Elevated concentrations of PM10 and, to a lesser extent PM2.5, were recorded across much of the UK between 7am 23rd January and 7am 24th January. Analysis of meteorological data, including air mass trajectory modelling, has shown that the most likely source of this particulate matter is the North Africa and the Sahara desert.

‘Moderate’ PM10 was recorded at all sites across London and the Home Counties. Additional local sources of PM10 pushed concentrations recorded at the majority of roadside and kerbside monitoring sites into the ‘high’ band. Peak concentrations were recorded at the kerbside site Marylebone Road, which recorded 99 ug m-3 as a running 24hr mean, just one point below the ‘very high’ band.

It is likely that this episode will result in two exceedences days of the EU Limit Value across the whole region (23rd and 24th January).

Such Saharan dust episodes are infrequent in the UK, occurring about once or twice per year. The last such episode was in March 2007.

Hourly mean particulate concentrations have now dropped back to more usual levels for the wet and windy weather we are currently experiencing.

The image is a back trajectory model plot showing the course of the air mass in 6 hour steps from the Sahara Desert on the 19th January, arriving over the UK on 23rd/24th January (source NOAA Hysplit model http://www.arl.noaa.gov/ready/).

Item date 24/01/2008