Scottish Air Quality Maps
Annual mean NOX, NO2, PM10 and PM2.5 modelling for 2022
| Authors | Jasmine Wareham, John Stedman, Rebecca Morris and David Hector, Ricardo |
| Compilation date | 14 October 2024 |
| Customer | Scottish Government |
| Approved by | John Stedman |
| Copyright | Scottish Government |
| EULA | Ricardo Report EULA |
| Contract reference | 688677 | Report reference | ED19050 - Issue Number 1 |
Executive summary
The UK PCM methodology has been applied to provide Scotland-specific air pollutant maps of annual mean background and roadside NOX, NO2, PM10 and PM2.5 concentrations for the Scottish Government for 2022. The methodology was tailored to represent air pollutant concentrations in Scotland, using Scotland-specific measurements from Scottish air quality monitoring sites to calibrate and verify the model.
Scottish air pollutant emissions were based on the UK National Atmospheric Emissions Inventory (NAEI) 2021 (Ingledew et al., 2023) and BEIS EEP 2022 energy and emissions projections (2022). The NAEI 2021 road transport emissions include:
- Department for Transport (DfT) traffic data
- Emission factors for NOX, PM10 and PM2.5 from COPERT 5.4 (2020)
- Updated DfT assumptions relating to the mileage splits by fuel type for passenger cars and LGVs, which include greater ambition on the uptake of electric vehicles
- Impact of the EU Directive on motorcycle emissions covering stricter emissions standards (2013)
In summary, the results of the 2022 Scotland-specific modelling exercise show:
- The modelled annual mean background NO2 concentrations from the Scotland-specific model provided reasonable agreement with the annual mean measured background NO2 concentrations. A similar degree of variability was seen when the modelled annual mean roadside NO2 concentrations from the Scotland-specific model were compared to the annual mean measured roadside NO2 concentrations. No exceedances of the Scottish annual mean NO2 air quality objective of 40 \(\mu\)g m-3 were modelled at background or roadside locations.
- The modelled annual mean background PM10 concentrations
from the Scotland-specific model provided good agreement with the annual
mean measured background concentrations. Reasonable agreement was seen
with the modelled annual mean roadside PM10 concentrations
provided by the Scotland-specific model. No exceedances of the annual
PM10 air quality objective of 18 \(\mu\)g m-3 were modelled at
background or roadside locations.
- The modelled annual mean background PM2.5 concentrations from the Scotland-specific model provided reasonable agreement with the annual mean measured background concentrations. Reasonable agreement was seen with the modelled annual mean roadside PM2.5 concentrations provided by the Scotland-specific model. No exceedances of the annual PM2.5 air quality objective of 10 \(\mu\)g m-3 were modelled at background or roadside locations.
Forward projections of NO2, PM10 and PM2.5 concentrations to future years are not carried out annually. Forward projections of background annual mean concentrations of NO2 and PM10 from a base year of 2018 to years up to 2030 are available for download (see the Data for Local Authority Review and Assessment purposes page on the Air Quality in Scotland website). Forward projections from a base year of 2021 are in progress and will replace the 2018 projections on the link above, when available. Due to the Scottish-specific nature of the data, it is more appropriate for anyone undertaking air quality studies in Scotland to use the Scottish background maps rather than data from the UK maps. The Scottish Government therefore expects Scottish local authorities to use the Scottish-specific modelling for LAQM purposes. Other organisations undertaking air quality studies in Scotland are directed to the Scottish-specific background modelled data. Please note the available projections from 2018 are based on assumptions that were applicable prior to the Covid-19 pandemic, and as such, do not reflect short- or long-term impacts of the pandemic and associated lockdowns on emissions in 2020 and beyond.
1 Introduction and overview
The air pollution concentration maps presented in this report for the year 2022 (see Sections 5.1, 5.2 and 5.3), have been prepared for Scotland under the Scottish Air Quality Database (SAQD) project, using Scotland-specific versions of the UK Pollution Climate Mapping (PCM) modelling methods (see Pugsley et al., 2024). The UK maps are published towards of the end of the year following the model year. The Scotland-specific modelling requires elements from the UK modelling and as such, the Scottish maps are published the year following the UK publication. The maps provide a representation of the spatial distribution of background and roadside annual mean concentrations of:
- NOX and NO2
- Gravimetric equivalent PM10
- Gravimetric equivalent PM2.5
Background concentrations are mapped at a spatial resolution of 1 km2 (1 km x 1 km grid) and roadside concentrations are mapped for urban major road links throughout Scotland.
The UK version of the PCM provides air pollution concentration maps in order to supplement measurements from the national monitoring networks and to satisfy the annual Air Quality Standards Regulations (AQSR) (AQSR, 2010a) reporting requirements for the Scottish Government, Defra, and the other Devolved Administrations. Components of the UK PCM model are calibrated by comparison with measurements from the national networks (chiefly the Automatic Urban and Rural Network, AURN), with calibration coefficients being representative of the scaling between these model components and measurements for the whole of the UK.
Scotland was previously identified as an area that may not conform as well as other regions with the general calibration approach performed for the UK PCM modelling, due to the limited number of urban areas in Scotland (clustered in the central belt and north east coast) and distinct meteorology. A Scotland-specific study was undertaken within the SAQD project where the PCM modelling was calibrated using measurements exclusively from Scottish air quality monitoring sites, and a Scottish meteorological data set was applied in the dispersion modelling, in order to improve the representativeness of the concentration maps.
An overview of the modelling methodology used to produce the 2022 air pollution concentration maps for Scotland is given in Section 2 of this report. The calibration and verification of the modelling using Scottish air quality monitoring data is described in Sections 3 and 4, respectively. As noted above, the air pollution concentration maps for 2022 are presented in Sections 5.1, 5.2 and 5.3.
Scotland is split into four zones (Highland, North East Scotland, Central Scotland, and the Scottish Borders) and two agglomerations (Edinburgh Urban Area and Glasgow Urban Area) for the purposes of air pollution monitoring, in accordance with the AQSR (AQSR, 2010a). Using the Scotland-specific modelled air pollutant concentration maps, an assessment has been made by zone of area, population, number of road links and road length exposed to specific concentrations above the Objective values adopted by the Scottish Government for NO2, PM10 and PM2.5. The results of the 2022 Scotland-specific modelling assessment in terms of exceedances of the objective values is given in Section 5.4. Calculations of population-weighted mean concentrations of NO2, PM10 and PM2.5 from the Scotland-specific model are summarised in Section 5.5.
Scotland-specific pollutant maps for NOX, NO2, PM10 and PM2.5 have been produced for 2022. The maps of modelled pollutant concentrations presented here are designed as an indicative, rather than absolute, measure of the annual mean NO2, PM10 and PM2.5 concentration at background and roadside locations.
2 Methodology
A detailed description of the UK PCM modelling methodology can be found in the annual report to the Scottish Government, Defra and the other Devolved Administrations for 2022 (Pugsley et al., 2024). An overview of the Scottish-specific PCM modelling methodology is presented here, including descriptions of:
- The emissions data applied (see Section 2.1)
- Specific aspects of the model tailored to represent air pollutant concentrations in Scotland (see Section 2.2)
- Impacts of the Covid-19 pandemic for aircraft and air support (see Section 2.3)
- Monitoring sites used for model calibration and verification (see Section 2.4)
- The methods for modelling concentrations of NOX and NO2 (see Section 2.5), PM10 (see Section 2.6) and PM2.5 (see Section 2.7)
- The availability of forward projections of background concentration maps for NO2, PM10 and PM2.5 (see Section 2.8)
2.1 Emissions data
Spatially disaggregated emissions information have been input to the PCM modelling system to produce the 2022 annual mean air pollutant concentration maps presented in this report. Scottish air pollutant emissions were based on the UK National Atmospheric Emissions Inventory (NAEI) 2021 (Ingledew et al., 2023) and BEIS EEP 2022 energy and emissions projections (2022). The treatment of emissions from aircraft and shipping within the PCM model is described in Appendix 6 of Pugsley et al. (2024). Estimates of the emissions from road transport sources include the following assumptions:
- Department for Transport (DfT) 2022 traffic data
- Emission factors for NOX, PM10 and PM2.5 from COPERT 5.4 (2020)
- Updated DfT assumptions relating to the mileage splits by fuel type for passenger cars and LGVs, which include greater ambition on the uptake of electric vehicles
- Impact of the EU Directive on motorcycle emissions covering stricter emissions standards (2013)
2.2 The Scotland-specific model
The UK PCM modelling methodology has been adapted for Scotland-specific modelling by incorporating air quality monitoring measurements from Scottish monitoring sites only in the model calibration (as shown in Section 2.4, Figure 1).
No attempt has been made to model hourly concentrations for comparison with the Scottish hourly or daily air quality objectives (AQOs) for NO2 and PM10 in this report. This is due to the considerable uncertainties involved in modelling at such a fine temporal scale.
2.3 Covid-19 pandemic impacts for aircraft and air support
National and regional lockdowns were introduced in 2020 due to the Covid-19 pandemic. As a result, air quality in 2020 was impacted because of the substantial changes in emissions activity. Activity levels returned to near-normal levels in 2021 for most sectors. For the 2022 Scotland-specific modelling, emissions estimates from the 2021 NAEI have been used to estimate emissions for 2022. Additional steps have been taken to capture the impact of the exceptional changes in activity in two related sectors: aircraft and air support, for which activity levels did not return to near-normal levels until 2022.
Levels of activity do not typically vary by a large amount from year-to-year. In previous years, emissions used in the modelling have been calculated by scaling forward emission estimates by one year based on emission projections from the NAEI. For the 2022 modelling, this approach has been used for all sectors except aircraft and air support, which did not return to normal levels of activity in 2021 and therefore there was a significant difference between activity levels in 2021 and 2022. The large changes in activity levels for these sectors during 2021 and 2022 have therefore been accounted for in the modelling by applying scaling factors for aircraft and air support of 2.155 and 2.094 respectively. These were derived from Civil Aviation Authority (CAA) aircraft movement statistics (CAA, 2022).
2.4 Air quality monitoring sites used for model calibration and verification
For 2022, as for previous years, both calibration and verification of the Scotland-specific model was undertaken. For the Scottish maps of NOX and NO2 for the year 2022, automatic measurements of these pollutants have been used in the calibration and verification of the background and roadside modelling. PM10 and PM2.5 monitoring data have been used in the calibration and verification of the background and roadside modelling for the 2022 PM10 and PM2.5 maps. The majority of the monitoring sites use FIDAS but where appropriate, a combination of FIDAS and Beta Attenuation Monitors (BAMs) data were used.
In recent years, the Scottish Government has invested significant effort into increasing the number of sites in the Scottish Air Quality Database (SAQD). The additional sites funded by the Scottish Government and placed on the SAQD were ratified in an equivalent manner to air pollutant measurements from the national Automatic Urban and Rural Network (AURN). Table 1 lists the Scottish air quality monitoring sites used to calibrate and verify the background and roadside NOX, NO2, PM10 and PM2.5 models.
Sites with a percentage data capture (%dc) less than 75% were not used to calibrate or verify the model output. Typically around half a dozen or more sites are required to calibrate the model, with remaining sites used to verify the model. Table 2 summarises the number of sites used to calibrate and verify the background and roadside models for each pollutant.
Measurements from AURN sites were used for model calibration in line with the UK PCM modelling method. Measurements from non-AURN air quality sites were split between sites for model calibration and model verification. The non-AURN air quality sites were partitioned on the basis of their location in order to provide a reasonable geographical spread of sites. Figure 1 shows the locations of calibration and verification sites for the NOX and NO2 models and the PM10 and PM2.5 models.
Table 1 is interactive. Columns can be sorted and filtered, and the entire table can be searched using the search box.
Table 1: Scottish air quality monitoring sites used to calibrate and verify the output of the 2022 Scotland-specific model.
Table 2: Number of sites used calibrate and verify each model.
| Pollutant | Site classification | Number of calibration sites | Number of verification sites |
|---|---|---|---|
| NOx, NO2 | Roadside | 14 | 30 |
| NOx, NO2 | Background | 8 | 4 |
| PM10 | Roadside | 9 | 31 |
| PM10 | Background | 9 | 4 |
| PM25 | Roadside | 9 | 30 |
| PM25 | Background | 9 | 4 |
Click on the markers to see site name and classification and zoom in and out to see more local detail. Select calibration and verification sites to display from the menu.
Figure 1: Location of Scottish air quality monitoring sites used to produce Scotland-specific background and roadside NOX, NO2, PM10 and PM2.5 maps for the year 2022, The coloured markers show the NOX and NO2 calibration and verification sites (green and orange, respectively), PM10 calibration and verification sites (red and blue, respectively) and PM2.5 calibration and verification sites (purple and yellow, respectively).
© Crown copyright. All rights reserved Defra, licence number 100022861 [2024].
2.5 NOX and NO2 modelling
Annual mean concentrations of NOX were modelled for Scotland for 2022 at background and roadside locations. Modelled annual mean NO2 concentrations were calculated from modelled NOX concentrations using a calibrated version of the updated oxidant-partitioning model (Jenkin 2004; Murrells et al. 2008; Jenkin 2012). This model is discussed in more detail in the 2022 UK mapping report (Pugsley et al., 2024) and the references therein. Briefly, the oxidant-partitioning model uses representative equations to account for the chemical coupling of O3, NO and NO2 within the atmosphere. A key advantage of this approach for modelling NO2 concentrations is that it allows different emission scenarios to be addressed by varying regional oxidant levels and/or primary NO2 emissions.
The modelling of the annual mean Scottish NOX and NO2 concentrations for 2022 were undertaken using the Scotland-specific adaptations to the UK PCM model noted in Section 2.2. The regional oxidant component of the NOX-NO2 model remains unchanged for the Scottish modelling as analysis commissioned for the UK PCM model has demonstrated an average value can be assigned to the UK as a whole, given that inter-zonal variations in regional oxidant are generally not statistically significant.
Scottish air quality objectives (AQOs) (AQSR, 2010b) for ambient NO2 concentrations are based on limit values set out in the Air Quality Standards Regulations (AQSR) (AQSR, 2010a). These have been specified for the protection of human health. The limit values are:
- An annual mean concentration of 40 \(\mu\)g m-3, and
- An hourly mean concentration of 200 \(\mu\)g m-3, with 18 permitted exceedances each year.
For the hourly objective, it is appropriate to have an allowed number of exceedances for the objective. This allows for situations where it is not practical to expect hourly average concentrations to always be within the allowed limit (for example, where bad weather conditions affect concentrations). Compliance with the annual mean limit value is considered to be more stringent than achieving compliance with the 1-hour limit value in the majority of situations (AQEG, 2004). This is illustrated in Figure 2 which shows the 2022 annual mean NO2 concentrations at all Scottish air quality monitoring sites (with an annual % data capture ≥75%) plotted against the 99.8th percentile (equivalent to 18 exceedances) hourly mean concentration for the same year. The plot shows no sites exceeded the annual mean limit value of 40 \(\mu\)g m-3 and no sites exceeded the 200 \(\mu\)g m-3 hourly limit value.
Figure 2: Plot of annual mean against 99.8th percentile hourly NO2 concentrations in 2022 for all Scottish air quality monitoring sites (% dc ≥ 75%).
2.6 PM10 modelling
2022 annual mean concentrations of PM10 were modelled for Scotland at background and roadside locations. The modelling methodology used to calculate the annual mean PM10 concentrations is broadly similar to that used for 2021; further detail can be found in the 2022 UK mapping report (Pugsley et al., 2024). As noted in Section 2.4, measurements from PM10 monitoring sites (FIDAS and BAM) were used to calibrate and verify the model.
It should be noted that many of the chemical components of the PM10 model are not affected by the Scotland-specific changes to the UK PCM model noted in Section 2.2. This includes the contribution to the total PM10 mass from the following components:
- Secondary inorganic aerosols (e.g. sulphate, nitrate, ammonium-based particles)
- Secondary organic aerosols (SOA)
- Primary particles from long-range transport
- Sea salt aerosol
- Iron and calcium-rich dusts
Scottish AQOs (AQSR, 2010b) for ambient PM10 concentrations for 2022 are more stringent than the limit values set out in the Air Quality Standards Regulations (AQSR, 2010a). In Scotland the PM10 objectives are:
- A 24-hour mean concentration of 50 \(\mu\)g m-3, not to be exceeded more than 7 times a year, and
- An annual mean value of 18 \(\mu\)g m-3.
2.7 PM2.5 modelling
2022 annual mean concentrations of PM2.5 were modelled for Scotland at background and roadside locations. The modelling methodology used is consistent with the PM10 model and further detail can be found in the 2022 UK mapping report (Pugsley et al., 2024). The 2022 maps have been calibrated using measurements from sites for which co-located PM10 measurements are also available.
Scottish AQOs (AQSR, 2010b) for ambient PM2.5 concentrations for 2022 include an annual mean value that is more stringent than the value set out in the Air Quality Standards Regulations (AQSR, 2010a). In Scotland the PM2.5 objective is:
- An annual mean value of 10 \(\mu\)g m-3.
2.8 Forward projections
Forward projections of air pollutant concentrations to future years are not produced annually. Projections are currently only available for NOX and PM10. The most recently available forward projections are from a base year of 2018 (Wareham et al., 2020) (see the Data for Local Authority Review and Assessment purposes page on the Air Quality in Scotland website). Forward projections from a base year of 2021 are in progress and will replace the 2018 projections on the link above, when available.
Due to the Scottish-specific nature of the data, it is more appropriate for anyone undertaking air quality studies in Scotland to use the Scottish background maps rather than data from the UK maps. The Scottish Government therefore expects Scottish local authorities to use the Scottish-specific modelling for LAQM purposes. Other organisations undertaking air quality studies in Scotland are directed to the Scottish-specific background modelled data.
Please note the available projections from 2018 are based on assumptions that were applicable prior to the Covid-19 pandemic, and as such, do not reflect short- or long-term impacts of the pandemic and associated lockdowns on emissions in 2020 and beyond.
3 Model calibration
3.1 Overview
Calibration of the 2022 area source components of the background NOX, NO2, PM10 and PM2.5 Scotland-specific modelling was undertaken using measurements from air quality monitoring stations situated within a range of background (rural, suburban, urban background and urban industrial) locations. Similarly, the roadside calibration was undertaken using air quality monitoring measurements from roadside (roadside and kerbside) sites. As noted in Section 2.4, only Scottish air quality monitoring data from background and roadside sites with an annual percentage data capture (%dc) ≥75% were used to prepare the calibration plots and to verify the 2022 Scotland-specific model, as discussed in Section 4.
The NOX, NO2, PM10 and PM2.5 area source and roadside calibration procedures are discussed below.
3.2 NOX and NO2 modelling
3.2.1 Annual mean background NOX and contributions from local area sources
Dispersion kernels generated with ADMS 5.2 were used to calculate the contribution of local area source emissions to ambient NOX concentrations at a central receptor location from area source emissions within a 33 km x 33 km square surrounding each receptor. 2022 hourly sequential meteorological data from the Weather Research and Forecasting (WRF) numerical weather prediction modelling system, were used to construct the dispersion kernels. More information on the meteorological input data to the model can be found in Appendix 4 of the 2022 UK modelling report (Pugsley et al., 2024).
The modelled NOX contributions from point sources, combustion in industry area sources and regional rural NOX sources have been subtracted from the measured annual mean NOX concentration at background sites. This concentration is compared with the uncalibrated modelled local area source contribution to annual mean NOX concentrations to calculate the calibration coefficient used in the area source modelling. Ambient NOX concentrations from eight background NOX monitoring sites were used to calibrate the model.
Figure 3 shows the calibration plot for the Scotland-specific NOX local area source model. The gradient of the line of best fit forced through the origin was 1.522, and this value was used as the Scotland-specific NOX area source calibration coefficient. The regression line has been forced through the origin to provide a reasonable model output without imposing an unrealistically high residual on the area source contribution. For each grid square in Scotland the uncalibrated modelled local area source contribution was multiplied by the Scotland-specific calibration coefficient to calculate the calibrated area source contribution. Point source, combustion in industry area source and regional rural contributions from distant sources were then added, resulting in a map of total background annual mean NOX concentrations for Scotland for 2022 as shown in Figure 24. A detailed description of the background modelling approach for NOX and NO2 can be found in Section 3 of the 2022 UK modelling report (Pugsley et al., 2024).
Figure 3: Calibration of the Scotland-specific background NOX model for the year 2022.
As noted above, background area source NOX model calibration coefficients applied in the Scotland-specific and UK PCM models are compared in Figure 4. This presents the calibration coefficients from 2008 to 2022 and highlights:
- the year-to-year variation in the Scotland-specific background area source NOX model calibration coefficient;
- the extent to which these values vary in both models.
From Figure 4 it can be seen that the 2022 Scotland-specific background area source NOX calibration coefficient was greater than the equivalent value used in the UK PCM model in 2022 due to the different mix of sites included in the UK and Scotland-specific PCM model calibrations. Overall the Scotland and UK coefficients are quite similar for each year.
Figure 4: Background model calibration coefficients applied in the Scotland-specific and UK models for NOX for 2008 to 2022.
3.2.2 Roadside NOX concentrations
It is assumed that the annual mean NOX concentration at roadside locations (\(NO_{X,roadside}\)) is made up of two components: the background concentrations as described above (\(NO_{X,background}\)) and a local roadside increment (\(NO_{X,roadside-increment}\)):
- \(NO_{X,roadside} = NO_{X,background} + NO_{X,roadside-increment}\)
The NAEI provided estimates of NOX emissions for major road links in Scotland for 2021 (Ingledew et al., 2023). Values for 2022 have been calculated using traffic data and emission factors for 2022. In 2022 the roadside increment was calculated using the PCM Roads Kernel Model (PCM-RKM) which was introduced into the UK PCM model in 2014. The PCM-RKM uses the ADMS-Roads 5.0 dispersion model to calculate the roadside increment to NOX concentrations on urban major roads. Individual model runs were carried out for 956 census points covering urban major roads in Scotland. Each model run is parameterised using specific input data for the census point:
- Road geometry
- Traffic speeds, emissions and traffic counts
- Meteorology
- Receptor locations
The input data used for this Scotland-specific modelling is consistent with the data used for UK PCM modelling and is described in detail in Appendix 8 of the 2022 UK modelling report (Pugsley et al., 2024). Modelled concentrations were derived from the ADMS-Roads model outputs at a distance of 4 m from the kerb, averaged across each side of the road for all road links modelled. The PCM-RKM represents a more process-based approach than the previous empirical method. It provides a more robust assessment, whilst retaining the link with measurement data provided by the use of measurement data to calibrate this component of the model.
The PCM-RKM modelled roadside increment is calibrated by comparison with an empirically calculated roadside increment (measured roadside NOX concentration minus modelled background NOX concentration) at Scottish roadside or kerbside monitoring sites. Calibration of the PCM-RKM is shown in Figure 5. The background NOX component at these roadside monitoring sites is taken from the background NOX map described in the previous section. The calibration coefficient derived is then applied to the roadside increment calculated using the PCM-RKM for each road link. The background NOX is added to the calibrated roadside increment, resulting in a map of total roadside annual mean NOX concentrations for Scotland for 2022.
Of the seventy four roadside NOX monitoring sites within the Scottish network, forty four had sufficient data capture for NOX (and NO2) in 2022 and were located on modelled major road links close to census points at which the traffic flow along the road was measured. The roadside NOX measurements from fourteen of these sites were used to calibrate the model. Scottish NOX air quality monitoring data from thirty roadside sites was used to verify the model output.
A total of three roadside NOX monitoring sites were excluded on the basis of low annual data capture (annual percentage data capture <75%). A further twenty six roadside sites were excluded because they were not located on a modelled urban major road link. The air quality monitoring sites outside of the AURN are typically installed at kerbside or roadside for AQMA purposes and may not be located on modelled major road links.
Figure 5: Calibration of the Scotland-specific roadside increment NOX model for the year 2022.
Figure 6 provides a comparison of the 2022 Scotland-specific and UK roadside NOX calibration coefficients. The Scotland-specific and UK roadside NOX calibration coefficients from recent previous years are also presented. From Figure 6 it can be seen that the 2022 Scotland-specific roadside NOX calibration coefficient was less than the value used in the 2022 UK PCM model, which as noted in relation to the background model is due to the different mix of sites included in the UK and Scotland-specific PCM model calibrations.
Figure 6: Roadside model calibration coefficients applied in the Scotland-specific and UK models for NOX for 2014 to 2022.
3.2.3 NO2 modelling
Maps of the estimated 2022 annual mean background and roadside Scottish NO2 concentrations were calculated from the modelled NOX concentrations using a calibrated version of the updated oxidant-partitioning model (Jenkin 2004; Murrells et al. 2008; Jenkin 2012). This model uses representative equations to account for the chemical coupling of O3, NO and NO2 within the atmosphere. A key advantage of this approach for modelling NO2 concentrations is that different emission scenarios can be addressed by varying regional oxidant levels and/or primary NO2 emissions.
This approach is discussed in detail in Section 3.4 of the 2022 UK modelling report (Pugsley et al., 2024). Briefly, the oxidant-partitioning model enables NO2 concentrations to be calculated using the following equations:
- \([NO_2] = [OX].f(NO_X)\)
- \([OX] = f{NO_2}.[NO_X] + [OX]_B\)
Where \(f(NO_X)\) expresses \([NO_2]/[OX]\) as a function of \([NO_X]\) and hence allows the calculation of the concentration of NO2 from the concentration of NOX when combined with \([OX]\), the total oxidant (the sum of NO2 and O3 concentrations). Here \(fNO_2\) is the primary NO2 emission fraction (defined as the proportion of, or volume ratio, of NOX that is directly emitted as NO2), and \([OX]_B\) is the regional oxidant. The regional oxidant component of the NOX-NO2 model remains unchanged from the UK PCM modelling (see Pugsley et al., 2024) as this is not affected by the Scotland-specific changes to the UK PCM model noted in Section 2.2. NOX, NO2, O3 and OX are all expressed as ppb in these equations: 1 ppb of O3 = 2 \(\mu\)g m-3; 1 ppb of NO2 = 1.91 \(\mu\)g m-3. By convention when NOX is expressed in \(\mu\)g m-3 it is expressed as “\(\mu\)g m-3 as NO2” therefore 1 ppb of NOX = 1.91 \(\mu\)g m-3 of NOX as NO2.
In Jenkin (2004), \([NO_2]/[OX]\) was calculated using two equations, one of which represented background locations and the other roadside locations. Updated equations for \([NO_2]/[OX]\) were subsequently developed in Murrells et al. (2008). More recently, Jenkin (2012) found that short term variability in NOX concentrations is a major cause of the scatter in the relationship between \([NO_2]/[OX]\) and \([NO_X]\). The ratio of the upper to the lower quartile of hourly concentrations is a good indicator of this variability and this ratio increases with decreasing NOX concentrations at roadside and background sites. This dependence has been used to interpolate between equations based on a constant NOX quartile ratio. This led to two equations for calculating \([NO_2]/[OX]\), one of which represents background locations and the other, roadside locations, derived from background and roadside monitoring sites respectively. The Jenkin (2012) equations are as follows:
- For background locations: \[ \begin{aligned} ~[NO_2]/[OX] & = -2.5124\times{10^{-13}[NO_X]^6} + 1.5805\times{10^{-10}[NO_X]^5} - 4.1429\times{10^{-8}[NO_X]^4} \\ & + 5.8239\times{10^{-6}[NO_X]^3} - 4.8076\times{10^{-4}[NO_X]^2} + 2.5916\times{10^{-2}[NO_X]} \end{aligned} \]
- For roadside locations: \[ \begin{aligned} ~[NO_2]/[OX] & = -2.0901\times{10^{-13}[NO_X]^6} + 1.5001\times{10^{-10}[NO_X]^5} - 4.2894\times{10^{-8}[NO_X] ^4} \\ & + 6.2659\times{10^{-6}[NO_X]^3} - 5.0720\times{10^{-4}[NO_X]^2} + 2.5322\times{10^{-2}[NO_X]} \end{aligned} \]
Where \([NO_2]/[OX]\) and \([NO_X]\) are in ppb. The equations for the Scottish maps are the same as are used for the UK PCM model for 2022 but the background and roadside calibration coefficients for NO2 are specific to the Scottish maps.
Detailed description of how the NO2 modelling approach is applied can be found in the 2022 UK modelling report (Pugsley et al., 2024), this includes:
- Derivation of the value for regional oxidant (\([OX]_B\)) for the UK
- Calculation of the local oxidant (\(f{NO_2}.[NO_X]\)) at background and roadside locations
- Calculation of \(f(NO_X)\) (i.e. \([NO_2]/[OX]\)) in the PCM model
- How the updated oxidant-partitioning model has been applied in the UK to background and roadside locations
The resultant maps of background and roadside annual mean NO2 concentrations for Scotland for 2022 are shown in Figure 25 and Figure 26, respectively. In terms of roadside locations, the roadside concentrations included in this report correspond only to the urban major road links (A-roads and motorways) that are reported as part of the annual compliance assessment for the UK as a whole.
3.3 PM10 modelling
3.3.1 Annual mean background PM10 and contributions from local area sources
The dispersion kernels applied in the background NOX modelling (as described in Section 3.2), have also been applied to calculate the contribution of local area source emissions to ambient PM10 concentrations for the year 2022. This provided the uncalibrated modelled Scotland-specific PM10 area source contribution.
The modelled PM10 contributions from point sources, combustion in industry area sources, secondary inorganic aerosol (SIA), secondary organic aerosol (SOA), iron and calcium-rich dust, long range transport (LRT) of primary PM10, sea salt and a residual contribution have been subtracted from the measured annual mean PM10 concentration at background sites. This concentration is compared with the uncalibrated modelled local area source contribution to annual mean PM10 concentrations to calculate the calibration coefficient used in the area source modelling. Ambient PM10 concentrations from nine background PM10 monitoring sites were used to calibrate the model.
Figure 7 shows the calibration plot for the Scotland-specific PM10 local area source model. The gradient of the line of best fit forced through the origin was 1.502, and this value was used as the Scotland-specific PM10 area source calibration coefficient. The regression line has been forced through the origin to provide a reasonable model output without imposing an unrealistically high residual on the area source contribution. For each grid square in Scotland the uncalibrated modelled local area source contribution was multiplied by the Scotland-specific calibration coefficient to calculate the calibrated area source contribution. The calibrated area source contribution was then added to the contributions from combustion in industry area sources, from secondary organic and inorganic particles, from point sources, from regional primary particles, from sea salt, from calcium and iron rich dusts and the residual, resulting in a map of total background annual mean PM10 concentrations for Scotland for 2022 as shown in Figure 27. In 2022 the residual was set to zero in the total background annual mean PM10 concentration. This value of the residual was found to provide the best fit to the monitoring data for PM10 in 2022, consistent with that used in the 2022 UK mapping. A detailed description of the background modelling approach for PM10 can be found in Section 5 of the 2022 UK PCM modelling report (Pugsley et al., 2024).
Figure 7: Calibration of the Scotland-specific background PM10 model for the year 2022.
As noted above, background area source PM10 model calibration coefficients applied in the Scotland-specific and UK PCM models are compared in Figure 8. This presents the calibration coefficients from 2008 to 2022 and highlights:
- the year-to-year variation in the Scotland-specific background area source PM10 model calibration coefficient;
- the extent to which these values vary in both models.
From Figure 8 it can be seen that the 2022 Scotland-specific background area source PM10 calibration coefficient was greater than the equivalent value used in the UK PCM model in 2022 due to the different mix of sites included in the UK and Scotland-specific PCM model calibrations.
Figure 8: Background model calibration coefficients applied in the Scotland-specific and UK models for PM10 for 2008 to 2022.
Since 2016, the Scotland-specific and UK calibration coefficients were lower than in earlier years and closer to 1. This is primarily a result of updates that improved the representation of two components of the model in 2016:
- Secondary Inorganic Aerosol (SIA): Monthly measurements are available for 28 rural monitoring sites within the UKEAP AGANet and NAMN networks and the measurements are interpolated to provide maps of SIA concentrations across the UK. The measurement method used within the AGANet was changed at the beginning of 2016. This revised method typically results in higher measured concentrations of sulphate and nitrate (Tang et al., 2015).
- Domestic wood combustion: The methods used to estimate the spatial distribution of emissions from domestic wood combustion were revised for the NAEI 2015 to incorporate new information from a survey of domestic wood use (BEIS, 2016). The revised spatial distribution places a larger proportion of these emissions in large urban areas than in previous assessments.
3.3.2 Roadside PM10 concentrations
As for NOX, it is assumed that the annual mean PM10 concentration at roadside locations is made up of two components: the background concentration as described above (\(PM_{10,background}\)) and a local roadside increment (\(PM_{10,roadside-increment}\)):
- \(PM_{10,roadside} = PM_{10,background} + PM_{10,roadside-increment}\)
The NAEI has provided estimates of PM10 emissions for major road links in the UK for 2021 (Ingledew et al., 2023). Values for 2022 have been calculated using traffic data and emission factors for 2022. In 2022 the roadside increment was calculated using the PCM Roads Kernel Model (PCM-RKM) which was introduced into the UK PCM model in 2014. The PCM-RKM uses the ADMS-Roads 5.0 dispersion model to calculate the roadside increment to PM10 concentrations on urban major roads. Individual model runs were carried out for 956 census points covering urban major roads in Scotland. For a short description of the application of the roadside increment modelling approach using PCM-RKM for Scotland-specific modelling see Section 3.2, and for a detailed description see Appendix 8 of the 2022 UK modelling report (Pugsley et al., 2024).
The PCM-RKM modelled roadside increment is calibrated by comparison with an empirically calculated roadside increment (measured roadside PM10 concentration minus modelled background PM10 concentration) at Scottish roadside or kerbside monitoring sites. Calibration of the PCM-RKM is shown in Figure 9. The background PM10 component at these roadside monitoring sites is taken from the background PM10 map described in the previous section. The calibration coefficient derived is then applied to the roadside increment calculated using the PCM-RKM for each road link. The background PM10 is added to the calibrated roadside increment, resulting in a map of total roadside annual mean PM10 concentrations for Scotland for 2022. It should be noted that the roadside concentrations included in this report correspond only to the urban major road links (A-roads and motorways) that are reported as part of the annual compliance assessment for the UK as a whole.
Of the seventy roadside PM10 monitoring sites within the Scottish network, forty had sufficient data capture for PM10 in 2022 and were located on modelled major road links close to census points at which the traffic flow along the road was measured. The roadside PM10 measurements from nine of these sites were used to calibrate the model. Scottish PM10 air quality monitoring data from thirty one roadside sites were used to verify the model output.
A total of three roadside PM10 monitoring sites were excluded on the basis of low annual data capture (annual percentage data capture <75%). A further twenty six roadside sites were excluded because they were not located on a modelled urban major road link. The air quality monitoring sites outside of the AURN are typically installed at kerbside or roadside for AQMA purposes and may not be located on modelled major road links.
Figure 9: Calibration of the Scotland-specific roadside increment PM10 model for the year 2022.
Figure 10 provides a comparison of the 2022 Scotland-specific and UK roadside PM10 calibration coefficients. The Scotland-specific and UK roadside PM10 calibration coefficients from 2014 are also presented. From Figure 10 it can be seen that the 2022 Scotland-specific roadside PM10 calibration coefficient was less than the value used in the 2022 UK PCM model, which as noted in relation to the background model is due to the different mix of sites included in the UK and Scotland-specific PCM model calibrations.
Figure 10: Roadside model calibration coefficients applied in the Scotland-specific and UK models for PM10 for 2014 to 2022.
3.4 PM2.5 modelling
3.4.1 Annual mean background PM2.5 and contributions from local area sources
The dispersion kernels applied in the background NOX modelling (as described in Section 3.2), have also been applied to calculate the contribution of local area source emissions to ambient PM2.5 concentrations for the year 2022. This provided the uncalibrated modelled Scotland-specific PM2.5 area source contribution.
The modelled PM2.5 contributions from point sources, combustion in industry area sources, secondary inorganic aerosol (SIA), secondary organic aerosol (SOA), iron and calcium-rich dust, long range transport (LRT) of primary PM2.5, sea salt and a residual contribution have been subtracted from the measured annual mean PM2.5 concentration at background sites. This concentration is compared with the uncalibrated modelled local area source contribution to annual mean PM2.5 concentrations to calculate a calibration coefficient, which is usually used in the area source modelling. However, for the year 2022, the background and roadside coefficients calculated for PM2.5, appeared unrealistic. The roadside coefficient was particularly low. The PM10 calibration coefficients were therefore used in the PM2.5 modelling instead to give a better representation of the contribution from area emission sources.
Figure 11 shows a plot comparing the uncalibrated area source contribution to the annual mean PM2.5 concentrations with the measured annual mean PM2.5 concentrations minus the non-calibrated sources. Ambient PM2.5 concentrations from nine background PM2.5 monitoring sites were included in the graph. The gradient of the line of best fit forced through the origin was 1.886. As detailed above, the PM10 background coefficient was used in the PM2.5 modelling instead of this value, and hence 1.502 was used. For each grid square in Scotland the uncalibrated modelled local area source contribution was multiplied by the Scotland-specific calibration coefficient to calculate the calibrated area source contribution. The calibrated area source contribution was then added to the contributions from combustion in industry area sources, from secondary organic and inorganic particles, from point sources, from regional primary particles, from sea salt, from calcium and iron rich dusts and the residual, resulting in a map of total background annual mean PM2.5 concentrations for Scotland for 2022 as shown in Figure 29. In 2022 the residual was set to zero in the total background annual mean PM2.5 concentration. This value of the residual was found to provide the best fit to the monitoring data for PM2.5 in 2022, consistent with that used in the 2022 UK mapping. A detailed description of the background modelling approach for PM2.5 can be found in Section 6 of the 2022 UK PCM modelling report (Pugsley et al., 2024).
Figure 11: Comparison of uncalibrated area source contribution to annual mean PM2.5 with measured annual mean PM2.5 minus non-calibrated sources (not used for calibration).
As noted above, background area source PM2.5 model calibration coefficients applied in the Scotland-specific and UK PCM models are compared in Figure 12. This presents the calibration coefficients from 2020 to 2022 and highlights:
- the year-to-year variation in the Scotland-specific background area source PM2.5 model calibration coefficient;
- the extent to which these values vary in both models.
From Figure 12 it can be seen that the 2022 Scotland-specific background area source PM2.5 calibration coefficient was greater than the equivalent value used in the UK PCM model in 2022 due to the different mix of sites included in the UK and Scotland-specific PCM model calibrations. This graph shows the coefficient that was used in 2022, which was chosen to be the same as the coefficient for PM10.
Figure 12: Background model calibration coefficients applied in the Scotland-specific and UK models for PM2.5 for 2020 to 2022.
3.4.2 Roadside PM2.5 concentrations
As for PM10, it is assumed that the annual mean PM2.5 concentration at roadside locations is made up of two components: the background concentration as described above (\(PM_{2.5,background}\)) and a local roadside increment (\(PM_{2.5,roadside-increment}\)):
- \(PM_{2.5,roadside} = PM_{2.5,background} + PM_{2.5,roadside-increment}\)
The NAEI has provided estimates of PM2.5 emissions for major road links in the UK for 2021 (Ingledew et al., 2023). Values for 2022 have been calculated using traffic data and emission factors for 2022. In 2022 the roadside increment was calculated using the PCM Roads Kernel Model (PCM-RKM) which was introduced into the UK PCM model in 2014. The PCM-RKM uses the ADMS-Roads 5.0 dispersion model to calculate the roadside increment to PM2.5 concentrations on urban major roads. Individual model runs were carried out for 956 census points covering urban major roads in Scotland. For a short description of the application of the roadside increment modelling approach using PCM-RKM for Scotland-specific modelling see Section 3.2, and for a detailed description see Appendix 8 of the 2022 UK modelling report (Pugsley et al., 2024).
The PCM-RKM modelled roadside increment is usually calibrated by comparison with an empirically calculated roadside increment (measured roadside PM2.5 concentration minus modelled background PM2.5 concentration) at Scottish roadside or kerbside monitoring sites. This comparison is shown in Figure 13. The background PM2.5 component at these roadside monitoring sites is taken from the background PM2.5 map described in the previous section. For 2022, the calibration coefficient derived appeared unrealistically low with a value of 0.2415 and so the PM10 roadside calibration coefficient (1.022) was used instead and was applied to the roadside increment calculated using the PCM-RKM for each road link. The background PM2.5 is added to the calibrated roadside increment, resulting in a map of total roadside annual mean PM2.5 concentrations for Scotland for 2022. It should be noted that the roadside concentrations included in this report correspond only to the urban major road links (A-roads and motorways) that are reported as part of the annual compliance assessment for the UK as a whole.
Of the sixty eight roadside PM2.5 monitoring sites within the Scottish network, thirty nine had sufficient data capture for PM2.5 in 2022 and were located on modelled major road links close to census points at which the traffic flow along the road was measured. The roadside PM2.5 measurements from nine of these sites were included in the graph. Scottish PM2.5 air quality monitoring data from thirty roadside sites were used to verify the model output.
A total of three roadside PM2.5 monitoring sites were excluded on the basis of low annual data capture (annual percentage data capture <75%). A further twenty six roadside sites were excluded because they were not located on a modelled urban major road link. The air quality monitoring sites outside of the AURN are typically installed at kerbside or roadside for AQMA purposes and may not be located on modelled major road links.
Figure 13: Comparison of the PM2.5 roadside increment from the roads kernel model with an empirically calculated PM2.5 roadside increment (not used for calibration).
Figure 14 provides a comparison of the 2022 Scotland-specific and UK roadside PM2.5 calibration coefficients. The Scotland-specific and UK roadside PM2.5 calibration coefficients from 2020 are also presented. From Figure 14 it can be seen that the 2022 Scotland-specific roadside PM2.5 calibration coefficient was less than the value used in the 2022 UK PCM model, which as noted in relation to the background model is due to the different mix of sites included in the UK and Scotland-specific PCM model calibrations. This graph shows the coefficient that was used in 2022, which was chosen to be the same as the coefficient for PM10.
Figure 14: Roadside model calibration coefficients applied in the Scotland-specific and UK models for PM2.5 for 2020 to 2022.
4 Model verification
The performance of the background and roadside Scottish-specific PCM modelling has been examined by comparison with measurements. For background model verification, modelled background annual mean pollutant concentrations were extracted for 1 km x 1 km grid squares containing background air quality monitoring sites. For roadside model verification, modelled roadside annual mean pollutant concentrations were extracted for urban major road links with roadside air quality monitoring sites. The modelled concentrations at Scottish air quality monitoring sites have been plotted against the measured pollutant concentrations in Figure 15 to Figure 22.
It should be noted that comparison of the modelling with measurements from air quality monitoring sites used to calibrate components of the model (background area sources and roadside increment) does not provide an independent indication of model performance. The model performance for sites used to calibrate the model (“calibration” sites) has therefore been evaluated separately to the model performance for those sites not used to calibrate the model which are a fully independent set (“verification” sites). A good agreement between the measured concentrations and the model outputs for “calibration” sites indicates that the calibration of the Scotland-specific model has worked well and there were no underlying problems with the model components, but it cannot provide confidence in the model result as a whole.
The UK PCM model is verified annually against independent air quality monitoring data available from sites located throughout the UK, which have not been used in the model calibration. These are typically air quality monitoring sites used for the purposes of LAQM, e.g., Local Authority sites located at roadside sites or in air pollutant hot-spot areas. The number of Scottish air quality monitoring sites used to verify the output of the Scotland-specific model in 2022, is summarised in Table 2 in Section 2.4. The number of sites available for verification of the Scotland-specific model is variable dependent on the air pollutant and site type (background or roadside). Most of the Scottish air quality monitoring sites used for model verification are in locations such that they would not be included in the UK model verification, for example, the sites might be close to a junction or more than 10 m from the road. This could help to explain some of the scatter for the model verification, shown in the sections below.
4.1 NOX and NO2 model verification
Model verification summary statistics for background and roadside sites are presented in Table 3 for NOX and NO2 for the Scotland-specific PCM modelling. The summary statistics presented in Table 3 are calculated for groups by calibration and verification sites, by pollutant and for background and roadside locations, and include:
- The mean of model estimates
- The mean of the corresponding measured values
- The number of sites included in the analysis for each metric
- The squared Pearson’s correlation coefficient, R2, indicating the correlation between measured and modelled data
- The percentage of sites at which modelled concentrations are outside of the DQOs
The percentage of monitoring sites for which the modelled annual mean concentration fell outside the data quality objectives was generally greater for NOX than for NO2.
Model verification plots are presented in Figure 15 and Figure 16 for background and roadside NOX, respectively. Figure 17 and Figure 18 show the verification plots for background and roadside NO2, respectively. Lines at ±30% are shown on the model verification plots. These lines represent the Data Quality Objectives (DQOs) specified in the Air Quality Standards Regulations (AQSR, 2010a) for modelled NO2 concentrations used for national reporting purposes. The plots show that the modelled values provide reasonable agreement with the measured annual mean concentrations, with some sites lying outside the DQOs.
Table 3: Summary statistics for comparison between modelled and measured NOX and NO2 concentrations at background and roadside sites (µg m-3, as NO2).
| Calibration/verification | Pollutant | Model | Mean of model estimates | Mean of measurements | R2 | % outside data quality objectives | Number of sites in assessment |
|---|---|---|---|---|---|---|---|
| calibration | NOx | Background | 14.5 | 16.3 | 0.678 | 37.5 | 8 |
| verification | NOx | Background | 13.3 | 15.9 | 0.980 | 25.0 | 4 |
| calibration | NOx | Roadside | 36.5 | 36.7 | 0.478 | 28.6 | 14 |
| verification | NOx | Roadside | 33.5 | 40.6 | 0.032 | 76.7 | 30 |
| calibration | NO2 | Background | 9.8 | 11.0 | 0.809 | 25.0 | 8 |
| verification | NO2 | Background | 9.1 | 10.0 | 0.985 | 25.0 | 4 |
| calibration | NO2 | Roadside | 18.5 | 18.6 | 0.589 | 14.3 | 14 |
| verification | NO2 | Roadside | 17.4 | 19.3 | 0.006 | 50.0 | 30 |
Figure 15: Scotland-specific annual mean background NOX model verification, for the year 2022.
Figure 16: Scotland-specific annual mean roadside NOX model verification, for the year 2022.
Figure 17: Scotland-specific annual mean background NO2 model verification, for the year 2022
Figure 18: Scotland-specific annual mean roadside NO2 model verification, for the year 2022.
4.2 PM10 model verification
Model verification summary statistics for background and roadside sites are presented in Table 4 for PM10. The summary statistics include the mean of model estimates, the mean of the corresponding measured values, the number of sites included in the analysis for each metric, the correlation (indicated by R2) between measured and modelled data, and the percentage of sites which fall outside of the DQOs.
The model verification plots for PM10 are presented in Figure 19 and Figure 20 for background and roadside sites, respectively. Lines at ±50% are shown on the model verification plots and represent the DQOs specified for modelled PM10 concentrations in the Air Quality Standards Regulations (AQSR, 2010a). The plots show that for PM10, the modelled annual mean concentrations provide good agreement with the measured annual mean concentrations and fall within the DQO specified by the AQSR.
Table 4: Summary statistics for comparison between gravimetric modelled and measured concentrations of PM10 at background and roadside sites (µg m-3, gravimetric).
| Calibration/verification | Pollutant | Model | Mean of model estimates | Mean of measurements | R2 | % outside data quality objectives | Number of sites in assessment |
|---|---|---|---|---|---|---|---|
| calibration | PM10 | Background | 9.2 | 9.5 | 0.307 | 0 | 9 |
| verification | PM10 | Background | 10.5 | 9.9 | 0.900 | 0 | 4 |
| calibration | PM10 | Roadside | 10.9 | 10.6 | 0.297 | 0 | 9 |
| verification | PM10 | Roadside | 11.0 | 11.3 | 0.007 | 0 | 31 |
Figure 19: Scotland-specific annual mean background PM10 model verification, for the year 2022.
Figure 20: Scotland-specific annual mean roadside PM10 model verification, for the year 2022.
4.3 PM2.5 model verification
Model verification summary statistics for background and roadside sites are presented in Table 5 for PM2.5. The summary statistics include the mean of model estimates, the mean of the corresponding measured values, the number of sites included in the analysis for each metric, the correlation (indicated by R2) between measured and modelled data, and the percentage of sites which fall outside of the DQOs.
The model verification plots for PM2.5 are presented in Figure 21 and Figure 22 for background and roadside sites, respectively. Lines at ±50% are shown on the model verification plots and represent the DQOs specified for modelled PM2.5 concentrations in the Air Quality Standards Regulations (AQSR, 2010a). The plots show that for PM2.5, the modelled annual mean concentrations provide good agreement with the measured annual mean concentrations and fall within the DQO specified by the AQSR.
Table 5: Summary statistics for comparison between gravimetric modelled and measured concentrations of PM2.5 at background and roadside sites (µg m-3, gravimetric).
| Calibration/verification | Pollutant | Model | Mean of model estimates | Mean of measurements | R2 | % outside data quality objectives | Number of sites in assessment |
|---|---|---|---|---|---|---|---|
| calibration | PM2.5 | Background | 5.0 | 5.5 | 0.242 | 0 | 9 |
| verification | PM2.5 | Background | 5.3 | 5.6 | 0.943 | 0 | 4 |
| calibration | PM2.5 | Roadside | 5.9 | 5.6 | 0.428 | 0 | 9 |
| verification | PM2.5 | Roadside | 6.0 | 5.8 | 0.055 | 0 | 30 |
Figure 21: Scotland-specific annual mean background PM2.5 model verification, for the year 2022.
Figure 22: Scotland-specific annual mean roadside PM2.5 model verification, for the year 2022.
5 Scottish pollutant maps and results for the year 2022
For the purposes of assessing compliance with air quality limit values, Scotland is divided into four zones (Highland, North East Scotland, Central Scotland, and the Scottish Borders) and two agglomerations (Edinburgh Urban Area and Glasgow Urban Area). The Scottish zones and agglomerations are mapped in Figure 23.
The zone and agglomeration map below is interactive. Click on the map to see the name of the zone or agglomeration and zoom in and out to see more local detail.
Figure 23: Zones and agglomerations in Scotland, 2022.