The weather can cause significant variation from year to year in the number of days of moderate or higher air pollution. The hot summer and other pollution episodes in 2003 and 2006 led to an unusually high number of pollution days.
In urban areas, air pollution in 2008 was recorded as moderate or higher on 26 days on average per site, compared with 23 days in 2007, and 59 days in 1993. This series has shown a high degree of year-on-year variability.
In rural areas, air pollution in 2008 was moderate or higher for 45 days on average per site, compared with 30 days in 2007. This series has also fluctuated significantly over time.
In the South West, Bournemouth and Bristol St Pauls were monitored urban sites, recording 65 and 33 days of poor air quality respectively. As with the national trend, these two sites have shown significant variability. However, between the number of poor air quality days increased by 11 days in Bournemouth between 2007 and 2008 and by 1 day in Bristol St Pauls.
Only Yarner Wood was monitored for rural air quality in the region in 2008, which recorded 36 days of poor air quality. Although this was eight days below the national average and seven days below the 2007 figure for the region, there is no clear trend in rural air quality due to significant year-on-year variability.
Instances of moderate and high air pollution in the region have occurred due to high levels of ground level ozone. Ozone is formed by chemical reactions between oxides of nitrogen and hydrocarbons in the lower layers of the atmosphere. Concentrations tend to be highest in the southern regions, near the coast and at high altitudes because high temperatures and sunshine levels promote the photochemical reactions that generate ground level ozone. Urban areas tend to have lower levels of ground level ozone due to higher levels of nitric oxide than rural areas, which can destroy ozone.
High levels of ground level ozone can be detrimental to health, causing irritation of eyes, nose and lungs. As a result, Defra's provisional health objective is that the daily maximum 8 hour running mean should not exceed a concentration of 50 parts per billion on more than 10 days a year at any site by 2005. There has been no clear trend in the average annual concentration of ground level ozone over the last 20 years nationally. The 6 monitoring sites in the South West also show no clear trend, however, all showed a significant decline between 2003 and 2005, an increase between 2005 and 2006 and decline again between 2006 and 2007.
In 2007 the number of days exceeding the maximum 8 hour mean ranged from 54 days in Plymouth Centre to 71 days in Yarner Wood. Yarner Wood and Somerton, the two rural monitoring sites in the region, were both above the UK average of 67 days. Whilst Bournemouth and Bristol St Pauls both registered above the UK average of 57 days, with 68 and 59 days respectively, Plymouth Centre recorded just below with 54 days.
