West London Particulate Episode
January 2023
1 Introduction
1.1 Purpose
On January 21st 2023, the Mayor of London issued a moderate air pollution alert for elevated pollution levels that Sunday, January 22nd. The alert attributed the potential event to:
…the cold, still, and foggy conditions in London resulting in poor dispersion of vehicle emissions.
This report summarises air quality in London in mid-January 2023, using data from 138 AQ monitoring stations across Greater London, which include sites from the Automatic Urban Monitoring Network, the Air Quality England network, and Londonair.
1.2 Monitoring Stations
The map can be zoomed in and out by using the scroll wheel, double-clicking, or through the plus and minus symbols to the top-left. Clicking and dragging will move the map viewing window. Different dates can be selected using the control menu at the top-right of the map. Clicking individual markers will show a popup detailing the site name, type, source network, and so on.
A summary of site information is presented in Figure 2, including the site name, code, type, network and pollutants measured. 32 sites are classed as “Urban Background”, 66 as either “Urban Traffic”, “Roadside” or “Kerbside”, and the remaining 35 are classed as a variety of other niche site types (suburban, airport, etc.). 50 of the sites are part of the Air Quality England (AQE) network, 16 the Automatic Urban and Rural Network (AURN), and the remaining 67, Londonair.
This document is based around unratified data, so the specific conclusions drawn may be subject to change.
1.3 Using this Document
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. For example:
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.
Maps 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.
Certain tables can be reordered, filtered and searched through.
Specific instructions for interactive elements can be found in boxes like these.
2 Daily Air Quality Index (DAQI)
2.1 What is the DAQI?
The Daily Air Quality Index (DAQI) tells you about levels of air pollution and provides recommended actions and health advice. The index is numbered 1-10 and divided into four bands, low (1) to very high (10), to provide detail about air pollution levels in a simple way, similar to the sun index or pollen index.
Click on the tabs below to learn more about the air quality index. Further information on air pollution banding can be found on UK AIR.
| Band | DAQI | O3 | NO2 | PM2.5 | PM10 |
|---|---|---|---|---|---|
| Low | 1 | 0-33 | 0-67 | 0-11 | 0-16 |
| 2 | 34-66 | 68-134 | 12-23 | 17-33 | |
| 3 | 67-100 | 135-200 | 24-35 | 34-50 | |
| Moderate | 4 | 101-120 | 201-267 | 36-41 | 51-58 |
| 5 | 121-140 | 268-334 | 42-47 | 59-66 | |
| 6 | 141-160 | 335-400 | 48-53 | 67-75 | |
| High | 7 | 161-187 | 401-467 | 54-58 | 76-83 |
| 8 | 188-213 | 468-534 | 59-64 | 84-91 | |
| 9 | 214-240 | 535-600 | 65-70 | 92-100 | |
| Very High | 10 | 241 or more | 601 or more | 71 or more | 101 or more |
| AP Band | Accompanying health messages for at-risk individuals | Accompanying health messages for the general population |
|---|---|---|
| Low (1-3) | Enjoy your usual outdoor activities. | Enjoy your usual outdoor activities. |
| Moderate (4-6) | Adults and children with lung problems, and adults with heart problems, who experience symptoms, should consider reducing strenuous physical activity, particularly outdoors. | Enjoy your usual outdoor activities. |
| High (7-9) | Adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms. People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion. | Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors. |
| Very High (10) | Adults and children with lung problems, adults with heart problems, and older people, should avoid strenuous physical activity. People with asthma may find they need to use their reliever inhaler more often. | Reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat. |
2.2 Aggregate Timeseries
Figure 3 shows the time series of the daily air quality index for ozone, nitrogen dioxide, and particulate matter. Each dot shows a unique combination of the DAQI and date, with their size proportionate to the number of sites that dot represents. The black line shows the mean DAQI on any given date, which illustrates the overall trend of the network. On January 22nd, the mean (+/- standard deviation) air quality index for Greater London was 4 (+/-2) for PM2.5, 3 (+/-1) for PM10, 2 (+/-0) for NO2, and 1 (+/-0) for O3.
Figure 4 illustrates the proportion of Greater London air quality sites at each daily air quality index in mid January. It is apparent that the peak of the episode was on January 22nd, which saw around 45% of PM2.5 sites experiencing at least a “moderate” episode (DAQI greater or equal to 4) and 19% of sites at least a “high” episode (greater than or equal to 7).
2.3 Individual Sites
Figure 5 shows the daily air quality index as an interactive map. This illustrates the geospatial characteristics of the episode. On 22nd January when the episode was at its most severe, the sites with the highest index were found predominantly in West London, particularly centred around Chiswick and Brentford.
This map can be controlled similarly to Figure 2. Clicking individual markers will show a popup detailing the site name and the DAQI for the four individual pollutants. The day can be controlled using the time slider at the bottom-left. Pressing “play” will automatically advance through the days, whereas the “skip” buttons will jump from one day to the next.
This same air quality index data is tabulated below.
Each column of this table can be independently sorted by clicking on the column name. Typing into the boxes labelled “All” beneath each column name will filter the columns. The whole table can be searched by typing into the search box at the top right.
3 Pollutant Concentrations
Figure 6 shows the hourly concentrations of the four pollutants over a short period in mid January, with the median trend overlaid. Of the visualised period, ozone shows a nadir and the other three pollutants a peak centred around the early morning of 23rd. Median PM2.5 has a maxima of 64.5 μg m-3 at 2023-01-22 22:00:00, PM10 66.5 μg m-3 at 2023-01-22 22:00:00, and NO2 86.4 μg m-3 at 2023-01-23 11:00:00.
If the pre-episode concentrations are treated as “baseline” conditions, Figure 6 appears to show the particulate species having the greatest deviation from that baseline, then nitrogen oxide, and then finally ozone. Figure 7 shows the same median time series shown in Figure 6 normalised to each pollutant’s respective values on the morning of January 20th (2023-01-20 00:00:00). The median PM2.5 concentration peaked at 9.1 times the baseline, the median PM10 at 5.9 times the baseline, and NO2 at 2.4 times the baseline. The three heightened pollutants show similar trends, although the early morning January 23rd peak is much greater in size than the early morning January 22nd peak for the particulates, whereas they are seen to be similar in size for nitrogen dioxide.
4 Air Mass Trajectories
Figure 8 shows data from the HYSPLIT forecast model (NOAA HYSPLIT). The plot illustrates where the air masses that affected London have travelled from over the 96-hour period up to several dates in mid January 2023. Air mass trajectories are an important factor on air pollution levels as they provide a useful indicator of the likelihood of transboundary air pollutants (pollutants such as PM2.5 and PM10 that can travel large distances in the air masses) affecting the UK. Air coming from the west is likely to be mainly clean and good pollution dispersion conditions (wind and precipitation), whilst air masses from the south and east can bring more settled conditions and also pick up background emissions from mainland Europe causing background pollution levels to become elevated. Figure 8 shows that, during the height of the episode, air masses arrived in London from Scotland and Northern Europe.
The map can be controlled in much the same way as Figure 2. By default, only the trajectories for 22nd and 23rd of January are displayed, but additional trajectories can be shown by toggling them on and off using the menu at the top-right. Clicking on the trajectory nodes will provide more information on the air masses.
5 Summary
London experienced an air pollution episode on January 22nd and 23rd 2023 owing to elevated particulate concentrations. The daily air quality index (DAQI) reached the maximum value of 10 at certain measurement sites, with around sixty percent of Greater London air quality measurement sites in the “moderate” DAQI band. Median concentrations of PM2.5 reached 64.5 μg m-3 across Greater London, approximately ten times as high as concentrations seen pre-episode on 20th January. Elevated concentrations and air quality indices were seen across all main site types.
Elevated concentrations may be attributed to cold, calm, high pressure conditions trapping particulates near the surface, potentially compounded by long range transport of particulates from Eastern Europe.
The episode appeared to cease on January 24th and, barring a small proportion of sites experiencing “moderate” PM10 concentrations towards the end of the month, almost all Greater London monitoring sites have remained in the “low” DAQI band (1-3) since.
6 References
Data were obtained from three primary sources:
Further Information
For further information, please contact:
| Name | Jack Davison |
| Address | Ricardo Energy & Environment, Gemini Building, Harwell, Didcot, OX11 0QR, United Kingdom |
| jack.davison@ricardo.com |