4.1 Summary statistics

The following tables present pollutant statistics for the period from the start of the year to the reported month. The Low, Moderate and High indicates the number of days for a particular pollutant when the concentrations are in the corresponding AQI bands from the beginning of the year until the end of the reported month.

4.2 AQ index distribution

The plots below illustrate the distribution of AQ index values for each site by pollutant. It shows the number of days that site concentrations are in each index. More information on the AQ Index is available from UK-Air ².

4.3 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.

4.4 Time series plot

The plots below show the time series of concentrations. Each pollutant is presented on a different tab and all sites are shown on each plot for comparison. A daily average resolution has been chosen as the most appropriate metric over a variety of different time windows. Zooming in on specific periods of the plot can be done by dragging a box over the section of the main plot frame or by using the sliders in the narrow plot frame below. To return to the default (all data) zoom level, double click the plot. Holding the mouse over the lines will highlight specific values and time stamp for that record for each station.

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 only the three months of the quarter or a dot showing one month depending on the time frame 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, NOx 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.

NO₂

Figure 15: Time variation of hourly NO₂ concentration.

O₃

Figure 16: Time variation of hourly O₃ concentration.

SO₂

Figure 17: Time variation of hourly SO₂ concentration.

Black Carbon

Figure 18: Time variation of hourly Black Carbon concentration.

VOCs

Figure 19: Time variation of hourly VOCs concentration.

4.6 Calendar plot

The plot below shows daily variation in concentrations by pollutant (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 date is coloured by the wind speed for that day. The actual value can also be seen by hovering the mouse on the cell.

4.7 Back trajectory analysis

The back trajectory plot shows data from the HYSPLIT model (NOAA HYSPLIT ³) run in analysis mode. This shows the 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 (Figure 29). This is useful to get an overview of air mass origins during the report period. Plots in Trajectories associated with top ten most polluted days provide information on the trajectory direction associated with the top 10 measured concentrations.

4.7.1 Trajectory clusters

4-day air mass back trajectories arriving at the sites for 2023 are grouped into 4 clusters.

Figure 29: Back trajectory clusters for the month reported.

Air mass back trajectories over these spatial scales do not vary locally so the receptor location used in this report has been selected from a range of national receptor locations maintained by Ricardo Energy & Environment. The receptor point is used here is London Bridge Place which is 46.2km away from the centre of all monitoring stations analysed in this report.

4.8 Polar plot map

The maps below show polar plots for each pollutant at each monitoring station superimposed on the region to help understand the relative strength and direction of sources. The distance from the plot origin shows the wind speed.

These plots are useful to help identify primary pollutants which are emitted directly into the atmosphere, especially when multiple plots are used in conjunction to ‘triangulate’ a common source. For pollutants with a secondary component (i.e. formed through chemical reactions in the atmosphere, e.g. NO₂, PM_2.5 and O₃) the directional signature seen in the measurements may not be as strong.