UK Summer 2023 Ozone
UK-wide elevated ozone in June and September 2023
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1 Introduction
1.1 Purpose
Throughout summer 2023, the United Kingdom experienced elevated ozone concentrations due to dry, sunny weather and light wind. Four Air Quality Information Bulletins were published within 5 days in June due to four sites - Bournemouth (BORN), Canterbury (CANT), Lullington Heath (LH), and St Osyth (OSY) - in the UK Automatic Urban and Rural Monitoring Network (AURN) exceeding the UK ozone public information threshold of 180 𝜇g m-3 (1 hour mean O3) (“UK AIR News” 2023). In September of the same year, two additional bulletins were published relating to exceedences from four more sites - London Bloomsbury (CLL2), London Harlington (HRL), Reading New Town (REA1), and Sibton (SIB). These are illustrated in Figure 1, which begins centred on the June episode.
“UK AIR News.” 2023. Defra. https://uk-air.defra.gov.uk/news.
This report more closely examines O3 concentrations across the whole of the AURN across summer 2023. The evolution of the O3 episode is examined by referencing each site’s daily air quality index (DAQI). In addition, measured concentrations are used to examine specific points of interest, including the effects of meteorology, variation across UK region, and closer assessment of the sites flagged in the UK-AIR bulletins. Arriving air masses will also be briefly examined.
1.2 Data
In 2023, the AURN included 90 sites measuring O3, 60 of which are urban background and 20 of which are rural background, with the remaining 10 being a mixture of suburban, traffic and industrial sites.
Figure 2 shows a map of all of these AURN sites which measure ozone in 2023. Links to each of their respective UK-AIR site information pages are included in each marker popup. The eight sites featured in the UK-AIR bulletins are shown in orange, and are all found in the South-East of England, many of them on the South-East coast.
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, low (1) to very high (10), and divided into four bands to provide detail about air pollution levels in a simple way, similar to the sun index or pollen index (“What Is the Daily Air Quality Index?” 2023).
“What Is the Daily Air Quality Index?” 2023. Defra. https://uk-air.defra.gov.uk/air-pollution/daqi.
| 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+ |
601+ |
71+ |
101+ |
| AP Band | Accompanying Health Messages | |
|---|---|---|
| at-risk individuals | 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 DAQI Statistics
Figure 3 shows the number of sites at each daily air quality index across summer 2023 in the UK. Up until the 7th of June 2023, all sites were found between 1 and 4, with the majority being at 3. From that point, the average DAQI increases to just under 5, with 7 being the highest index reached. The average DAQI returns to 3 on the 19th of June, slowly decreasing to 2 as the year progresses through July.
The average O3 DAQI remains low up until the first of September, when it begins to climb sharply until it peaks on the 7th. Once again, sites reach a DAQI of 7, although the overall episode is shorter compared to the June peak.
Thankfully, no sites reached the “very high” DAQI band for O3 during the summer. Bournemouth, Canterbury, Cardiff Centre, High Muffles, London Bloomsbury, London Harlington, London N. Kensington, Lullington Heath, Reading New Town, Sibton, St Osyth, Telford Hollinswood, and Yarner Wood all reached the “high” band, and all but one of the rest of the sites in the AURN peaked in the “moderate” band. The one site which peaked in the “low” band, London Eltham, had poor data capture owing to power issues so was not measuring during the episodes themselves. It is possible that, had London Eltham been operational, it too would have reached the “moderate” or “high” DAQI band.
Note that the “very high” and “high” DAQI bands are distinct from the public information threshold (refer to Table 1).
Low
1
Moderate
75
High
13
Very High
0
2.3 Geographic Distribution
Figure 4 shows the evolution of the ozone episode, starting with June. By pressing the play button at the bottom-right, the daily air quality index of the AURN sites will begin to change over time. The June episode appears to start in the South West before progressing east and then north until it fully covers the majority of the UK. The September episode is much more focused on England, and appears to be the most intense in the South-West.
3 Measured Concentrations
3.1 Air Quality Bulletins
As mentioned in Section 1.1, eight sites were named in air quality bulletins in June and September 2023 for exceeding the UK ozone public information threshold of 180 𝜇g m-3 (1 hour mean O3). These were Bournemouth (BORN), Canterbury (CANT), Lullington Heath (LH), St Osyth (OSY), London Bloomsbury (CLL2), London Harlington (HRL), Reading New Town (REA1), and Sibton (SIB). Table 3 provides an overview of some key summary statistics for these eight sites.
| site | mean | min | max | median | Max 8hr Rolling Mean | DAQI | WEB | ||
|---|---|---|---|---|---|---|---|---|---|
| value | days >100 | days >120 | |||||||
| Bournemouth | 67.95 | 2.99 | 191.14 | 67.60 | 178.30 | 26 | 12 | link | |
| St Osyth | 66.91 | 3.14 | 190.89 | 66.61 | 161.05 | 21 | 13 | link | |
| Lullington Heath | 65.89 | 9.78 | 181.41 | 65.53 | 165.96 | 17 | 10 | link | |
| Sibton | 63.24 | 6.09 | 185.20 | 62.74 | 169.72 | 21 | 10 | link | |
| Canterbury | 61.65 | −1.06 | 198.10 | 63.06 | 177.50 | 19 | 13 | link | |
| London Bloomsbury | 59.77 | 2.69 | 185.15 | 58.30 | 167.73 | 17 | 13 | link | |
| London Harlington | 56.52 | 1.86 | 190.68 | 55.68 | 173.54 | 20 | 11 | link | |
| Reading New Town | 51.77 | 0.00 | 183.31 | 50.94 | 166.77 | 15 | 9 | link | |
The hourly ozone concentrations of these sites are shown in Figure 5 and Figure 6, both initially zoomed in on the ozone peaks which exceeded the public information threshold. The three of the four sites which exceeded the threshold in June (excluding Lullington Heath) were very close to exceeding it again on the 9th of September. Similarly, three out of the four sites which exceeded the threshold in September (excluding Sibton) also nearly exceeded it back on the 10th of June.
The eight sites which exceeded the public information threshold are naturally not the only sites with elevated O3 concentrations. The following sites were all within 10 𝜇g m-3 of the threshold at some point in summer 2023 (with the difference between their maximum O3 concentration and 180 𝜇g m-3 shown in parentheses); Birmingham Ladywood (0.44), Rochester Stoke (0.99), Southend-on-Sea (3.03), High Muffles (3.63), Telford Hollinswood (4.08), Cardiff Centre (4.38), London Westminster (4.43), Thurrock (7.17), Charlton Mackrell (7.92), Salford Eccles (8.07), Leamington Spa (8.82), York Bootham (9.47), and Portsmouth (9.92).
The full distribution of maximum hourly ozone concentrations throughout the AURN is shown in Figure 7, binned in 5 𝜇g m-3 intervals. Most sites reached maxima of roughly 160-165 𝜇g m-3, around 15-20 𝜇g m-3 from the public information threshold. Most of these high O3 concentrations happened in June.
3.2 Zones & Agglomerations
The UK has been divided into zones and agglomerations for the purposes of air pollution monitoring, in accordance with EC Directive 96/62/EC. There are 16 zones. They match:
The boundaries of England’s Government Offices for the Regions; and
The boundaries agreed by the Scottish Executive, National Assembly for Wales, and Department of the Environment in Northern Ireland.
There are 28 agglomerations in the UK (“UK AIR Glossary” 2023). An agglomeration is defined as any urban area with a population greater than 250,000.
“UK AIR Glossary.” 2023. Defra. https://uk-air.defra.gov.uk/air-pollution/glossary.php?
Figure 8 through Figure 11 show the mean, median and interquartile range of hourly O3 concentrations in the UK by zone and agglomeration during each episode. For this purpose, the June episode is defined as lasting from the 8th to the 19th of June, and the September episode from the 1st to the 12th of September. Note that Figure 8 and Figure 10 include all sites within the zone, including sites also within an agglomeration.
4 Meteorology
4.1 Temperature Dependence
Every site in the AURN has accompanying modelled meteorological data obtained using the Weather Research & Forecasting (WRF) Model by Ricardo (Lingard et al. 2013), which can be combined with the measured meteorological data to demonstrate the temperature dependence on ozone. Figure 12 shows that daily mean O3 concentrations initially increase gradually from daily mean temperatures of 5 to 15 ℃, then more steeply from 15 to 20 ℃, and then more rapidly still from 20 ℃ onwards.
Lingard, Justin, Lorenzo Labrador, Daniel Brookes, and Andrea Fraser. 2013. “Statistical Evaluation of the Input Meteorological Data Used for the UK Air Quality Forecast (UK-AQF).” Ricardo-AEA. https://uk-air.defra.gov.uk/library/reports.php?report_id=770.
4.2 Stratospheric Ozone
Of particular interest to the September ozone episode is the behaviour of stratospheric ozone. Figure 13 illustrates that the September ozone roughly fell in a nadir in stratospheric ozone over the UK. The increased amount of UV radiation permitted by low stratospheric ozone will promote atmospheric chemical reactions which promote the formation of secondary tropospheric ozone (“UK Stratospheric Ozone and UV Measurements” 2023).
“UK Stratospheric Ozone and UV Measurements.” 2023. Defra. https://uk-air.defra.gov.uk/research/ozone-uv/.
4.3 Air Mass Trajectories
Figure 14 and Figure 15 show data from the HYSPLIT forecast model (Stein et al. 2015). The plot illustrates where the air masses that affected the UK have travelled from during the peaks of the June and September episodes. Readers can progress through different arrival days using the control at the bottom-right of the maps.
Stein, A. F., R. R. Draxler, G. D. Rolph, B. J. B. Stunder, M. D. Cohen, and F. Ngan. 2015. “NOAA’s HYSPLIT Atmospheric Transport and Dispersion Modeling System.” Bulletin of the American Meteorological Society 96 (12): 2059–77. https://doi.org/10.1175/BAMS-D-14-00110.1.
Air mass trajectories are an important factor on air pollution levels as they provide a useful indicator of the likelihood of transboundary air pollutants, many of which could be ozone precursor species. 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.
In the period shown in Figure 14, chosen as it coincides with the highest peaks in the UK DAQI in June, many of the air masses arriving in the UK have travelled over Northern Europe. These have potentially picked up ozone-producing pollutants from industrial activity, exacerbating the ozone episode experienced by the UK. Figure 15 tells a similar story, although many of the air masses arriving in Ireland and Scotland in particular appear to be arriving from Southern Europe (Spain and Portugal).
5 Summary
In June and September 2023 the UK experienced elevated ozone levels. Effectively all AURN sites which measure ozone reached at least a “moderate” band of daily air pollution, with 12 reaching the “high” band (Figure 3, Figure 4).
Elevated ozone can be attributed to dry and sunny weather (Figure 12), with easterly wind carrying ozone precursors over from mainland Europe (Figure 14, Figure 15). The September episode, while smaller than its June counterpart, is of particular interest. September was unseasonally hot, with an unprecedentedly protracted heatwave affecting the UK. Low stratospheric ozone (Figure 13) also led to increased UV intensity, which promotes tropospheric ozone formation.
From the 12th of September, daily O3 concentrations in the UK have fluctuated within the “low” air quality index band. The changing meteorology which coincides with the UK entering the winter months makes any further ozone episodes in 2023 unlikely.