Air quality study: assessing variations in roadside air quality with sampling height

Executive summary

The Scottish Government commissioned a study to undertake a mobile air quality monitoring study. The two key aims of the study were:

• Determine the relationship between height from pavement and air quality under a range of conditions; and
• Investigate the relationship between concurrent air quality sampling obtained from mobile and fixed sampling stations.

The study primarily focussed on particulate matter with mean aerodynamic diameters of 2.5 m (PM _2.5) and 10 m (PM ₁₀), ultrafine particles ( UFP) between the sizes of 10 nm and 300 nm and black carbon ( BC). In addition, the following pollutants were monitored:

• Nitrogen dioxide (NO ₂);
• Sulphur dioxide (SO ₂);
• Carbon monoxide (CO);
• Benzene (C ₆H ₆);
• Particulate matter with a mean aerodynamic diameter of 0.5 m (PM _0.5);
• Particulate matter with a mean aerodynamic diameter of 1.0 m (PM _1.0);
• Particulate matter with a mean aerodynamic diameter of 5.0 m (PM _5.0);
• Total particulate matter ( TPM).

The study was carried out in Glasgow City Centre with a total of 11 colocation exercises to evaluate agreement between samplers; and 8 mobile monitoring exercises to evaluate potential concentration gradients between 0.80 m and 1.68 m. All monitoring exercises were carried out between February 2014 and September 2014. In order to maximise the range of conditions captured (e.g. traffic and weather conditions), the monitoring exercises were carried out over 6 weekdays (2 x Fridays) and 2 weekend days.

Contour plots of pollutant concentrations throughout the mobile monitoring route were produced in order to help analyse the spatial distribution of pollutants and assess the influences of local factors, such as traffic density.

The findings of the Glasgow study demonstrated the complexity of ambient air quality within the urban environment and the challenges of monitoring a range of important pollutants within such an environment. Within a limited duration exercise, the study provided valuable insights into not only spatial variations of key air pollutants in Glasgow City centre but also the influence of exposure height on the concentrations observed. In addition, the study also helped to elucidate correlations between observed concentrations of different pollutants at different exposure heights and also provide useful information regarding the application of mobile monitoring and the application of personal sampler/ sensor technology for assessing human exposure to air pollutants in an urban environment.

In brief summary, the study identified that spatial and temporal variations in pollutant concentrations in Glasgow City centre are generally species specific and can also be influenced by a wide range of other environmental parameters, such as the proximity to pollution sources. Whilst a large number of initial conclusions have been drawn, several of the findings are considered to merit further attention. These were:

• It was identified that consistently higher average concentrations of PM _2.5 (42% higher) and PM ₁₀ (47-63% higher) were recorded by the mobile monitoring trolley when monitoring along the study route than those reported at the fixed Glasgow Kerbside AURN monitoring site.
• Average concentrations of PM ₁₀ were shown to be consistently higher (up to 12.6% higher) at 0.80 m than at 1.68 m throughout the mobile monitoring trolley study route. This finding is significant, as it appears to indicate that in the urban environment children may be exposed to higher concentrations of PM ₁₀ on average than adults. However, this relationship was not observed for concentrations of PM _2.5.
• For NO ₂, at lower ambient concentrations, no consistent influence of exposure height was identified on observed concentrations. However, at higher ambient concentrations, such as that may be observed in close proximity to busy road junctions, significantly higher concentrations were observed at adult breathing height (1.68 m) than at child breathing height (0.80 m).
• Visualisation of pollutant concentrations monitored through the application of the mobile monitoring trolley demonstrated potential for the identification or confirmation of pollutant hotspots within urban environments. Through the application of such a mobile monitoring 'screening' approach, authorities may be able to confirm the presence and extent of pollution hotspots, and thus investigate, design and implement appropriate mitigation measures.

Overall, the study provided valuable insights into spatial (vertical and horizontal) variations of concentrations of key air pollutants in Glasgow City centre. The study has generated numerous interesting findings which with further research could help to inform and guide future air quality research and policy in Scotland.