4.1 Summary statistics

The following tables present some basic pollutant statistics for the period selected in the report. The Low, Moderate and High indicates the percentage of times when pollution concentrations are in the DAQI bands described above. Comparisons have been made with the UK Objectives as well. The Praxis/Cube sensor is not a reference equivalent monitor but it is certified as indicative for particulate matter measurements. Although it does not have indicative certification for gaseous measurements. However, these results are still useful in assessment against air quality objectives and in studies such as this.

In 2024, the periodic means measured by the Swale Teynham sensor for PM and NO₂ are below the annual mean stated in the UK objectives. For PM₁₀, there were 6 exceedances of 24-hour UK objectives, but this is still within the allowance of exceedances (35 times). The 90.4 percentile for 24-hour PM₁₀ is 23 μg m^-3, which is less than 50 μg m^-3.

In 2025, the periodic means measured by the Swale Teynham sensor for PM and NO₂ are still below the annual mean stated in the UK objectives. For PM₁₀, there were 19 exceedances of 24-hour UK objectives, but this is still within the allowance of exceedances as well. The 90.4 percentile for 24-hour PM₁₀ is 65.9 μg m^-3, which is over 50 μg m^-3. It is likely that PM₁₀ episodes occurred between January and March 2025. To evaluate the variation of air pollutant concentrations across different months, time variation plots are recommended. As only three months of data are available for 2025, this dataset is insufficient to capture full annual variation. Therefore, the 2024 results are considered more representative of typical annual conditions.

There were no exceedances for NO₂. For PM_2.5 24-hour average, it is not stated in the UK objectives, so no comparison was made.

To estimate the annual mean for Swale Teynham, annualisation is performed. The estimated annual means for Swale Teynham still comply with the UK objectives for all pollutants. The details of the calculation are provided below.

4.2 Time series plot

The plots below show the time series of concentrations each hour. Each pollutant is presented on a different tab and all sites are shown on each plot for comparison. Both PM plots include a rolling 24-hour average plot with shading by the range of DAQI index, and an hourly average plot as a reference. The NO₂ plot includes only an hourly average plot, also shaded by the range of DAQI index.

The plot is interactive and you can change what is shown by the following:

• Clicking on the “Current month” button shows the data for the reporting period. All data since the sensor installed can be seen by clicking on the “All” button.
  
• Zooming in on specific periods can be done by dragging a box over the section of the main plot.
  
• Holding the mouse over the lines will highlight specific values and time stamp for that record for each site.
  
• Double-clicking on a site in the legend will show the time series for the selected site.
  
• To return to the default (all data), double click the plot.

4.3 AQ index distribution

The plots below illustrate the distribution of AQ index values for each site by pollutant. It shows the number of occasions (hours) that each site reported concentrations are in each index. More information on the AQ Index is available from UK-Air. Note that this summary presents data at hourly resolution which differs from Defra’s Daily AQ Index (DAQI) which takes the worst across all pollutants and hours to represent each day.

4.4 Boxplots

The plots below are box and whisker plots to show the distribution in concentrations for each monitoring station. The boxes demarcate the lower quartile, median and upper quartile. The whiskers extend to the maximum and minimum values within median ± 1.5 times interquartile range (IQR). Values outside the median ± 1.5 times IQR are generally considered as outliers.

Swale Teynham has lower median and quartile values in PM₁₀ and PM_2.5, compared to the other two locations. The outliers are more contained, without extreme high values as seen at the other two sites.

4.5 Time Variation plot

These plots show concentrations over different time intervals such as diurnal, day of week and month of year. The plot showing seasonal variation will show the months from the sensor installed to the month covered in this report. The topmost frame shows the concentrations as they vary by hour of the day and day of the week. The hour of the day variation is summarised on its own in the lower left pane and the variation by day of the week is shown in the lower right pane. These plots often help explain variations in concentration according to the emissions activity associated with them. For example, NO_x concentrations at roadside sites tend to exhibit peaks according to morning and evening traffic rush hours and tend to decline over weekends when there is generally lower traffic volumes.

Swale Teynham shows lower PM concentrations and less pronounced diurnal patterns compared to the other two sites. The monthly averages are generally lower as well, although PM₁₀ levels increase between October and December, reaching or even exceeding those observed at Swale Newington 4. This increase may be related to domestic burning or other seasonal activities. Swale Teynham recorded higher PM₁₀ concentrations between January and March compared to Swale Newington 4. However, other monitoring sites also experienced elevated PM₁₀ concentrations during February and March, suggesting that seasonal factors may have contributed to the observed increase across locations.

For NO₂, the diurnal patterns are similar across the four sites. Swale Teynham’s diurnal concentrations fall approximately in the middle range relative to the others. However, its monthly averages drop slightly from November to December, making it the site with the lowest NO₂ concentrations during those months.

4.6 Calendar plot

The plots below show daily variation in concentrations by pollutant and site (one on each tab) across the period of the report, as laid out in a calendar style. This allows intuitive viewing of day to day headline trends in the wider context of the period. The background colours shown for each day relate to the concentration. The actual concentrations, along with the wind speed, can also be seen by hovering the mouse on the cell.

4.7 Polar plot map

The map shows polar plots for each pollutant at each monitoring station. The bivariate polar plots determine how the concentrations are varied by wind speed and wind direction.The distance from the plot origin shows the wind speed. The concentration of the pollutant is shown by different colour scale by the concentration range of that station. To look into a specific site, pan and zoom into areas of interest to get greater detail.

Swale Teynham shows that there are likely local emission sources, as PM and NO₂ exhibit higher concentrations when wind speeds are lowest or medium wind speeds from northeast. PM₁₀ also shows a high concentration when wind speeds are highest from the southwest. This pattern is not observed for PM_2.5 and NO₂, suggesting that the canyon effect is unlikely. It is possible that a distant source from that direction is contributing to the PM₁₀ levels in Swale Teynham when the wind speed is very high.

4.8 Back trajectory analysis

The back trajectory plot shows data from the HYSPLIT model (NOAA HYSPLIT) run in analysis mode. This shows the 72-hour air mass back trajectories for the period covered by the report. Two different kinds of plot are shown. One statistically groups the trajectories into similar clusters and shows the proportion of time during the report period that each represents. This is useful to get an overview of air mass origins during the report period. A plot of the trajectories associated with the top 10 measured concentrations is also presented.