LAQN Pollution Episodes

Pollution Notification released on 05/12/2019

The frosty calm conditions overnight from Tuesday 3rd to Wednesday 4th saw a strong inversion form. An inversion is where the air close to the ground is colder than the air above it and this cap of warmer air traps all local emissions in the ground layer.

With all local emissions being trapped in the layer of air close to the ground we saw primary pollutants, PM10 particulate and Oxides of Nitrogen/Nitrogen Dioxide, rise markedly in the morning rush hour.

The busy roadside locations of Wandsworth - Putney High Street and Brent- Ikea recorded single hours of moderate Nitrogen Dioxide, based on provisional data, at 8 and 9am respectively. Putney High Street was less than 1ppb over the threshold. In previous years these sorts of conditions resulted in many hours of moderate at multiple sites.

The sunny conditions warmed the ground and air above it until the inversion was broken late morning at which point the dispersal into a large volume of air brought PM10 and Nitrogen dioxide levels down.

At this point PM sites across London displayed two different sets of characteristics. For most sites PM10 levelled off and the average across London was around 35 ug/m3. At some roadside sites however after the initial fall levels started to rise rapidly. It is thought that this was due to the drying of the roads in the sun and the subsequent resuspension of road salt. Later in the day, around 3pm when the sun had lost it’s heat, the inversion started reforming and PM10 levels at these sites fell rapidly, probably due to road surfaces becoming damp again and reducing resuspension. A short while later the PM10 and Oxides of Nitrogen start to rise as local emissions once built up in the capped layer.

An increase in wind broke the inversion just before midnight.

Moderate PM10 was recorded at City of London - Upper Thames Street and Ealing -Western Avenue, both of which showed a likely road salt signature. Moderate was also recorded at Lambeth Vauxhall Bondway interchange with a likely additional contribution from local construction. High PM10 was recorded at Brent Ikea. Whilst for most of the day it followed the same signature of road salt impact as seen in Ealing an additional large peak occurred from 11:30 with a 15 minute average of 682 ug/m3 at 11:45. The source of this has not been identified but a faint smell of burning was reported from site around 1pm.

The current situation can be followed at:

* including sites in Berkshire, Essex and Surrey.

Timothy Baker and Erica Winn
Duty forecasters

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