Pollution Episodes
Details of major pollution episodes including peak values recorded across
Ealing Council.
Please select an episode to view from the list below:
Widespread 'Moderate' Ozone – Beginning Wednesday 11th June 2025 and Continuing
A prolonged episode of widespread 'moderate' ground-level ozone pollution began Wednesday 11th June across London. Elevated ozone levels have been recorded each subsequent day and current meteorological forecasts suggest this episode is likely to continue in the coming days.
This period of increased ozone pollution has been driven by a combination of meteorological and chemical conditions conducive to ozone formation. As temperatures began to rise and clear skies dominated the region, photochemical reactions accelerated. Ground-level ozone is not directly emitted but is formed in the atmosphere through complex reactions between precursor pollutants such as nitrogen oxides (NOX), volatile organic compounds (VOCs), and carbon monoxide (CO), under the influence of ultraviolet (UV) radiation from sunlight.
Seven monitoring sites in London have so far recorded 'moderate' ozone pollution levels.
It is crucial to understand that ozone is a regional-scale pollutant. Unlike nitrogen dioxide (NO2), which can vary significantly near traffic sources, ozone concentrations tend to be more uniform across wide areas. Therefore, measurements at a single site often reflect exposure levels affecting vast numbers of people across the city and surrounding regions. The UK's Daily Air Quality Index (DAQI) for ozone is based on the maximum running 8-hour mean concentration observed during a 24-hour period.
This pollution episode may persist until a shift in the weather pattern occurs. A return to cooler, cloudier, and windier conditions—such as those typically associated with Atlantic frontal systems—would help disperse pollutants and suppress ozone formation by reducing solar radiation and atmospheric stability, this change may happen Friday looking at forecast data currently available.
Erica Withers
Forecast Duty Officer.
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