Air quality modelling

Air quality is not static. There are many different sources of air pollution that change over time.

Air quality is also influenced by factors including:

• short-term variations in weather
• longer-term effects of climate change

Air quality modelling is the use of mathematical calculations to simulate the formation, transportation and deposition of pollutants, as well as the chemical interactions between pollutants.

It provides information on past, present and future air quality.

Why Defra uses air quality modelling

The Department for Environment, Food and Rural Affairs (Defra) uses air quality modelling to complement measurements that are gathered from air pollution monitoring networks.

Modelling:

• improves the spatial coverage of air quality assessments – this allows us to assess levels of pollutants where monitoring does not take place
• provides information about sources of pollutants – this helps develop policies to reduce pollution
• allows us to assess both current and future levels of pollutants, so we can continue to improve air quality in the UK
• provides a wide assessment of the state of air quality across the UK, both in terms of airborne concentrations and potential human exposure, and the deposition of acidifying and eutrophying pollutants which affect ecosystems

It is an important tool for informing legislation including the UK’s annual compliance assessment under the Air Quality Standards Regulations (2010).    

The models that Defra uses

Defra uses several different models as no single model can meet all modelling requirements.

Each model has different strengths. They may:

• model different types of pollutants
• provide information at a detailed local or national scale
• have different computer and run time requirements
• produce information in different formats

For example, the CMAQ-Urban (Community Multi-Scale Air Quality Urban) modelling system provides modelled air pollutant concentrations across an area and at the roadside at the level of detail needed to assess compliance for all regulated pollutants.

The UK Integrated Assessment Model (UKIAM), on the other hand, is a faster running model. It enables the amount of emissions from different sources to be varied to quickly assess the impact of policies at a national scale for the pollutants of most concern.

The main models currently used by Defra are outlined on this page.

The CMAQ-Urban modelling system

The CMAQ-Urban modelling system is used for the annual national air quality compliance assessment.  

It is run for Defra by Imperial College London’s Environmental Research Group.

The CMAQ-Urban modelling system seeks to represent general exposure to air pollutants.

It predicts background pollutant concentrations across the UK on a grid of 1km x 1km resolution. In addition, concentrations of NO2, PM10, PM2.5 and benzene are modelled at the roadside of the UK’s major urban roads.

The UK Integrated Assessment Model (UKIAM)

The UKIAM is a national-scale model used to estimate how air quality may change under different scenarios to inform policy development.

It is developed and run for Defra by Imperial College London’s Integrated Assessment Unit.

The UKIAM produces information on future concentrations of key pollutants by bringing together projected UK emissions with information on natural and imported pollution.

Tools associated with the UKIAM analyse the modelled concentrations to enable health benefits to be quantified and expressed in economic terms, the assessment of ecosystem risks, and evaluation of progress towards legal targets.

The European Monitoring and Evaluation Program Unified Model for the UK (EMEP4UK)

The EMEP4UK is used to simulate how air pollutants are transported and chemically changed in the atmosphere.

It is used to validate the UKIAM and provide information on the impact of different meteorology and atmospheric environments on air quality.

It also is used to produce maps of nitrogen deposition, enabling assessment of critical loads exceedances in sensitive habitats.

The model is run for Defra by the UK Centre for Ecology and Hydrology (UKCEH).

Modelling outputs

Modelling outputs are:

• part of published reports, such as Air Pollution in the UK which evaluates compliance against legislation
• included in policy documents, for example the UKIAM is used to produce the target trajectories which are included in the Air quality Environment Act target delivery plan
• published as data for use by local authorities and other stakeholders

Compliance data

Download the modelled outputs for the annual compliance assessment.

Source apportionment 

Pollutants are released into the air from many sources, including:

• human activities, such as industrial processes, transport or domestic burning
• natural sources, such as sea salt or wind-blown dust

Modelling provides information on the sources of pollutants.

Data on source apportionment is produced annually after the publication of the national air quality compliance assessment.

Future projections 

Defra produces projections of pollutant concentrations in future years.

This supports government policies to improve air quality in the UK.

The most recent projections are from a 2018 reference year for concentrations of:

• nitrogen dioxide (NO2)
• oxides of nitrogen (NOx)
• particulate matter (PM2.5 and PM10)

Defra’s modelling provides maps of background pollutant concentrations at 1x1km resolution. These maps support local air quality management (LAQM).

The latest maps use reference data from 2021 to provide background pollution concentrations for the years 2021 to 2040.

Research 

Modelling is used in research to explore what causes air pollution and how changes in emissions sources might affect future air pollution.

The evidence published as part of the Environment Act target consultation provides information on how modelling can be used to explore the potential impact of different policy or future trends.

Historic modelled air quality data

Data is available on National Archives for years 2019 to 2023.