Appendix II. Strategies to reduce deposition of nitrogen from agricultural sources on sensitive ecosystems: spatial considerations

MODULE 8 Interface with parallel investigation by DETR and scenario analysis H M ApSimon, T Loh, T Oxley, A Grossinho Department of Environmental Science and Technology, Imperial College London

Introduction Concerns about the contribution of nitrogen deposition to ecosystem change, through processes of acidification and eutrophication, have led to international action to control emissions. In this context, European emissions of reduced nitrogen in the form of ammonia (NH3) are comparable with those of oxidised nitrogen (NOx) from stationary and mobile sources combined. A large proportion of the reduced nitrogen comes from agricultural sources particularly livestock excreta and manures. Under the Convention on Long-Range Transboundary Air Pollution of the UN Economic Commission for Europe (the Gothenburg protocol), and subsequent EC directives, national emission ceilings for each member country have been set on pollutants contributing to transboundary acidifcation, eutrophication and formation of excess ozone. These ceilings are to be attained by 2010, and include NH3 emissions, where for the UK the ceiling set is 297 kilotonnes per year of ammonia as NH3 (equivalent to 244 kt of nitrogen in reduced form). This represents a 19% reduction relative to NH3 emissions in 1990, compared with much larger reductions of 59% and 85% re NOx spectively for NOx and SO2. It is however much harder to reduce NH3 emissions than SO2 and NOx where very efficient “end-of-pipe” control technologies are available. The subject of this Appendix is to propose alternative ways of controlling the deposition of ammoniacal nitrogen in areas that are sensitive to N deposition, which can complement direct control of emissions. This involves spatial separation of emissions and sensitive areas “on the ground”, hence diluting the concentrations and deposition where damage can occur. This is analogous to the effect of tall stacks in reducing ground level concentrations from major combustion sources to complement measures to control the quantities emitted. These ideas will be illustrated in the context of the UK, but should also be applicable in other countries. Thus the current analysis of the UK situation will be followed by case studies of a more generic nature involving different ecosystem configurations relative to area and point sources of NH3. Simple modelling studies will illustrate how low emission strips round such ecosystems can limit the nitrogen deposition upon them. Finally the value of combining such measures to separate source areas and sensitive ecosystems with direct emission abatement measures will be explored, and suggestions made for further investigation.

Current analysis of the situation in the UK The UK, in parallel with other countries, is developing strategies for attainment of these emissions ceilings. The far shorter range of NH3 emissions and greater local deposition of reduced nitrogen as compared with oxidised nitrogen means that for protection of UK ecosystems control of reduced nitrogen deposition is particularly important. Thus based on 1996-1997 UK emissions of N in oxidised form being 615 kt, compared with 178 kt deposited in the UK, much of this originating from outside the UK. By comparison, emission of reduced N as NH3 was 282 kt, of which 236 kt was from agriculture; with 209kt of reduced N deposited, most of which originated in

1 the UK (NEGTAP, 2001). Moreover, NH3 emissions are widespread and patchy, and can occur at high densities in rural areas close to the sensitive natural ecosystems at risk. The aim is therefore to develop strategies that are cost-effective in controlling emissions in order to achieve the ceilings, but at the same time maximise protection of UK ecosystems. The criteria for protection are based on the definition of critical loads as the maximum annual levels of deposition sustainable in the long term without adverse effects. A great deal of effort has been put into quantifying and mapping these critical loads for different types of ecosystem. For acidification this involves a combination of sulphur and nitrogen deposition, whereas, for eutrophication, it is only the nutrient nitrogen deposition from both NOx and NH3 emissions combined that is of importance. Detailed critical load maps have been compiled for the UK on a 1 x 1 km grid by CEH Monkswood (Hall et al. 2003). Comparing maps of deposition with the critical load data indicates where there is exceedance, and hence where ecosystems may be at risk. Figure 1a illustrates the situation for 1996 emissions with respect to effects of eutrophication in the UK. Modelling studies are now being used to investigate potential strategies to reduce such exceedance, and improve ecosystem protection, that are also cost effective. These include integrated assessment modelling to bring together data on emissions, atmospheric transport, critical load data, and the potential measures to reduce emissions with their associated costs. Thus, the UKIAM model is being developed at a UK scale (distinguishing emissions from different counties and using a 5 x 5 km grid for concentrations and deposition), in an analogous way to the ASAM and RAINS models were used during development of the Gothenburg protocol to investigate cost-effective strategies to reduce transboundary air pollution on a European scale. The emissions from agriculture have been compiled by a working group on emissions (Misselbrook et al. 2000, Dragositis 1998). These account for 84% of the ammonia emissions in the UK, with the remaining non-agricultural emissions arising from such sources as horses (classified as non-agricultural animals), vehicle exhausts, fertiliser production and other industrial processes, landfill and sewage sludge disposal, and mapped accordingly. Modelling of atmospheric transport is based on the FRAME model of CEH Edinburgh (Singles et al. 1998), which simulates the transport of columns of air across the UK, picking up emissions of NH3 and other pollutants, and simulating the processes of vertical diffusion and chemical transformation to ammonium particulate matter, and dry and wet deposition. This has been used to derive source-receptor matrices for use in the UKIAM model for rapid assessment of different emission reduction measures, and selection of abatement strategies that reduce ecosystem exceedance of critical loads as much as possible with increasing levels of expenditure. Typically such analysis picks out measures which make a large improvement at low cost, and measures which do little to improve the situation or are of very high cost. For NH3 the potential abatement measures involve reduction of emissions from livestock excreta and manures from animal housing, subsequent storage, and eventual spreading. Avoiding use of urea as a fertiliser can also be effective. On the European scale these were investigated in the MARACCAS project (Cowell and ApSimon 1998), using estimates of the efficiency, applicability depending on agricultural practices and conditions, and the costs of each measure. Cost curves were compiled for each country summarising the emission reductions possible in order of increasing cost per ton abated. In the UK, the NARSES project is investigating potential reductions in a similar manner but addressing the spatial variability, using

2 GIS to assemble detailed data relating to emissions and agricultural conditions in order to assess the geographical variation in the applicability of measures and their capacity to reduce emissions and associated costs. This is just the information required on NH3 by UKIAM to consider combined strategies for abatement of ammonia, oxides of nitrogen, and sulphur dioxide in the UK Unfortunately, initial assessments have indicated that approximate simulation of the maximum feasible reduction of UK NH3 emissions (a total reduction of the order of 15 to 20%) still leaves a large proportion of UK ecosystems with exceedance of the critical loads (see Figure 1b). This exceedance is not removed by simultaneous scaling of NOx emissions to achieve the Gothenburg protocol ceiling. However, such assessment is based on averaging emissions and deposition spatially within each 5 x 5 km grid cell. In practice, both will be very patchy and non-uniform. Hence this modelling approach may either significantly overestimate or underestimate exceedance of critical loads. This problem of uncertainty due to spatial resolution is addressed in the sections below. In order to investigate deposition patterns over shorter distances, we have used the LADD model developed jointly by Imperial College and CEH Edinburgh. Like the FRAME model this is a Lagrangian model assuming columns of air advected across the area of interest, picking up emissions on the way and simulating vertical diffusion, chemical transformation and deposition processes (Hill 1998). It has been applied successfully at a farm scale in a landscape with major poultry units, and used to illustrate the spatial variability in deposition in typical situations (e.g. Dragositis et al. 2002)

Case studies of a receptor-oriented approach We shall now illustrate a simple source-apportionment approach for deposition of nitrogen and consider this relative to a typical critical load for protection from eutrophication. This will illustrate the importance of the local emissions within the same grid square as the ecosystem. We can then consider different simplified situations to show how deposition on an ecosystem varies between different situations. Overall the results suggest that buffer zones between emissions and sensitive ecosystems could help to reduce exceedance and hence decrease the proportion at risk. Critical loads for eutrophication range from around 5 to 35 kg N ha-1 yr-1. For example, changes in ground flora and mycorrihzae have been observed in acidic coniferous forests with deposition between 7 and 20 kg N ha-1 yr-1. Similarly, for lowland dry heaths, transition from heather to coarse grass can occur at deposition rates between 15 and 20 kg N ha-1 yr-1 (e,g, NEGTAP report 2001, chapter 7)1. For illustrative purposes therefore, we shall consider an idealised ecosystem with a critical load of 15 (+/- 50%) kg N ha-1 yr-1. Now let us consider the deposition averaged over a 5 x 5 km grid square, and the relative contributions from different surrounding areas. Thus, imagine a 5 x 5 km grid square embedded in a larger 25 x 25 km grid square with background concentrations imported into this larger square from the rest of the UK and overseas. Let us simplify the wind-rose for the moment and assume the wind strength and direction are isotropic. The concentrations of reduced N arriving from beyond the larger grid square are likely to be of the order of 1 g m-3. Assuming typical rates of deposition, this would typically yield an average of about 3 kg of N per hectare per

1 Note that some critical loads have been revised downwards following critical review in the UN ECE programme under the Convention on Long-Range Transboundary Air Polution.

3 year to sensitive ecosystems such as heathland. Even if this was twice as large, and there was some additional wet deposition of ammonium aerosol, this imported contribution would still be much less than the critical load, implying that there needs to be a significant contribution from more local emissions to cause exceedance. Now let us consider the contribution of the 25 x 25 km grid square to the 5 x 5 km grid square embedded in it. Suppose we assume a rather intense average emissions density for such an area, based on inspection of the mapped emissions over the UK, of 25 kg of reduced N ha-1 yr-1. With such an emission density distributed uniformly over the 25 x 25 km square outside the 5 x 5 km smaller grid square at the centre, the LADD model indicates a deposition rate of 4.5 kg N ha-1 yr-1 at the centre. However, if we now assume the same emission density within the central 5 x 5 km grid square, the corresponding contribution to deposition at the centre is 13.1 kg N ha- 1 yr-1. This contribution is almost twice as large as the combined contribution from the surrounding large square and longer range, and on its own is comparable with the critical load (see Table).

Source area N deposition at centre (kg N ha-1 yr-1) Transboundary and national-> Concentration ~1g m-3 on boundary 3 of 25 x 25 km area Emission of 25 kg of N ha-1 yr-1 over 25 x 25 km area outside 5 x 5 km 4.5 square in centre Emission of 25 kg of N ha-1 yr-1over 5 x 5 km area in centre 13.1

Different configurations within the central 5 x 5 km grid square The analysis illustrates the dominant contribution of local emissions within a 5 x 5 km area, containing a sensitive ecosystem or SSSI, to the deposition in situations leading to exceedance of critical loads (in this case about two thirds of the total reduced N deposition, but in other cases it could be an even greater proportion). Without such a major local contribution, it is difficult to see how such exceedance would occur except in a few particularly sensitive ecosystems with lesser tolerance. It is therefore important to consider carefully how such local deposition is assessed, illustrated below by different case studies for the local 5 x 5 km grid square. We have already referred to the very uneven distribution that can occur within such a grid square down to the field scale, as illustrated in Dragositis et al. (1998). In the analysis above it has been assumed that the emissions are homogeneously mixed and spread across the whole grid square. This is as though the ecosystem area is embedded as tiny pockets with the agricultural emissions distributed round them uniformly right up to their edge. However this may be far from the true situation, and there are several different possible situations. For example, from an ecosystem perpsective we can have i) a very small ecosystem area as a small patch within an agricultural area, considered below as if located as a small area of order 100 x 100 m at the centre of the square with area emissions surrounding it - case A

4 ii) alternatively the ecosystem area may be large, bordering on an agricultural area. This can be illustrated by considering a strip along the side of the square downwind of emissions over the rest of the square - case B iii) the opposite of case A is a small area with intensive agriculture or major poultry or pig units embedded within a sensitive ecosystem area - case C We will now consider these different possibilities, and variations upon them.

Case A - small ecosystem enclosed in agricultural area

If calculated as in Table 1, assuming a uniform emission density of 25 kg N ha-1 yr-1 over the whole 5 x 5 km grid area, and a small ecosystem of the order of 100 x 100 m at the centre, then the estimated deposition to this ecosystem using the LADD model is 13.1 kg N ha-1 yr-1. (NB an isotropic wind rose has been assumed for simplicity). Now let us consider the effect of eliminating emissions from a small strip, just 50 m wide surrounding the ecosystem. This reduces the deposition on the ecosystem area by 17% to 10.9 kg of N per year, which is of the same order as the maximum feasible reduction in UK emissions using all available measures [not the same as feasible measures. A further calculation was made in which emissions were set to zero in a broader strip 150 m wide surrounding the ecosystem. This makes a greater reduction of 34% in the deposition on the central ecosystem, to 8.6 kg of N per hectare per year. This is a substantial reduction. By contrast, consider an ambitious level of 30% reduction in emissions by applying abatement measures to reduce emission from land spreading etc. This would reduce the deposition on the central ecosystems correspondingly by 30% to 9.2kg N ha-1 yr-1. Note that, in many areas, depending on the farming activities and applicability of measures, overall emission reductions of this magnitude would not be feasible. An alternative scenario is to reduce emissions locally by this amount in a region round the ecosystem. For example, if emissions were reduced by 30% within 500 m of the ecosystem area, then this gives a comparable reduction of 17% to having zero emissions in a narrower strip just 50 m wide. This might be achieved by land- use patterns with, for example, grazing or lesser emitting areas close to the sensitive ecosystem areas, or by limiting the amounts of manure that can be applied to arable land in such areas.

Case B - large ecosystem area bordering on square By contrast, let us consider a larger ecosystem area adjacent to farming land, which can be illustrated by assuming that the ecosystem area overlaps down one side of the 5 x 5 km square in a band 500 m wide. We can assume the same emission density of 25 kg N ha-1 yr-1, as in the previous case, over the 90% of the gird area that is agricultural. If the prevailing wind is more from the direction of the agricultural area towards the ecosystem area, then deposition will be greater than that calculated using an isotropic wind rose. So calculations for this situation have been calculated both for an isotropic wind-rose and with a more typical wind-rose for the UK with more frequent winds from the south-west. As the base case, the maximum deposition on the ecosystem area assuming that the agricultural emissions are uniform over the rest of the grid-square right up to the edge of the ecosystem, was calculated to be 7.1 kg N ha-1yr-1 for the isotropic wind rose, and 9.3 kg N ha-1 yr-1 for the wind-rose more typical of UK conditions. If now, emissions are removed from a 50 m wide strip along the border of the ecosystem, then

5 the deposition is reduced to 5.6 and 7.5 kg of N ha-1 yr-1 respectively- or by approximately 20% for both. As in the previous Case A, we can compare with an ambitious reduction in emission by 30% over the whole square by applying abatement measures, giving a corresponding 30% reduction in the resulting deposition to 5.0 and 6.5 kg of N ha-1 yr-1. If such a 30% reduction is limited to a band 500 m wide along the perimeter of the ecosystem area, then this yields a corresponding reduction of around 19% to 5.7 and 7.6 kg of N ha-1 yr-1 for the isotropic and anisotropic wind- roses respectively.

Case C - Farm area enclosed within a large ecosystem area We can now consider the case where some intensive agricultural activities are embedded in a large sensitive ecosystem area, which will be illustrated by a farm area 1 x 1 km in extent at the centre of a 5 x 5 km grid, of which the remainder is heathland or equivalent natural ecosystem area. It will be assumed that the agricultural area is intensively farmed with, for example, large amounts of manure/slurry applied, giving en emission density of 50 kg of N ha-1 yr-1. This is twice that of the previous cases but over a much smaller area. If we again start by assuming that the emissions are uniform over the whole area surrounding the farm land, then the greatest deposition over the surrounding ecosystem area again differs in magnitude and location when an assumed isotropic wind rose is replaced by the wind-rose more characteristic of the UK. The LADD model yields values of 8.4 and 11.2 kg of N ha-1 yr-1 respectively for these two situations. If now, emissions are set to zero along a 50 m wide strip round the edge of the farm area, that is next to the ecosystem boundary, then there is a substantial reduction of 37% assuming the wind is isotropic, and 34% using the UK wind rose, giving reduced deposition values of 5.3 and 7.5 kg of N ha-1 yr-1. Part of the reduction is due to a change in the total emissions of 18% because, in this situation, the area over which the emissions are cut is a much larger proportion of the total emitting area. By contrast, if emissions were reduced uniformly by the larger amount of 30%, the reduction in emission would still be slightly less than for the perimeter strip example – that is deposition values of 5.9 and 7.9 kg of N ha-1 yr-1. However, an alternative situation is that of a major very concentrated source located somewhere within the pocket of farmland. Thus, suppose that instead of a dispersed source, there is a poultry unit generating around 6.3 tons of ammoniacal-N per year embedded in a 1 x 1 km farm area generating more moderate area emissions of 25 kg of N ha-1 yr-1, as in cases A and B. If the total resulting emission of 8.8 tons was treated as if distributed uniformly over the 1 x 1 km farm area, the calculated maximum deposition at the edge of the ecosystem becomes 14.8 and 19.7 kg N ha-1 yr- 1, with the uniform and more typical UK wind-rose respectively. By contrast, consider the situation if 6.3 tons of the emission is treated as a concentrated source with dimensions more typical of a large poultry unit, but located 100 m from the north and east farmland boundaries. This gives much greater values for localised deposition at the edge of the ecosystem of 18.3 and 28.1 kg of N.ha-1 yr-1 for the 2 wind rose assumptions. Moving the poultry unit further away, so that it is 300 m from the north and east boundaries, more than halves this deposition to 9.0 and 13.3 kg of N ha -1 yr-1 respectively. Finally, putting the poultry unit at the centre of the farmland, 500 m from each boundary, gives the very much smaller values of 6.0 and 7.7 kg of N ha-1 yr-1. This illustrates how critically the occurrence of deposition in excess of critical loads within the ecosystem area will be very dependent on the separation distance

6 from major pig/poultry unit sources. In the situations considered here, the deposition could be more or less than the values calculated if the emission was evenly spread over the farmland area. But the implication here is that deposition and exceedance decrease sharply with relocation of a concentrated source by a few hundred metres.

Table 2: Deposition to ecosystem (maximum) due to emissions in 5 x 5 km grid square Isotropic UK wind- wind rose Case A: small ecosystem at centre of agricultural area i) Uniform emission density 25 kg N ha-1 yr-1 over grid 13.1 kg N ha-1 yr- % figures ii) zero emission within 50 m of ecosystem 1 in italics iii) zero emission within 150 m 10.9 (17%) are iv) 30% reduction in emissions 8.6 (34%) percentage v) emissions 30% less within 500 m 9.2 (30%) reductions 10.9 (17%) Case B: large ecosystem bordering on agricultural area i) uniform emission density 25 kg N/ha/y except over 7.1 9.3 500 m wide band down side of square kg N/ha/y ii) zero emissions in strip within 50 metres 5.6 (21%) 7.5 (19%) iii) uniform 30% reduction in emissions 5.0 (30%) 6.5 (30%) iv) 30% reduction within 500 metres 5.7 (20%) 7.6 (18%) Case C: farm area enclosed in large ecosystem area i) uniform emission 50 kg N ha-1 yr-1 over 1 x 1 km farm 8.4 11.2 ii) zero emissions 50 m strip round perimeter 5.3 (37%) 7.5 (34) iii) 30% reduction across area 5.9 (30%) 7.9 (30%) iv) uniform. emission = 25 kg N ha-1 yr-1 +6.3 tons 14.8 19.7 spread out over 1 x 1 km farm area v) move source 6.3 t yr-1 to 100 m from N and E edge 18.3 28.1 vi) move source 6.3 t yr-1 to 300 m from N and E edges 9.0 13.3 vi) move source 6.3 t yr-1 to 500 m from N and E edges 6.0 7.7 i.e. to centre of 1 x 1 km farm area

Discussion For ease of comparison, the illustrative cases assumed above are summarised in Table 2. The general conclusion is that avoiding or reducing emissions close to the perimeter of sensitive ecosystem areas can effectively reduce the deposition of reduced N on them by as much as the application of stringent abatement measures to reduce emissions over the whole of the local 5 x 5 km grid square. A combination of both spatial separation and abatement measures could yield an even larger reduction. The maps below give a very crude indication of how this might affect exceedance. Figure 1a shows a map of the exceedance of critical loads for eutrophication due to the combined nitrogen deposition from NOx and NH3 in 1996. Figure 1b gives the corresponding map assuming a uniform 20% reduction in NH3 emissions across the country. This is somewhat unrealistic, in that it would be easier

7 to reduce NH3 emissions in some areas than others, but is close to the reduction required to attain the national emission ceiling, and could be achieved by some combination of abatement measures and a small reduction in animal numbers. It can be seen however that this still implies a large proportion of the original areas of exceedance. Parallel NOx reductions would help a little, but do not change this picture significantly, because of the longer range transport and export of UK emissions. However, as discussed above these maps are based on smoothing out emissions and deposition over 5 x 5 km grid cells. We have shown that spatial separation of emissions and sensitive ecosystems can easily introduce an effective further reduction in reduced N deposition on the latter by a further 20%. Figure 1c therefore considers a combined reduction of 40% overall, due to both spatial separation and applying abatement techniques. It is clear that this makes a much bigger improvement in reducing areas of exceedance.

Conclusions and further work The above analysis suggests that avoiding emissions close to sensitive ecosystems, even along a narrow band of width 50 m along the perimeter, can improve the protection of ecosystems and help to reduce exceedance and associated effects. Simple cases have been used to illustrate situations for small and large ecosystem areas. These are somewhat idealise but indicate the deviation from traditional modelling assumptions. The potential for exploiting reduction of deposition to sensitive ecosystem areas by restricting emissions close to their perimeters should be investigated further. In modelling deposition and exceedance, the effect of alternative assumptions about contributions from within the local grid square also needs to be addressed.

Acknowledgements This work has been funded by the UK Department of Environment, Food and Rural Affairs. We should also like to thank CEH Edinburgh for provision of data from the FRAME model, and CEH Monkswood for the compilation and supply of critical load maps and assistance in calculation of exceedance.

References:

Cowell DA, ApSimon HM (1998) Cost effective strategies for the abatement of ammonia emissions from agriculture. Atmos. Env. 32, 573-580.

Dragositis U, Sutton MA, Place CJ, Bayley A (1998) Modelling the spatial distribution of ammonia emissions in the UK. Environ. Polln 102 S1, 195-203.

Dragositis U, Hill J, ApSimon HM, Sutton MA (2002) Ammonia emissions, deposition and impact assessment at the field scale. Environ. Polln 117(1), 147-158.

Hall J et al. (2003) Status of UK critical loads- critical loads methods and map. UK National Focal Centre for Critical Loads http://critloads.ceh.ac.uk

Hill J (1998) Application of computational modelling to ammonia dispersion from agriculture PhD Thesis, University of London.

8 Missellbrook et al (2000) Ammonia emission factors for UK agriculture. Atmos. Env. 34, 871-880.

NEGTAP(2001) Report of National Expert Group on Transboundary Air Pollution. DEFRA.

Singles R, Sutton MA, Weston K (1998) A multi-layer model to describe the atmospheric transport and deposition of ammonia in Great Britain. Atmos. Env. 32, 393-399.

9 Exceedance due to NH3 and NOx emissions (1996).

Exceedance keq/yr 0 - 25 25 - 200 200 - 500 500 - 1000 1000 - 6000

0 100 200 Kilometers

Figure 1a

10 Exceedance after 20% reduction in NH3 emissions

Exceedance keq/yr 0 - 25 25 - 200 200 - 500 500 - 1000 1000 - 6000

0 100 200 Kilometers

Figure 1b

11 Exceedance after 40% reduction in NH3 emissions

Exceedance keq/yr 0 - 25 25 - 200 200 - 500 500 - 1000 1000 - 6000

0 100 200 Kilometers

Figure 1c

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