Widespread 'Moderate' Ozone – Beginning Saturday 27th April 2025 and Continuing
A significant and prolonged episode of widespread 'moderate' ground-level ozone pollution in 2025 began on Saturday, 27th April, across London and southeastern England. 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 (NO?), volatile organic compounds (VOCs), and carbon monoxide (CO), under the influence of ultraviolet (UV) radiation from sunlight.
On Sunday 27th April, ten monitoring sites in London recorded 'moderate' ozone pollution levels. Monday 28th saw eleven sites record ‘moderate’ and Tuesday 29th saw thirteen. The meteorological conditions on that day included strong solar radiation, air temperatures in the mid 20s Degree C, and very light winds, which led to poor dispersion of local emissions. These stagnant conditions are ideal for enhancing the photochemical reactions that generate ozone.
Additionally, the presence of a continental air mass likely introduced further precursor pollutants from mainland Europe, amplifying the regional ozone burden. These transboundary contributions can be significant during stable high-pressure systems, where air stagnation allows pollutants to accumulate and interact over large areas.
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
Duty Officer.
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