Oxfordshire Summer 2023 Ozone

Elevated ozone in Oxfordshire during Summer 2023

Authors

Jack Davison

Approved by

Jenny Thomas

Customer

Oxfordshire County Council

Compilation Date

October 19, 2023

Ricardo Energy & Environment

EULA

Ricardo EULA

Contact

Jenny Thomas at jenny.thomas@ricardo.com.

Using this document

This is an interactive document produced using Quarto, an open-source scientific publishing system. As well as providing a traditional report narrative, many of the elements within this document can be interacted with in a similar way to a data dashboard. For example, plots can show tooltips and maps can be zoomed and panned around. More specific instruction can be found near interactive elements by hovering over tooltips like the one in this box below.

Tooltip

1 Introduction

1.1 Purpose

Throughout summer 2023, the United Kingdom experienced elevated ozone concentrations due to dry, sunny weather and light wind. This report closely examines O₃ concentrations measured at the Oxford St Ebbes monitoring station in Oxford to assess how Oxfordshire was impacted by this ozone episode. The evolution of the O₃ episode is examined by referencing the daily air quality index (DAQI). In addition, measured concentrations are used to examine specific points of interest, including the effects of meteorology and arriving air masses.

1.2 Data

In 2023, Oxfordshire’s air quality network 7 AURN/Air Quality England sites. Figure 1 shows a map of these sites. Links to each of their respective OxonAir site information pages are included in each marker popup. Oxford St Ebbes, the only site which measures ozone, is highlighted in orange.

Interactive Maps

Figure 1: A map showing all air quality measurement sites in Oxfordshire. Oxford St Ebbes is highlighted in orange.

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 (“Daily Air Quality Index (DAQI)” 2023).

“Daily Air Quality Index (DAQI).” 2023. Oxfordshire County Council. https://www.oxonair.uk/about-air-quality/daily-air-quality-index-daqi.

Tabsets

Table 1:
Defra DAQI boundaries for particulate matter. Boundaries are based on daily mean concentrations for NO₂ and PM, and 8-hour mean concentrations for O₃.
Band	DAQI	O₃	NO₂	PM_2.5	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+

Table 2:
Suggested actions and health advice at different DAQI boundaries.
AP Band	Accompanying Health Messages
AP Band	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 2 shows the evolution of the ozone DAQI across summer 2023 at Oxford St Ebbes. There are two clear periods during which ozone concentrations spiked; a June episode from roughly the 9^th to the 18^th, and a shorter September episode from the 4^th to the 9^th. The Oxford St Ebbes monitoring station reached a DAQI of 5 during June and 6 during September, both of these indices falling in the “moderate” band.

Interactive Plots

Figure 2: The daily air quality inex (DAQI) for O₃ at Oxford St Ebbes over time. The y-axis shows the relevant statistic (maximum 8-hour running mean) and the colour the DAQI that day.

Thankfully, Oxford St Ebbes did not reach either the “high” or “very high” bands during the summer ozone episodes. It spent around 12.1% of days from May to September in the “moderate” band, which has associated health guidance for at-risk individuals (Table 2).

Low

131

Moderate

High

Very High

3 Measured Concentrations

3.1 Air Quality Statistics

Table 3 provides a monthly summary for ozone measured at Oxford St Ebbes, including a distribution of the daily air quality index for each month.

Table 3:
A statistical summary of ozone concentrations in May to September 2023 measured at Oxford St Ebbes.
month	mean	min	max	median	Max 8hr Rolling Mean			DAQI
month	mean	min	max	median	value	days >100	days >120	DAQI
May	60.50	6.44	111.61	62.02	103.07	1	0
June	67.45	3.53	161.20	67.30	140.40	12	5
July	43.63	1.80	94.95	43.73	90.22	0	0
August	41.90	0.20	94.65	43.56	86.26	0	0
September	44.39	−0.30	164.65	43.91	155.53	6	2

3.2 Hourly Mean Values

The hourly ozone concentrations measured at Oxford St Ebbes are shown in Figure 3, initially zoomed in on the June episode.

Interactive Plots

Figure 3: Hourly ozone concentrations in Summer 2023, initially zoomed in on June.

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 4 shows a hexagonally binned scatter plot of hourly modelled air temperature against ozone with a smooth trend fitted to the data. It shows that high ozone concentrations are broadly associated with high temperatures, with a particularly high gradient starting at roughly 20 ℃.

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.

Figure 4: The relationship between hourly mean O₃ and hourly modelled temperature. The white line represents a smooth fit to the underlying data.

4.2 Stratospheric Ozone

Of particular interest to the September ozone episode is the behaviour of stratospheric ozone. Figure 5 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/.

Figure 5: Stratospheric ozone measured at Lerwick in 2023. The rough bounds of the two ozone episodes are marked as filled bands, and a smooth fit is shown in orange.

4.3 Air Mass Trajectories

Figure 6 and Figure 7 show data from the HYSPLIT forecast model (Stein et al. 2015). The plot illustrates where the air masses that affected Oxfordshire 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.

The maps show that many of the air masses arriving in Oxfordshire have mainly travelled over Northern Europe during the peak of the episode. These have potentially picked up ozone-producing pollutants from industrial activity, exacerbating the ozone episode experienced in the UK.

Tabsets

Interactive Maps

Figure 6: A map showing airmass arrivals at Welsh receptors during the June episode.

Interactive Maps

Figure 7: A map showing airmass arrivals at Welsh receptors during the September episode.

5 Summary

In June and September 2023 the UK experienced elevated ozone levels. This included the Oxford St Ebbes Automatic Urban and Rural Monitoring Network site in Oxford town centre, which reached “moderate” pollution levels in both June and September (Figure 2).

Elevated ozone can be attributed to dry and sunny weather (Figure 4), with easterly wind carrying ozone precursors over from mainland Europe (Figure 6, Figure 7). 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 5) also led to increased UV intensity, which promotes tropospheric ozone formation.

Since September, O₃ concentrations in Oxford 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.