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National Atmospheric Emissions Inventory

5.2 NOx Emission estimates

Since 1970 there has been a small reduction in emissions of around 13%, however the variation in emissions has not been smooth (Figure 5.1). Up to 1984 the NOx emission profile was relatively flat with small peaks in 1973 and 1979, as seen previously for CO2, which were due largely to the cold winters in those years. However, from 1984, emissions rose markedly as a result of the growth in road traffic reaching a peak in 1989 (Table 5.2). Since 1989, total emissions have declined by 26% as a result of a 42% reduction from power stations and 27% decrease from road transport.

Table 5.2 UK Emissions of Nitrogen Oxides by UNECE1 Source Category and Fuel (kt)

  1970 1975 1980 1985 1990 1993 1994 1995 1996 1996%
By UNECE Category
Comb. in Energy Prod & Transf.
    Public Power 714 702 747 659 781 577 524 494 449 22%
    Petroleum Refining Plants 40 40 45 38 40 45 45 47 47 2%
    Other Comb. & Trans. 62 43 45 67 64 67 78 54 60 3%
Comb. in Comm/Inst/Resid/Agri
    Domestic 61 58 64 68 64 72 68 66 75 4%
    Other 57 42 46 44 37 38 38 38 39 2%
Combustion in Industry 357 299 244 188 188 174 177 171 169 8%
Production Processes 17 16 15 17 13 9 8 5 5 0%
Extr./Distrib. of Fossil Fuels 0 0 0 1 1 0 1 1 1 0%
Solvent Use 0 0 0 0 0 0 0 0 0 0%
Road Transport
    DERV 364 385 427 445 536 494 484 448 423 21%
    Petrol 401 477 532 583 747 670 625 582 543 26%
Other Transp & Mach.
    Off-Road 124 120 105 98 87 88 86 81 86 4%
    Military 48 48 48 49 54 44 40 37 35 2%
    Railways 35 30 28 25 21 21 21 21 20 1%
    Shipping 76 84 81 81 88 88 81 78 85 4%
    Civil Aircraft 5 8 10 11 12 15 15 16 17 1%
Waste Treatment & Disp. 5 5 11 9 8 7 7 8 8 0%
Agricult/Forest/Land Use Change 7 7 12 14 9 0 0 0 0 0%
Nature 0 0 0 0 0 0 0 0 0 0%
By Fuel
Solid 817 696 781 656 801 564 515 484 428 21%
Petroleum 1425 1488 1467 1499 1707 1575 1501 1397 1339 65%
Gas 105 154 174 203 204 240 251 234 259 13%
Non-fuel 27 27 39 40 40 29 30 31 33 2%
Total 2375 2365 2460 2398 2752 2409 2297 2145 2060 100%

1 UK emissions reported in IPCC format (Salway, 1998) differ slightly due to the different source categories used.
2 Railways, civil aircraft, shipping, naval vessels and military aircraft.

The spatially disaggregated emission inventory for the UK based on a 1km2 grid is shown in Figure 5.2 images and data files are also available from the NAEI's internet site http://www.environment.detr.gov.uk/airq/aqinfo.htm. A large fraction (~29%) of the total emission is concentrated in a few grid squares containing the point sources. For NOx road transport dominates with 33% of the UK NOx emission (70% of the total UK road transport) deriving from major road links. Vehicles travelling at high speeds contribute most. As a result the major route-ways (e.g. Motorways and primary routes) are clearly defined on the map. Conurbation's and city centres show high emissions resulting from large volumes of road transport, residential and commercial combustion. A combination of high national shipping emission and relatively few large ports result in significant localised emissions from shipping in port areas.

Figure 5.1 Emission Profile NOx Emissions


Figure 5.2 Mapped NOx Emissions


5.2.1 Transport

The major source of NOx emissions in the UK is the transport sector with road vehicles and off-road vehicles contributing 48 % and 4%, respectively, to the total emission. Road emissions rose steadily between 1970 and 1989 reflecting the overall growth in road traffic in the UK. During this period emissions from petrol vehicles, predominantly cars, rose by 87% compared to the 1970 level and emissions from diesel vehicles rose by 56%. Figure 5.3 clearly shows the growth in the vehicle fleet during this period. Since 1989 there has been a steady decline in emissions due to the introduction of catalytic converters on cars and stricter regulations on truck emissions.

Figure 5.3 Emissions of NOx from Road Transport by Vehicle Type

Figure 5.4 shows the average NOx emissions per vehicle kilometre for different vehicle types. Various emission regulations on new petrol cars which have come into effect in stages since 1976 have led to the gradual reduction in emission rates from petrol cars; the more rapid decline in emissions from 1992 is due to the penetration of cars fitted with three-way catalysts. Limits on emissions from diesel cars and LGVs did not first come into effect until 1993/94. Overall emissions per kilometre from HGVs showed a small rise from 1970-1987 due to the increasing usage of larger HGVs for freight movement. Limits on emissions from HGVs first came into effect in 1988 leading to a gradual reduction in emission rates as new HGVs penetrated the fleet, accelerated by tighter limits on emissions from new HGVs in 1993/94.

Figures 5.4 NOx Emissions per Vehicle km by Vehicle Type

Figure 5.5 shows emissions per passenger km and by tonne km of freight. Technological improvements to HGVs have almost halved the emissions per tonne of freight moved in 1996 compared with 1970. Emissions per passenger km from cars, vans and taxies have reduced by over 30% since 1970 due mainly to the introduction of the catalyst in 1992 now penetrating the car fleet. Per passenger km emissions from buses and coaches rose by almost 70% from 1970 to 1993. This was due to the gradual decrease in occupancy rate of buses and their under utilisation over this period. Since 1993, this rise in per passenger km emissions has been halted by the penetration of buses meeting tighter emission standards into the fleet.

Figure 5.5 Emissions of NOx by Mode for Passenger and Freight Transport

Other transport and machinery contribute a further 12% to total UK NOx emissions. Of these only those from civil aircraft have grown steadily over the period 1970 to 1996; however, they only comprise around 1% of the total emission.

5.2.2 Power Generation

Emissions from power stations have declined over the period 1970-1996 by around 37%. Emissions in the seventies were fairly flat with peaks in severe winters. Since 1979 emissions have declined with a minimum at the time of the miners strike. Prior to 1989 this decline was due to the increased use of nuclear power and an increase in the average efficiency of the thermal power stations. Since 1988 the electricity generators have adopted a programme of progressively fitting low NOx burners to their 500 MWe coal fired units. More recently the increased use of nuclear generation and the introduction of CCGT plant burning natural gas (See Section 2.2.2) have further reduced NOx emissions. The emissions from the low NOx turbines used are much lower than those of pulverised coal fired plant even when low NOx burners are fitted. Given that these trends continue, power station emissions are expected to fall further.

5.2.3 Industry

The emissions from industrial combustion have declined by 53% since 1970 and they currently contribute 8% to total UK emissions. This is due to the decline in coal use in favour of gas and electricity.