September is normally a good month for air pollution. By this time of year the sun is starting to weaken, making it hard for summertime smog to form and the nights are not yet long, or cold, enough for wintertime smog to form. Warm sunny weather at the start of the month led to some moderate levels of ground-level ozone across England as far north as Walsall. London and the south-east also suffered some particle pollution problems at this time.
However the big pollution event of the month happened around 25 September as a high pressure system over Belgium caused air to circulate slowly over the UK and the near continent. Polluted air settled over most of England as far north as Manchester, Leeds and Stockton and as far west as Bristol. In London particulate pollution reached level nine on the ten point UK air quality index. The city of Paris also issued a pollution alert. Later in the month nitrogen dioxide caused pollution problems in central Glasgow and industrial pollution affected Port Talbot in south Wales.
During September the UK sent its annual air pollution assessment to the EU. For air pollution purposes the UK is divided into 43 areas. During 2012, 38 of these areas breached the EU limits for nitrogen dioxide. These limits were set in 1999 but concentrations in London are still two and a half times the legal requirement. Only Preston, Blackpool and rural areas of Scotland and Northern Ireland achieved the standard.