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Air quality methodology

This paper outlines the approach taken for inclusion of air quality in the version of the 2050 Calculator released in December 2011.

Summary

In this methodology air quality (AQ) emissions are calculated for four different air quality pollutants: nitrogen oxides (NOX), Particulate Matter (PM10), sulphur dioxide (SO2) and non-methane volatile organic compounds (NMVOCs). The 2050 Calculator is split into different modules covering energy supply, energy demand and non-energy sectors. Estimates of emissions of air quality pollutants are derived through the use of a set of AQ emission factors (EFs) specific to the different sectors in the 2050 Pathways Calculator.

The EFs are based on the UK National Atmospheric Emissions Inventory (NAEI) programme , which is funded by the Department for Environment, Food and Rural Affairs (Defra) and the Devolved Administrations. The steps involved in deriving the AQ EFs are summarised as below:

  • Assess a list of modules in the 2050 Calculator that have potentially significant impact on air quality (see Table 1).
  • Map the sectors in the 2050 Pathways Calculator against the NAEI source sectors. If a sector in the 2050 Pathways Calculator does not correlate exactly or is considered in the NAEI, the nearest source sector (based on expert judgement) or emission factors from an alternative data source is used.
  • Develop appropriate EFs expressed in Mtonne of emissions per TWh of energy consumed. Assumptions behind the development of these aggregated EFs are discussed below
  • Calculate emissions from non-energy related sources that are currently not covered in the 2050 Calculator because they do not have a direct impact on CO2 emissions (e.g. solvent use).
  • Calculate the final AQ emission projections for the 2050 Calculator by adding emissions from these missing sources to the AQ emissions calculated within the 2050 Calculator (using the AQ EFs developed above).
  • Convert the AQ emissions into an air quality health impact index

Figure 1 shows how the AQ EFs are combined with the energy consumption data calculated within the 2050 Calculator to produce the final AQ emission and resulting air quality health impact index.

Figure 1: Diagram outlining the air quality methodology

Derivation of emission factors

Three sets of emission factors were developed for the different scenarios used in the Pathways Calculator:

  • 2010 emission factors consistent with current day emissions
  • 2050 low innovation emission factors, based on projected emission factors to 2025 which are then assumed to be held constant
  • 2050 high innovation emission factors, based on projected emission factors in 2025 and assumptions about further improvement and innovation in air pollution abatement technologies

All the emission factor use the most recent set of NAEI projections based on the UEP38 energy projections and the 2009 NAEI base year as a basis. Although these projections have not been published, the methodology report can be found on the NAEI website.

Annex A lists all the emission factors derived for use in estimating air quality emissions from pathways in the 2050 Pathways Calculator.

2010 emission factors

The national inventory for emissions of air quality pollutants in 2010 had not been completed when this methodology was produced and emission factors were derived from the UEP38 (2009 NAEI) projections. If a sector in the 2050 Pathways Calculator does not correlate exactly or is considered in the NAEI, the nearest source sector (based on expert judgement) or emission factors from an alternative data source is used. For instance, the NAEI does not consider gas used for cooking separately and so default emission factors for small scale gas combustion taken from the 2009 EMEP/CORINAR Guidebook is used.

2050 low innovation emission factors

These emission factors represent a worst case scenario for air pollution whereby there is no further deployment or innovation in pollution abatement technology between now and 2050 beyond planned measures. Emission factors were derived from the UEP38 (2009 NAEI base year) projections used to derive the 2010 emission factors which estimate emissions of air quality pollutants from 2010 to 2025 based on the expected deployment of firm and funded pollution abatement technologies and actions. 2025 emission factors were kept constant to derive 2050 low innovation emission factors.

2050 high innovation emission factors

These emission factors represent a best case scenario whereby innovation radically improves pollution abatement technology between now and 2050 and it is fully deployed. The emission factors were derived from the UEP38 (2009 NAEI) and the application of expert judgement taking into account the level of emission reductions already achieved in that sector and the likely scope for further emission reduction technologies. It is assumed that all future abatement techniques are fully deployed across all sectors.

Emission sources that are not currently covered in the 2050 Calculator

Some non-energy related sources are currently not covered in the 2050 Calculator. The following missing sources were identified by comparing the sources/modules included in the 2050 Calculator against the sources included in the UK Emissions Inventory:

  • Off-road mobile machinery (industrial off-road mobile machinery, agriculture mobile machinery, house and garden machinery and aircraft support vehicles)
  • Lubricants use in road, aircraft and marine engines
  • Other Mobile (aircraft military and shipping naval)
  • Solid fuel transformation
  • PM emissions from housed agriculture livestock
  • Agriculture - stationary combustion (from straw and operations)
  • Clinical and industrial waste incineration
  • Cremation
  • Small-scale waste burning
  • Other (accidental fires, bonfire night and fireworks)
  • Fugitive emissions from fuels (exploration production (transport), refining/storage, distribution of oil products and venting and flaring)
  • Paint application
  • Other product use
  • Solid waste disposal on land

Total emissions from missing sources were calculated for 2010 and 2020 (the latest year we have emissions for based on NAEI projections UEP38 (2008 NAEI base year) which are publicly available and are consistent with the recent National Emission Ceiling Directive (NECD) data submission . As the NAEI projections are not available beyond 2020, in the absence of better available data the 2020 emissions data were applied to the low and high innovation 2050 emission estimates.

Calculation of the air pollution health impact index

The total emissions calculated using the methods above are converted into an air pollution health impact index using known relationships between changes in emissions of pollutants NOX, particulate matter, SO2 and health impacts developed by the Interdepartmental Group on Costs and Benefits – air quality subject group. These tools estimate the marginal change in years of life lost caused by each additional tonne of pollutant emitted - or conversely the years of life gained through reducing one tonne of a pollutant. More information on how these estimates are developed is available from http://archive.defra.gov.uk/environment/quality/air/airquality/panels/igcb/index.htm

For a limited range of source sectors the ratio between the emissions from that sector and the population weighted mean exposure arising from that emission is derived using dispersion modelling. The population average exposure is then converted, using the IGCB agreed parameters, into a health impact.

The index uses 2010 as the baseline year and give this a number of 100 with all changes in years of life gained and lost being proportionate to the total number of years of life lost from air pollution estimated for 2010. A number lower than 100 indicates a reduction in average air pollution and associated health impacts, whilst a higher number indicates an increase. This index reflects changes in the average concentration of air pollutants across the UK. It does not provide information on the number or severity of pollution hotspots.

Limitations of the AQ Impacts

It should be noted that the AQ EFs derived for the 2050 Calculator have been aggregated (i.e. generalised) according to the aggregated fuel and sector breakdown as currently set up in the 2050 Calculator. For instance, the 2050 Calculator calculates total energy demand for liquid hydrocarbons consumed by four types of domestic passenger transport (CAR, BUS, RAIL and AIR) and consequently, aggregated AQ EFs have been developed based on this same breakdown. In this example, liquid hydrocarbons cover both petrol and diesel fuels while the transport mode ‘CAR’ covers all cars, vans and motorcycles. However, the level of AQ emissions are dependent on the vehicle type and fuel type, hence the aggregated EFs will only provide approximate estimates of AQ emissions if future choices of vehicle fuel change from the current mix. This limitation in the accuracy of AQ emissions results calculated by the 2050 Calculator applies to other sectors. For example, energy demand by industrial processes is disaggregated into gaseous, liquid and solid hydrocarbons, and is further disaggregated by industrial sub-sector, but emissions of AQ pollutants are also highly dependent upon the type of combustion device being used (boilers, furnaces, engines, gas turbines etc, and sub-divisions thereof.), as well as the presence of any abatement systems, and the 2050 Calculator does not disaggregate energy use to this extent. Individual fuel types within the three categories of fuel used in the 2050 Calculator can also have very different emission characteristics, particularly for SO2. For example, heavy fuel oil (high sulphur) and LPG (very low sulphur) are both liquid hydrocarbons in the 2050 Calculator.

Furthermore, the methodology used to account for emissions from combustion of biofuels is also limited in accuracy as it does not take account of differing combustion technologies and fuel type mixes in sectors, however since the model applies a universal percentage of biofuel usage across all sectors this is considered an appropriate approximation. The structure of the model also does not allow for the effect of combustion of blended fuels, which in some sectors (e.g. road transport) has been shown to have a significant impact on emissions.

The Calculator does not currently split renewable heat between combustion and non-combustion sources. The assumptions used assume that all renewable heat is biomass combustion. It may be the case that in future heat pumps and solar thermal heat technologies provide a significant fraction of the renewable heat and the air quality emissions would be reduced proportionately. There already exist technologies which lead to a significant reduction in emissions which are available to be implemented at an additional cost to the operator.

The emissions of the sources currently not covered within the model are ‘static’ emissions which will not change with different pathways. Thus these emissions are indicative values only and do not reflect any change in the ambition level with a change in the CO2 pathway.

The analysis of future air quality makes a number of assumptions about the future which will in all likelihood not be borne out by events. However they are based on the information available now and are usable without adding excessive computational complexity to the model.

Examples of such issues are that the currently the majority of electricity generation occurs distant from the major centres of population for historic reasons. Future combustion derived electricity may be generated in city centres and hence if emitting the same amount of pollution would lead to a higher impact on the population average exposure to that pollutant. This potential change in the spatial relationship between emission location and population is not addressed in the calculator for any sector.

By 2050 it is possible that climate change may have begun to had significant impacts on both the climate and the chemical and dispersion behaviour of pollutants. These issues are not addressed again in the absence of appropriate information on which to base assumptions.

The air quality index as a measure of health impacts is based on current knowledge, population sensitivities and age profile which will change over the period to 2050 in ways which may alter the relationship between emission of pollutants and their impacts on health.

Any analysis looking forward is subject to uncertainty. At this distance it is hard to predict the changes in society that may lead to changes in the impact of air pollution. The calculator is designed to enable users to explore the routes of reducing greenhouse gas emissions and to inform the user of what the wider impacts of these choices may be on air quality.

Annexes

Annex A: emission factors used in the 2050 Calculator spreadsheet for 2010 and 2050 (high and low innovation scenarios): http://2050-calculator-tool-wiki.decc.gov.uk/AirQualityMethodologyAnnexeA.pdf

Annex B: further information on the air quality emission factors by sector: http://2050-calculator-tool-wiki.decc.gov.uk/AirQualityMethodologyAnnexeB.pdf

Water quality impacts

The Government is also considering adding water quality impacts into the 2050 Calculator. Our proposed methodology for doing so is set out here: water quality methodology. Comments are welcome on this.