The number of automatic monitoring sites in Wales has increased greatly in recent years. While this helps to improve our understanding of air quality across the country, it potentially complicates the investigation of how air quality has changed over time. If such investigations are based on all available data, discontinuities and false trends may be introduced because of changes in the composition of the network. Here, investigation of changes has been based on subsets of long-running sites rather than on every site in the network. This should lead to a more robust assessment.
In Wales (and the rest of the UK), the most widely exceeded limit value is the annual mean nitrogen dioxide concentration (40 μg m-3). The figure below shows how annual mean nitrogen dioxide concentrations have varied with time. Urban background sites are represented by the longest running site of this type (Cardiff Centre) from 1992, and a subset of four long-running sites which have all been in operation since 2003, with annual data capture of at least 50% – Cardiff Centre, Cwmbran, Newport St Julians and Port Talbot (replaced by the nearby Port Talbot Margam site in 2007 – the two Port Talbot sites are treated as one for the purpose of the graph). Cardiff Centre shows a clear decrease from 1992 to around 2000, after which the downward trend appears to have levelled off. The mean for the long-running sites shows a slight decrease through the 2000s, although 2010 was a higher year. Urban traffic sites (those within 10 m of a major road) are represented by the longest-running roadside site (Swansea Morriston) from 2001, and a subset of two long-running sites that have been in operation since 2002 – Swansea Morriston and Wrexham. Neither Swansea Morriston nor the mean of the two long-running sites shows any clear pattern of increase or decrease in nitrogen dioxide concentration in recent years.
The figure below shows how annual mean concentrations of particulate matter up to 10 μm in size (PM10) have generally decreased in recent years at urban background and urban traffic sites. Urban non-roadside sites are represented by the mean of three long-running sites from 2001 (Cardiff Centre, Cwmbran, and Port Talbot/Port Talbot Margam – again, the latter two are treated as one site for this purpose). Please note that Port Talbot/Port Talbot Margam is classified as urban industrial rather than urban background as it is located in the vicinity of a large steelworks. It has been included because there are few long-running urban non-roadside sites. Cardiff Centre (which has operated for longer than any other site) is also shown individually. All sites have at least 70% annual data capture except Cardiff Centre in 2010. Urban traffic sites are represented by the mean of two longrunning sites from 2002 – Rhondda-Cynon-Taf Nantgarw and Wrexham. Wrexham (the longest-running traffic site) is also shown individually.
Ozone concentrations tend to be highest at rural locations. The figure below shows how annual mean rural ozone concentration has changed over time. This is based on the mean concentration measured by three long-running sites in Wales (shown by the grey line) – Aston Hill, Marchlyn Mawr and Narbeth. All have been in operation since 2003 or earlier, with data capture of at least 70%. Also shown is Aston Hill alone – this site has been monitoring ozone since the late 1980s. Although there are no clear trends, concentrations vary considerably from year to year because of variation in meteorological factors.