User guide – Environment

User guide – Environment

Notes and definitions


The atmospheric pollutants listed in Table 13.1 have been selected because they are considered to be a threat to human health, and transport is understood to be a significant contributor to emissions of these pollutants. The Air Quality Strategy for England, Scotland, Wales and Northern Ireland contains air quality objectives for nine pollutants (benzene, carbon monoxide, lead, nitrogen dioxide, ozone, particulates (PM10 and PM2.5), sulphur dioxide, 1,3-butadiene and polycyclic aromatic hydrocarbons (PAHs)). The objectives are policy targets expressed as a maximum ambient concentration to be achieved, either without exception or with a permitted number of exceedances, within a specified timescale. The table below sets out the agreed air quality objectives (for pollutants which transport is understood to contribute to significantly). PM10 are small particulates less than 10 microns in diameter while PM2.5 are less than 2.5 microns in diameter.

Air Quality Objectives for Scotland



Date to be achieved by



Measured as:




running annual mean

31 Dec 2010

Nitrogen dioxide2



annual mean

hourly mean not to be exceeded more than 18 times a year

31 Dec 2005

31 Dec 2005

Particles (PM10)3





annual mean

24-hour mean not to be exceeded more than 35 times a year

annual mean

24-hour mean not to be exceeded more than 7 times a year

31 Dec 2004

31 Dec 2004

31 Dec 2010

31 Dec 2010

Particles (PM2.5)


annual mean




daily maximum (measured as an 8 hour running mean) not to be exceeded more than 10 times a year

31 Dec 2005

Carbon Account for Transport

The Carbon Account for Transport (CAT) is published on an annual cycle and contains:

  • Scotland's annual transport emissions from 1990 to 2017;
  • emissions efficiency estimates across different modes of transport;
  • emissions efficiency of road vehicles registered in Scotland;
  • comparison of Scotland's emissions to those of the UK as a whole;
  • key leading transport emissions indicators.

The Climate Change (Scotland) Act 2009 requires Scottish Ministers to lay a report in Parliament setting out their proposals and policies for meeting annual emissions reduction targets. The Climate Change Plan, published February 2018, is the Scottish Government's third report on proposals and policies for meeting its climate change targets. It sets out how Scotland can deliver its target of 66% emissions reductions, relative to the baseline, for the period 2018–2032. In April 2019 the First Minister acknowledged that Scotland – like the rest of the world – faces a Climate Emergency and confirmed that the Scottish Government would accept the recommendations of the UK Committee on Climate Change to set a target of net zero greenhouse gas emissions by 2045 with interim reduction targets of 70% by 2030 and 90% by 2040. The Scottish Government has committed to updating the Climate Change Plan within six months of the Climate Change Bill receiving Royal Assent so that it reflects the more ambitious targets being established.

While the UK emissions return to the UN does not include emissions from international aviation and shipping (IAS), the Climate Change Scotland Act 2009 explicitly includes this category of emissions in its calculation of total Scottish emissions and the required reduction in emissions to fulfil the terms of the Act. International aviation and shipping emissions are shown in the national emissions Inventory as an additional, outside scope, item.

Ultra Low Emission Vehicles (ULEV)

An ULEV emits extremely low levels of carbon dioxide (CO2) compared to conventional vehicles fuelled by petrol/diesel. They typically also have much lower or virtually nil emissions of air pollutants and lower noise levels. Since 2009, the Office for Low Emission Vehicles has considered ULEVs as new cars or vans that emit less than 75 grams of CO2 from the tailpipe per kilometre driven, based on the current European type approval test.

Plug in Grant

Since January 2011, UK motorists purchasing a qualifying ultra-low emission car have been able to receive a grant of 25% towards the cost of the vehicle, up to a maximum of £5,000. The Plug-in Car Grant has been designed to help make the whole-life costs of a qualifying car more comparable with petrol or diesel equivalents. The terms of this scheme were modified in early 2016.[fn]


Pollutants and air quality objectives

The information on air pollutant emissions is taken from the publication Air Quality Pollutant Inventories for England, Scotland, Wales and Northern Ireland: 2005 – 2019, published in September 2021 on the National Atmospheric Emissions Inventory website. Emissions estimates are modelled and revisions may be made to the time series each year where revised figures are available. The most recent report provided revisions from 2005 only. Emissions for 1990-2004 have therefore not been revised and are taken from the previous report Air Quality Pollutant Inventories for England, Scotland, Wales and Northern Ireland: 1990 – 2018 published in October 2020. The year 2005 is now used as the point of reference in response to the new national emission reduction commitments (ERCs) which are applicable from 2020 and 2030 onwards for SO2, NOX, NMVOC, NH3, and PM2.5 to cut the health impact attributed to air pollution by approximately half when compared to 2005.

2 A sensitive parameter in the emission calculations for petrol cars is the assumption made about the proportion of the fleet with catalyst systems that have failed, for example due to mechanical damage or failure of the lambda sensor. Following discussions with DfT, it is assumed that the failure rate is 5% per annum for all Euro standards, and that up to 2008 only 20% of failed catalysts were rectified properly, but those that were rectified were done so within a year of failing. The revisions are based on evidence on fitting of replacement catalysts. According to DfT there is evidence that a high proportion of replacement catalysts were not Type Approved and do not restore the emission performance of the vehicle to its original level (DfT 2009). This is being addressed through the Regulations Controlling Sale and Installation of Replacement Catalytic Converters and Particle Filters for Light Duty Vehicles (LDVs) for Euro 3 (or above) LDVs after June 2009. Therefore a change in the repair rate is taken into account for Euro 3 and above petrol LDVs from mid-2009, assuming all failed vehicles are rectified properly.

The methodology for estimating emissions from shipping was revised in the Air Quality Pollutant Inventories for England, Scotland, Wales and Northern Ireland: 1990-2016 ( Full details of the revision are given in the report. As a result of the revision there has been a large apparent increase in emissions from shipping compared with the previous inventory which particularly affects the NOx figures. The percentage of NOx emissions allocated to transport in 2015 increased from 45% in the 2017 inventory to 53% in the 2018 inventory.

In the inventory Air Quality Pollutant Inventories for England, Scotland, Wales and Northern Ireland: 1990-2017 ( there was a major revision to the emission factor for gas oil combustion on locomotive trains for all years after 1998. Additional revisions are due to minor refinements to the shipping methodology which now produces uses pollutant-specific techniques to disaggregate UK emissions, but this is minor compared to changes to the emissions for Railways.

In the inventory Air Quality Pollutant Inventories for England, Scotland, Wales and Northern Ireland: 1990-2018 (, emission and fuel consumption factors for different train classes have been revised based on newly available data, leading to a reduction in emissions for NFR code 1A3c (Railways: intercity, regional and freight.

In the inventory Air Quality Pollutant Inventories for England, Scotland, Wales and Northern Ireland: 1990-2020 ( were significant revisions to navigation estimates resulting from a correction to the calculations for the allocation of emissions for tug vessels and vessels serving the offshore oil and gas sector. The consequence of this is an increase in the emissions allocated to coastal shipping compared to the 2019 inventory

Detailed information on all sites in the Scotland Air Quality Database are available from the data section of the "Air Quality in Scotland" website ( and the Scottish Air Quality Database – Annual Report 2020. The air quality objectives are taken from The Air Quality Strategy for England, Scotland, Wales and Northern Ireland: Addendum. Summary statistics for all sites are available from the "Scottish Environment Statistics Online" website (Scottish Environment Statistics Online (SESO): index - ( Please note that this website is no longer being updated as of 30 September 2017.

Emissions of greenhouse gases from transport allocated to Scotland

The majority of the Scottish emissions tables shown here are based on emissions estimates reported in Greenhouse Gas Inventories for England, Scotland, Wales & Northern Ireland 1990-2017, compiled by Aether/Ricardo-AEA under contract to the Department for Business, Energy and Industrial Strategy, the Scottish Government, the Welsh Government and the Northern Ireland Department of Environment. Data from other sources, such as Scottish Transport Statistics, are also presented in the report. In this inventory:

In line with the methodology used to report against the Climate Change (Scotland) Act 2009, emissions from transport only include those at the point of use, also known as tailpipe emissions. Lifestyle and displaced emissions, such as emissions from generating the electricity to power electric trains, are not included. The all sources figures given in Table 13.2 take account of removals of carbon dioxide as a result of Land Use, Land Use Change and Forestry (LULUCF).

The way in which emissions are allocated to the different countries within the UK are described in the Greenhouse Gas Inventories report. In summary, the bases of the different estimates are:

  • road transport - the estimated volume of traffic on the roads within each country. The estimates for carbon dioxide are constrained so that the total for the four countries agrees with the internationally-reported overall total for the UK as a whole (which was calculated from the total volume of fuel sold within the UK);
  • railways - emissions from railway locomotives in Great Britain are disaggregated based on diesel oil consumption data for passenger services and National Atmospheric Emissions Inventory (NAEI) estimates for freight services. In addition, emissions associated with the use of coal for steam locomotives are also included within the calculations. The data used in the 2006 inventory was reported for each railway company, whose area of operation can in most cases be allocated to one of the four constituent countries;
  • civil aviation - estimates of emissions from domestic aviation are calculated based on aircraft movement data from the UK's major airports. The total number of domestic flights from each of the devolved administration areas has been calculated, and based on this, a fraction of the total UK emission has been allocated to each constituent country. This approach is also used to allocate emissions from aircraft support vehicles;
  • national navigation - the disaggregation of emissions from navigation and coastal shipping has been derived in a similar way to the approach used for aviation, based on port movements in each constituent country.

Road transport carbon dioxide (CO2) emissions are estimated using vehicle kilometre data constrained so the sum of the UK areas equate to the total for the UK inventory (where that total is derived from fuel sales data of petrol and DERV within the UK as specified in the reporting guidelines of the Intergovernmental Panel on Climate Change). A criticism of this method is that the presentation of results does not always provide a CO2 emission trend that is directly consistent with the vehicle kilometre trend data, as the fluctuations in UK fuel data have a more significant impact on the resultant emission trends. As an alternative, road transport CO2 emissions from the constituent countries of the UK may be estimated solely by vehicle kilometre data unconstrained to the UK total derived from fuel consumption data.

The difference in results between the constrained and unconstrained methods at Devolved Administration level largely reflects the difference in the results at UK level between bottom-up calculated fuel consumption using vehicle km data and fuel consumption factors and the fuel sales data in the Digest of UK Energy Statistics (DUKES). The reason for a disparity has previously been attributed to cross-border fuel sales ("fuel tourism") although model uncertainty was always emphasised as an additional, and probably a major explanation for the differences.

Any change in the methodologies or the factors used to calculate fuel consumption will affect the magnitude of the difference between calculated fuel consumption at national level and sales figures from DUKES and so, in turn, it will affect the disparity between the Devolved Administration CO2 emissions from the constrained and unconstrained approaches.

Carbon dioxide emissions per passenger-kilometre

The figures are taken from the new Greenhouse Gas Conversion Factor Repository created for Defra.

Figures are consistent with the factors used in the compilation of the UK's National Atmospheric Emissions Inventory (NAEI) and in the Greenhouse Gas Emissions Inventory compiled for Scotland and other constituent countries in the UK by Ricardo - AEA.

Figures within the repository are estimated using data for GB/UK as a whole and so do not relate specifically to Scotland. There are no estimates of emissions per passenger-kilometre for Scotland alone. The basis of each estimate is as follows:

Road Transport

The factors used are estimated values for the average petrol and diesel car fleet travelling on average trips in the UK. This has been divided by an average car occupancy rate of 1.50 passengers to calculate average emissions per passenger kilometre.


The national rail estimate refers to an average emission factor for diesel, electric and steam trains. The light rail and tram factors are based on an average of the annual electricity consumption and passenger kilometre data provided by network operators, and a CO2 emission factor for electricity generation on the national grid from the UK Greenhouse Gas Inventory.


The emission factor is an aggregate representation of typical CO2 emissions from illustrative types of aircraft for the three types of air services – domestic, short haul and long haul. Broadly speaking the definition of domestic flights, are those within the UK, short-haul are those within Europe and long-haul are outside of Europe. In keeping with evidence from the IPCC, a 8% uplift factor has been applied to allow for sub-optimal routing and stacking at airports during periods of heavy congestion.

Vehicle Licensing data

Data used in tables 13.6 to 13.10 is provided by the Department for Transport Vehicle Licensing team. More information can be found in Chapter 1 of STS or on the DfT website.

Further information

Within Scottish Transport Statistics, further information can be found in:

  • Chapter 1 – Road transport vehicles
  • Chapter 5 – Road Traffic

Other Transport Scotland Publications:

Transport and Travel in Scotland – includes more detailed analysis of SHS data, in particular:

  • Table 2 – Fuel costs
  • Table 7 – Mode of transport for travel to work
  • Table 11 – Car sharing
  • Table 18b – Car Access
  • Table 20 – Frequency of driving
  • Table 28 – Frequency of train use

Scottish Household Survey Travel Diary, published as part of Transport and Travel in Scotland – includes detailed tables using the Travel Diary dataset, in particular:

  • Table 2 – journeys by mode of transport
  • Table 2a – journey distance by mode of transport
  • Table 4a – mode of transport by journey distance
  • Table 5a – distance summary statistics by mode of transport

SHS Local Authority Results, published as part of Transport and Travel in Scotland – provide breakdowns of SHS data by Local Authority, Regional Transport Partnership and Urban Rural Classification. In particular:

  • Table 1 – Travel to work by mode of transport
  • Table 2 – Travel to school by mode of transport
  • Table 16 – Journeys by mode of transport

The Department for Transport produces a number of related publications:

Department of Business, Energy & Industrial Strategy:

Scottish Government:

Transport Scotland:


1. Revised terms of Plug-in-Grant scheme

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