1.1.1 Transport can also have air pollution impacts over larger areas than those that are described in this guidance as being local. Most notably, these impacts are acidification, excess nitrogen deposition and generation of tropospheric ozone (Box 1). These are regional issues, affecting areas with distance scales of some 10s to 100s of km. For these effects, the impacts may be felt by humans or ecosystems at considerable distances from the source of the emissions. Partly as a consequence of this, it is difficult to quantify impacts associated with emissions from particular sources. The only sensible approach is to assess the change in emissions associated with alternative transport schemes.

Non-Local Air Quality Effects

Some pollutants are associated with environmental impacts on a regional scale. Emissions of NO_x and VOCs are responsible for disturbing the photochemical equilibrium of the atmosphere and creating excess tropospheric ozone in summer. Ozone is an important pollutant in the context of human health, damage to agriculture and some materials. Emissions in the UK can affect other European states and vice versa. Emissions of NO_x, SO₂ and NH₃ are associated with regional acid deposition, which can have consequences for vulnerable ecosystems in certain parts of the UK and Europe. They can also lead to the formation of secondary aerosol particles, important in the context of human health.

Persistent pollutants (ie are not transformed chemically) can have impacts on a global scale if they remain airborne as they are then effectively mixed throughout the atmosphere. Industrial releases of organic compounds such as dioxins and PCBs are examples of such pollutants. In practice, pollutants of this kind are not strongly associated with transport.

The UK has international commitments to reduce national emissions of NO_x, S, VOCs, NH₃, heavy metals and persistent organic pollutants through UNECE protocols. The European Union's Auto-Oil programme of research and legislation is very influential in defining emission limits for road transport and is driven by the need to reduce emissions on a European scale.

1.1.2 A consideration of total emissions is intrinsically useful in the context of regional air pollution. The UK is a signatory to six UNECE protocols outlined in Box 1. These require the UK to reduce emissions of these pollutants in line with specific national targets.

1.1.3 For the purposes of appraisal, NO_x is taken to be an indicator of the potential for regional air pollution impacts arising from options as this regional pollutant is the most strongly linked with transport.

1.2 Approach to be Adopted for Studies

1.2.1 The calculation of total emissions should be undertaken in the same way as described in The Local Air Quality Sub-objective (TAG Unit 3.3.3), methodology for strategies. The primary source of road traffic emission factors for this calculation should be consistent with those set out in the DMRB Volume 11. Emission factors for rail are discussed below.

1.2.2 It should be noted that in terms of total transport emissions, rail transport accounts for less than 1% of the total. Therefore, even with the most rail orientated transport options, perhaps doubling the rail kilometres, the potential for any significant impact on emissions will lie mainly with the saving in emissions from road transport brought about by modal transfer, rather than those generated by rail. Hence, it is suggested that emissions from rail sources can be scoped out in most cases. However, where options perform similarly in terms of their total road traffic emissions alone, emissions from rail should be included in the total and used as a determining factor.

1.2.3 Approximately 70% of energy used on the railways is derived from diesel, the remaining 30% comes from electrical energy generated in power stations (DETR, 1998). The balance between diesel and electric power trains varies considerably throughout the UK. Some areas are almost exclusively electrified (e.g. commuter services south of London and to the coast) and some are exclusively diesel (e.g. south west from Bristol). Other areas are mixed with electric trains tending to dominate in the south of England and diesel in the north. Diesel power tends to dominate for local services in towns and cities, except for Liverpool, Glasgow, Birmingham and some services in Leeds and Manchester.

1.2.4 Using DfT transport statistics (DETR, 1999) and information from the National Atmospheric Emissions Inventory a generic emission factor for all rail types for NO_x (as NO₂) of 89g/km has been derived (based on total NO_x (as NO₂) attributable to rail transport of 35,000 tonnes NO_x (as NO₂) divided by total rail distance travelled (passenger and freight) 391 million train-kilometres, latest available figures are for 1994/5. Source: Transport Statistics, Great Britain). This includes a contribution from both electric and diesel trains. More specific emission factors are available for diesel trains, which are generally more efficient at converting fuel into useful energy than electric trains. Where options are likely to affect mainly diesel trains, the emission factors for diesel trains shown in Table 1 can be used to calculate more accurately the total emission from rail.

Table 1: Summary of Rail Emission Factors

Diesel Locomotive Type	Power Cars/Train (most frequent number per train)	NO_x Range^(a)	NO_x Factor^(b)
Passenger DMU	1 - 6 (2)	12 - 31	40
Passenger HST 125	2 (2)	-	97
Passenger Loco	1 (1)	-	64
Freight	1 - 4 (1)	51 - 170	170
(a) Grams per kilometre per powered car. (b) Gram per kilometre per train, based on likely powered cars per train - this factor can be varied if details are known.

1.2.5 The emissions per train will be dependent on the number of power cars per train. For rail freight, single power car trains are becoming more common as the new, more powerful locomotives are introduced (see Box 2).

Composition of Diesel Train Units

Diesel powered passenger trains are variable in their make-up in terms of unpowered and powered carriages. The Inter-City 125s may have two power cars and many operators are currently re-engining these trains - and hence emissions will decrease from these trains over the coming years. Many local train services and provincial long-distance services run with Diesel Multiple Units (DMUs) where the power comes from engines sited under most carriages - there may be 1-6 carriages per train and up to 6 of these carriages would have their own power. A number of train operating companies operating InterCity type services are acquiring new Diesel Multiple Units and these may operate longer trains with more powered vehicles. Freight trains have historically used locomotives from the 1970s or earlier although many operators are replacing their existing rolling stock with the latest locomotives or re-engineering locomotives which are generally less polluting. Freight trains may have one or more powered locomotives although the latest classes of locomotive are powerful enough to have one per train in most cases.

1.2.6 However, in the absence of any data to enable a more accurate figure to be determined, NO_x emissions from diesel can be taken to be in the order of 80 g km-1 per train.

1.2.7 Worksheet 1 is provided to assist in developing the information required.

Qualitative comment

1.2.8 The results of this assessment should be incorporated into the qualitative column of the local air quality row in the AST. It should set out the change in mass emissions of NO_x and PM₁₀ in tonnes/year, for each option relative to the do-minimum in the assessment year. Where options perform similarly in terms of total emissions from road traffic, account should be taken in the totals of emissions from rail. A comment should be made to indicate any special features of the appraisal, including whether rail emissions have been taken into account and, where they have, the basis of the calculations.

2. Application of TAG to Highway Schemes

This section provides advice on the links between TAG's treatment of Regional Air Pollution and the advice given in Volume 11 of the Design Manual for Roads and Bridges (DMRB), which deals with the environmental assessment of highway projects.

2.1 Methods and Worksheets

2.1.1 The method for assessment of Regional Air Pollution in TAG is consistent with the regional impact assessment in DMRB 11.3.1. Worksheet 1 shows the information that should be derived.

2.2 DMRB Stages and TAG

2.2.1 The regional impact assessment is generally undertaken at DMRB Stage 3.

Worksheet 1 Regional Air Quality - Strategy and Plan Level

3. References

Highways Agency Design Manual for Roads and Bridges (DMRB)

DETR (1999). DfT transport statistics

National Atmospheric Emissions Inventory www.naei.org.uk/emissions/index.php

DfT (2003). Guide to Producing Regional Transport Strategies

4. Document Provenance

This Transport Analysis Guidance (TAG) Unit is based on Appendix E of Guidance on the Methodology for Multi-Modal Studies Volume 2 (DETR, 2000).

Technical queries and comments on this TAG Unit should be referred to:

Integrated Transport Economics and Appraisal (ITEA) Division
Department for Transport
Zone 3/08 Great Minster House
33 Horseferry Road
London
SW1P 4DR
itea@dft.gsi.gov.uk
Tel 020 7944 6176
Fax 020 7944 2198

Updated: April 2009

Documents

Guidance documents - Expert

3.3: The Environment Objective

TAG unit 3.3.4: Regional Air Pollution

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