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How is Air Quality Predicted?

19 August 2015

There’s an awful lot of information about local air quality out there.  We measure it and we can model it.  Much like the weather forecast, we can get daily air quality forecasts from Defra (Department for Environment, Food and Rural Affairs).  A quick internet search can tell you whether air pollution is likely to be low, medium, high or very high in your region and across the nation.

Defra’s air quality forecast provides air quality information looking at five pollutants; ozone, nitrogen dioxide, sulphur dioxide, PM2.5 and PM10 particles.  The forecasts use the Daily Air Quality Index (DAQI) which is numbered 1-10 and divided into four bands ranging from low to very high to indicate air pollution levels.  The bands have health information and advice associated with them to be used by the general public and those with conditions that make them more sensitive to air pollution.

Pollution levels obviously vary from place to place and depend on a number of conditions.  The monitoring stations represent both rural and urban locations and so may not capture actual pollution levels close to a busy road, where you would expect pollution levels to be higher.  The forecasts themselves may not be entirely accurate.  Just like weather forecasts, the further into the future you predict, the more uncertain.  Either way, the forecasts should give a general indication of the conditions.

These forecasts are produced using the Met Office weather forecast and climate prediction model, including air quality forecasting.  Air quality is determined by emissions of pollutants, transport and dispersion of pollutants by winds, chemical reactions amongst reactive gases and aerosols and removal processes such as rain and surface deposition, which are used to predict forecasts.  The Met Office model uses UK and European maps of annual average pollutant emissions to simulate conditions to predict how they will behave and affect air quality.  They look at how chemical species are released into the atmosphere and how they react based on chemical concentration, temperature and sunlight.  Winds and observations of air quality from across the UK are then taken into account to improve the forecast.

All that information is very helpful and it especially assists people with medical conditions that may be worsened by high levels of pollution.  But is the data we collect and report accurate?  Does it reflect what we actually breathe in and experience at ground level?  For example, most vehicle emissions occur less than one metre above the ground.  Current sampling and monitoring strategies do not take into account the variations in air quality with height above the ground and existing monitoring infrastructure is unlikely to provide the spatial resolution to get an accurate idea of street level emissions.  The height of current monitoring station sampling points varies depending on local conditions and the type of equipment installed.  This means that the pollution levels that we are monitoring are different to what people actually experience.  This is a fairly under researched area with a lot of questions still to be answered about how pollution behaves and travels.

Air pollution comes from a variety of sources including industry and traffic and affects the quality of the air we breathe.  There are a variety of methods we use to measure and predict air quality.  However, there are a number of issues that affect how accurate this data is.  The situation and resolution of these monitoring stations can give varying results and it is unclear whether what is recorded is what people actually experience at ground level.  This is an area that can be addressed to more accurately predict air pollution but, the best solution will be to reduce air pollution overall.

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