A series of graphs of air quality data in Scotland have been prepared using the latest figures up to and including 2022 and are presented below. This trend analysis summarises how air quality in Scotland has changed over recent years and focuses on pollutants NO2, PM10, PM2.5 and O3.
The trend analysis has been carried out using Openair; the air pollution analytical tool available via the Openair page on this website. The analyses were carried out using the ‘TheilSen’ tool. This uses the Theil-Sen statistical method to determine trends in pollutant concentrations over several years. The trend analysis is based on monthly mean pollutant concentrations. The data used in these trend graphs has been ‘de-seasonalised’ (i.e. the data has been statistically modify to remove the influence of seasonal cycles, thus providing a clearer indication of the overall trend over the relevant time).
In these plots, the trend line is shown by a solid red line, with 95% confidence intervals for the trend shown by dotted red lines. The trend is given at the top of the plot in green, with confidence intervals shown in square brackets. The trend is given as units (i.e. µg m-3) per year, over the period shown. This may be followed by a number of stars:
* indicating that the trend is statistically significant at the 0.05 level
** indicating significance at the 0.01 level and
*** indicating significance at the 0.001 level.
The symbol + indicates that the trend is significant at the 0.1 level.
All the sites featured in this section have been in operation for a minimum of five consecutive years, as this is usually considered to be the minimum required to assess long-term trends at a monitoring site.
Use the links below to navigate to the section you are interested in. Clicking on the graphs will open them in a new window.
- Annual Mean trends for Nitrogen Dioxide (NO2)
- Annual Mean Trends for Particulate Matter (PM10)
- Annual Mean Trends for Particulate Matter as (PM2.5)
- Annual Mean Trends for Ozone (O3)
Further information on air quality trends for a range of pollutants is reported in more detail within the SAQD Annual Report and Local Air Quality Management (LAQM) reports; available on the Technical Reports page.
Annual Mean Trends For Nitrogen Dioxide (NO2)
In Scotland (as elsewhere in the UK), the largest number of AQMAs have been declared in response to exceedances of objectives for nitrogen dioxide (NO2). This is also reflected in the number of monitoring stations reporting exceedances for this pollutant. In particular, the objective of 40 µg m-3 for annual mean NO2 concentration is the most widely exceeded. Therefore, it is important to understand how concentrations of this pollutant vary with time.
NO2 at Urban Non-Traffic sites
There are relatively few long-running urban non-traffic sites monitoring stations in Scotland due to the nature of the pollutant and its main source (vehicles). However it is important to obtain background levels of NO2 to help ascertain what concentrations away from the source are like, as this helps identify general conditions affecting the population and also the extent of the issue at areas of concern. Five of these sites have been in operation for the past 15 years. These are as follows: Aberdeen Errol Place (closed in September 2021), Edinburgh St Leonards, Fort William, Glasgow Anderston and Grangemouth. For clarification, Fort William is classified as a ‘suburban’ site, Grangemouth is an ‘urban industrial’ site, and the other three are ‘urban background’. Please note that we have described these sites as Urban Non-traffic sites along with all other rural, background and industrial site types.
Figure 1 provides NO2 trend for these sites from 2008 to 2022. All sites display statistically highly significant negative trends (at the 0.001 level) over this time period.
Figure 1 - Trends in NO2 Concentrations at Five Long-Running Urban Non-Roadside sites, 2008-2022
Figure 2 takes into consideration analysis from all urban Non-traffic site in Scotland over the past five years to identify whether the trend is consistent over more recent times. As can be seen the decreasing trend continues across all sites in recent years however with varying statistical significancy.
Figure 2 - Trends in NO2 Concentrations at All Urban Non-Traffic sites, 2018-2022
NO2 at Urban Traffic Sites
Recent years have seen a substantial increase in the number of monitoring stations at urban traffic (roadside and Kerbside)-related sites in Scotland. There are now over 50 urban traffic (roadside and Kerbside) automatic monitoring sites in Scotland that have been in operation for 10 years or more and are still in operation. To identify where these sites are, please go to the Latest & Forecasts page.
This is a large number of sites, so we have selected eight sites from the list of long-running sites. These are:
|Aberdeen Union Street
|Edinburgh St John’s Road
|Aberdeen Wellington Road
|Glasgow Kerbside (Hope Street)
|Dundee Lochee Road
|North Lanarkshire Chapelhall
|Perth Atholl Street
Figure 3 shows the trend plots for these sites from 2013 to 2022. All eight sites show statistically highly significant decreasing trends.
Trends over the most recent five complete years, 2018 – 2022, have also been examined for these sites (Figure 4). Comparing the 10-year and five-year trends, the patterns are similar with statistically highly significant decreasing trends . Notably, for the Aberdeen and Dundee sites, the decreasing trend has become greater in magnitude over the past five years.
Figure 3 - Trends in NO2 Concentrations at Eight Long-Running Urban Traffic Sites with Exceedances, 2013-2022
Figure 4 - Recent trends in NO2 Concentration at Eight Long-Running Urban Traffic Sites with Exceedances, 2018-2022
Particulate Matter (PM10 and PM2.5)
This pollutant is of particular interest because:
- Current evidence suggests that there is no safe level of particulate matter in terms of human health effects.
- Scotland’s current annual mean PM10 objective is 18 µg m-3, which is more stringent than the objective of 40 µg m-3 adopted in the rest of the UK.
- In 2016 Scotland opted to make its annual mean PM2.5 objective more stringent,by reducing it from 12 µg m-3 to 10 µg m-3 in line with the World Health Organization guideline.
PM10 at Urban Traffic Sites
There are 51 PM10 monitoring sites in Scotland that have been monitoring for over 10 years. Trends in de-seasonalised monthly mean PM10 concentrations for eight traffic-related sites in operation since 2010 or earlier are shown in Figure 5. The sites selected for this analysis are Aberdeen Wellington Road, East Dunbartonshire Bearsden, Edinburgh Salamander, Fife Cupar, Glasgow Anderston, Glasgow Byres Road, Perth Crieff and West Lothian Broxburn. These sites were chosen to be analysed because of the length of time they have been monitoring (10 years or more), historical exceedances of the annual mean objective and geographical coverage.
Figure 5 illustrates that all sites, except Glasgow Byres Road, show statistically highly significant decreasing trends (at the 0.001 level). These trends indicate that PM10 concentrations over the past 10 years is, in general, decreasing year on year at these urban traffic locations. The Glasgow Byres Road site trend analysis indicates no real change in concentration.
Trends in monthly mean PM10 concentrations for the same eight sites (plus Edinburgh Queensferry Road), for the most recent five complete years 2018 – 2022, are shown in Figure 6. The analysis shows that the decreasing trend is plateauing at a number of sites with varying levels of statistical significancy. In contrast to the 10-year analysis, the five-year analysis for Glasgow Byres Road shows that concentrations are now decreasing with highly statistical significancy.
Figure 5 - Trends in PM10 Concentration at Eight Long-Running Urban Traffic Sites, 2013–2022
Figure 6 - Recent Trends in PM10 Concentrations at Eight Urban Traffic Sites, 2018–2022
Particulate Matter PM2.5
There are currently 83 sites monitoring PM2.5 in Scotland. However, the vast majority of these sites started monitoring in the last six years with the introduction of the PM2.5 objective and the requirement for local authorities to measure the pollutant. By the end of 2022 there were four sites with 10 consecutive years of PM2.5 data. These sites are: Aberdeen Errol Place (urban background), Auchencorth Moss (rural), Edinburgh St Leonards (urban background), and Grangemouth (urban industrial). The trend plot for these sites is shown in Figure 7.
The Analysis shows that all sites show a slight decreasing trend over the 10-year time period at varying levels of statistical significancy.
Trend analysis for a selection of urban traffic sites (chosen for their geographical coverage) that have been monitoring for the past 5 years is provided in Figure 4.8. As can be seen, all nine sites are very similar with slight decreasing trends at varying statistical significance. This is relatively consistent with the 10-year analysis.
Figure 7 - Trends in PM2.5 Concentrations at Four Long-Running Monitoring sites, 2013-2022
Figure 8 - Trends in PM2.5 Concentrations at Urban traffic sites, 2018-2022
Three of Scotland’s rural air quality monitoring stations have been monitoring ozone for 32 years, 1986 – 2022. These are Bush Estate, Eskdalemuir and Strath Vaich. Figure 9 shows long-term trends in de-seasonalised monthly mean ozone (O3) concentrations at these three exceptionally long-running rural monitoring sites. Bush Estate and Eskdalemuir both show small but statistically highly significant increasing trend in monthly mean rural ozone concentrations over this period. For Strath Vaich, there has been neither an increasing or decreasing trend over the same period with concentrations generally staying the same. The charts also show a significant amount of fluctuation; this may reflect the fact that ozone is formed by reactions involving other pollutant gases, in the presence of sunlight. Thus, ozone concentrations depend substantially on weather conditions. There is also evidence that the “hemispheric background” concentration of O3 has increased since the 1950s due to the contribution from human activities.
Six sites have been in operation for over 10 years. These are the above three sites, plus Auchencorth Moss, Glasgow Waulkmillglen Reservoir and Lerwick. Trends in ozone concentration at these six sites are shown in Figure 10. The ten-year trend analysis shows that three sites have increasing trends in O3 concentrations at varying levels of statistical significance. The other three sites (Bush Estate, Eskdalemuir and Strath Vaich) have slight decreasing trends; however Bush Estate and Eskdalemuir shows no statistical significance and Strath Vaich is significant to 0.05.
Figure 9 - Trends in O3 Concentrations at Long-Running Rural Sites, 1986–2022
Figure 10 - Trends in O3 Concentrations at Six Long-Running Rural Sites, 2013–2022