity » and the usual surface resist$ ance R on the other hand. The dry deposition velocity » is # $ defined by (left part of the equation):
A B
1 !F(x) " . (8) » (x, z ), $ 3 s (x, z ) R (z )#R #R (x) ! 3 " # ! 3 The right part of the equation is the same simple ‘‘big leaf type’’ resistance model. From equations (10) and (15) the following relation can be found between » and » : $ % s (x) » (x, z )"» (x, z ) 1! 463&!#% . (9) $ 3 % 3 s (x, z ) ! 3 From this equation it can be seen that the difference between » and » depends on the ratio between the surface concen% $ tration and the air concentration. » is only equal to » when % $ the surface concentration is 0. This resistance model is rather simple and it may well be appropriate to use more complicated models, although these become more difficult to validate. The relation between R and R@ is # # R (x)"[s (x, z )R@ #s (x) (R (z )#R )]/ # ! 3 # 463&!#% ! 3 " [s (x, z )!s (x)]. (10) ! 3 463&!#% From this equation it can be seen that R equals R@ when # # s is 0. Moreover, it can be seen that R approaches 463&!#% # infinity when s approaches s and that the relationship 463&!#% ! of R to R@ is a function of R (z ) and R . # # ! 3 " In the model it is possible to include a reaction of the gas with another component, described by a first-order reaction rate. Moreover, it is possible to include dry deposition of the secondary component in the model. The finite difference technique is used to solve the set of equations. DEPO1 was tested against the results of other models describing concentrations around line/point sources presented in Asman and
A
B
Factors influencing local dry deposition of gases van Jaarsveld (1990). The results of DEPO1 were within 30% of the results of these models. DEPO1 is in principle a two-dimensional model. It describes the diffusion in the vertical and the resulting exchange at the surface as a function of the distance to the source well. For point sources it is easily possible to make the results of the two-dimensional model three-dimensional by redistributing the calculated crosswind integrated concentrations using a Gaussian distribution: )2/2p (x)2] #%/53% y (11) J2n p (x) y where for p (x), the crosswind standard deviation of the y Gaussian distribution (m), Briggs’ parameterization is chosen (Pasquill and Smith, 1983); y is the y coordinate #%/53% of the plume centre (m). For line sources and area sources the model can give reasonable results in cases where the effect of the crosswind diffusion can be neglected: a line or area source of infinite crosswind length, or an area source of finite crosswind length in the area closely downwind to the downwind edge of the source. The model is rather general, but its use will be demonstrated with NH as an example. This means that in the 3 following the properties of NH are used in the calculations, 3 i.e. that the R for NH has been used and that examples of " 3 sources also are appropriate to NH . An overview of dry 3 deposition velocities for NH is presented in Sutton et al. 3 (1994). In an atmospheric transport model a typical estimate of the surface resistance (R ) of 30 s m~1 has been used for # NH (Asman and van Jaarsveld, 1992). In many examples in 3 the next sections a R@ -value of 25 m s~1 has been adopted. # About half of the NH emissions in Western Europe come 3 from animal housings and manure storage facilities (ECETOC, 1994). The other half originates from manure and fertilizer after application to bare soil or low crops. This means that there are two distinct source categories: (a) housings and storage facilities, which are point sources, have source heights of the order of 1—5 m and are more or less continuous sources and (b) ground level area sources, which are active only during a few months per year. The roughness length z used in the model calculations should be a local 0. roughness length, because a substantial fraction of the emission may be deposited close to the source. It is difficult to know which roughness should be used for category (a). The air flow is influenced by the housings and storage facilities giving rise to more turbulence but also to special, wind direction dependent, air flows. The model is unable to describe this airflow, which also depends on the characteristics of the individual building. Moreover, very often other obstacles are found near these sources: other buildings, trees, hedges etc. For this reason the mesoscale roughness length of 0.1 m was used for this type of source. It is representative for arable land with height variations in the crops. For source category (b) a local roughness length of 0.025 m was used. These choices are somewhat arbitrary. In Section 3 source type (a) is discussed. In Section 4 source type (b) is discussed. In all calculations presented below, 10, logarithmically spaced layers from ground-level to the mixing height were chosen with boundaries at 0.0, 0.70, 1.32, 2.49, 4.70, 8.87, 16.93, 31.57, 59.56, 112.37, 212.01 and 400 m; 10 layers were chosen because a sensitivity study showed that the results did not depend on the number of (logarithmically spaced) layers if it were 10 or larger. A mixing height of 400 m was chosen because the results up to 2000 m downwind from low sources, described here, will not be influenced by the mixing height. Moreover, at such a distance in most situations not much NH will have reacted and reaction is therefore 3 neglected here. This makes it possible to present the results in a more general fashion. It should be noted that the source height mentioned in this paper is an approximate one, because in the model the emission is distributed instantaneously over the layer to which the source emits. s (x, y, z)"s (x, z) ! !
exp[!(y!y
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3. ACCUMULATED DEPOSITION NEAR A POINT SOURCE: SENSITIVITY STUDY
This section focuses on the sensitivity to variations in the parameters that determine the accumulated dry deposition of NH3 as a function of distance to a point source. The accumulated dry deposition up to a distance x downwind of a point source is expressed as fraction Fr(x) of the emission from the source. This means that Fr(x) can be 1 at maximum (all emitted NH is then deposited). Fr(x) depends in principle on: 3 the source height H4063#% , wind speed (in this case referenced to a height of 10 m), the concentration corrected surface resistance R@# , the atmospheric stability in the form of the Monin—Obukhov length ¸, the surface roughness length z0. , the distance to the source x, the concentration at the surface s , the 463&!#% mixing height and the reaction rate of NH3 . In some examples a value of 0 is adopted for R@# . This value is then taken to calculate the maximum possible value of Fr(x). 3.1. Source height Figure 1 shows Fr(x) as a function of the source height for neutral atmospheric conditions (¸" 20 000 m), z0."0.1 m, s463&!#%"0 and R@#"0 and R@#"25 s ms~1. A wind speed at 10 m of 3.46 m s~1 (corresponding to a u of 0.3 m s~1) is chosen because * it is representative of the average wind speed in agricultural areas in countries with a high ammonia emission density like Denmark, Belgium and the Netherlands. In continental Europe the average wind speed is lower. The figure shows that deposition close to the source is quite high. This is caused by the fact that R@# is small and the source height is low which results in high concentrations at ground level at a short distance from the source (Janssen and Asman,
Fig. 1. Fraction of the NH emission that is dry deposited as 3 a function of distance to a point source; u(10 m)" 3.46 m s~1, neutral atmospheric conditions (¸"20 000 m), z "0.1 m and s "0. The thick lines and the thin 0. 463&!#% lines indicate the results for R@ "0 and R@ "25 s m~1, # # respectively.
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1988). Figure 1 illustrates that Fr(x) deceases strongly with increasing source height. For that reason it is very important to use representative source heights in atmospheric transport models for NH3 . The figure shows also that the Fr(x) for a 10 m high source is very small in the beginning, which is caused by the fact that the plume has not reached the ground. The accumulated deposition up to 100 m from the source Fr(100 m) is 31, 13 and 0.6% of the emission for sources heights of 1, 3 and 10 m, respectively, and a R@# of 0 s m~1. For an R@# of 25 s m~1 this is 20, 6.5 and 0.4%, respectively. 3.2. ¼ind speed, surface resistance and atmospheric stability Figure 2 shows how the accumulated dry deposition up to 2000 m from a 3 m high source Fr(2000 m) varies with wind speed for various R@# values and neutral conditions. This figure shows that Fr(2000 m) does not depend on the wind speed if R@#"0. This means that the maximum possible fraction of the emission that is deposited in this area, or the amount that remains for long-range transport, does not depend on the wind speed. The reason for this is that the air concentration is proportional to 1/(wind speed)" 1/u* , and the exchange velocity »% is proportional to the wind speed or u . The exchange flux F (depos* $ ition only because s463&!#% is set to 0), which is equal to !»% (z3)s! (z3), is then independent of the wind speed because the effects of the wind speed on »% (z3) and s! (z3 ) compensate each other. Fr(2000 m) under the same stable/unstable conditions is also independent of the wind speed if R@ "0 for the same value of ¸. # Another phenomenon which can be seen in Fig. 2 is that, at very low wind speeds, Fr(2000 m) calculated with different R@# values approaches the value of Fr(2000 m) for R@#"0. This is simply caused by the fact that there is so little turbulence at such wind speeds that the overall resistance is dominated by R!#R" . On the contrary R@# becomes increasingly important with larger wind speed. In general it can be concluded that there is more local deposition at low wind speeds and more long-range transport at higher wind speeds. Figure 3 shows the effect of atmospheric stability on Fr(x). H "3 m, u(10 m)"3.46 m s~1, 4063#% z0."0.1 m for R@# values of 0 (thick lines) and 25 s m~1 (thin lines). The Monin—Obukhov lengths ¸ taken for a neutral, stable and unstable atmosphere were 20 000, 30 and !30 m, respectively; u* is then 0.3, 0.221 and 0.345 m s~1, respectively. Figure 3 shows that Fr(x) is larger for stable conditions and lower for unstable conditions. This is caused by slower mixing under stable conditions which gives less diluted plumes. 3.3. Surface roughness length At about 60 m above the earth’s surface, the wind speed is no longer influenced by local roughness variations (Wieringa and Rijkoort, 1983). To show the effect of differences in z0. on the accumulated dry
Fig. 2. Fraction of the NH emission that is dry deposited at 3 2000 m from a 3 m high point source for varying wind speeds and various R@ values; neutral atmospheric conditions # (¸"20 000 m), z "0.1 m and s "0. 0. 463&!#%
Fig. 3. Fraction of the NH emission that is dry deposited as 3 a function of distance to a point source for different atmospheric conditions (s"stable atmosphere, n"neutral atmosphere, u"unstable atmosphere); u(10 m)"3.46 m s~1, z "0.1 m and s "0. The thick lines and the thin 0. 463&!#% lines indicate the results for R@ "0 and R@ "25 s m~1, # # respectively.
deposition under neutral atmospheric conditions a wind speed of 4.80 m s~1 at 60 m was assumed, which corresponds to a wind speed of 3.46 m s~1 at 10 m height for a surface roughness length of 0.1 m. For this run the following parameters were set: H "3 m, neutral atmospheric conditions (¸" 4063#% 20 000 m), R@#"0 s m~1 and s463&!#%"0. This gives an indication of the maximum values of Fr in relation to z0. . The surface roughness heights chosen were: 0.005 m (very short grass; u(10 m)"3.88 and u*" 0.204 m s~1), 0.025 m (rough grass; u(10 m)"3.70 and u "0.247 m s~1), 0.1 m (arable land with height vari* ations in crops; u(10 m)"3.46 and u*"0.3 m s~1), 0.25 m (young coniferous forest; u(10 m)"3.23 and u*"0.350 m s~1) and 1 m (deciduous forest;
Factors influencing local dry deposition of gases
Fig. 4. Fraction of the NH emission that is dry deposited as 3 a function of distance to a point source for different surface roughness lengths; H "3 m, u(60 m)"4.80 m s~1, neu4063#% tral atmospheric conditions (¸"20 000 m), R@ "0 s m~1 # and s "0. 463&!#%
u(10 m)"2.70 and u*"0.469 m s~1). Figure 4 shows that Fr(x) increases with roughness length. This is caused by two effects: (a) the near surface wind speed decreases with roughness length, which leads to increased concentrations, and (b) the turbulence is greater, which leads to an increase in dry deposition velocity. Figure 4 indicates also that the dry deposition to forests close to farm buildings can be very high. It should be noted here, however, that the real situation is likely to be somewhat different as the forest usually does not begin near the farm door. If a R@# value of 25 s m~1 is chosen Fr(2000 m) will be only 0.18 (0.22), 0.22 (0.28), 0.25 (0.36), 0.28 (0.42) and 0.33 (0.59) for roughness lengths of 0.005, 0.025, 0.1, 0.25 and 1 m, respectively. The values in brackets are the values for a R@ value of 0 s m~1, # shown in Fig. 4. 3.4. Compensation point Figure 5 shows the vertical NH3 flux in the plume centre as a function of distance from a 3 m high point source at x"0 m for different compensation points (s , for stomatal exchange this has been termed 463&!#% s4 ) of the agricultural crops surrounding the source. The emission of the point source represents a housing and storage facility for 500 pigs, with an annual emission of 2.52 kg NH3 per animal (Asman, 1992), which corresponds to 4.0]104 kg NH3 s~1; u(10 m)" 3.46 m s~1, neutral atmospheric conditions (¸" 20 000 m), z0."0.1 m. R@# for downwind deposition fluxes was taken 25 s m~1, whereas the R@# for downwind emission fluxes was taken at 100 s m~1 (stomata only). This somewhat arbitrary difference in R@# was adopted because dry deposition may take place on the surface of the leaves. A positive flux indicates surface emissions into the atmosphere. The vertical flux shows a minimum (maximum deposition) at about 20 m from the source (not shown in Fig. 5). This
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Fig. 5. Vertical NH flux (emission is positive) as a function 3 of distance from a 3 m high point source and as a function of the compensation point of the surrounding area. The source is equivalent to a stable/storage complex with 500 pigs; u(10 m)"3.46 m s~1, neutral atmospheric conditions (¸"20 000 m) and z "0.1 m, R@ for deposition is 0. # 25 s m~1 and R@ belonging to the compensation point is # 100 s m~1 (stomata).
minimum varies from !6.48 (s4"0 kg m~3) to !6.41 (s4"10 kg m~3) kg m~2 s~1. The distance at which the flux is zero is the distance at which the deposition component of the net flux caused by the point source is balanced by the emission component of the net flux caused by the existence of a compensation point. This distance varies from 450 m (s4" 10 kg m~3) to 1450 m (s4"1 kg m~3). It should be noted here that the balance will be reached at shorter distance from the source at some lateral distance from the plume centre. 3.5. »ariations in the modelled accumulated dry deposition due to real meteorological variations Figure 6 shows the modelled frequency distribution of the accumulated dry deposition of NH up to 3 2000 m from a 3 m high point source at Kastrup Airport, Denmark (55°39@ N and 12°40@ E). Kastrup is situated at the east coast of the island Zealand, and the local surface roughness length for momentum (z ) is 0.03 m. The calculations were made with the 0. meteorology (u(10 m) and ¸) for each hour for two 1974 and 1975. s is set to zero. Figure 6 shows 463&!#% two frequency distributions: one for a R@ value of # 0 and one for a R@ value of 25 s m~1. The frequency # intervals are 0.01. The highest peaks on the right side are caused by an artefact, which occurs for about 20% of all hours. The values for u and ¸ at Kastrup were calculated from * routine meteorological data using a meteorological preprocessor (Olesen and Brown, 1992). The preprocessor uses an iterative procedure to find u and * ¸ and in order that this process converges for stable conditions it is necessary to set a lower limit to ¸ of 100*z "3 m. For very stable situations ¸ will 0. therefore be set to this value. Although this is an artefact, it points to an important problem. These are
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Fig. 6. Frequency distribution of the fraction of the NH 3 emission that is dry deposited at 2000 m from a 3 m high point source based on Kastrup meteorology for 1974 and 1975 with s "0 for R@ "0 and R@ "25 s m~1. 463&!#% # #
the situations where little turbulence is present. In these cases the model does not adequately describe the situation. One reason is that the diffusion in the x-direction can no longer be neglected. Moreover, the situation can be so stable by cooling of the Earth’s surface that a plume released at some height may actually never reach the ground. These situations occur mostly at night and in the early morning hours. This is a serious problem because it can be expected that these situations will occur even more frequently at stations that are situated more inland than Kastrup. The other peaks indicate Fr(2000 m) for neutral atmospheric conditions. For a R@ value of # 0 s m~1 the peak occurs at Fr(2000 m)"0.30, while higher values are all associated with stable conditions and lower values with unstable conditions. For a value of R@ of 25 s m~1 the peak occurs at 0.19. It # can also be concluded that at one site variations in Fr(2000 m) of a factor two can easily occur, depending on meteorological conditions.
4. SOME ASPECTS OF AREA SOURCES
The micrometeorological mass balance method has been used to measure the NH emission from rota3 tionally grazed swards in the field (Jarvis et al., 1989; Bussink, 1994). The dung and urine patches cover about 10% of the area and their average size is of the order of 1 m2. The emission has also been measured in the laboratory (Whitehead et al., 1989; Vertregt and Rutgers, 1991). A question is whether these experiments are comparable. One aspect of this is that in the laboratory experiments there is only one patch, whereas in the field there are patches with grass between which deposition can take place. The mass balance method therefore measures the effect of both emission and deposition. Model calculations were made with three area sources (‘‘patches’’) each 1 m wide in the downwind direction, with 9 m grass in between. The crosswind
Fig. 7. Vertical NH flux (emission is positive) as a function 3 of distance from the upwind edge of a field with three urine patches; u (10 m)"3.70 m s~1, neutral atmospheric conditions (¸"20 000 m), z "0.025 m, R@ "25 s m~1 (for 0. # emission and deposition), s of the patches is 305 lg 463&!#% NH m~3, outside the patches s is 0. The lines in3 463&!#% dicated with E above the peaks indicate the areas with emission.
diffusion is neglected. The value of s was ad463&!#% justed to 305 lg NH m~3 so that the emission at the 3 most upwind edge of the patch was realistic: about 5.6 lg NH m~2 s~1 (D. W. Bussink, Research Station 3 for Cattle, Sheep and Horse Husbandry, Lelystad, the Netherlands). Other conditions were: u(10 m)" 3.70 m s~1, neutral atmospheric conditions (¸" 20 000 m), z "0.025 m, R@ "25 s m~1 (for emission 0. # and deposition). Outside the patches s "0. 463&!#% Figure 7 shows several important features. The emission flux within the patch decreases with increasing downwind distance. The NH concentration in the air 3 over the patch becomes so high that the emission flux at the downwind edge is reduced (the flux depends on s !s ). This is a very general phenomenon: 463&!#% ! the larger the alongwind emission area, the less becomes the emission flux if s is the same every463&!#% where in this area. The deposition caused by the first patch in the area between 1 and 10 m is about 15% of the emission; up to 30 m another 5% is deposited. Figure 7 shows also that the deposition downwind of the second and third patch is slightly larger. This is caused by the accumulated effect of the previous patches.
5. CONCLUSIONS
The results presented in this paper illustrate clearly that the dry deposition of a component up to 20—100 m from a low source may be considerable and that it depends on the source height, wind speed, surface resistance, atmospheric stability, surface roughness length and surface concentration (compensation point). Equally, these factors also determine the fraction of the emitted gaseous component transported
Factors influencing local dry deposition of gases
over long distances. For these reasons it is very important to take the above-mentioned factors into account in designing local and long-range transport models. This is especially important for NH , which is 3 released mainly from low sources and frequently has a small surface resistance. It should be noted here that for NH not only 3 does the dry deposition depend on these factors, but also the emission flux from ground sources (e.g. applied manure and chemical fertilizer). The model results discussed in Section 3.5 suggest that the dry deposition close to a source cannot be calculated for at least 20% of the time, because the atmosphere is so stable that the current theory is unlikely to be adequate. Modelling of local NH dry deposition close to 3 a source should be improved, taking into account the air flow and turbulence around agricultural buildings and the NH concentration in the surface. Moreover, 3 farm buildings and fields are two distinct source types with their own properties and they should therefore be treated separately in atmospheric transport models. Acknowledgements—The development of the model was inspired by the modelling work done by Hans Maas and the author at the National Institute of Public Health and Environmental Protection (RIVM), Bilthoven, The Netherlands. Ruwim Berkowicz (NERI) is thanked for his comments on the draft version of this paper. This research was partly supported by the Danish Environmental Research Programme and the Danish Environmental Protection Agency (Project ‘‘Spredning og effekter af ammoniak’’). The author is grateful to the referees and the editor for their critical comments and to Wim Bussink, Research Station for Cattle, Sheep and Horse Husbandry, Lelystad, The Netherlands for the information on urine patches.
REFERENCES
Arya S. P. (1988) Introduction to Micrometeorology. Academic Press, San Diego, USA. Asman W. A. H. (1992) Ammonia emission in Europe: updated emission and emission variations. Report 228471008, National Institute of Public Health and Environmental Protection (RIVM), Bilthoven, The Netherlands. Asman W. A. H. (1995) Parameterization of below-cloud scavenging of highly soluble gases under convective conditions. Atmospheric Environment 29, 1359—1368.
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Asman W. A. H. and van Jaarsveld J. A. (1990) A variableresolution statistical transport model applied for ammonia and ammonium. Report 228471007, National Institute of Public Health and Environmental Protection (RIVM), Bilthoven, The Netherlands, 46. Asman W. A. H. and van Jaarsveld J. A. (1992) A variableresolution transport model applied for NH in Europe. x Atmospheric Environment 26A, 445—464. Bussink D. W. (1994) Relationships between ammonia volatilization and nitrogen fertilizer application rate, intake and excretion of herbage nitrogen by cattle on grazed swards. Fertilizer Research 38, 111—121. ECETOC (1994) Ammonia emissions to air in Western Europe. Technical Report No. 62, ECETOC, Brussels, Belgium. Hicks B. B., Baldocchi D. D., Meyers T. P., Hosker R. P. and Matt D. R. (1987) A preliminary multiple regression routine for deriving dry deposition velocities from measured quantities. ¼ater Air and Soil Pollution 36, 311—330. Ho¨gstro¨m U. (1979) Initial dry deposition and type of source in relation to long-distance transport of air pollution. Atmospheric Environment 17, 295—301. Horst T. W. (1977) A surface depletion model for deposition from a Gaussian plume. Atmospheric Environment 11, 41—46. Janssen A. J. and Asman W. A. H. (1988) Effective removal parameters in long-range air pollution transport models. Atmospheric Environment 22, 359—367. Jarvis S. C., Hatch D. J. and Roberts D. H. (1989) The effects of grassland management on nitrogen losses from grazed swards through ammonia volatilization; the relationship to excretal N returns from cattle. Journal of Agricultural Science Cambridge 112, 205—216. Olesen H. R. and Brown N. (1992) The OML meteorological preprocessor. Report MST LUFT-A122, National Environmental Research Institute (NERI), Roskilde, Denmark. Pasquill F. and Smith F. B. (1983) Atmospheric Diffusion. Ellis Horwood, Chichester, U.K. Sutton M. A., Asman W. A. H. and Schjoerring J. K. (1994) Dry deposition of reduced nitrogen. ¹ellus 46B, 255—273. Van der Hoven I. (1968). Deposition of particles and gases. In: Meteorology and atomic energy 1968, (edited by Slade D.), pp. 202—208. U.S. Atomic Energy Commission, Office of Information Services, Oak Ridge, Tennessee, U.S.A. Vertregt N. and Rutgers B. (1991) Ammonia emissions from grazing. In Odour and Ammonia Emissions from ¸ivestock Farming (edited by Nielsen V. C., Voorburg J. H. and L’Hermite P.), pp. 177—184. Elsevier, London, U.K. Whitehead D. C., Lockyer D. E. and Raistrick N. (1989) Volatilization of ammonia from urea applied to soil: influence of hippuric acid and other constituents of livestock urine. Soil Biology and Biochemistry 21, 803—808. Wieringa J. and Rijkoort P. J. (1983) ¼indklimaat van Nederland (Wind climate of The Netherlands). Staatsuitgeverij, The Hague, The Netherlands.
Atmospheric Environment Vol. 32, No. 3, pp. 415— 421, 1998 ( 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain S1352–2310(97)00166–0 1352—2310/98 $19.00#0.00
FACTORS INFLUENCING LOCAL DRY DEPOSITION OF GASES WITH SPECIAL REFERENCE TO AMMONIA WILLEM A. H. ASMAN National Environmental Research Institute (NERI), Frederiksborgvej 399, 4000 Roskilde, Denmark (First received 15 November 1995 and in final form 17 January 1997. Published February 1998) Abstract—A model is developed that is used to compute the dry deposition of gases as a function of the downwind distance from a source. The model is applied to examine the fraction of the ammonia emission (Fr) from a point source that is deposited within different distances from the source in relation to factors affecting dispersion and deposition. The results show that Fr at 2000 m distance from the source may be as large as 60% for a 3 m high source when ammonia deposits to mature forest at rates limited only by atmospheric transfer. It is shown how the dry deposition of ammonia depends on source height, wind speed, atmospheric stability, surface resistance, surface roughness length and compensation points. The model’s application to area sources is illustrated with urine patches. Differences between emissions measured in the field and in the laboratory are attributed to dry deposition in between the urine patches. ( 1998 Elsevier Science Ltd. All rights reserved. Key word index: Dry deposition, model, NH , source height, wind speed, atmospheric stability, surface 3 resistance, surface roughness, compensation point, urine patch.
1. INTRODUCTION
A certain fraction of the emission of gases from a source of pollution is dry deposited near the source. It is important to know this fraction for two reasons: (a) If the emission rate of this source is relatively high and the release height is low and the dry deposition velocity of the component is high, it may determine the total deposition near the source to a large extent; (b) The dry deposition fraction determines how much gas is left for long-range transport. The last reason is especially important in international negotiations on the effects of air pollutants that are transported over long distances. Van der Hoven (1968) and Horst (1977) studied dry deposition as a function of the distance to sources of varying heights and under differing atmospheric conditions. Ho¨gstrom (1979) and Janssen and Asman (1988) described dry deposition close to the source in order to derive correction factors that can be used in atmospheric transport models that assume instantaneous mixing of the emitted component over the entire mixing layer. All these publications refer either to neutral atmospheric conditions, to a few source heights or to average conditions, or use a description of the dry deposition velocity in which there is no relation between the dry deposition velocity and the wind speed and atmospheric stability. Asman and van Jaarsveld (1992) developed a variable-resolution transport model for NH and NH` (ammonium) in which there 3 4
was a relation between the dry deposition velocity and the meterological conditions. However, only average results for one source height and a few results for meteorological situations with a different atmospheric stability were published (Asman and van Jaarsveld, 1990). There are two types of plume dispersion models that can take into account depletion due to dry deposition: source depletion models and surface depletion models. In the former model the material deficit, caused by dry deposition at the earth’s surface, is spread instantaneously over the whole plume. This is done by reducing the source strength by the amount deposited. Such a model (Van der Hoven, 1968) gives reasonable results for components with a relatively low dry deposition velocity, but yields depositions near the source that are too high for components with a relatively high dry deposition velocity, at least for neutral and stable atmospheric conditions (Horst, 1977). This is caused by the higher rate of removal of material from the plume at the earth’s surface than the rate at which material can be supplied from the plume. This means that the implicit assumption of instantaneous spreading in source depletion models is no longer justified. For this reason a surface depletion model was developed, based upon the work of Maas and Asman (Asman and van Jaarsveld, 1990). It is a K-model named DEPO1. The special features of the model are that the dry deposition is described in a consequent way with the same parameters as the diffusion, so that they are not treated independently
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as e.g. in Horst (1977). Moreover, dry deposition/ emission can be described in terms of differences between the airborne concentrations and the concentrations in the surface and a first order reaction rate can be taken into account. This paper will focus on the fraction that is dry deposited close to the source as a function of the source height, wind speed, atmospheric stability, surface resistance and surface concentration (compensation point) of the depositing component with NH as an example. 3 2. SETUP OF THE MODEL
The DEPO1-model is a steady-state K-model with two dimensions: the downwind distance (x-direction) and the height (z-direction). This means that only crosswind integrated concentrations/depositions can be calculated with the model. The model consists of a variable number of layers in the vertical direction. The diffusion in the x-direction is neglected, as well as the vertical wind velocity. The diffusion in the z-direction is described with the eddy diffusivity. As the model is used specifically to study dry deposition, wet deposition is not taken into account. Removal by precipitation occurs only 5—10% of the time, but can be quite effective for highly soluble gases like NH (Asman, 1995). The model 3 is based on the following equation: u(z)
C
D
Ls (x, z) L Ls (x, z) ! " K (z) ! #Q(x, z)!S(x, z) (1) H Lx Lz Lz
where u(z) is the wind speed at height z (m s~1), s (x, z) the ! crosswind integrated concentration (units m~2), K (z) the H eddy diffusivity (m2 s~1), Q(x, z) the concentration change due to sources of the component (emission, reaction) (units m~2 s~1), S(x, z) the concentration change due to sinks of the component (dry deposition, reaction) (units m~2 s~1). In this case units could be mol or kg. Dry deposition can take place only from the lowest layer; no transport is possible across the top of the upper layer (height of the top of this layer is equal to the mixing height). The wind speed is a function of height and is therefore different for each layer (Arya, 1988):
CA B
AB
A BD
z z z u 0. !( #( (2) u(z)" * ln M ¸ M ¸ z k 0. where z is the surface roughness length for momentum (m), 0. u the friction velocity (m s~1), k the von Karman’s constant * (dimensionless) and ¸ the Monin—Obukhov length (m), which defines the atmospheric stability. The values of the stability function ( follow equation (11.14) of Arya (1988). M The eddy diffusivity for heat, which is also taken for airborne material, is described by ku z K " * (3) H / H where the values of / follow equation (11.11) of Arya (1988). H The exchange flux between the surface and the atmosphere for both dry deposition and emission is described by a ‘‘big leaf’’ model, where exchange with the surface is assumed to take place through single bulk resistances. The actual direction of the flux depends on the concentration difference between the air concentration which would be in equilibrium with the concentration in the surface (s , also called 463&!#% ‘‘compensation point’’) and the concentration in the air (s (z )) at the centre of the lowest layer (which is z , the ! 3 3 reference height in the calculations). For NH s can 3 463&!#% have a value larger than 0, because NH can be present in 3 vegetation or in applied manure or chemical fertilizer. For
most other components s will be 0. The flux is defined 463&!#% by F(x)"!» (x, z ) (s (x, z )!s (x)) (4) % 3 ! 3 463&!#% where » (z )"exchange velocity (m s~1). The flux in % 3 this equation can be split up into a deposition flux F (x)"!» (x, z ) s (x, z ) and an emission flux F (x)" $ % 3 ! 3 % » (x, z ) s (x). The exchange velocity is given by % 3 463&!#% 1 » (x, z )" (5) % 3 R (z )#R #R@ (x) ! 3 " # where R is the the aerodynamic resistance at a certain ! reference height given by: (ln (z /z )!( (z /¸)#( (z /¸)) 3 0. H 3 H 0. R (z )" (6) ! 3 ku * and the formulation of ( follows equation (11.14) of Arya H (1988). The laminar boundary layer resistance for gases R (s m~1) can be approximated by (Hicks et al., 1987): " 2 Sc 2@3 R" (7) " k u Pr * where Sc is the Schmidt number, defined by Sc"l/D with ' l being the kinematic viscosity of air (1.44]10~5 m2 s~1 at 10°C) and D the diffusivity of the gas (m2 s~1). Pr is the ' Prandtl number (0.72). It should be noted that although both l and D are temperature dependent, their ratio is not, in that ' they vary in the same way with temperature. As a result R is " not a function of temperature. The resistance R@ (s m~1) is not the usual surface resist# ance R , but is a surface resistance without the presence of # a concentration of the gas in the surface (called ‘‘concentration corrected surface resistance’’ hereafter). R@ depends on # the properties of the gas and the surface. Its meaning can be illustrated best by comparing » and R@ on the one hand, % # with the deposition velocity » and the usual surface resist$ ance R on the other hand. The dry deposition velocity » is # $ defined by (left part of the equation):
A B
1 !F(x) " . (8) » (x, z ), $ 3 s (x, z ) R (z )#R #R (x) ! 3 " # ! 3 The right part of the equation is the same simple ‘‘big leaf type’’ resistance model. From equations (10) and (15) the following relation can be found between » and » : $ % s (x) » (x, z )"» (x, z ) 1! 463&!#% . (9) $ 3 % 3 s (x, z ) ! 3 From this equation it can be seen that the difference between » and » depends on the ratio between the surface concen% $ tration and the air concentration. » is only equal to » when % $ the surface concentration is 0. This resistance model is rather simple and it may well be appropriate to use more complicated models, although these become more difficult to validate. The relation between R and R@ is # # R (x)"[s (x, z )R@ #s (x) (R (z )#R )]/ # ! 3 # 463&!#% ! 3 " [s (x, z )!s (x)]. (10) ! 3 463&!#% From this equation it can be seen that R equals R@ when # # s is 0. Moreover, it can be seen that R approaches 463&!#% # infinity when s approaches s and that the relationship 463&!#% ! of R to R@ is a function of R (z ) and R . # # ! 3 " In the model it is possible to include a reaction of the gas with another component, described by a first-order reaction rate. Moreover, it is possible to include dry deposition of the secondary component in the model. The finite difference technique is used to solve the set of equations. DEPO1 was tested against the results of other models describing concentrations around line/point sources presented in Asman and
A
B
Factors influencing local dry deposition of gases van Jaarsveld (1990). The results of DEPO1 were within 30% of the results of these models. DEPO1 is in principle a two-dimensional model. It describes the diffusion in the vertical and the resulting exchange at the surface as a function of the distance to the source well. For point sources it is easily possible to make the results of the two-dimensional model three-dimensional by redistributing the calculated crosswind integrated concentrations using a Gaussian distribution: )2/2p (x)2] #%/53% y (11) J2n p (x) y where for p (x), the crosswind standard deviation of the y Gaussian distribution (m), Briggs’ parameterization is chosen (Pasquill and Smith, 1983); y is the y coordinate #%/53% of the plume centre (m). For line sources and area sources the model can give reasonable results in cases where the effect of the crosswind diffusion can be neglected: a line or area source of infinite crosswind length, or an area source of finite crosswind length in the area closely downwind to the downwind edge of the source. The model is rather general, but its use will be demonstrated with NH as an example. This means that in the 3 following the properties of NH are used in the calculations, 3 i.e. that the R for NH has been used and that examples of " 3 sources also are appropriate to NH . An overview of dry 3 deposition velocities for NH is presented in Sutton et al. 3 (1994). In an atmospheric transport model a typical estimate of the surface resistance (R ) of 30 s m~1 has been used for # NH (Asman and van Jaarsveld, 1992). In many examples in 3 the next sections a R@ -value of 25 m s~1 has been adopted. # About half of the NH emissions in Western Europe come 3 from animal housings and manure storage facilities (ECETOC, 1994). The other half originates from manure and fertilizer after application to bare soil or low crops. This means that there are two distinct source categories: (a) housings and storage facilities, which are point sources, have source heights of the order of 1—5 m and are more or less continuous sources and (b) ground level area sources, which are active only during a few months per year. The roughness length z used in the model calculations should be a local 0. roughness length, because a substantial fraction of the emission may be deposited close to the source. It is difficult to know which roughness should be used for category (a). The air flow is influenced by the housings and storage facilities giving rise to more turbulence but also to special, wind direction dependent, air flows. The model is unable to describe this airflow, which also depends on the characteristics of the individual building. Moreover, very often other obstacles are found near these sources: other buildings, trees, hedges etc. For this reason the mesoscale roughness length of 0.1 m was used for this type of source. It is representative for arable land with height variations in the crops. For source category (b) a local roughness length of 0.025 m was used. These choices are somewhat arbitrary. In Section 3 source type (a) is discussed. In Section 4 source type (b) is discussed. In all calculations presented below, 10, logarithmically spaced layers from ground-level to the mixing height were chosen with boundaries at 0.0, 0.70, 1.32, 2.49, 4.70, 8.87, 16.93, 31.57, 59.56, 112.37, 212.01 and 400 m; 10 layers were chosen because a sensitivity study showed that the results did not depend on the number of (logarithmically spaced) layers if it were 10 or larger. A mixing height of 400 m was chosen because the results up to 2000 m downwind from low sources, described here, will not be influenced by the mixing height. Moreover, at such a distance in most situations not much NH will have reacted and reaction is therefore 3 neglected here. This makes it possible to present the results in a more general fashion. It should be noted that the source height mentioned in this paper is an approximate one, because in the model the emission is distributed instantaneously over the layer to which the source emits. s (x, y, z)"s (x, z) ! !
exp[!(y!y
417
3. ACCUMULATED DEPOSITION NEAR A POINT SOURCE: SENSITIVITY STUDY
This section focuses on the sensitivity to variations in the parameters that determine the accumulated dry deposition of NH3 as a function of distance to a point source. The accumulated dry deposition up to a distance x downwind of a point source is expressed as fraction Fr(x) of the emission from the source. This means that Fr(x) can be 1 at maximum (all emitted NH is then deposited). Fr(x) depends in principle on: 3 the source height H4063#% , wind speed (in this case referenced to a height of 10 m), the concentration corrected surface resistance R@# , the atmospheric stability in the form of the Monin—Obukhov length ¸, the surface roughness length z0. , the distance to the source x, the concentration at the surface s , the 463&!#% mixing height and the reaction rate of NH3 . In some examples a value of 0 is adopted for R@# . This value is then taken to calculate the maximum possible value of Fr(x). 3.1. Source height Figure 1 shows Fr(x) as a function of the source height for neutral atmospheric conditions (¸" 20 000 m), z0."0.1 m, s463&!#%"0 and R@#"0 and R@#"25 s ms~1. A wind speed at 10 m of 3.46 m s~1 (corresponding to a u of 0.3 m s~1) is chosen because * it is representative of the average wind speed in agricultural areas in countries with a high ammonia emission density like Denmark, Belgium and the Netherlands. In continental Europe the average wind speed is lower. The figure shows that deposition close to the source is quite high. This is caused by the fact that R@# is small and the source height is low which results in high concentrations at ground level at a short distance from the source (Janssen and Asman,
Fig. 1. Fraction of the NH emission that is dry deposited as 3 a function of distance to a point source; u(10 m)" 3.46 m s~1, neutral atmospheric conditions (¸"20 000 m), z "0.1 m and s "0. The thick lines and the thin 0. 463&!#% lines indicate the results for R@ "0 and R@ "25 s m~1, # # respectively.
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W. A. H. ASMAN
1988). Figure 1 illustrates that Fr(x) deceases strongly with increasing source height. For that reason it is very important to use representative source heights in atmospheric transport models for NH3 . The figure shows also that the Fr(x) for a 10 m high source is very small in the beginning, which is caused by the fact that the plume has not reached the ground. The accumulated deposition up to 100 m from the source Fr(100 m) is 31, 13 and 0.6% of the emission for sources heights of 1, 3 and 10 m, respectively, and a R@# of 0 s m~1. For an R@# of 25 s m~1 this is 20, 6.5 and 0.4%, respectively. 3.2. ¼ind speed, surface resistance and atmospheric stability Figure 2 shows how the accumulated dry deposition up to 2000 m from a 3 m high source Fr(2000 m) varies with wind speed for various R@# values and neutral conditions. This figure shows that Fr(2000 m) does not depend on the wind speed if R@#"0. This means that the maximum possible fraction of the emission that is deposited in this area, or the amount that remains for long-range transport, does not depend on the wind speed. The reason for this is that the air concentration is proportional to 1/(wind speed)" 1/u* , and the exchange velocity »% is proportional to the wind speed or u . The exchange flux F (depos* $ ition only because s463&!#% is set to 0), which is equal to !»% (z3)s! (z3), is then independent of the wind speed because the effects of the wind speed on »% (z3) and s! (z3 ) compensate each other. Fr(2000 m) under the same stable/unstable conditions is also independent of the wind speed if R@ "0 for the same value of ¸. # Another phenomenon which can be seen in Fig. 2 is that, at very low wind speeds, Fr(2000 m) calculated with different R@# values approaches the value of Fr(2000 m) for R@#"0. This is simply caused by the fact that there is so little turbulence at such wind speeds that the overall resistance is dominated by R!#R" . On the contrary R@# becomes increasingly important with larger wind speed. In general it can be concluded that there is more local deposition at low wind speeds and more long-range transport at higher wind speeds. Figure 3 shows the effect of atmospheric stability on Fr(x). H "3 m, u(10 m)"3.46 m s~1, 4063#% z0."0.1 m for R@# values of 0 (thick lines) and 25 s m~1 (thin lines). The Monin—Obukhov lengths ¸ taken for a neutral, stable and unstable atmosphere were 20 000, 30 and !30 m, respectively; u* is then 0.3, 0.221 and 0.345 m s~1, respectively. Figure 3 shows that Fr(x) is larger for stable conditions and lower for unstable conditions. This is caused by slower mixing under stable conditions which gives less diluted plumes. 3.3. Surface roughness length At about 60 m above the earth’s surface, the wind speed is no longer influenced by local roughness variations (Wieringa and Rijkoort, 1983). To show the effect of differences in z0. on the accumulated dry
Fig. 2. Fraction of the NH emission that is dry deposited at 3 2000 m from a 3 m high point source for varying wind speeds and various R@ values; neutral atmospheric conditions # (¸"20 000 m), z "0.1 m and s "0. 0. 463&!#%
Fig. 3. Fraction of the NH emission that is dry deposited as 3 a function of distance to a point source for different atmospheric conditions (s"stable atmosphere, n"neutral atmosphere, u"unstable atmosphere); u(10 m)"3.46 m s~1, z "0.1 m and s "0. The thick lines and the thin 0. 463&!#% lines indicate the results for R@ "0 and R@ "25 s m~1, # # respectively.
deposition under neutral atmospheric conditions a wind speed of 4.80 m s~1 at 60 m was assumed, which corresponds to a wind speed of 3.46 m s~1 at 10 m height for a surface roughness length of 0.1 m. For this run the following parameters were set: H "3 m, neutral atmospheric conditions (¸" 4063#% 20 000 m), R@#"0 s m~1 and s463&!#%"0. This gives an indication of the maximum values of Fr in relation to z0. . The surface roughness heights chosen were: 0.005 m (very short grass; u(10 m)"3.88 and u*" 0.204 m s~1), 0.025 m (rough grass; u(10 m)"3.70 and u "0.247 m s~1), 0.1 m (arable land with height vari* ations in crops; u(10 m)"3.46 and u*"0.3 m s~1), 0.25 m (young coniferous forest; u(10 m)"3.23 and u*"0.350 m s~1) and 1 m (deciduous forest;
Factors influencing local dry deposition of gases
Fig. 4. Fraction of the NH emission that is dry deposited as 3 a function of distance to a point source for different surface roughness lengths; H "3 m, u(60 m)"4.80 m s~1, neu4063#% tral atmospheric conditions (¸"20 000 m), R@ "0 s m~1 # and s "0. 463&!#%
u(10 m)"2.70 and u*"0.469 m s~1). Figure 4 shows that Fr(x) increases with roughness length. This is caused by two effects: (a) the near surface wind speed decreases with roughness length, which leads to increased concentrations, and (b) the turbulence is greater, which leads to an increase in dry deposition velocity. Figure 4 indicates also that the dry deposition to forests close to farm buildings can be very high. It should be noted here, however, that the real situation is likely to be somewhat different as the forest usually does not begin near the farm door. If a R@# value of 25 s m~1 is chosen Fr(2000 m) will be only 0.18 (0.22), 0.22 (0.28), 0.25 (0.36), 0.28 (0.42) and 0.33 (0.59) for roughness lengths of 0.005, 0.025, 0.1, 0.25 and 1 m, respectively. The values in brackets are the values for a R@ value of 0 s m~1, # shown in Fig. 4. 3.4. Compensation point Figure 5 shows the vertical NH3 flux in the plume centre as a function of distance from a 3 m high point source at x"0 m for different compensation points (s , for stomatal exchange this has been termed 463&!#% s4 ) of the agricultural crops surrounding the source. The emission of the point source represents a housing and storage facility for 500 pigs, with an annual emission of 2.52 kg NH3 per animal (Asman, 1992), which corresponds to 4.0]104 kg NH3 s~1; u(10 m)" 3.46 m s~1, neutral atmospheric conditions (¸" 20 000 m), z0."0.1 m. R@# for downwind deposition fluxes was taken 25 s m~1, whereas the R@# for downwind emission fluxes was taken at 100 s m~1 (stomata only). This somewhat arbitrary difference in R@# was adopted because dry deposition may take place on the surface of the leaves. A positive flux indicates surface emissions into the atmosphere. The vertical flux shows a minimum (maximum deposition) at about 20 m from the source (not shown in Fig. 5). This
419
Fig. 5. Vertical NH flux (emission is positive) as a function 3 of distance from a 3 m high point source and as a function of the compensation point of the surrounding area. The source is equivalent to a stable/storage complex with 500 pigs; u(10 m)"3.46 m s~1, neutral atmospheric conditions (¸"20 000 m) and z "0.1 m, R@ for deposition is 0. # 25 s m~1 and R@ belonging to the compensation point is # 100 s m~1 (stomata).
minimum varies from !6.48 (s4"0 kg m~3) to !6.41 (s4"10 kg m~3) kg m~2 s~1. The distance at which the flux is zero is the distance at which the deposition component of the net flux caused by the point source is balanced by the emission component of the net flux caused by the existence of a compensation point. This distance varies from 450 m (s4" 10 kg m~3) to 1450 m (s4"1 kg m~3). It should be noted here that the balance will be reached at shorter distance from the source at some lateral distance from the plume centre. 3.5. »ariations in the modelled accumulated dry deposition due to real meteorological variations Figure 6 shows the modelled frequency distribution of the accumulated dry deposition of NH up to 3 2000 m from a 3 m high point source at Kastrup Airport, Denmark (55°39@ N and 12°40@ E). Kastrup is situated at the east coast of the island Zealand, and the local surface roughness length for momentum (z ) is 0.03 m. The calculations were made with the 0. meteorology (u(10 m) and ¸) for each hour for two 1974 and 1975. s is set to zero. Figure 6 shows 463&!#% two frequency distributions: one for a R@ value of # 0 and one for a R@ value of 25 s m~1. The frequency # intervals are 0.01. The highest peaks on the right side are caused by an artefact, which occurs for about 20% of all hours. The values for u and ¸ at Kastrup were calculated from * routine meteorological data using a meteorological preprocessor (Olesen and Brown, 1992). The preprocessor uses an iterative procedure to find u and * ¸ and in order that this process converges for stable conditions it is necessary to set a lower limit to ¸ of 100*z "3 m. For very stable situations ¸ will 0. therefore be set to this value. Although this is an artefact, it points to an important problem. These are
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W. A. H. ASMAN
Fig. 6. Frequency distribution of the fraction of the NH 3 emission that is dry deposited at 2000 m from a 3 m high point source based on Kastrup meteorology for 1974 and 1975 with s "0 for R@ "0 and R@ "25 s m~1. 463&!#% # #
the situations where little turbulence is present. In these cases the model does not adequately describe the situation. One reason is that the diffusion in the x-direction can no longer be neglected. Moreover, the situation can be so stable by cooling of the Earth’s surface that a plume released at some height may actually never reach the ground. These situations occur mostly at night and in the early morning hours. This is a serious problem because it can be expected that these situations will occur even more frequently at stations that are situated more inland than Kastrup. The other peaks indicate Fr(2000 m) for neutral atmospheric conditions. For a R@ value of # 0 s m~1 the peak occurs at Fr(2000 m)"0.30, while higher values are all associated with stable conditions and lower values with unstable conditions. For a value of R@ of 25 s m~1 the peak occurs at 0.19. It # can also be concluded that at one site variations in Fr(2000 m) of a factor two can easily occur, depending on meteorological conditions.
4. SOME ASPECTS OF AREA SOURCES
The micrometeorological mass balance method has been used to measure the NH emission from rota3 tionally grazed swards in the field (Jarvis et al., 1989; Bussink, 1994). The dung and urine patches cover about 10% of the area and their average size is of the order of 1 m2. The emission has also been measured in the laboratory (Whitehead et al., 1989; Vertregt and Rutgers, 1991). A question is whether these experiments are comparable. One aspect of this is that in the laboratory experiments there is only one patch, whereas in the field there are patches with grass between which deposition can take place. The mass balance method therefore measures the effect of both emission and deposition. Model calculations were made with three area sources (‘‘patches’’) each 1 m wide in the downwind direction, with 9 m grass in between. The crosswind
Fig. 7. Vertical NH flux (emission is positive) as a function 3 of distance from the upwind edge of a field with three urine patches; u (10 m)"3.70 m s~1, neutral atmospheric conditions (¸"20 000 m), z "0.025 m, R@ "25 s m~1 (for 0. # emission and deposition), s of the patches is 305 lg 463&!#% NH m~3, outside the patches s is 0. The lines in3 463&!#% dicated with E above the peaks indicate the areas with emission.
diffusion is neglected. The value of s was ad463&!#% justed to 305 lg NH m~3 so that the emission at the 3 most upwind edge of the patch was realistic: about 5.6 lg NH m~2 s~1 (D. W. Bussink, Research Station 3 for Cattle, Sheep and Horse Husbandry, Lelystad, the Netherlands). Other conditions were: u(10 m)" 3.70 m s~1, neutral atmospheric conditions (¸" 20 000 m), z "0.025 m, R@ "25 s m~1 (for emission 0. # and deposition). Outside the patches s "0. 463&!#% Figure 7 shows several important features. The emission flux within the patch decreases with increasing downwind distance. The NH concentration in the air 3 over the patch becomes so high that the emission flux at the downwind edge is reduced (the flux depends on s !s ). This is a very general phenomenon: 463&!#% ! the larger the alongwind emission area, the less becomes the emission flux if s is the same every463&!#% where in this area. The deposition caused by the first patch in the area between 1 and 10 m is about 15% of the emission; up to 30 m another 5% is deposited. Figure 7 shows also that the deposition downwind of the second and third patch is slightly larger. This is caused by the accumulated effect of the previous patches.
5. CONCLUSIONS
The results presented in this paper illustrate clearly that the dry deposition of a component up to 20—100 m from a low source may be considerable and that it depends on the source height, wind speed, surface resistance, atmospheric stability, surface roughness length and surface concentration (compensation point). Equally, these factors also determine the fraction of the emitted gaseous component transported
Factors influencing local dry deposition of gases
over long distances. For these reasons it is very important to take the above-mentioned factors into account in designing local and long-range transport models. This is especially important for NH , which is 3 released mainly from low sources and frequently has a small surface resistance. It should be noted here that for NH not only 3 does the dry deposition depend on these factors, but also the emission flux from ground sources (e.g. applied manure and chemical fertilizer). The model results discussed in Section 3.5 suggest that the dry deposition close to a source cannot be calculated for at least 20% of the time, because the atmosphere is so stable that the current theory is unlikely to be adequate. Modelling of local NH dry deposition close to 3 a source should be improved, taking into account the air flow and turbulence around agricultural buildings and the NH concentration in the surface. Moreover, 3 farm buildings and fields are two distinct source types with their own properties and they should therefore be treated separately in atmospheric transport models. Acknowledgements—The development of the model was inspired by the modelling work done by Hans Maas and the author at the National Institute of Public Health and Environmental Protection (RIVM), Bilthoven, The Netherlands. Ruwim Berkowicz (NERI) is thanked for his comments on the draft version of this paper. This research was partly supported by the Danish Environmental Research Programme and the Danish Environmental Protection Agency (Project ‘‘Spredning og effekter af ammoniak’’). The author is grateful to the referees and the editor for their critical comments and to Wim Bussink, Research Station for Cattle, Sheep and Horse Husbandry, Lelystad, The Netherlands for the information on urine patches.
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