Last night PM2.5 and PM10 levels reached new highs. PM2.5 was around the 100ug/m³ level for 7 hours until 0600 GMT this morning, 23 Feb 2019. Levels above 71 are considered very high on DEFRA's DAQI system. Peak PM2.5 was 110ug/m³ at around 2330 yesterday.

PM2.5 for 24h until morning of 23 Feb 2019

The sustained high level of pollution inevitably means a high 24h average figure. This is still rising as I type (1930 GMT) but the present 24h value is 66ug/m³.

Cause is unknown still at this point. It was misty/foggy all day yesterday on the coast, so that does not appear to be the cause of the high readings. The high pressure and slow winds may have been enough to bring in pollution from shipping in the Channel (again), but I am just speculating.

Update: 2000 GMT - This pollution bulge may even have come from Paris! See this amazing GIF from MetClim. PM10 forecast Looks like we should expect high levels of pollution for most of the rest of the week, courtesy of France and Channel shipping. This Tweet from MetClim:
For more info see this Tweet from MetClim