Primary NO₂ Emission Factors for Road Vehicles

Nitrogen oxides (NO_x) are emitted in the form of nitric oxide (NO) and nitrogen dioxide (NO₂). The fraction emitted directly as NO₂ (f-NO₂) is of particular interest for air quality modelling. Road transport is the major source of primary NO₂ emissions especially in urban areas and different vehicle types emit different proportions of NO_x as NO₂. Evidence has shown that diesel vehicles are particularly prone to high f-NO₂ values and especially those vehicles fitted with certain types of catalyst systems for controlling other pollutant emissions such as oxidation catalysts and diesel particulate filters for controlling carbon monoxide, hydrocarbons, and particulate matter. Thus, diesel vehicles meeting more recent Euro standards tend to have higher f-NO₂ values.

Values of f-NO₂ have been developed from recent real-world roadside vehicle emissions remote sensing measurements of NO₂/NO_x ratios compiled by Ricardo Energy and Environment and the University of York. Factors were developed for different vehicle types and Euro standards, with the exception of buses. The f-NO₂ factors for buses were taken from the EMEP/EEA Emissions Inventory Guidebook (2019) and previous roadside remote sensing studies by Carslaw et al (2016). Analysis of the Ricardo and University of York roadside vehicle remote sensing data suggests that f-NO₂ factors for Euro 3, 4 and 5 diesel cars and LGVs are lower than in the EMEP/EEA Emissions Inventory Guidebook. The f-NO₂ factors for Euro IV and V HGVs derived from roadside remote sensing are lower than in the EMEP/EEA Guidebook, while for Euro VI the remote sensing factors suggest higher factors than the EMEP/EEA Guidebook.

Values of f-NO₂ for each main vehicle type and Euro standard are provided in the spreadsheet “By Euro” tab. These figures refer to the mixing ratio of NO₂ in emitted NO_x, i.e., the volume fraction of NO₂ in the emitted NO_x.

Other worksheets provide weighted values of f-NO₂ for years between 2013 and 2035. The weighting has been done in different ways for use when the user does not have detailed information on the mix of Euro standards in the fleet. The weighting of the factors are based on the NO_x emissions from each detailed vehicle category.

The Primary NO₂ factors up to 2021 are based on the methodology and input data for the NO_x inventory reported in March 2023 . The NO_x emissions inventory uses fleet composition and vehicle kilometre data derived from the Department for Transport (DfT) to estimate the NO_x emissions by each Euro standard. Details on the method for estimating NO_x emissions from road transport are given in the UK’s official Informative Inventory Report submitted under the National Emission Ceilings Regulations (NECR, 2018) and the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Transboundary Air Pollution (CLRTAP). Fleet composition uses evidence from DfT on the survival rate and mileage with age of vehicles from licensing and Ministry of Transport (MOT) data. The emission factors for NO_x are those derived from COPERT 5.4.

Primary NO₂ factors from 2022 -2035 are based on the latest NAEI projections on road transport NO_x emissions updated to base year 2021. The key assumptions behind the forecasts in weighted f-NO₂ factors are those that define the composition of the future vehicle fleet, future traffic activity and the corresponding NO_x and f-NO₂ factors for current and future vehicles.

The composition of the future vehicle fleet is based on future sales data provided by DfT in January 2023 for new cars and vans, and on updated assumptions made by the NAEI on future sales, annual mileage and survival rates of HGVs and buses derived from analysis of past trends. Outside of London, a fleet turnover model is used to calculate the future fleet composition using vehicle survival rates derived from trends in historic licensing data. Traffic growth assumptions for each main vehicle type come from DfT (provided in January 2023) for Great Britain (GB) projected to 2040 and set to a base year of 2015 as this is the only historic year with data to align to in the DfT traffic growth data. Fleet composition and vehicle activity forecasts for London were provided by Transport for London in January 2023.

The projections account for the introduction of new vehicles up to Euro 6/VI standards. Euro 6 standards for diesel cars and LGVs are introduced in 3 stages from 2015/16 according to COPERT 5.4. DfT’s traffic forecasts have not yet considered the recovery in traffic activity from the pandemic. Traffic forecasts for GB reflect the Renewable Transport Fuel Obligation (RTFO), latest fuel efficiency policies for cars, vans, HGVs and PSVs (buses), rail electrification and active travel spending. However, the traffic forecasts do not include the measures to phase out Internal Combustion Engines (ICE) vehicles from 2030.

‘Fleet-avg by area_road_type’. This sheet provides values for each main vehicle class weighted by NO_x emissions by each fuel type and Euro standard in the fleet. The different values for cars and taxis on urban, rural and motorway roads reflect the different mix of NO_x emissions coming from petrol and diesel cars on each road type. The values for different parts of London reflect the different proportions of NO_x emissions coming from diesel taxis and cars in each part of London. The different values for cars in Northern Ireland reflect the different proportions of NO_x emissions coming from diesel and petrol cars in Northern Ireland due to the different petrol/diesel car fleet in this country. The different values for LGVs, HGVs and buses for London and the rest of the UK reflect the different fleet age mix of these vehicles in London. For London, traffic projections are based on TfL’s current reference case and were provided by Transport for London in January 2023. These forecasts were used as an index to derive growth factors which were applied to DfT’s London vkm for historic years. Updated forecasts up to 2040 on future vehicle fleet composition data for London were also provided by TfL in December 2022. These fleet forecasts account for zero emissions from TfL Buses by 2034 (consistent with current business plan), uptake of zero emission capable (ZEC) and zero emission (ZE) taxis, but don’t yet account for the Londonwide ULEZ.

‘Fleet-avg by_vehicle_fuel_type’. This sheet shows values weighted by NO_x emissions occurring from each Euro class in the mix of vehicles on all roads outside London, but provides separate values for cars and LGVs by fuel type. This should be used when the user knows the fuel mix of vehicles on the road(s) being modelled, but not the mix of Euro standards. The factors for each vehicle type are weighted by the NO_x emissions coming from the mix of Euro standards on all roads.

‘Fleet-avg all_traffic’. This sheet provides the most aggregated values of f-NO₂ which can be used when the user does not know the mix of vehicles on the roads being modelled. The factors for individual vehicle types are weighted by the relative amounts of NO_x emissions occurring from the mix of vehicle types on urban, non-urban and for all UK roads combined.

These Base 2023 Projection figures are an update of the March 2022 f-NO₂ factors (PrimaryNO₂_factors_NAEIBase_2022.xlsx). The major reasons for recalculations include revised projected DfT vkm fuel split projections and updated taxi and bus fleet projections from TfL. These factors will be updated annually after submission of each version of the NAEI’s UK inventory figures.