Pollution Episodes
Details of major pollution episodes including peak values recorded across
Ealing Council.
Please select an episode to view from the list below:
First Widespread Moderate Ozone Episode of 2026
Tuesday 28th April 2026
Episode Overview
With the arrival of warmer weather, London experienced its first widespread 'Moderate' ozone episode of 2026 yesterday. While isolated 'Moderate' readings were recorded at single sites over the past week, the peak of the episode occurred on Monday 27th April, where seven monitoring sites across the capital reached 'Moderate' levels. During this period, weather patterns remained similar, characterized by air feeds initially arriving from the Atlantic before shifting to a Northeast feed that clipped Northern Europe.
The elevated concentrations were driven by the combination of local environmental conditions and significant transboundary influences and a baseline import of ozone from the Atlantic. Strong sunshine and temperatures reaching low twenties °C on the 27th acted as a primary catalyst for photochemical reactions, fuelling the production of ground-level ozone from both local and continental precursor emissions. This process was further intensified by atmospheric stability; light breezes resulted in the poor dispersion of these local pollutants, allowing them to accumulate and react with greater intensity. Additionally, a continental air-feed on Monday likely imported a secondary stream of precursor pollutants from mainland Europe, providing the final boost necessary to reach these 'Moderate' levels across the capital.
Health & Monitoring Impact - The national Daily Air Quality Index (DAQI) for ozone is calculated based on a running 8-hour mean. It is important to note that ozone is an area-wide pollutant. Although monitoring sites are sparse, they represent wider population exposure. A 'Moderate' reading at a single site can indicate that hundreds of thousands of people across the surrounding area have been exposed to similar concentrations.
Outlook - Similar weather conditions are forecast to persist throughout the week. With high pressure remaining dominant and the potential for further continental air-feeds, it is predicted that more 'Moderate' ozone levels will be seen in the coming days. We will continue to monitor the situation closely and update our forecast as the episode develops.
Duty Officer
Erica Withers
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