This report is the sixth of a series summarising air pollution monitoring activities associated with the Highways Agency (HA) long term roadside monitoring network. TRL has been operating this network on behalf of the HA since its commission in the early 1990s. This report provides an analysis of data measured during 2006. Historic trends in individual pollutant concentrations are presented and a range of data mining tools, including bivariate polar plots, pairs plots and generalised additive modelling (GAM) are used to further investigate trends and source contributions.
In 2006, all sites exceeded the Air Quality Strategy (AQS) nitrogen dioxide (NO2) annual mean objective although concentrations of NO2 were lower than in 2005 except at the M25. This might have been due to changes in traffic flows following completion of road widening activities. The M60 site was the only site to breach the two ozone objectives. This was likely to be driven by area wide contributions and episodes during the summer months. It is noted that the AQS objectives do not apply at these sites, as there is no public exposure.
A combination of traffic flow data and standard emission factors were used to derive emissions estimates at each site. Emissions of regulated pollutants from across the HA network have significantly declined over the years, except for CO2 which has increased. However, on a year-to-year basis, these changes are subject to considerable variability. Source analysis identified that concentrations of CO were the most heavily influenced by emissions from the motorway, particularly at the M4 and M25. There was no such clear pattern for other pollutants, which is not unexpected given that non-motorway sources are likely to be a major contributor to concentrations.

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