Widespread ‘Moderate’ PM10 and PM2.5 Particulate Pollution Incident Notification for Saturday 11st and Sunday 12th January 2025.
Widespread ‘Moderate’ PM10 and PM2.5 particulate air pollution were measured across the London Air Quality Network on the weekend of Saturday 11th and Sunday 12th January. On Saturday 11th, 15 sites measured ‘Moderate’ for PM10 particulates, 33 sites measured ‘Moderate’ for PM2.5. On Sunday 12th ‘Moderate’ PM10 and PM2.5 particulate pollution was measured at 6 and 20 sites respectively. A few of these sites recorded ‘High’ PM2.5 on Saturday 11th, although there is some uncertainty surrounding those measurements.
The weekend was characterised by very light wind activity and very cold weather. Freezing temperatures occurred each night and for a significant part of each day from Friday 10th until mid-day on Sunday 12th. Particulate levels began rising during Friday evening, peaking just before midnight, before reducing, but generally remaining elevated into Saturday. From early evening on Saturday, particulate levels again began to increase, with the highest concentrations measured just after midnight. Levels then decreased gradually during Sunday.
Over the weekend there was a very cold northerly air feed. Usually, an air feed along this path is relatively ‘clean’ and an unlikely source of any significant pollution. Aethalometer particulate measurements from Friday evening through to Sunday showed an increase in both Ultraviolet Particulate Matter (UVPM) and Black Carbon (BC) concentrations, as well as increased UVPM concentrations relative to BC, indicating an increased contribution of those particulate species present in domestic wood and solid fuel burning emissions. Additionally, the daily observed pattern of pollution was characteristic of the typical timing for domestic solid fuel burning, from evening to late at night. The build-up of particulates generated would have been facilitated both by poor dispersion due to little wind activity, and temperature inversion, which can result as colder air becomes trapped at lower altitude in settled high atmospheric pressure conditions.
This episode finished as ambient temperature and wind activity increased from later on Sunday 12th, overnight and into Monday 13th.
John Casey (Duty Forecaster)
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