LAQN Pollution Episodes

Pollution Notification released on 14/05/2024

Widespread 'Moderate' Ozone Thursday 9th May to Sunday 12th May 2024.

With the arrival of warmer summer weather, the first episode of Widespread 'moderate' ozone in 2024, occurred across London from Thursday 9th through to Sunday 12thMay. Over the four days, ground-level ozone production was driven by rising temperatures and sunshine acting on local and continental precursor emissions such as oxides of nitrogen, Volatile Organic Compounds (VOCs) and carbon monoxide.

'Moderate' ozone was reached in London at four sites on Thursday 9th, two sites on Friday 10th, one site on Saturday 11th and 4 sites on Sunday 12th May. On Thursday 9th May there was strong sunshine and temperatures reached 24 degrees Celsius. A very light breeze resulted in poor dispersion of local emissions which probably then played a significant role in increased ozone production. The breeze picked up after Thursday 9th increasing dispersion, however Sunday 12th May was the warmest day with strong sunshine and temperatures reaching 26 degrees Celsius. In addition, a continental air-feed on Sunday likely also contributed pollutants required for ozone production.

It is important to remember that ozone is an area wide pollutant, and whilst monitoring sites are relatively sparse compared to those monitoring nitrogen dioxide they represent the wider population exposure, so a single site may represent the ozone concentrations that hundreds of thousands of people have been exposed to. The national Daily Air Quality Index for ozone is measured on a running 8 hour mean.

The episode finished with the arrival of a cooler and cleaner Atlantic air-feed on Monday 13th May, with temperatures falling back to the mid to high teens and significantly increased cloud cover further inhibiting ozone production.

John Casey (Duty Forecaster)