LAQN Pollution Episodes

Pollution Notification released on 24/03/2015

Particulate Pollution Incident Summary for 17th March to 20th March 2015

Between Tuesday 17th and Friday 20th March, a high pressure system centred over Scandinavia resulted in settled conditions in south-east England and light easterly to south-easterly winds. Consequently, air arriving from the north of continental Europe mixed with local emissions to produce a widespread particulate episode across the whole region.

The episode was dominated by PM2.5 particulate with, at times, almost 90% of the measured PM10 particulate being made up of this smaller size fraction. Further analysis shows that the particulate was mainly composed of nitrate with a strong ammonium signal too. This is likely to have been due to the springtime application of slurry and fertiliser in agricultural regions on the near-continent. The presence of some sulphate in the measured particulate indicated a contribution from both local and distant combustion sources. This type of long-range transport episode has become common at this time of year.

The first day of the incident - Tuesday 17th - was also the most severe, with ‘High’ levels of PM2.5 particulates at urban background locations across London reflecting the long-range transport nature of the episode and the relatively small contribution of local sources. ‘Very High’ PM2.5 particulates were also measured in Greenwich and Tower Hamlets. Elevated PM10 particulate concentrations were recorded too; ‘Moderate’ levels occurred across Greater London and Essex, with ‘High’ concentrations at sites in central and south London including roadside locations in Sutton and Merton.

In London, the highest PM2.5 levels occurred during the evening of 17th as polluted air which had passed over industrialised areas of continental Europe continued to feed into London and combine with local emissions. However, as the air continued to travel west it was not until the early hours of the 18th that concentrations in Southampton peaked. As a result, Southampton only measured ‘Moderate’ for PM2.5 on the 17th.

Particulate pollution was recorded for the remainder of the working week although concentrations were lower than on Tuesday 17th. During Wednesday 18th, ‘Moderate’ PM2.5 particulate levels occurred throughout the capital with the Bloomsbury background site in central London reaching the ‘High’ banding. A somewhat different pattern was evident for PM10 measurements, with ‘Moderate’ levels being largely confined to central and inner south London boroughs. PM10 concentrations in Essex and east London by and large remained in the ‘Low’ banding.

By Thursday 19th, the episode weakened as a change in wind direction to north easterly brought in somewhat ‘cleaner’ air which had passed over the North Sea. A slight increase in wind speed also improved pollutant dispersion, although ‘Moderate’ levels of PM10 and PM2.5 particulate were still recorded at a number of locations in central and inner London. With reduced importation of particulates at the time, the ‘Moderate’ levels recorded were most probably driven by locally-emitted pollutants.

Friday 20th marked the final day of the episode with a considerable build-up of particulates throughout the evening in the still conditions. As a result, ‘Moderate’ PM10 occurred at a number of roadside locations and extended to the outer London boroughs of Merton, Sutton and Croydon, and also Sevenoaks in Kent. Back trajectory analysis for the 20th indicates that the air affecting Greater London during the evening had previously passed over urbanised areas of the West Midlands and Greater Manchester. As these areas had themselves experienced particulate pollution during the week, there was probably some longer-range import of particulates into London as a result; indeed, PM2.5 particulate concentrations reached ‘Moderate’ in a band running from Camden in the north west to Bexley in the south east.

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