A variable-resolution transport model applied for NHχ in Europe

A variable-resolution transport model applied for NHχ in Europe

Atmospheric Environment Vol. 26A, No. 3, pp. 445464, 1992. 0004-6981/92 S5.00+0.00 © 1991 Pergamon Press plc Printed in Great Britain. A VARIABLE-R...

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Atmospheric Environment Vol. 26A, No. 3, pp. 445464, 1992.

0004-6981/92 S5.00+0.00 © 1991 Pergamon Press plc

Printed in Great Britain.

A VARIABLE-RESOLUTION TRANSPORT MODEL APPLIED F O R N H x IN E U R O P E WILLEM A. H. ASMAN* a n d HANS A. VAN JAARSVELD National Institute for Public Health and Environmental Protection, P.O. Box l, 3720 BA Bilthoven, The Netherlands (First received 18 October 1990 and in final form 7 June 1991) Abstract--The statistical atmospheric transport model TREND has been applied for the calculation of

concentrations and depositions of ammonia (NH3) and ammonium (NH2). The model is capable of describing dispersion, conversion and deposition from both local sources (necessary to obtain good results for NHa) and from more distant sources(necessaryto obtain good results for NH2). Model results show that in western Europe 44% of the emitted NH3 from a 1m high source is dry deposited as NH3, 6% is wet deposited as the contribution of NH 3 to the wet deposition of NHx, 14% is dry deposited as NH~ and 36% is wet deposited as the contribution of NH2 to the wet deposition of NH~. The model results agree wellwith measured NH 3 and NH~" concentrations--the latter in both aerosol form and precipitation--in The Netherlands, Belgium, Denmark, the U.K., the F.R.G., Sweden and other parts of Europe. Vertical concentration profiles of NH3 and NH2 at Cabauw, The Netherlands are also reproduced well, as well as the diurnal variation of the NH 3 concentration at Elspeetsche Veld, The Netherlands. Key word index: Ammonia, ammonium, air, precipitation, statistical transport model.

I. INTRODUCTION Ammonia (NH3) and ammonium (NH,~) are important atmospheric components. NH 3 is the most abundant alkaline component in the atmosphere. A substantial part of the acid in the atmosphere generated by the oxidation of sulphur dioxide and nitrogen oxides is neutralized by NHa. As a result, NH2 is a major component in aerosols and in precipitation. NH 3 and NH~ act as fertilizers and deposition of these substances have unfortunate effects (Roelofs et al., 1985) or can lead to a change in the composition of the vegetation (Nilsson and Grennfeit, 1988). Oxidation of NH~ in the soil to nitric acid may lead to acidification of the soil. For these reasons the interest in the atmospheric behaviour of NH 3 and NH2 is increasing. NH 3 is mainly emitted from animal manure, but also production and application of fertilizers may lead to NH 3 emission (Buijsman et al., 1987). NH~ is not emitted in significant quantities. All NH~ found in the atmosphere originates from NH 3. NH 3 is released mainly from scattered low-level sources, which results in a rapid decrease in concentration and deposition with distance from the source, due to dilution. NH 3 is not transported over very long distances as it is rapidly converted to NH~ aerosol at a rate of about 30% h - 1. NH~ aerosol is not dry deposited very well and is therefore transported over long distances. The conversion rate is, however, not so large that the NH~ * To whom correspondence should be addressed. Present affiliation: National Environmental Research Institute (NERI), Frederiksborgvej 399, 4000 Roskilde, Denmark.

aerosol concentration is influenced to a large extent by very local emissions. For NH3, the horizontal concentration gradients at ground level are usually steeper than for SO2. This is partly caused by the fact that SO2 is released from higher sources, so that a SO 2 plume is spread over a substantial part of the mixing layer before it reaches the Earth's surface, and the gradient at the surface is then not so large as near the source for NH 3. Another reason is that SO2 is converted to a secondary product at a much lower rate (about 1% h -1) than is NH 3. This means that for SO 2 there is also a substantial contribution from more distant sources, as compared with NH 3. A model for the atmospheric behaviour of NHx (NH 3 and NH2) should be able to describe both the atmospheric transport of NH3 near a source as well as the transport of NH~ over long distances. This is difficult. If a model developed for the short range should be used for longer distances, it should take phenomena like the mixing height and wind shear into account. And often running such a model will take much computer time as it also should be able to describe the detailed situation near the source. An attempt to model NH 3 and NH2 has been made by Fisher (1984). His work was severely handicapped by lack of data on the emission of NH3, on the transformation rate of NH3 to NH~, and on the dry deposition velocity of NH3. Subsequently more information became available on the emission (Buijsman et al., 1987), transformation rate (Erisman et al., 1988), dry deposition velocity (Duyzer et al., 1987), long-range transport modelling (Asman and Janssen, 1987) and mesoscale modelling (Asman and Maas,

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WILLEMA. H. ASMANand HANSA. VANJAARSVELD

1986), and more long-range models were made suitable to include NHx (Fisher, 1987; Derwent, 1988; Hov et al., 1988). The only model which was used to model the atmospheric behaviour of NH x on both short and long range was the statistical reactive plume model developed by Asman and Maas (1986). At RIVM, a statistical transport model named TREND (van Jaarsveld and Onderdelinden, 1991) was developed and succesfully applied for SO 2 and NOx. This model is able to describe both short- and long-distance transport, and average concentrations and depositions can be computed for time-scales from 1 day to more than 10 years. The model was subsequently adapted to describe the atmospheric behaviour of NHx as well. In the following sections, the atmospheric processes for NH~ and their parameterizations are described as well as their inclusion into the TREND model.

diurnal variations in the aerodynamic resistance and temperature. Preliminary model calculations showed that adoption of a diurnal variation in the NH 3 emission rate results in a 20% reduction of the annually averaged concentrations of NH 3 and NH~- aerosol. It was therefore in principle decided to incorporate a 10 year averaged diurnal variation of the emission rate in the model (Asman, 1990a). The TREND model is unique in that it can account for both point sources of various height and area sources of various shapes and heights and that the sources need not to be distributed on a regular grid. It yields realistic results both within area sources and near point sources as well as at long distances from sources.

3. DRY DEPOSITION

3.1. Dry deposition of ammonia 2. EMISSION OF AMMONIA

The emission inventory of Asman (1990a) was used in the model with an annually averaged total emission for 16 European countries of about 8 x 106 tonne NH 3 a-~. This emission was computed from information on livestock and consumption of fertilizers for the year 1987. Moreover, emissions by industrial ammonia plants were included in the inventory. Buijsman et al. (1987) estimated that the natural NH 3 emissions in Europe are maximally 0.75 × l 0 6 tonne NH 3 a - 1. As no information was available on the geographical distribution of these sources, and as their contribution is much less than the inherent uncertainty of 30-40% in the annually averaged emissions from livestock, application of fertilizer and ammonia plants, and natural emissions were not included in the inventory. Most model calculations were made with a detailed emission inventory for The Netherlands (Erisman, 1989), Belgium and the western part of the F.R.G., (Asman, 1990a), and with emissions on IE-grid elements (75 × 75 km 2) for the rest of Europe (Asman, 1990a). But some computations for stations in The Netherlands were made with emissions on IE-grid elements for all countries and with emissions on EMEP-grid elements for all countries. This is to investigate the influence of the spatial resolution of the emission on the model results. In one case, concentration gradients over Denmark were computed using a detailed (5 x 5 km 2) emission inventory for Denmark (Asman, 1990b) and emissions on IE-grid elements for other countries. Asman (1990a) found that the seasonal variation in the NH 3 emission rate derived from measurements did not agree with the variation derived from agricultural practice. Therefore no seasonal variation in the NH3 emission rate was adopted in the model. Asman (1990a) found a substantial diurnal variation in the NH 3 emission rate, which could be explained from

Depending on whether the difference between the NH 3 concentration in the air and that in the soil and vegetation is positive or negative, respectively, dry deposition or emission from the surface will occur. This means that there is no dry deposition to surfaces which contain a substantial amount of NH 3, such as fields within a few weeks after spreading of manure and meadows where cattle are grazing. The flux will depend on the concentrations in the soil and vegetation, which in agricultural areas will depend on the applied amount of manure or fertilizers, the properties of the soil and vegetation, and the meteorological circumstances. It is known that NH 3 emission occurs from leaves of senescent plants and from grass after it has been cut (Whitehead and Lockyer, 1989). Asman (1986) estimated that averaged annually the reduction of the overall net dry deposition velocity on fields where manure has been spread, compared to fields where no manure has been spread, could be of the order of 20%. For meadows where cattle are grazing the reduction could be up to 50%. Farquhar et al. (1980), Lemon and van Houtte (1980), and Horv~ith (1982, 1983) found that when the NH 3 concentration in the ambient air was below the so-called 'compensation point', emission occurred, while above it deposition occurred. But it is likely that there is not one general compensation point, as it will depend on the concentration ofNH 3 in the soil and in the vegetation, which also can vary with time. Moreover, the values of the 'compensation point' mentioned in the literature sometimes refer to laboratory measurements under laminar flow conditions. Such values are not representative of the field concentrations at reference height under more or less turbulent conditions. Dry deposition of material from the atmosphere is conceptually thought to take place through three resistances in series. The aerodynamic resistance r a describes the resistance due to turbulent diffusion of

Variable-resolution NHx transport model material from the atmosphere to a very shallow (about 1 mm) layer near the surface. The aerodynamic resistance increases with height and is therefore usually given for a reference height (1 m). The laminar boundary layer resistance r b describes the resistance due to molecular diffusion (gases) or Brownian diffusion (particles) through the very shallow laminar layer near the surface. The surface resistance r c described the ability of the surface to receive the airborne material and depends therefore both on properties of the surface and on properties of the material. The relation of the resistance (s m-1) to the dry deposition velocity vd (m s 1) is: vd=(ra + rb + re)- 1.

The flux of material to the surface, F (mol m - z s- 1) is given by: F

= UdC

where c is the concentration of material (mol m - 3). Duyzer et al. (1987) measured the dry deposition of NH 3 in nature reserves and could not find any indication of the existence of a compensation point, not even at concentrations as low as 1/~gNH 3 m 3. They found that the annually averaged surface resistance was very low, 23 s m - 1, which means that the transport was almost to a large extent determined by turbulence. Deposition also occurred to grass seemingly dead (van Aalst, MT-TNO, Delft, The Netherlands, 1986, pers. comm.). Duyzer et al. (1987) found a climatologically averaged (all year, all hours) dry deposition velocity for NH 3 to heather/purple moor grass vegetation at a reference height of 1 m of 1.6 × 1 0 - 2 m s -1" Apart from these measurements there exist other measurements of the NH 3 dry deposition velocity (see Asman and Janssen, 1987, for an overview). But either no information on the meteorological conditions during the measurements is given, or the measurements are made over vegetation which also emits NH 3. This means that these data cannot be used to parameterize dry deposition of NH 3 in atmospheric transport models because it is not sufficiently known which conditions they represent. The dry deposition velocity given by Duyzer et al. (1987) is a maximum dry deposition for low vegetation such as grass and heather. If any NH 3 exists in the ground, as is the case during at least part of the time for agricultural surfaces, the dry deposition velocity of NH 3 will be lower. To account for some reduction in the average dry deposition velocity of NH 3 in the model due to this effect, a surface resistance of 30 s m - 1 was adopted in the model, which is somewhat higher than the value given by Duyzer et al. (1987). This value was used for the whole modelling area, as the model is not very well suited to incorporate different dry deposition velocities for different sites. Therefore the model will overestimate the dry deposition in agricultural areas, where during part of the time no deposition occurs. On the other hand, the dry deposition to forests will be

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underestimated, at least close to forest edges, as the dry deposition velocity in forests is higher than over moorland, where Duyzer et al. (1987) measured, because of the larger surface roughness in forests. The dry deposition velocity applied in the present model is computed using a surface resistance of r c = 30 sm 1, but with values of r a and r b which are different for each meteorological class. Using these values and meteorological statistics for a 10-year period an effective (concentration weighted) average dry deposition velocity of 1.22 × 10 2 m s- 1 was calculated for The Netherlands. 3.2. Dry deposition of ammonium aerosol Duyzer et al. (1987) measured the dry deposition velocity of particles containing NH~ on heather/purple moor grass vegetation and found a value of about 1.8 × 10 -3 ms -1, with a large uncertainty. No other measurements of the dry deposition velocity of N H f exist. For aerosols the laminar boundary layer resistance is large. This phenomenon could not easily be fitted into the model concept. To obtain the same dry deposition velocity as if a correct laminar boundary layer resistance were taken, a high surface resistance of 600 m s-1 was adopted. This leads to an effective dry deposition velocity of 1.4 × 10 -3 ms -1.

4. WET DEPOSITION NH 3 is very soluble in water. Moreover, as water droplets in the atmosphere are usually acidic, the uptake of NH 3 will be enhanced by the reaction of NH 3 with H + to form NH~. Cloud droplets have a large surface to volume ratio and a relatively long residence time. As a result of the properties of NH 3 and cloud droplets, most of the NH 3 within the cloud will be found in the water phase and will be in equilibrium with a very low NH 3 concentration in the interstitial air. This concentration will be much lower than the concentration at that level in the atmosphere before the cloud was formed. In-cloud processes lead to the formation of raindrops. When the raindrops fall down, also NH 3 below the cloud will be washed out. The NH 3 concentration below a cloud is much higher than the very low interstitial concentration within the cloud. As a result most raindrops will not become saturated with NH 3 relative to the concentration at the Earth's surface before they hit the Earth's surface. The below-cloud uptake of NH 3 will be a function both of the diffusion constant of NH 3 at that temperature, and of the drop size as a measure for the surface to volume ratio. Particles containing NH2 will act as condensation nuclei and will in this way efficiently be incorporated in cloud droplets. Also below-cloud scavenging occurs and is dependent on the size distribution of the particles and raindrops. In general, below-cloud scavenging and in-cloud scavenging occur at different rates

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WILLEM A. H. ASMAN and HANS A. VAN JAARSVELD

for the same component. Therefore it is necessary to differentiate between them. Near the source, before the plume enters the clouds, only below-cloud scavenging will occur. For this case a below-cloud scavenging coefficient is used in the model which is based on parameterizations of the results of the below-cloud scavenging model of Janssen and ten Brink (1985). In these parameterizations the below-cloud scavenging coefficient is a function of the precipitation rate (and thereby of the size distribution of the raindrops) as well as either the diffusion coefficient when the plume material is a gas or the size distribution when it consists of particles. A more detailed description of the incorporation of the wet deposition process in the TREND model is given by van Jaarsveld and Onderdelinden (1991). At some distance from the source, a plume may enter clouds, and then also in-cloud scavenging will occur. Further away from the source ( > 30 km), incloud scavenging is the most important scavenging process. At such distances, the overall effect of both processes is described in the model by an overall scavenging coefficient: Ao= SR/H where A o is the overall scavenging coefficient (s- 1), S is the scavenging ratio, i.e. the ratio between the concentration in the precipitation and that in the air (mol/mol) (for NH 3 and particulate NH,~ the same value of 1 x 106 was used), R is the rainfall rate (m s - 1), H is the mixing height (m). Close to the source, scavenging is solely described by the below-cloud scavenging coefficient. But with increasing distance from the source, the fraction which is scavenged by in-cloud scavenging is increased in the model. Contrary to dry deposition, wet deposition takes place only during part of the time. In a statistical transport model, concentrations and depositions are not computed hour by hour, but by using a continuous wet deposition rate computed with an effective scavenging coefficient. This rate should give the same removal of components over a longer period (including dry periods) as the intermittent removal by precipitation computed with the normal scavenging coefficient (van Egmond, 1984): P A=--.(1-e

-row)

Tw

where .~ is the effective scavenging coefficient (s- 1), p is the probability of precipitation, zw is the average duration of the precipitation event (s), A is the belowcloud scavenging coefficient or overall scavenging coefficient (s-1). Both P and zw are extracted from measurements of the Royal Netherlands Meteorological Institute (KNMI) for 12 stations in The Netherlands for the period to be modelled. As no such precipitation statistics are available for other parts of Europe, it is assumed that the same values of P and z~ are valid for Europe as a whole. This means that, e.g.

orographic effects on the amount of precipitation cannot be taken into account. Most model calculations were made using an annual amount of precipitation of 725 mm, which is the average for The Netherlands for the period 1979-1988.

5. REACTION

Under European conditions most NH 3 will react with H2SO 4 containing aerosol to form NH,~ containing aerosol. This is a one way reaction and this aerosol will not evaporate again. A minor part will react with gaseous H N O 3 or gaseous HC1 to form NH4NO 3 or NH,CI containing aerosol (Stelson and Seinfeld, 1982; Allen et al., 1989). Both the formation and the evaporation of these ammonium salts depend strongly on temperature and relative humidity. The concentrations of the gaseous components will not always be high enough to form the aerosol. The measured concentration products [NH3][HNO3] or [NHa][HCI'] are sometimes higher than the concentration products at equilibrium derived from thermodynamics (Erisman et al., 1988; Allen et al., 1989). There seems to exist kinetic limitations to the reaction rate of the gaseous components which may explain the fact that the gaseous concentration products do not always correspond to their expected values at equilibrium (Harrison and MacKenzie, 1990). NH 3 is predominantly released from low-level sources, whereas acid aerosol, H N O 3 and to some extent HC1 are reaction products which are formed throughout the whole mixing layer. It is therefore likely that the rate at which NH 3 is converted to NH2 aerosol will depend on the presence of acids in the atmosphere and thereby on the emission of the precursors of those acids. The conversion rate is also likely to depend on the mixing in the atmosphere, because mixing increases the encounters of NH 3 and the acidic components together. The setup of the model permits only a description of the reaction rate by a pseudo-first-order reaction rate constant k (s- 1). The reaction rate is then given by: d[NH3]/dt=k[NH3], k is not a real constant but depends on the presence of acidic components and on the atmospheric mixing characteristics. As the circumstances which favour the conversion of NH 3 are different at different locations, under different conditions and in different seasons, the conversion rate is likely to vary with space and time. For Europe as a whole, enough acid precursors (SO2, NO~) are released to neutralize all emitted NH 3 (Asman and Janssen, 1987). There exist only a few measurements of the conversion rate k for NH 3. Lenhard and Gravenhorst (1980) measured concentrations of NH3 and NH2 aerosol over western Germany at 100 and 700m above ground level. From their measurements and vertical eddy diffusivities they estimated an average value of k at 400m height of 1.2x 10 - s s -1 in winter and 2.1

Variable-resolution NH x transport model x 10- 5 s - 1 in summer. They noted also that k is likely to be greater at lower levels. Erisman et al. (1988) measured vertical concentration profiles of NH 3 and NH,~ aerosol on a meteorological tower in an emission area in The Netherlands. They calculated an average conversion rate of 1 x 10 -4 s - 1 for daytime periods and 5 x 10 -5 s- 1 for nighttime periods from the concentration profiles and the meteorological measurements on the tower, Asman and Janssen (1987) used a value of 8 × 10- s s - 1 in their long-range transport model, because modelled concentrations compared favourably with measured concentration when this value for k was used. In our model too, a value of 8 x l0 -5 s-~ was adopted. The conversion rate was taken to be constant because not enough information was available to make it a function of the time of the day, of the season or of other factors. NH a can also react with OH, O and O(aD). Levine et al. (1980) found that reaction with OH was most important. When adopting a constant relatively high OH-concentration of 4 x 106 molecules cm- 3 (Logan et al., 1981) a pseudo-first-order reaction rate of 5.4 x 10 -7 s- 1 for NH 3 is found. This is much lower than the conversion rate caused by reaction with acidic components. Therefore the reaction with OH is neglected in our model.

6. DISPERSION AND T R A N S P O R T

A detailed description of the model formulation is given by van Jaarsveld and Onderdelinden (1991). In this section an overview will be presented. The model is a statistical long-term transport model, To make the model more efficient, instead of computing concentrations for each hour of the time period under consideration, computations are only made for a limited number of meteorological situations (meteorological classes) with a representative meteorology for each class. Amongst the discretizations, a total of 12 wind-direction sectors and 6 atmospheric stability classes are distinguished. Another simplification is that no spatial variation is accounted for in the frequency distribution of the meteorological parameters and that of the removal and conversion processes within the modelling area. The basis of the model is the Gaussian plume formulation for a point source. It is assumed that the plume reflects only once at the Earth's surface and at the top of the boundary layer. Moreover, it is assumed that the plume at larger distances from the source is vertically distributed homogeneously over the whole boundary layer, apart from an attenuation near the Earth's surface due to dry deposition. The horizontal dispersion in the direction of the wind is neglected, as this is only important for non-continuous releases. The horizontal dispersion perpendicular to the wind direction is not described with a Gaussian formula. It is simply assumed that within each wind-direction AE(A)

26:3-G

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sector the frequency distribution of the wind direction is homogeneous. The model uses straight trajectories. However, the curvature of a trajectory is simulated by making its direction a function of the distance between source and receptor point. The straight trajectories used are computed from curved trajectories based on groundlevel measurements and measurements on television towers in The Netherlands. The model takes into account that the mean transport height varies during the transport as a function of the time of the day and that wind direction and wind speed are height dependent. The mixing height is computed from meteorological observations and shows hourly variations. Break-up of inversion layers is simulated so that fumigation can be adequately described. Dry deposition, wet deposition and conversion is accounted for by reducing the apparent source strength with the removed or converted amount. Dry deposition is modelled in such a way that depletion due to dry deposition is larger near the Earth's surface than higher up in the atmosphere. To account for this effect, a vertical concentration gradient is introduced in the model. This is done by adopting a constant downward flux in the lower part of the boundary layer. The concentration is then computed for each height from the constant flux and the height dependent aerodynamic resistance r a. The resulting dispersion and dry deposition compared well with results from two other models. They were a surface depletion model with Gaussian plume formulation (Horst, 1977, 1984; Asman et al., 1989) and a K-model developed by Maas and Asman at RIVM (Asman and van Jaarsveld, 1990).

7. MODEL RESULTS 7.1. General results 7.1.1. Fate of NHx in the atmosphere. The fate of NHx in the atmosphere is illustrated in Fig. 1, where the cumulative deposition of different kinds of NH x is presented as a function of downwind distance from a source with a source height of 1 m. This figure shows clearly that NH x is both a local and a long-distant pollutant. NHx is mainly removed from the atmosphere in the form of dry deposition of NH 3 close to the source and in the form of wet deposition of N H g at some distance from the source, partly because NH~ is a reaction product which is first formed after some time. It appears that 44% of the emitted NH a is dry deposited as NH 3, 6% is wet deposited as the contribution of NH 3 to the wet deposition of NHx, 14% is dry deposited as NH~ aerosol and 36% is wet deposited as the contribution of NH~ aerosol to the wet deposition of NHx. The rather high dry deposition of NH3 close to the source and the high conversion rate of NH 3 to NH2 aerosol imply that, in general, the bulk of the dry

450

WILLEM A. H. ASMAN and HANSA. VAN JAARSVELD dry

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deposition of NH 3 in a country originates from emissions within that country itself, whereas a substantial part of the wet deposition of NH~ comes from abroad. 7.1.2. Influence of reduction of emission around a receptor point. In some parts of Europe detrimental effects of high loads of NHx are observed. It is therefore important for environmental planners to know where NH 3 emissions have to be reduced and by how much in order to avoid any damage. A detailed study on this subject was made for The Netherlands by Maas (1988). But such a study is quite specific for one area because it depends on the local distribution of NH 3 sources, the local distribution and size of the areas to be protected, the defined deposition threshold values and the technical possibilities and political willingness to reduce the emissions. It was therefore felt more appropriate to present a more general and illustrative situation for which the effect of a reduction of the emission around a receptor point on the depositions at that point can be demonstrated. The receptor point is located in the middle of 'Ammonialand', a square country of a size of 2952 x 2952 km 2 with a uniform N H 3 emission density of 0 . 1 # g m - 2 s- 1 (this emission density is about half the average emission density for The Netherlands). Outside this country no emission takes place. The emissions are now stepwise removed from larger and larger squares around the receptor point (Fig. 2a). For each situation the different kinds of deposition in the receptor point are computed and graphically displayed as points in Fig. 2b. This figure shows the different kinds of deposition as a function of the size of the side of the emission-free square (the shortest distance between receptor point and nearest emission area is half this size). At each step a larger area is made emission-free until the boundaries of 'Ammonialand' are reached. It shows that local emission reductions will lead to a reduction in the dry deposition of NH3 and to a lesser extent in the contribution of NH 3 to the wet deposition of NHx in the receptor point. Dry deposition of NH 3 is reduced relatively fast compared to the contri-

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bution of NH 3 to the wet deposition of NHx. This is caused by the fact that the NH 3 plume is more spread out over the mixing layer when it reaches the receptor point when the distance between receptor point and nearest emitting area is larger. Wet removal takes place over the whole mixing layer and does therefore not show this effect. If the side of the emission-free square becomes larger than 300 km almost no NH3 will reach the receptor point because it has then already been converted to NH2 aerosol. Deposition of NH,~ cannot be reduced by local emission reductions. Dry and wet deposition of NH2 in the receptor point become less if the emission-free area becomes larger than 300 km. This is partly caused by the limited size of 'Ammonialand'. 7.2. Results for The Netherlands Figure 3 shows the modelled spatial distribution of the NHx components for The Netherlands. The spatial distribution of the NH 3 concentrations (Fig. 3a) shows more or less the same picture as the NHa emission. The spatial distribution of NH,~ aerosol (Fig. 3b) shows a large gradient over the country. This results partly from the fact that the reaction of NH 3 to NH2 is so fast, that part of the NH 3 has already reacted to

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Fig. 3. Concentrations and depositions in The Netherlands.

N H 2 before it is diluted over the whole mixing layer. The dry deposition of NHx (not shown) is dominated by the dry deposition of NHa because of the relatively high dry deposition velocity of NH 3 compared to NH2 aerosol. As a result it shows more or less the same distribution as for the concentration of NH 3. The contribution of NH 3 to the wet deposition of NHx (Fig. 3c) shows a more smoothed distribution than the NH 3 concentration. This difference can be explained by the fact that this contribution is caused by scavenging of NH3 over the whole mixing layer. It reflects therefore not only the contribution of local sources as is the case for the NH3 concentration at ground level. The contribution of N H 2 to the wet deposition of NH~ does not show such a steep gradient over the country as the N H 2 aerosol. This is caused by the fact that scavenging of NH~- takes place over the whole mixing layer. The contribution of NH2 to the wet deposition of NHx (Fig. 3d) reflects thus more the average concentration over the whole mixing layer. In the areas with the highest emission density, the contribution of N H 3 to the wet deposition of NH~ is as large or even larger than the contribution of NH2. The wet deposition of NH~ (Fig 3e) is simply the sum of the contributions of NH 3 and NH~ which is reflected in its spatial distribution. The spatial distribution of the total deposition of NHx (Fig. 3f) is dominated by the dry deposition of

NH3 and the contribution of NH a and NH~ to the wet deposition of NHx, of which the first one is the most important. 'Hot spots' in the NH 3 emission can therefore clearly be recognized in total deposition of NHx but they are more spread out. In the areas with the highest emission densities the total deposition of NHx is over 5000 mol h a - 1 a - 1. Apart from the coastal areas dry deposition of NHx is more important than wet deposition of NHx. Although for The Netherlands as a whole, foreign sources only contribute for 22% to the total deposition of NH~, this is certainly not true for all components and for all parts of The Netherlands. For a more locally determined component, like NH3, more than 50% of the concentration in the extreme SW part of The Netherlands is caused by foreign sources. In the coastal areas more than 60% of the total deposition of NH x is caused by foreign sources. The computed contribution from sources in The Netherlands to the total deposition in The Netherlands is 72%. This result agrees well with results from other models (see Derwent et aL, 1989; Asman and van Jaarsveid, 1990). 7.3. Results for Europe Figure 4 shows the computed concentrations and depositions o f N H x components for Europe. Figure 4a shows that the NH 3 concentration decreases rapidly when going from land to sea. In fact this decrease is more rapidly than shown in the picture because the

Variable-resolution NH~ transport model

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emission used in this computation was spread out over 75 x 75 km 2 grid elements. This means that in the model part of the emission occurs at sea, causing too high computed concentrations for the sea area within the grid element. The N H ~ aerosol concentration (Fig. 4b) and the wet deposition of NHx (Fig. 4d) do

not decrease so fast when going from land to sea. Figure 4e shows that dry deposition of NHx is only more important than wet deposition of NHx in areas with a high emission density. At sea wet deposition of NHx is much more important than dry deposition of NHx.

454

WILLEMA. H. ASMANand HANSA. VANJAARSVELD

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8. SENSITIVITY ANALYSIS

8.1. Influence of variations parameters

in some basic model

The sensitivity of the model results to changes in the parameters was investigated for three locations. These three locations were chosen to be representative of different situations. Vredepeel in The Netherlands (51°32 ' N, 5°52 , E) was selected because it is situated in an area with probably the highest NH 3 emission density in Europe. Petten in The Netherlands (52046' N, 4°40' E) is a coastal site in an area with a relatively low emission density. Velen in Sweden (58°46 ' N, 14°18' E) was chosen because it is situated in a rural area with a low emission density far away from areas with high emission densities. The sensitivity of the model results to variations in the emission density was not investigated because an increase of the emission of one source will lead to the same relative increase in the contribution of this source to concentrations and depositions at all receptor points (the model is linear). The sensitivity study was made both for the concentration as well as for the dry deposition of a component, because they change in a different way when the deposition velocity is changed. The concentration near the Earth's surface is not the same for different dry deposition velocities, but will decrease with increasing dry deposition velocity. The amount dry deposited will, however, increase with the dry deposition velocity. In this study the value of one of the parameters was varied with regard to the base case for each model run.

This does not mean, however, that all used values of parameters are even likely to occur. For the base case the following parameter values are used: rc(NHa) = 30 s m - 1 (Vd= 1.22 X 10- 2 m s- 1), rc(NH~) = 600 s m-1 (Vd= 1.4 × 10 -3 ms-1), a scavenging ratio of 1 × 106 for both NH 3 and NH~ and a conversion rate of 8 x 10 -5 s -1 for NH 3. Some runs were made for r¢(NH3) values of 0 sm -1 and 120 sm - I leading to effective (concentration weighted) dry deposition velocities of, respectively, 1 . 9 2 x 1 0 - 2 m s -1 and 5.5 x 1 0 - 3 m s -1 and for rc(NH~) values of 270sin -1 and 1200 s m -1, leading to effective dry deposition velocities of, respectively, 3.0x 10-3ms -1 and 8.0 x 10 -4 m s - L Moreover, model calculations were made for two combinations of parameter values which all either gave the lowest or the highest concentrations at Vredepeel (combination 1 and 2). To investigate the influence of different types of meteorology, one calculation was made with coastal meteorology and one with inland meteorology for The Netherlands. Normally the calculations are made with the meteorological statistics averaged over The Netherlands. Some uncertainty exists too in the height of the sources to be applied in the model. For that reason some calculations were made with fixed source heights of 1 and 5 m instead of a height which is dependent on the contribution of different emission categories to the emission. It is not clear whether only deposition of NH 3 can occur at sea, or that NH 3 also can be emitted from the sea. To get an idea of the possible consequences of this uncertainty calculations were done with a zero dry deposition velocity for NH3 at sea. The results of the sensitivity study are presented in Tables 1-3. For Vredepeel, the dry deposition of NHx is in all cases much higher than the wet deposition of NHx. For Petten, dry deposition of NHx is in almost all cases more important than wet deposition. But for Velen the wet deposition of NHx is always higher than the dry deposition of NHx. For all stations and for all cases except one for Velen the dry deposition ofNH 3 is larger than the dry deposition of NH~. The relative contribution of the dry deposition of NH3 to the dry deposition of NH x is largest for Vredepeei, followed by Petten and then by Velen. For Vredepeel the contribution of NH 3 to the wet deposition of NH~ is in almost all cases greater than the contribution of NH~. But in all cases in Petten and Velen the contribution of NH~ to the wet deposition of NH x is more important than the contribution from NH 3. The range in the total deposition of NHx, when only one parameter is changed, is largest in Vredepeel (+ 30% difference from the base case) and is about + 15% from the base case for Petten and Velen. The range in the results for the total deposition of NH x is not so large as in the dry and wet deposition of NH 3, NH~ and NH~ or in the concentration of NH3 and NH~-. This is caused by the fact that a change in one parameter values will result in a change in dry depos-

8.1 6.2 10.2 8.3 8.1 7.0 8.9 8.9 8.0 5.3 11.9 6.9 9.9 7.9 8.5 8.2 7.9 7.8

20.2 14.8 26.4

Conc. NH,~

20.2 16.2 23.9 20.3 20.1 20.2 20.2 20.2 20.2 21.8 18.0 16.9 25.0 22.6 20.1

Conc. NH 3

150 101 167

149 88 193 119 159 144 152 126 153 142 160 142 151 144 154

Conc. NH~pr

4583 6115 2667

4581 6544 2429 4612 4574 4581 4581 4581 4581 4895 4163 4145 5213 5127 4566

Dry dep. NH 3

202 365 104

201 153 250 205 201 336 119 223 198 131 293 175 238 195 209

Dry dep. NH~

4785 6480 2771

4783 6697 2679 4817 4776 4918 4701 4805 4780 5026 4456 4320 5451 5323 4776

Dry dep. NH~

* rc(NH3) = 0 , rc(NH~)= 270, sc(NH3)= 5 × 106, s c ( N H ~ ) = 5 x 106, conv. r a t e = 1.6 x 10 -4. t rc(NH3)= 120, rc(NH,~)= 1200, s c ( N H 3 ) = 2 × 105, sc(NH,~)= 2 × 105, cony. rate =4.0 × 10-5.

Base rc(NH3) = 0 rc(NH3) = 120 sc(NH3) = 2.0 x 105 sc(NH 3) = 5.0 x 106 r, ( N H ~ ) = 270 r~(NH~ ) = 1200 sc(NH,~) = 2.0 x 105 s c ( N H ~ ) = 5.0 x 106 Cony. rate =4.0 x 10 -5 Cony. r a t e = 1.6 × 10 -4 Coastal meteorology Inland meteorology Source height 1 m Source height 5 m No dry deposition NH 3 N o r t h Sea Combination 1" Combination 2t

Case

611 311 812

610 347 786 372 679 610 610 610 610 717 481 572 623 585 626

Wet dep. NH3

478 421 405

475 295 618 492 473 439 498 304 499 312 683 439 505 459 493

Wet dep. NH,~

1090 733 1217

1086 642 1405 865 1153 1050 1108 915 1109 1030 1165 1012 1129 1045 1120

Wet dep. NHx

5875 7214 3989

5870 7339 4085 5682 5929 5968 5809 5720 5890 6056 5622 5333 6580 6368 5896

Tot dep. NH~

Table 1. Sensitivity study for Vredepeel. Concentrations in air in pg m 3, concentration in precipitationin p m o l / - ~ and depositions in mol h a - ' a '

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355 464 205 359 355 355 355 355 355 417 304 333 384 395 353

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447 535 321 454 447 501 411 467 444 483 428 422 481 485 449

Dry dep. NH~

* rc(NHa) =0, r c ( N H 2 ) = 270, sc(NH3)= 5 x 106, sc(NH +) = 5 x 106, conv. rate = 1.6 x 10 -4. t rc(NH3)= 120, r c ( N H 2 ) = 1200, sc(NH3)= 2 x 105, s c ( N H 2 ) = 2 x 105, cony. rate =4.0 x 10 -5.

Base rc(NH3)=0 r¢(NH3) = 120 sc(NH3) = 2.0 x 105 sc(NH3) = 5.0 x 106 rc(NH2) = 270 rc(NH+) = 1200 sc(NH2) = 2.0 x l0 s sc(NH2) = 5.0 x 106 Cony. rate =4.0 x 10 -~ Cony. rate = 1.6 x 10 -4 Coastal meteorology Inland meteorology Source height 1 m Source height 5 m No dry deposition N H 3 North Sea Combination l* Combination 2t

Case

49 14 99

42 20 61 28 44 42 42 42 42 69 24 46 39 40 44

Wet dep. NH 3

276 224 265

260 168 343 275 258 228 279 176 266 187 346 257 264 252 269

Wet dep. NH +

326 239 364

302 189 405 303 303 271 322 218 308 257 370 303 303 293 313

Wet dep. NH x

799 810 683

750 725 726 758 750 772 733 686 753 740 799 726 785 778 763

Tot dep. NH~

Table 2. Sensitivity study for Petten. Concentrations in air in/zg m - 3, concentration in precipitation in/zmol d - ~ and depositions in mol h a - ~ a - 1

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0.199 0.087 0.405

Conc. NH2

0.199 0.134 0.281 0.203 0.198 0.199 0.199 0.199 0.199 0.271 0.127 0.187 0.218 0.200 0.205

Conc. NH 3

16 12 19

15 12 19 16 15 13 17 14 15 13 18 16 15 15 16

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44 32 42

44 47 29 46 44 44 44 44 44 60 29 44 44 45 45

Dry dep. NH 3

23 37 15

22 18 26 23 22 35 13 37 21 16 29 22 23 22 23

Dry dep. NH,~

68 69 57

67 66 56 69 67 80 58 82 65 76 58 66 68 67 69

Dry dep. NH~

* rc(NHa) = 0 , rc(NH+) = 270, s c ( N H 3 ) = 5 x 106, s c ( N H 2 ) = 5 x 106, conv. rate = 1.6 x 10 -4. t rc(NH3)= 120, r c ( N H 2 ) = 1200, s c ( N H a ) = 2 x 105, s c ( N H ~ ) = 2 x 105, conv. rate = 4.0 x 10 -5.

Base rc(NH3)=0 r0(NH3) = 120 sc(NH3) = 2.0 × l0 s sc(NH3) = 5.0 x 106 rc(NH 2) = 270 r¢(NH+) = 1200 so(NH2) = 2.0 x 105 sc(NH2) = 5.0 x 106 Conv. r a t e = 4 . 0 x 10 -5 Cony. r a t e = 1.6 x 10 -4 Coastal meteorology Inland meteorology Source height 1 m Source height 5 m N o dry deposition NH 3 N o r t h Sea Combination 1" Combination 2f

Case

14 4 29

13 8 19 9 14 13 13 13 13 21 7 13 13 12 14

Wet dep. NH 3

105 87 114

101 79 123 109 100 85 111 94 98 75 129 100 102 99 104

Wet dep. NH,~

119 92 144

115 87 142 118 115 99 125 108 111 97 136 114 116 112 119

Wet dep. NH~

187 162 202

183 154 198 188 182 179 184 191 177 173 195 180 184 180 188

Tot dep. NH~

Table 3. Sensitivity study for Velen. Concentrations in air in/~g m - 3 , concentrations in precipitation in #mol f - t and depositions in mol h a - 1 a -

O

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o-

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458

WILLEMA. H. ASMANand HANSA. VANJAARSVELD

ition of NHx which is to some extent compensated for by a change in wet deposition of NH x in opposite direction. The range in the total deposition of NH x for Vredepeel is maybe largest because it is situated in a relative small area with probably the highest emission density of NH 3 of Europe. A change in parameter values will lead to a change in the ratio Import/export for this area, because all surrounding areas have a lower emission density. The study shows further that taking a different meteorology has a substantial effect on the concentrations (up to + 2 0 % difference from the base case), but taking a somewhat different source height has only a minor effect. Adopting no dry deposition of NH 3 to the North Sea leads to about 6-8% higher concentrations and depositions for a coastal station like Petten. Even concentrations and depositions at Velen are then up to 3% higher. At Vredepeel the influence is negligible. Taking extreme and therefore unlikely combinations of parameter values (combinations 1 and 2) leads to up to +_30% difference from the base case in the total deposition of NH x at Vredepeel, but only to a _+10% difference at Petten or Velen. The results of the combination runs show the following differences from the base case for concentrations and depositions which can be measured: NH 3 concentration, 30100%; NH~ aerosol concentration, 5-30%; and wet deposition of NHx, 10-30%. In general the conclusion from this sensitivity study is that the uncertainty in the model results caused by the uncertainty in the model parameters, as far as it has been investigated, will be of the order of 20%. One should, however, bear in mind that the uncertainty in the emissions is at least 30-40%. The influence of variations from month to month and from year to year in the meteorological parameters was also investigated (Asman and van Jaarsveld, 1990). In general the results show substantial monthly variations (up to a factor 3) but lesser annual variations (< 20%). 8.2. Influence of adoption of a diurnal variation in the emission rate It is also possible to compute diurnal variations in the ground-level concentrations and depositions with the model. Van den Beld and R6mer (1990) measured the diurnal variation in the NH 3 concentration during 1 year in the nature area 'Elspeetsche Veld' in The Netherlands (52°17 ' N, 5°48 ' E). For this site the diurnal variation in the NH 3 concentration was modelled with the diurnal variation in the emission rate and with a constant emission rate. Figure 5 shows that the results obtained when adopting a diurnal variation in the emission rate (see section 2) agrees very well with the measured diurnal variation in the NH3 concentration at 1 m height. The modelled diurnal variation in the NH 3 concentration when adopting a constant emission shows a pattern opposite to the measured one.

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This means that the model gives good results with the diurnal variation which was adopted, if a diurnal variation in the reaction rate is not large. Measurements at more sites are needed, however, before any general conclusions can be drawn. The concentration computed with a constant emission rate shows the highest values during nighttime. This is the result of increasing atmospheric stability during nighttime. which reduces the wind speed and thereby the dilution and the dry deposition velocity. The results when a diurnal variation in the emission rate is taken into account show a maximum around noon, showing that the effect of increased stability during nighttime is more than compensated for by the pronounced peak in the emission rate during daytime.

9. C O M P A R I S O N

OF MODEL RESULTS WITH

MEASUREMENTS

In this section model results will be compared with measurements for different parts of Europe. Because no detailed meteorological statistics are known for other parts of Europe, meteorological statistics for the 'average Netherlands' have been used in the model calculations. In general the most detailed inventory available was used to compute concentrations and depositions. Although NH~ is one of the bulk components in precipitation, it is one of the most difficult components to measure (Buijsman and Erisman, 1988), because dry deposition of N H 3 in bulk collectors, bird droppings (Asman et al., 1982), and biological activities (Ridder et al., 1984) contribute to the measured NH,~ concentrations. Therefore, some of the measured values that were used to validate the model were reduced by 25% (see Asman and van Jaarsveld, 1990, for details). In Fig. 6 model results are compared with measurements (see Table 4 for references). In general the agreement is good. Exceptions occur near the boundaries of the model area, e.g. northern Scandinavia,

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460

WILLEMA. H. ASMANand HANSA. VANJAARSVELD Table 4. Measurements used to compare with model results

Component

Country

NH 3 NHx in air

The Netherlands Belgium Sweden The Netherlands The Netherlands

NH3, NH~ vertical profiles NH~ aerosol NHx wet deposition

Europe (EMEP) The Netherlands Denmark U.K. F.R.G. Sweden Europe

Reference(s) Diederen (1984), Duyzer et al. (1989), Erisman et al. (1986, 1988) Muylle and Verduyn (1988) Ferm, pers. comm. (1990)* Erisman et al. (1988) Diederen (1984), Erisman et al. (1988), van der Meulen, pers. comm. (1989)t Schaug et al. (1987) see e.g. Buishand et al. (1988) Grundahl and Hansen (1990), Hovmand (1990) Campbell et al. (1988, 1989) FBW (1989) Ferm, pers. comm. (1990),* Granat (1989) Schaug et al. (1987)

* Swedish Environmental Research Institute, Gothenburg, Sweden. f National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands.

eastern Poland and the western U.S.S.R. This is due to the fact that concentrations and depositions at these locations partly derive from emissions outside the model area, which cannot be taken into account. For The Netherlands the calculations were made with emissions inventories on a 5 x 5 km 2 grid, 75 x 7 5 k m 2 grid (IE grid) and 1 5 0 x l 5 0 k m 2 grid (EMEP). For NH a a good agreement between model results and measurements was only obtained using the 5 x 5 km 2 inventory (Fig. 6a). Results computed with the emissions on an IE grid (Fig. 6b) show some correlation, but the concentrations are underpredicted. Therefore NH3 concentrations for individual sites should not be computed using an IE or larger grid. This is because the NH a concentration is always determined by very local emissions (Asman et al., 1989). This means that modelled NH 3 concentrations can only be compared with measured concentrations for those areas for which a very detailed NH3 emission inventory is known (e.g. on a 5 x 5 km 2 scale). The NH a measuring sites should be situated at least a few hundred metres from the nearest source to avoid a noticeable influence of nearby sources on annually averaged concentrations. Emissions on an IE or EMEP grid may well be used to calculate average values for larger areas (Asman and van Jaarsveld, 1990). For N H 2 aerosol and wet deposition of NHx, model results are far better when using a 5 x 5 km 2 grid than an IE or an EMEP grid, but even using these large grids the model performs reasonably well. No systematic differences were observed between modelled and measured wet NHx depositions for parts of the U.K. (Fig. 6i). Metcalfe et al. (1989) had found such differences, but they used a different emission inventory for the U.K. (Kruse et al., 1986) and another model approach (Derwent e t al., 1988). The model underpredicts wet deposition of NHx in Sweden (Fig. 6k), although a clear correlation exists between modelled and measured results. This underprediction is probably due to the difference between

N H 4 + NH4+ model, meas.

NH 3

Nm~d~el. meas.

1ooo

800

600

400 ~

l

~

,

200

0

~.

0

2

4

oonoentrstion

6

8

:

10

(ug/m3)

Fig 7. Modelled vs measured vertical concentration profiles of NH 3 and NH + at Cabauw (/~gm- 3).

Sweden and The Netherlands of the wind directions at which most precipitation occurs (due to the effect of the Norwegian mountains). Not onty ground-level model results were compared with measurements. Figure 7 shows that the model is able to reproduce well the vertical concentration gradients of NH3 measured at the 213 m high meteorological tower of the Royal Netherlands Meteorological Institute at Cabauw (51o55 , N, 4o55 , E). The agreement for the concentration gradient for NH~ aerosol is not as good but still reasonable considering that the number of measurements is rather limited. A sensitivity study was made for both NH3 and NH4+ vertical gradients. The conclusion could be drawn that reasonable gradients can be computed with a wide range of parameter values (Asman and van Jaarsveld, 1990).

Variable-resolution NHx transport model 10. CONCENTRATION AND D E P O S I T I O N GRADIENTS IN DENMARK

Denmark is a country with islands. No significant NH3 emission will occur from the sea areas in between the islands. This gives an excellent opportunity to demonstrate how the annually averaged NH 3 and NH~ aerosol concentration and NHx wet deposition gradients change as a function of the emission density. Model calculations were made for a west-east crosssection of Denmark using a detailed emission inventory (Asman, 1990b). Figure 8 shows the emission density gradients, and the concentration or deposition gradients. The NH3 concentration pattern is much like the NH3 emission pattern but with a reduced amplitude and shows very low concentrations over the sea areas. The NH~ aerosol concentration does not 'see' the individual islands, but does see Denmark as a whole. This reflects the fact that a large fraction of the NH~ aerosol concentration over Denmark is caused by foreign sources and that NH~ aerosol is a reaction product, i.e. it is first formed some time after the emission of NHa. For wet deposition of NHx the situation is slightly different. This component originates both from scavenging of more local NH 3 as well as more distant NH~ aerosol. Therefore some influence of the islands on the deposition pattern can be seen. The fact that the annually averaged wet NH~ deposition is relatively high compared to the NH2 aerosol concentration in the eastern part of Denmark reflects the fact that the predominant wind direction during precipitation is south-west. 11. DISCUSSION AND CONCLUSIONS

It has been shown that the TREND model is able to describe the annually averaged concentrations of NHx

461

components well, both on a local and on a regional scale. This despite the fact that the meteorological statistics used in the model are only valid for The Netherlands. The model results compare favourably with the results of other long-range transport models as the Harwell model, the EMEP model and the IE model (Derwent et al., 1989). It should be mentioned here that it is extremely important not to assume that NH 3 is mixed instantaneously over the whole mixing layer after emission, as some models do. Janssen and Asman (1988) have shown that this assumption may lead to an underprediction of the NH 3 concentration at ground level by about 65% if equilibrium between emission and deposition is assumed. The model is also able to reproduce vertical concentration profiles. Moreover, if enough information on variation in emissions and conversion rate is present also diurnal, seasonal and annual variations can be calculated. This has already been demonstrated with a similar model version for SO2 and NO~. The model is able to perform so well on a local scale because emission inventories of a different setup (gridded and non-gridded) and of different spatial resolution can be incorporated easily into the model. It has been shown that if model calculations are made using not very detailed emission inventories, e.g. on a 150 x 150 km 2 grid, deposition of NHx to sea-areas can be grossly overpredicted. Sensitivity analysis shows that the choice of the removal parameters greatly influences the ratio between NH 3 and NH~ concentrations as well as the ratio between dry and wet deposition of NHx. However, it appears that the total deposition of NHx, especially at remote sites, is not very sensitive to large variations in the removal parameters. The reason for this is simply that the model is mass-conservative: any change in dry deposition leads to a correspondingly

NH, co. . log ..... ........

7tO

NH4 conc. L,u9 m ) NH, wet aep. ( 1 0 0 tool ha -I a-t1 NH.~ emlsslon (tonne NHa km -2 o - )

.~6ul

E~_ C 0 4 -

0

x~;

k= ' - ' -

(3-3-

.... . .....

.___._

._1

"IO .'2U C: 0 u1

0 400

_ . - ; : ..... ..o ....

500

x (krn,

600

UTM z o n e

32

7oo

coordinates)

Fig. 8. Computed annually averaged emission density, concentration and deposition of NH~ components over a W-E transect in Denmark at 55033' N.

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WILLEM A. H. ASMANand HANSA. VAN JAARSVELD

reverse change in wet deposition. Therefore, this model approach is preferable to an approach in which the dry deposition is the product of measured concentrations and constant deposition velocities measured under different conditions. Although it is now possible to calculate concentrations of N H x components on local and regional scales, still much more information is needed to describe behaviour of NHx. In general more carefully performed and evaluated measurements are needed at sites with different concentration levels to calibrate models. The measurements should give information on the diurnal and seasonal variations in concentration. Measurements of N H 3 in air are needed at sites with different emission densities. But measured NH3 concentrations can only be used to calibrate models if spatially detailed emission inventories are available. Moreover, information is needed to N H 3 sources and concentrations in urban areas, although this seems to be more of academic interest as this is quantitatively not likely to be very important. For N H ~ aerosol also additional measurements are needed. If wet deposition of N H x is measured either daily samples should be taken and/or wet-only samplers should be used to avoid dry deposition of N H 3 in the samplers. Moreover, light should be excluded from the sample bottles and during storage in the laboratory. It should also be mentioned in this respect that the sites should be chosen carefully, i.e. not close to sources and likely to be representative of larger areas. Locations chosen to measure 'background' concentrations of industrial pollutants are not always suitable for this purpose. The diurnal and seasonal variations of the emission rate and conversion rate of N H 3 should be investigated for different chemical and meteorological regimes. Acknowled#ements--We are greatly indebted to Jan H. Baard and Menno P. Keuken (Netherlands Energy Research Foundation, Petten, The Netherlands), to Anneke Houdijk (Catholic University, Nijmegen, The Netherlands), to Dr Jimi G. Irwin (Warren Spring Laboratory, Stevenage, U.K.), Dr Martin Ferm (Swedish Environmental Research Institute, Gothenburg, Sweden), Dr Lennart Granat (University of Stockholm, Sweden), Dr Dirk Wintermeyer (University of Dortmund, F.R.G.), the 'Forschungsbeirat Waldsch/iden/ Luftverunreinigung der Bundesregierung und der L/inder' (F.R.G.), and to Lone Grundahl and Mads F. Hovmand (National Environmental Research Institute, Roskilde, Denmark) for allowing us to use their measurement results. Nicky Brown (National Environmental Research Institute, Roskilde, Denmark) is gratefully acknowledged for correcting the English. This investigation has been carried out on behalf and for account of the Netherlands Directorate-General of the Environment, within the framework of the project 228471: 'Acid Deposition'.

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