Pollutionwatch: February 2007

The first week of February saw an area of high pressure bringing still, cold and clear conditions over much of the UK. Over this week, particle concentrations built up in many locations across the country. On the 3rd and 4th, a dozen monitoring sites in London, the midlands and Northern Ireland recorded moderate concentration of particles. Over the next two days, concentrations dropped slightly before shooting up on the morning of the 7th and remaining elevated for the rest of the day before dropping sharply in the early hours of the morning on the 8th. This sudden and short-lived episode on the 7th was most evident at roadside locations in London (including 3 sites where the ‘high pollution’ threshold was breached). However, Glasgow also recorded ‘high’ concentrations and many other towns measured peaks in the ‘moderate pollution’ band. The fact that it was mainly roadside locations showing this sudden increase suggests that rather than being directly caused by weather conditions, as most pollution episodes are, this was a secondary effect - the particles being measured were largely a result of councils gritting the roads in anticipation of the snow that had been forecast. This grit gets ground down and thrown into the air by the action of passing cars. Under normal conditions it is estimated that roughly 50% of particle pollution associated with vehicles is related to resuspended dust from road surfaces, so the deliberate gritting of roads makes a significant additional impact. In the early hours of the next morning the snow arrived, washing the dust out of the air and trapping it to the ground, before eventually melting and washing much of it away down the drains.