PM10 and PM2.5 – the stuff of which dreams are made.......

Posted by Geoff Dollard, Practice Director: Air & Environmental Quality on 1 February 2012

...Maybe it was a spooky coincidence that I was dreaming about last night’s weather forecast when I awoke about 4 am - the time of AEA’s daily download of meteorological data from NOAA. (http://en.wikipedia.org/wiki/Global_Forecast_System). Whatever caused it, I did wake early with the image in mind of cold easterly air over my roof in the south of England doing battle with warmer westerly air – with the eventual skirmish line making the difference of several degrees in temperature..... so what about the pollution I thought?

No frost on the car, good – off to our Harwell Office. AEA’s team of forecasters undertake a daily, 24/7 forecast of air pollution on behalf of Defra for towns and cities across the UK. Pollution sources are widespread and the pollutant levels we are exposed to when we wake each day and go about our business have global, regional and local sources. In order to forecast these we need to utilise global weather data (hence the 4 am download) and combine this with on-the-ground measurements of pollutants we collect from national monitoring networks and with assessments or projections from large scale models of the atmosphere.

 

Our forecasters collect the data and then run the models; so, by 10 am my dream was broken and I could see on my pc that we are expecting high levels of particle pollution over the next couple of days.

Latest pollution summary - http://uk-air.defra.gov.uk/latest/
Interactive AQ map - http://uk-air.defra.gov.uk/interactive-map 
UK weather map -     http://uk-air.defra.gov.uk/forecasting/wrf-summary?view=today&region=uk

This is where the air quality scientist has to honestly confess to often saying things are “interesting”. However, we are vigilant and forecast air pollution on a daily basis because it is harmful to health, and many individuals who suffer from health related conditions that may be exacerbated by high pollution can use the information to help manage their condition e.g. to modify their planned activity.

The current pollution forecast is predicting high levels of particle pollution. Following research and advice from experts on health impacts the UK now has 2 measures of this material – PM10 and PM2.5. These terms relate to the effective size of the particles and research on their health impacts on people indicates the smaller size (PM2.5) is the component of the material in the air we breathe that is most strongly linked to adverse health effects.

The UK is now using a new pollution indicator scale based upon the PM2.5 measure – interestingly the current pollution is mostly this smaller, PM2.5 material. This can indicate the particles have originated from more distant sources beyond the UK. Close to the sources of particles there is a broad range of sizes; with time, coarser or larger particles settle out under the influence of gravity leaving those that are small enough (to effectively behave like gases) remaining airborne and available to travel long distances “on the wind”. Given a higher proportion of the smaller particles are present in this forecast event we can infer a poorer quality of air – and it is reassuring to see this reflected in the higher index indicated for PM2.5 rather than by the measure of the larger fraction (PM10). So we are seeing the use and impact of what is effectively a more sensitive measure of air pollution in action.