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The first week of September marks the end of the normal risk period for summertime smog in the UK. Summertime, or Los Angeles type, smog is caused by the sun's energy interacting with air pollution from plants, vehicles and industry. It takes several days to form so summertime smog is rare on Mondays, Tuesdays and Wednesdays due to lower weekend pollution emissions. In heatwave years ground-level ozone was linked to hundreds of UK deaths and it also affected London in the run up to the 2012 Olympic games, but there have been few serious problems with ground-level ozone from summertime smog this year; just a single day of high air pollution across eastern England in mid-July.
Autumn is often the least polluted time of year in the UK. Although sunlight weakens, the nights are not sufficiently long to allow strong temperature inversions to form and cause a build-up of air pollution in our cities. Spring and autumn experience similar sunlight and temperatures, however autumn is not affected by the pollutant load that builds up over the whole northern hemisphere during the winter. Additionally there is less use of agricultural fertilisers in autumn compared to that early in the growing season and there are therefore less agricultural ammonia emissions to contribute to particle pollution.
So, even with a hoped for Indian summer, the weakening sunlight and shorter days limit how much summertime smog can form. For instance the record breaking October temperatures of 2011 only caused moderate levels of air pollution.
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