- 1 Introduction
- 2 Monitoring stations
- 3 Data description
- 3.1 Relevant pollution Limit Values
- 4 Data Analysis
- 4.1 Summary statistics
- 4.2 AQ index distribution
- 4.3 Polar plot map
- 4.4 Boxplots
- 4.5 Time series plot
- 4.6 Time Variation plot
- 4.7 Calendar plot
- 4.8 Back trajectory analysis
Waltham Forest Monthly Report
Monthly report for February 2025
| Authors | Louisa Kramer |
| Compilation date | 04 March 2025 |
| Customer | London Borough of Waltham Forest |
| Approved by | Tim Bevington |
| Copyright | Ricardo |
| EULA | http://ee.ricardo.com/cms/eula/ |
| Contract reference | ED62657 | Report reference | February 2025 |
1 Introduction
This is a monthly summary report for London Borough of Waltham Forest for the period 2025 February.
This is a dynamic report containing embedded data from which the report tables, plots and graphs are generated. The embedded data allows the reader a level of interaction with some of the report findings, providing additional insight. This approach enables a more easily navigated and streamlined report providing an engaging and intuitive reader experience.
Maps for example can be panned and zoomed with different layers and markers turned on and off and with popup information by clicking on markers or hovering the mouse over them. Tables may contain much more information than initially displayed and can be set to show different numbers of rows, and can be filtered sorted or searched to display only specific information of interest. Some time series plots can be tracked with the mouse cursor to obtain specific time/date stamps and values and the reader can zoom into a specific time window by dragging with the cursor and double left clicking to return to the full plot extent.
The report is easily navigable using the floating table of contents on the left pane which tracks with the reader’s progress through the report and expands and collapses to a level of detail related to the subheadings used. The layout of the report is also dynamic, with some sections split into specific ‘tabs’ (e.g. per pollutant or per site) for ease of access to those sections.
2 Monitoring stations
A summary of site information is presented below in Figure 1 and Table 1. Full site information can be obtained using the URL hyperlink in the popup for each monitoring station on the map.
Figure 1: An interactive map showing the location of air quality sites included in this report.
3 Data description
Data contained within this report is managed by Ricardo and stored in a dedicated, secure database. The data within this report is provisional and subject to change, and as such should be treated with caution.
Some of the plots provided in this report use associated meteorological data (typically wind speed and wind direction) in order to show measured pollutant concentrations in a dispersion context. Very few monitoring stations provide quality controlled met data, therefore these reports import data from the Met Office DataPoint 1.
Gaseous pollutant mass units are at 20 °C and 1013mb. NOx mass units are NOx as NO2 μg m-3. Particulate matter concentrations are reported at ambient temperature and pressure.
3.1 Relevant pollution Limit Values
The European Air Quality Directive and Fourth Daughter Directive set out legal limits for different pollutants as Limit Values, Target Values or Long Term Objectives to protect human health. With the UK’s exit from the EU the UK’s Air Quality Strategy (AQS) is no longer tied to that of the EU, however the current objectives are at least as stringent as the EC limit values.
The UK AQS limits are summarised in the table below. Local authorities don’t typically measure ozone, benzene, B[a]P or metals that are captured within Defra’s national networks. All pollutants measured have been included in this data summary for completeness, irrespective of their significance for local authority policy interests.
4 Data Analysis
4.1 Summary statistics
The below two tabs contain summary statistics for the reported collection of measurement sites. “Cumulative Statistics” contains all data from the beginning of the year up until the reported month, whereas “Monthly Statistics” contains data for only the reported month.
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.
The following tables present pollutant statistics just for the reported month. For public data, these statistics will match those made available on Air Quality England. The Low, Moderate and High indicates the number of days for a particular pollutant when the concentrations are in the corresponding AQI bands during the reported month.
4.2 AQ index distribution
The plots below illustrate the distribution of AQ index values for each site by pollutant. They show the number of days that site concentrations are in each index. More information on the AQ Index is available from UK-Air 2.
“Cumulative AQI” visualises all air quality indices from the beginning of the year up until the reported month, whereas “Monthly AQI” visualises the air quality index for only the reported month.
Figure 2: Distribution of AQI for NO2.
Figure 3: Distribution of AQI for PM10.
Figure 4: Distribution of AQI for PM2.5.
Figure 5: Distribution of AQI for NO2.
Figure 6: Distribution of AQI for PM10.
Figure 7: Distribution of AQI for PM2.5.
4.3 Polar plot map
The map below shows 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. Different pollutants can be shown using the “layer control” menu at the top-right of the map.
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, PM2.5 and O3) the directional signature seen in the measurements may not be as strong.
Figure 8: An interactive map displaying polar plots for each pollutant and monitoring station.
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.
Figure 9: Boxplot for NO2 concentration.
Figure 10: Boxplot for PM10 concentration.
Figure 11: Boxplot for PM2.5 concentration.
4.5 Time series plot
The plots below show the time series of concentrations for this year. 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.
Figure 12: Time series plot of daily average NO2 concentration.
Figure 13: Time series plot of daily average PM10 concentration.
Figure 14: Time series plot of daily average PM2.5 concentration.
4.6 Time Variation plot
These plots show concentrations over different time intervals such as diurnal, day of week and, if relevant, month of year. A plot showing seasonal variation in the three months of the quarter will be shown if multiple months are 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.
Figure 15: Time variation of hourly NO2 concentration.
Figure 16: Time variation of hourly PM10 concentration.
Figure 17: Time variation of hourly PM2.5 concentration.
4.7 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.
Figure 18: NO2 calendar plot.
Figure 18: NO2 calendar plot.
Figure 18: NO2 calendar plot.
Figure 19: PM10 calendar plot.
Figure 19: PM10 calendar plot.
Figure 19: PM10 calendar plot.
Figure 20: PM2.5 calendar plot.
4.8 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 21). 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-day air mass back trajectories arriving at the sites for 2025 are grouped into 4 clusters.
Figure 21: Back trajectory clusters for the period 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. The receptor point is used here is London Bridge Place which is 12.4 km away from the centre of all monitoring stations analysed in this report.
The average daily concentration for each pollutant across all the sites is calculated, with the top 10 most polluted days identified and linked to its back trajectory data in the plot below.
Figure 22: Trajectory plot for top ten highest daily NO2 concentration.
Figure 23: Trajectory plot for top ten highest daily PM10 concentration.
Figure 24: Trajectory plot for top ten highest daily PM2.5 concentration.
| Name | Tim Bevington; David Madle |
| Address | Ricardo, Gemini Building, Harwell, Didcot, OX11 0QR, UK |
| Telephone | 01235 753 125 |
| Tim.bevington@ricardo.com; David.madle@ricardo.com |