Emissions of Air Pollutants in the North West Region of England J.W.S

Emissions of air pollutants in the north west region of England

J.W.S. Longhurst, S.J. Lindley, D.E. Conlan,

A.F.R. Watson

Atmospheric Research and Information Centre,

Department of Environmental and Geographical

Sciences, Manchester Metropolitan University,

Chester Street, Manchester Ml 5GD, UK

ABSTRACT

An emissions estimate for SO,, NO,, CO, CO,, VOCs, black smoke and lead in the North West of England is presented for 1987 and 1991. Calculations are made on a pro-rcita basis and the value of this approach is discussed within the framework of alternative emission estimation methodologies. Estimates, their spatial characteristics and temporal trends are then presented for each species considered. Finally, the relative position of the North West is considered within the context of the United Kingdom as a whole.

INTRODUCTION

Most emission estimates are made at the national level. There is, however, a need for complimentary estimations at regional and local scales as a key component in the formulation of successful air quality management plans [1]. Studies concerning emissions of pollutants form an important pre-cursor to modelling and the understanding of ambient concentrations at the local scale as well as providing an input

to local air quality management plans [2, 3].

This work updates a previous paper which estimates emissions of SO?, NO*, HC1 and NH, from the North West region of England for the year 1987 using a pro-rota approach [1]. The aim has been to expand upon the number of species considered, develop a more refined pro-rota methodology and to provide a temporal comparison

between the initial base year of 1987 and a new base year of 1991. A comparison of the basic and refined methodologies was undertaken for 1987 using original data sources which are then updated for a new base year of 1991 for which all necessary statistical data could be accessed [4, 5]. In order to examine temporal trends a revised calculation of emissions in 1987 was made using the same data source as for the 1991 estimate. This was necessary as the methodology used to calculate national emissions is constantly revised as new information becomes available and estimates for each year

re-calculated on this basis [6].

Estimates are made for a number of key pollutants; carbon monoxide (CO), carbon dioxide (CO,), volatile organic compounds (VOCs), black smoke and lead from transportation in addition to sulphur dioxide (SO?) and oxides of nitrogen (NO,). It was

100 Pollution Control and Monitoring

not possible to include HC1 and NH, in this study due to the lack of suitable data for 1991. As new data becomes available these will be re-introduced into the emissions database.

The North West region is considered due to it being the most densely populated region of the UK [7] and also as a supplement to current research into air quality in Greater Manchester [e.g. 3, 8, 9]. For continuity, the North West region used here is that defined in Lee and Longhurst [1] comprising Lancashire, Greater Manchester, Merseyside, Cheshire and the High Peak district of Derbyshire ^Figure 1).

EMISSION ESTIMATION METHODOLOGIES

Estimates of emission can be made by taking either a 'bottom-up' or 'top-down' approach. The 'bottom-up' approach provides estimates for a particular region by utilising local datasets with appropriate emission factors. Providing that a careful choice of these factors is made then this method can provide the best representation of emissions from a specified locality. It does, however, have the disadvantage of being relatively complicated and time consuming. Alternatively, the 'top-down' approach involves dissaggregating national emission estimations to a local level through the use of indicators of the proportion of a particular polluting activity occurring in the specified region. This method is the simplest to carry out and the least expensive of time and resources but can still provide a good indication of local emission magnitudes and characteristics [1].

Lee and Longhurst [1] estimate emissions on a per capita basis pro-rota with nationally derived emissions estimates for 1987 except for the source area of power stations [4]. This and the refined method used in this study are shown schematically in Fig. 2 with data and sources shown in Table 1. National emissions estimates from power station sources are spatially disaggregated according to the contribution of North

West fossil fuelled plants to the total UK fossil fuelled electricity generation. Emissions from other sources are summed and apportioned according to the percentage of the UK's population resident in the region.

A refined method was designed in an attempt to reduce some of the uncertainties associated with the per capita pro-rota approach. Whereas the power station contribution could be estimated with some certainty, other estimates based on population as a surrogate were less certain. Therefore, in addition to isolating power stations as a source, use was also made of the other source breakdowns to provide a more sensitive representation of activities within the North West region. This was possible for air traffic, shipping, refineries and road transport which are all significant activities within the region. This is important in estimating pollutants such as CO, as its dominant source is road transport as opposed to the predominance of power station sources in the emission of species such as SO2. Again, the remaining source categories for which individual calculations could not be made were summed and apportioned according to population. The refined method allows a more reliable estimate of emissions to be made although there are still a number of uncertainties. Firstly, there is the issue of data availability. It may be difficult to find appropriate regional data eg. North west vehicle registrations are used to estimate road transport emissions when vehicle km travelled may be a better indicator Another problem arises when data is

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Major road KEY communications

A Fossil fuelled power stations in operation in 1987 and 1991

. Fossil fuelled power

stations in operation in 1987 but not 1991 *' Closed 1993 *2 Closed pre-1991 *3 Closed pre-1991

(R) Stanlow oil refinery

(A) Manchester Airport

6%j Urban counties

Figure 1: A Map to show the North West Region of England.

Detailed National Statistical Indicators from the DoE, DoT,DTI etc. NATIONAL ATMOSPHERIC EMISSIONS ESTIMATES

North West Component of Errissions Calculable I>y using Regional Statistical Indieators with a Pro-Rata / 11 Approachi 1'he Original Pro-Rata The Idefined Pro-Rata AMethod (Method 1)[1] Method (Method 2) 41 1 1 4 4 1 1 F'ower Power ~~! RO£;n Other Refinery Shipping Aircraft Other Station Station Sourc < TrafBe Sources Source Sources ss sources Sources Sources Sources

GWh GWh Tonnes of Pass- electricity \ NW electricity Tonnes of enger Vehicle 1 NW : i oil frcmNW regiona 1 from NW i produced i cargo traffic : registr- regional • bssil popula- i fossil ftomNWi from i ations in : popula- : fromNW fueled . tion fueled refinenes I docks NW the IsfW tion F)lants plants airportS

II II 1 1 1 1 T Original NW Refined NW Emissions Emissions Estimate Estimate Figure 2: A Schematic Representation of the Original and Refined Regional Emission Estimation Methodologies.

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Table 1: Statistics and sources used in the calculation of North West emissions.

National North West North West Data Source Level of Certainty Emission Statistical Share of UK Source Indicator Source Total Category (%)

1987 1991

Power GWh energy from 5.8 6.3 [11,111], [12], High Stations fossil fuelled plants. [13], [14] 1987- 12105 1991- 14467

Refineries Stanlow Oil 14.3 13.8 [13], [14] High Refinery. 1987- 13 Mt oil. 1991- 12 Mt oil. Aircraft Manchester Airport 11 11 [15] Lower, no data for passenger traffic smaller airports &

general aviation traffic Shipping Liverpool Docks 5.5 7.6 [15] Lower, no data for cargo smaller docks 1987- 25.3 Mt, 1991- 37.4 Mt.

Road Vehicle registrations 10.6 10.6 [15] Lower, no data for Transport 1991 - 2,348,000 vehicle use Others Population 11.5 11.7 [1], [16] High for population, 1987- 6,435,230 lower as surrogate 1991-6,540,600 for other activities

no longer available in the public domain eg. the privatisation of the electricity supply industry has resulted in much information becoming confidential and only available at the discretion of individual power generating companies. Secondly, there is the problem of data quality. Statistical indicators chosen have a degree of uncertainty associated with them and this may vary between data sources. The same is also true of the original national emission calculations where the level of confidence ranges from + /- 25% to + /- 50% [5]. Statistics may also change their levels of uncertainty over time [4, 5, 6]. The best regional estimates can only have the level of confidence associated with the original national estimates and in practice are likely to be much less confident.

EMISSION SOURCES AND STRENGTHS IN THE NORTH WEST.

North West emissions estimated for each species in 1987 and 1991 are given in Table 3 and discussed below. A comparison is made of the results from the original method (1) used by Lee and Longhurst [1] and the refined method (2) used by this study for 1987 and 1991. Two estimates for each method were made for North West emissions in 1987, one using the original national emissions data source and the other

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using the same source used to estimate the 1991 regional emission [4, 5]. This was necessary to allow a valid temporal comparison to be made.

Table 2: Estimates of Emissions for 1987 and 1991 using the original (Ml) and refined (M2) methodologies, in kt (except CO; in Mt of C).

Species 1987 North West 1987 North West 1991 North West Estimate (Original Estimate (Revised Estimate Data) Data) M 1 M 2 M 1 M 2 M 1 M 2

SO, 283.18 287.35 286.85 286.64 280.43 279.65 NO, 218.81 210.85 252.09 236.66 282.29 262.36

CO 608.59 568.58 694.43 646.75 784.01 718.17 CO% 57.87 57.59 15.14 14.99 15.84 15.53

VOCs 269.73 263.85 267.78 259.08 280.32 269.23 Black Smoke 60.68 59.19 59.30 57.75 56.82 54.51 Lead - 0.32 0.21

Sulphur Dioxide SOz is emitted during fossil fuel combustion and its largest national source in 1991 is power generation at 71 % [5]. The 1987 emission estimate of SOj in the North West is 283.18 kt [1]. Use of the refined method for 1987 gives a new total of 287.35 kt. There is little difference between the results for the two methodologies largely attributable to the fact that the dominant source is power generation. The results for 1991 were 280.43 kt (method 1) and 279.65 kt (method 2). A comparison of the estimates for 1987 and 1991 indicates that emissions of SOj in the North West have reduced by 7 kt (2.4%) between 1987 and 1991, indicating a slight downward trend, from 286.64 to 279.65 kt. This is less pronounced than the national reduction of 8.5% which may be due to the relative increase in the proportion of power generating activity occurring in the North West from 5.8% to 6.31% between 1987 and 1991. Spatially, power generation is concentrated in a central band across the region and due to the closure of 2 plants, production has become concentrated in a fewer number of point sources. The location of these point sources to the west of Greater Manchester may have an important influence over air quality in this densely populated area due to prevailing south-westerly winds (see Fig 1).

Nitrogen Oxides The principal sources of NO% are fossil fuel combustion, the two largest sources being transportation and power generation at 51% and 26% of UK emission totals respectively for 1991 [5]. Original emission estimates for 1987 suggest that the North West region emitted 218.81 kt (method 1) and 210.85 kt (method 2), the disparity of

8 kt being due to the original method overestimating transportation sources. Estimates for 1991 were 282.29 kt (method 1) and 262.36 kt (method 2). The difference in these results is simply a magnification of the disparity in 1987 due to the relative rise in total emissions at the national scale and a further rise in the importance of the transportation sector. There has been an increase in emissions of NO* in the North West of 25.7 kt over the period 1987 to 1991, representing a 10.9% increase compared to a national

104 Pollution Control and Monitoring

rise of only 5.5%. Spatially, the sources of NOx emissions are more disperse than those of SO; despite 20% being from power station point sources. Transportation emissions have increased in importance from 56% to 62% of North West emissions between 1987 and 1991 and these are likely to be largely associated with the regions main high-speed transportation routes, such as the M56, M63, M6 and M62 and the two large metropolitan areas of Merseyside and Greater Manchester.

Carbon Monoxide Emissions of CO are overwhelmingly from road transport sources (89%) and principally from petrol engined vehicles. Estimates of CO for 1987 using the original method were 608.59 kt and 568.58 kt using the refined method. For CO, therefore, there is a wider disparity between the results from each methodology due to the overestimation of transport emissions from the basic pro-rota method (89.31% of emissions at 11.68% instead of 10.60% in 1991). This would suggest that source characteristics of the individual species under consideration should be the primary determinant of the methodology chosen. In the case of CO, it is more important to provide a relatively more accurate statistical description of regional vehicle use with less importance attached to other source areas. For 1991, North West emissions were estimated as 784.01 kt (method 1) and 718.17 kt (method 2). CO emissions have increased by 11.0% between 1987 and 1991. Spatially, CO emission is very strongly linked to road transportation and so will be associated with major road routes and large urban areas as already discussed in relation to N0% emission. In contrast to NO%, higher emissions of CO occur at low speeds and are highest in areas of traffic congestion.

Carbon Dioxide The importance of CO; is due to its contribution to the enhanced greenhouse effect. Fossil fuel combustion is the main emitter with power generation being the main source. Results for 1987 from the two methods are again reasonably consistent, 57.87 Mt (method 1) compared to 57.59 Mt as CO; (method 2). Subsequent national emission estimates measured CO; as tonnage of C, making the new 1987 North West total 14.99 Mt (method 2) compared to a 1991 estimate of 15.84 Mt (method 1) and 15.53 Mt (method 2). Emissions of carbon dioxide in the North West have increased by 0.54 Mt from 14.99 to 15.53 million tonnes between 1987 and 1991. This 3.6% rise is greater than at the national level where emissions have remained almost static.

Volatile Organic Compounds Industrial processes and solvents account for almost half of national and regional emissions with another third from road transport. VOC estimates from the original data for 1987 attribute 269.73 kt (method 1) and 263.85 kt (method 2) to the North West region. A better estimate may be calculable through the use of a more appropriate indicator of industrial activity in the North West, for example GDP, and this could be included in future pro-rota methodologies. Estimates for 1991 are 280.32 kt (method 1) and 269.23 kt (method 2). Regional VOC emissions determined using the more recent national data indicates a 3.9% rise over the 4 years compared to a slight decrease at the national scale. The distribution of emissions is strongly related to the urban areas of the region.

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Black Smoke

Black smoke is one of the most extensively measured paniculate pollutants. Its sources are varied but with large contributions historically from combustion especially domestic sources and more recently from diesel fuelled vehicles [10]. North West black smoke emissions are calculated as 60.68 kt in 1987 (method 1) and 59.19 kt (method 2). Domestic sources contribute to over half of this nationally and regionally.

1991 estimates of black smoke in the North West are 56.82 (method 1) and 54.51 (method 2). Emissions of black smoke are in decline at both national (-5.9%) and regional (-5.6%) scales. As with SO], the reduction in this region is less pronounced than in the country as a whole. There is also a clear rise in the importance of road transportation to black smoke emission, almost a 10% rise over the four years primarily as the result of an increase in the numbers of diesel vehicles. The spatial distribution of these emissions are strongly associated with urban areas.

Lead from Transportation Lead is primarily emitted from petrol engined vehicles where it is used as an anti- knock agent [10]. Lead emission from transportation sources is most significant in areas where the road network is most dense, such as in congested urban areas. The reduction in the amount of lead now permitted in petrol coupled with the rise in consumption of unleaded petrol has caused a reduction in the emission of lead from transportation from 3 to 2 kt nationally between 1987 and 1991 and from 0.32 to 0.21 kt in the North West, a reduction of over a third of the 1987 emission total.

SUMMARY AND CONCLUSION

As it can be seen from Table 3, emissions of the traditional pollutants, smoke, SO2 and lead are continuing to reduce whereas the others are either showing no substantial change or are rising. In the majority of cases, the regional trend mirrors the national trend but the rate of change is often different. The rate at which emissions of SO] and black smoke are reducing is slower in the North West compared to the UK. Additionally the rate at which other pollutants are increasing in the North West is larger than for the country as a whole. The consequential impact is that absolute and relative air quality is likely to be poorer in the North West than in many other UK regions.

Table 3: Emissions trends in the North West region between 1987 and 1991.

Species National Change Regional Change (%) (%) SO, -8.5 - 2.4

NO, 5.5 10.9 CO 11.0 11.0

CO, 0.6 3.6 voc -0.3 3.9 Smoke -5.9 - 5.6

Lead - 33.3 - 33.3

The pro-rota methodology has given an indication of the magnitude of emissions to be expected and the rate at which these are changing over time. The basic methodology could be criticised for relying too heavily on population as an indicator but does seem to provide a similar estimate to the more refined method at least in the case of the North West region. However, this may not necessarily be the case if the methodology were to be applied to a different region. Further improvements to the refined pro-rota methodology presented here could take better account of those source categories not individually considered. For example, in future estimates, use will be made of GDP data as a surrogate for industrial activity in the region and an attempt made to improve estimates for forestry, railways and offshore oil and gas.

The emission estimation procedures presented here provide the first detailed emission totals for a region of the UK. While each of the techniques has limitations, they are relatively easy to carry out once the original data sources have been identified and the necessary statistics obtained. Consequently, they have a role to play in the provision of cost-effective atmospheric emissions estimates to feed into air quality management plans. Such plans will allow a greater role to be played by local decision- makers in finding a solution to the air quality problems of their region.

REFERENCES

1. Lee, D.S. and Longhurst, J.W.S. 'Estimates of Emission of SCK NO%, HC1 and NHg From a densely populated region of the UK' Environmental Pollution 79 pp. 37-44. 1993. 2. Loibl, W., Orthofer, R. and Winiwater, W. 'Spatially Disaggregated Emissions Inventory for Anthropogenic NMVOCs in Austria' Atmospheric Environment 16 pp. 2575-2790. 3. Longhurst, J.W.S, Lindley, S.J. and Conlan, D.E. Towards Local Air Quality Management in the UK' in Air Pollution 2 (Ed. Brebbia, C.A., Baldasano, J.H. and Zanetti, P.) Accepted, Proceedings of the 2nd Int. Conf. Air Pollution 94 Barcelona, Spain. Computational Mechanics Publications, Southampton, 1994. 4. Department of the Environment Digest of Environmental Protection and Water Statistics No. 11 HMSO, London, 1988. 5. Department of the Environment Digest of Environmental Protection and Water Statistics

No. 15 HMSO, London, 1992. 6. Eggleston, H.S. and Mclnnes, G. Methods for the Compilation of UK Air Pollution Inventories Warren Spring Laboratory Report LR634 (AP) M 1987. 7. Central Statistics Office Regional Trends HMSO, London, 1993. 8. Conlan, D.E., Raper, A.W., Lindley, S.J., and Longhurst J.W.S. Temporal and Spatial Variability of NO? in Greater Manchester' in Air Pollution 2 (Eds. Brebbia, C.A., Baldasona, J.H. and Zanetti, P.) Accepted. Proceedings of the 2nd Int. Conf. Air Pollution 94 Barcelona, Spain, 1994. 9. Conlan, D.E. and Longhurst, J.W.S. 'Spatial Variability in Urban Acid Deposition, 1990. Results from the Greater Manchester Acid Deposition Survey (GMADS) Network in the UK' Science of the Total Environment 128 pp. 101-120 1990. 10. Quality of Urban Air Review Group The Quality of Urban Air in the UK HMSO, London, 1993. 11. PowerGen Annual Report 1993 Solihull, 1993. 12. National Power Personal Communication Harrogate,1994. 13. Department of Energy Digest of Energy Statistics HMSO, London, 1988. 14. Department of Trade and Industry Digest of Energy Statistics HMSO, London, 1993.

15. Department of Transport Digest of Transport Statistics HMSO, London, 1993. 16. Rusbridge, B.J. Municipal Yearbook 1993 Municipal Journal, London, 1993.