Air Quality Trends for Scotland

A series of graphs of air quality data in Scotland have been prepared using the latest figures up to and including 2009 and are presented below.

Annual Mean trends for Nitrogen Dioxide

Plots for annual mean trends for NO₂ at Roadside and Kerbside and background sites are presented in Figure 1:

The data presented in Figure 1 indicates a long-term decline in ambient concentrations of nitrogen dioxide concentrations in urban areas of Scotland. Data prior to 2001 should be treated with caution due to the smaller number of monitoring locations that were in place at this time.

Roadside and Kerbside NO₂

Ratified monitoring data for 2009 shows lowest average concentrations of NO₂ to-date across Scotland. This provides encouragement in that average roadside concentrations of NO₂ have now been below 40 µg m^-3 for two consecutive years. This is despite accepted evidence of a levelling-off in the reduction in concentrations in recent years which UK experts (AQEG 2007) believe may be due to:

• An increase in the proportion of NO₂ emitted directly to the atmosphere. This is due to the increased market penetration of diesel cars and the retrofitting of pollution control devices, such as catalytically regenerative traps to buses.
• Increasing background O³ which promotes the oxidation of emitted NO to NO₂.

Urban Background NO₂

In 2009, the calculation was based on 8 monitoring sites in 7 different urban areas. This represents a relatively small dataset, as most of the recently introduced NO₂ monitoring stations in Scotland have been focused on roadside or kerbside locations. This is because most background locations already meet the 40 µg/m³ air quality objective.

Taking into consideration 2009 average NO₂ concentrations, the data set continues to show a downward trend.

Annual Mean Trends for Particulate Matter (PM₁₀)

Trends in Particulate Matter (PM₁₀) across Scotland are illustrated in Figure 2. These trends are of great interest since:

• Scotland has adopted a more stringent annual mean PM₁₀ objective (18 µg/m³) than the rest of the UK (40 µg/m³) for 2010.
• Scientists do not believe that there is actually a safe level of this pollutant in terms of human health effects.

Figure 2 demonstrates that there has been a general reduction in urban background PM₁₀ concentrations since 1992, but that for the last few years concentrations have hovered around the 18 µg m^-3 annual mean Objective. For roadside sites the trend is similar but with more year-to-year variation than background concentrations. There are two possible reasons for this:

• Concentrations at roadside monitoring stations are more likely to be affected by localised and perhaps short-term pollution issues caused by road works, building construction or demolition, or temporary changes to traffic flow or intensity.
• From 2004 onwards there has been a dramatic increase in the number of roadside monitoring sites in Scotland in each consecutive year as illustrated in Table 1 below. Current policy is to include all data in the analysis, but this may affect the robustness of the underlying trend.

Once again there is encouragement as the 2009 data shows concentrations are, for the first time, below the 18 µg m^-3 objective for both mean background and Roadside PM₁₀.

Table 1: Increasing number of PM₁₀ Kerbside/Roadside Monitoring Sites in Scotland: 2004-2009

Year	2004	2005	2006	2007	2008	2009
Number of Sites	5	13	18	27	41	49

Annual Mean Trends for Ozone (O₃)

Figure 3 presents the trend in average annual ozone concentrations at rural and urban background monitoring site across Scotland. The data illustrates increases in both urban background and rural concentrations of ozone. This increase in ozone is small but detectable at rural and remote locations, but dramatic and clear at urban background locations in Scotland. This clear increase in urban areas is largely due to the reduction in NO_x concentrations.

Whilst the reduction of ozone concentrations is outside the control of local authorities, and even Scottish Government to a certain extent, it is an extremely important pollutant in terms of health impacts. The increasing concentrations in urban areas are of particular concern.