Air quality forecast

What do we mean by air quality?

The air we breathe contains small amounts of trace gases and particles which can be harmful to our health. Pollutants such as sulphur dioxide, nitrogen oxides and particulate matter are routinely released into the atmosphere as a result of the combustion of fossil fuels and industrial processes. Chemical interactions amongst these primary pollutants, under the influence of heat and sunshine, can give rise to additional secondary pollutants, such as ozone. Individually or together, these common pollutants degrade the quality of the air we breathe and an index has been devised to quantify the air quality and its impact on human health. The Air pollution (DAQI) is the standard index defined by Defra for characterising air quality. Air quality can also be degraded by the release of other, less common pollutants, for example during an accidental release of a harmful chemical. This unexpected type of air pollution is not covered by the routine air quality forecast described here.

How do we forecast air quality?

The build-up of all of these pollutants in the atmosphere is determined by the following factors:

• Emissions of pollutants
• Transport and dispersion of pollutants by winds
• Chemical reactions amongst reactive gases and aerosols
• Removal processes, such as rain and deposition on surfaces.

The Met Office weather forecast and climate prediction model has been developed, as part of the UKCA project, to include these processes. The model uses UK and European maps of annual average pollutant emissions to simulate the release of chemical species into the atmosphere. These are then allowed to chemically react according to prescribed reaction rates which depend on factors including the concentration of the species, the temperature and the amount of sunlight. Species are then transported and dispersed within the model according to the winds and the concentrations are re-evaluated. Using the concentrations calculated in this way throughout the forecast period, the Daily Air Quality Index is calculated as an average over prescribed time periods. We have found that our Air pollution forecasts can be improved by using recent observations of air quality from across the UK, together with our forecast model to give an adjusted forecast. We therefore use observations from the Automatic Urban and Rural Network (AURN) as part of our forecast system.

How do we check the accuracy of our forecasts?

Defra maintains the AURN network of air quality observation sites which provide measurements of pollutant concentration on an hourly basis across the UK. At the Met Office we use these observations to compare our forecasts against. In this way we are able to continuously monitor our forecast performance and to investigate reasons behind poor forecasts. This might occur, for example, if a new source of air pollution appears which is not accounted for in the annual average emission maps we use. In view of the fact that our model uses average emission maps and has grid boxes which are large compared to the road widths, our forecast does not represent the very localised increases in pollution that one might find close to roads in the urban environment. The forecast represents the background and regional air quality away from these strong sources of pollution.