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The high temperatures, clear skies, and slack winds are contributing to a widespread "moderate" air pollution event across eastern and central UK right now. 🧵#airquality #airpollution 
Typical of heatwave conditions, urban background and rural monitors are detecting elevated surface #ozone pollution across Midlands, North West, N Wales, Yorkshire, Scottish Borders and Eastern regions.
What is interesting though, is that the "moderate" air pollution in #London and the south east is being driven mainly by enhanced particulate matter (PM10 and PM2.5).
This seems to be due to two factors: 1) higher particulate concentrations in the SE, and 2) a more pronounced ozone diurnal cycle in the SE (which reduces the running 8-hr mean ozone used for the air quality index). e.g. compare Rochester Stoke (SE) and Sheffield.
The meteorological situation helps us understand what may be happening. A high pressure established to the east allows slack transport of regional European pollution into the SE, combining with local emissions --> enhanced PM.
These conditions are v similar to those that tend to drive springtime pollution episodes during the UK in Feb/Mar. Those tend to be characterised by widespread PM2.5 enhancement and low ozone. An interesting contrast.
https://twitter.com/steverarnold/status/1098954320169631744?s=20
Hot summer temperatures mean a likely different aerosol composition and stability (e.g. dominance of nitrate during springtime events, penetrating inland to central UK) and conditions much more conducive to ozone formation, despite similar large-scale met.
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