How the Air Pollution Forecasts are produced

The air pollution forecasts are by a team of air quality experts from AEA Energy & Environment and the Met Office.

A forecast of air pollution for the following 24 hours is prepared each afternoon, for inclusion in the 16:00 air pollution bulletin. A revised forecast is also issued at 11:00 if HIGH (index >=7) air pollution is being measured or is expected. If conditions are changing rapidly, a new or revised forecast may be issued at any time.

Air pollution forecasts are based on information from a number of sources. They are prepared with reference to all available information and on the basis of a number of years of 'hands on' experience of UK air pollution monitoring and forecasting. Sources of information we use include:

• On-line measured concentrations of all pollutantsfrom the UK air pollution monitoring networks. Data are averaged for comparison with the relevant health effects criteria in the air pollution index (i.e. 15-minute, hourly, 8-hourly or 24 hourly averages).
• Weather forecasts for the following day from the Met Office, including specific meteorological parameters for a range of UK air pollution monitoring site locations for two days ahead.
• 'Real time' results from the ozone trajectory model. This model is run each day, taking the results of the Met Office numerical weather prediction models as its input. Results are presented to the forecasters in the form of UK concentration maps for up to three days ahead. The model also provides estimates of ozone concentration for a list of twenty specific monitoring locations.
• Ozone data from selected monitoring sites in North West Europe are available to forecasters each day via email and the world-wide web. The results of a series of pan-European ozone forecasting models are also available on the internet from a number of international research groups. These are a useful comparison for the UK's modelling results.
• Results from the urban pollution forecasting models, NAME and BOXURB, provided by the Met Office. The models provides estimates of NOx, NO₂, CO, SO₂ and PM₁₀ concentrations for eleven sites for one-day ahead.
• Results from a trajectory model adapted to forecast particulate sulphate are used in the forecasting of secondary PM₁₀. Primary PM₁₀ can be forecast using the results from the urban pollutants forecasting models.

The model is run each day; it is used to predict episodes of photochemically generated pollutants in the summer, where long-range transport is an important factor in producing high UK concentrations. It uses the output of the Met Office numerical weather prediction models as its input, and predicts how air masses have been transported to the UK over the preceding 96 hours. These pathways are known as "back trajectories". The model uses a simplified chemical scheme to predict the formation of ozone as the air travels to the UK. It uses UK emissions from the latest National Atmospheric Emissions Inventory, and European emissions from the latest EMEP emissions inventory.

Concentrations of the secondary particle contribution to PM₁₀ are also predicted by this model.

The following pictures show examples of back trajectories and the ozone model output:

1 Animated 5-day pollution forecast

2 Forecast and measured concentrations compared

From April 2000 onwards, the results of the NAME urban air pollution forecasting model (Ryall, D.B. and Maryon, R.H., 1996 and 1998) - also provided by the Met Office - have been made available to the Netcen forecasting team for routine forecasting. The main features of NAME are;

• A sophisticated Lagrangian particle dispersion model.
• Based on up-to-date 3-D meteorology from The Met. Office's Unified Model (UM) (Cullen, 1993).
• It uses the latest emission inventories (NAEI and EMEP) and takes account of daily traffic flows and large point source emissions.
• The PM₁₀ forecast is comprised of three elements; U.K.-only primary PM₁₀ particles, non-U.K. European primary PM₁₀ particles and the sulphate component of secondary PM₁₀ particles.
• The U.K. emissions are modelled using the high-resolution (~11 km) mesoscale meteorology from the Unified Model. The non-U.K. primary PM₁₀ particles and secondary sulphate pollutants are modelled using a coarser meteorological grid (~60 km) also from the Unified Model.

Twice daily, (approximately 4am and 9am) the 0-47 hour U.K.-wide forecasts for NOx, CO, SO₂ and PM₁₀ particles are updated and sent to Netcen for inclusion in the forecasting process.

These forecasts are available as hour-by-hour animated UK maps of the overall air pollution index, as well as for each individual pollutant. An example of the map output is shown below:

The forecast is also available as a tabular timeseries of hourly averaged values for each of twenty specified air pollution monitoring locations (including the 16 agglomerations defined for air pollution forecasting).

This model predicts urban pollutant concentrations, and is used as a backup to NAME to forecast episodes that occur in cities during the winter months. It is useful for modelling localised episodes which are due to the poor dispersion of nitrogen dioxide, sulphur dioxide, carbon monoxide and PM₁₀ particles. It uses emissions estimates from the National Atmospheric Emissions Inventory and weather forecasts from the Met Office for the next day.