Consistent with previous work (e.g, Karamchandani et al., 2017; Lupaşcu and Butler, 2019; Butler et al., 2020), the hemispheric O3 level, represented here by the LB source region, dominated the monthly O3 concentrations in the UK during the entire study period with a mean relative contribution of ∼ 71 %, exhibiting a maximum in May (mean 76 %), a minimum in June (mean 66 %), and an increase in August (mean 72 %); see Fig. 3. The mean contribution from the Europe (Eu) super-region (FRA, GER, NET, LUX, BEL, NOS, Rest_CEu, and Rest_Eu; see Fig. 1a for delineation of regions and Table 2 for abbreviation definitions) accounts for nearly 16 % of the simulated monthly mean O3. The largest Eu super-region contributions are observed in the UK locations that are closer to continental Europe and that together contain about 40 % of the UK population (East Anglia, the London area, the South East, and Yorkshire and Humberside). The smallest Eu super-region contributions are observed over Scotland (May and June) and Ireland (July and August). Emissions from UK sources, on the other hand, accounted for about 13 % of the simulated monthly O3. The domestic contributions tend to increase in June and decrease again in August. This monthly variation in the O3 contributions is mainly caused by larger photochemical activity taking place during the summer months (e.g. Monks, 2005). Under these conditions, O3 is formed by reactions involving the oxidation of NMVOCs in the presence of NOx and under the influence of solar radiation.

The spatial distribution of the monthly O3 concentrations and the absolute contribution of the source regions (UK, LB, and Eu super-region) to surface O3 in the UK are shown in Fig. 4. The first column shows that the monthly surface O3 concentrations are higher in May than during the summer months, in particular over Ireland, most of the Atlantic Ocean, north of the UK, Scandinavia, and the northern North Sea. This is consistent with the Northern Hemisphere mid-latitude spring maximum (Monks, 2000), which is characteristic of remote locations and attributed to both transport of O3 from the stratosphere to the troposphere (Monks, 2000) and transport of O3 produced from anthropogenically emitted precursors (NOx and VOCs) (Monks, 2000; Butler et al., 2018). By contrast, the south-east of the UK, the southern North Sea, and continental Europe exhibit sustained high O3 mixing ratios throughout the entire analysed period (May to August), reflecting a spring–summer maximum that is frequently attributed to photochemical O3 production (Monks, 2000).

A marked latitudinal gradient is observed in the monthly O3 mixing ratios, in particular during June, July, and August. Over the ocean areas, O3 concentrations tend to be higher at the lower latitudes and in the North Sea; see Fig. 4e, i, and m. Mean O3 mixing ratios as low as 22 ppbv (Fig. 4i) are observed in most of the UK and Scandinavia in July, while mixing ratios as high as 32 ppbv (Fig. 4e) are calculated for southern locations in the UK and western Europe in June. This is consistent with previous estimates such as those in Butler et al. (2018). Part of the latitudinal gradient in surface O3 over land can be attributed to the changing O3 mixing ratios arriving from the Atlantic Ocean. Moreover, high mixing ratios in the south-east UK during summertime are generally associated with photochemical production of O3 (Monks et al., 2005), in particular from anthropogenic NOx and biogenic NMVOC emissions (Atkinson, 2000; Butler et al., 2018) as well as transport of O3 rich air masses from continental Europe during anticyclonic conditions (Jenkin et al., 2002; Lee et al., 2006; Francis et al., 2011; Romero-Alvarez et al., 2022). Low mean O3 mixing ratios (as low as 20 ppbv), on the other hand, are observed in the vicinity of the main urban centres, e.g. Greater Manchester, the Midlands, and the London area (first column, Fig. 4). This is because strong titration by excessive local NOx emissions takes place over the main urban centres, whereas high O3 production rates are expected in the outskirts following the progressive reduction in NOx concentration relative to that of NMVOCs (Jenkin, 2008). Note that the latitudinal gradient across the UK is not evident in high-altitude areas in Wales and northern England, which have relatively high concentrations of O3. This is because much of the time high-altitude areas are above the shallow boundary layers that form over the lower-lying land and experience therefore larger exposure to O3.

A suitable way to identify the areas influenced by fresh NOx emissions is comparing the mixing ratios of O3 and Ox (i.e. O3 + NO2). Ox is considered a conservative quantity as it is, to a large extent, free from the titration effect of NO + O3 → NO2 (Kley et al., 1994). The effect of titration for July is evident in Fig. 5. When Ox is considered, the mixing ratios tend to increase over the main urban centres such as London, Birmingham, Nottingham, Sheffield, and Greater Manchester and to a lesser extent over Edinburgh and Glasgow. The NO titration effect is also observed within the main urban centres in continental Europe and along the shipping lanes over the North Sea and the English Channel due to the high NO content of ship emissions compared with those from NMVOCs (Aulinger et al., 2016).

The decrease in the monthly average O3 mixing ratios towards the summer months over the Atlantic Ocean and most of the British Isles coincides with a progressive reduction in the contribution from LB O3 over the same areas (second column in Fig. 4). Over remote marine areas, it is likely that the decrease in total O3 is due to an increase in the photochemical activity and concentration of water vapour during the summer months. O3 concentrations over land, on the other hand, are likely to be altered by both the changing background contribution from over the ocean and processes occurring at the regional and local scale (Jenkin, 2008). Such processes include O3 scavenging near emission sources, changes in meteorology (wind direction influencing transport and temperature and radiation influencing photochemical production of O3), and planetary boundary layer stability (influencing vertical mixing and deposition) (Pilling et al., 2009). In addition, LB O3 can be chemically lost near emission sources, e.g. the Midlands and London area, shipping lanes, and over an extended area on the southern part of the Atlantic Ocean, as shown in the map of the net midday (11:00–14:00 UTC) near-surface LB O3 chemical production rate in Fig. 6. The figure also shows how the absolute contribution from the LB decreases southwards and eastwards. Over the Atlantic, part of this can be attributed to a greater chemical O3 sink due to the increase in photolysis of O3 and subsequent production of OH radicals from water vapour (Johnson et al., 1999). Transport of O3 from the stratosphere might also influence the spatial gradient in the contributions.

A marked reduction in LB O3 is observed over the UK in column 2, Fig. 4 (e.g. a decrease of ∼ 10 ppbv between LB O3 over the ocean and the UK). Depletion of surface O3 by dry deposition and chemical loss processes within the UK, such as the reaction of O3 with NO, may help explain the observed spatial gradient. Reductions in LB O3 due to the effect of local O3 scavenging by reaction with NO in the urban centres as illustrated in Fig. 7 might be an additional cause.

Whereas the absolute contribution from the LB over the UK tends to decline with distance towards the south-east, the absolute contribution of UK anthropogenic NOx emissions to the mean surface O3 over the UK (Fig. 4, third column) decreases from the south-east to the north and west. The mean contribution of the UK-to-UK surface O3 concentrations is marginal. Over most of the north and the west, the UK contributions ranged from 1–3 ppbv. By contrast, maximum UK contributions can reach up to 7 ppbv in the east and the Midlands during the summer months. These areas tend to be drier, warmer, and sunnier than those regions further west and north, features that are conducive to photochemical O3 formation (e.g. Coyle et al., 2002; Jenkin, 2008). Furthermore, these regions contain some of the UK's largest cities (e.g. London, Birmingham, Nottingham, Manchester, and Leeds), which can lead to net O3 formation downwind of the emission sources where the NOx titration effect is reduced. Indeed, overall, the south and east of the UK exhibit the highest midday (11:00–14:00) O3 chemical production from UK anthropogenic sources; see Fig. 8a. Lower O3 chemical production is instead observed in the west and the north beyond Yorkshire and Humberside, as shown in Fig. 8a. On the other hand, the UK makes a positive contribution to O3, of around 4–8 ppbv, downwind over continental Europe.

The contribution from European NOx emissions to the mean surface O3 over the UK (Fig. 4, fourth column) is comparable to that observed from the UK contribution and tends to be higher along much of the eastern, southern, and south-west borders, reaching up to 10 ppbv in East Anglia during July. This reflects the effective transport of continental O3 by south-easterly winds during high-O3-pollution events. The European contributions then decrease towards the northern and western areas of the UK, with a minimum (1–3 ppbv) over Scotland and Ireland. Figure 8b demonstrates that surface O3 from anthropogenic sources from the Eu super-region is mainly produced outside the UK. This indicates that the contribution from EU emissions to UK surface O3 is predominantly due to transport of O3 rather than its NOy precursors. Also, O3 from EU sources is chemically lost near the largest cities in the UK (e.g. the London area, Birmingham, Nottingham, Manchester, and Leeds) and in the English Channel and North Sea, as shown in the plot of the net midday surface chemical O3 production rate from European anthropogenic NOx emissions in Fig. 8b. Chemical production is generally concentrated over central Europe and the Baltic Sea. By contrast, chemical loss happens within the main urban centres; near point sources; and along the shipping routes around western Europe, the North Sea, and the English Channel, e.g. sites previously identified to be influenced by NOx titration.

The modelled contributions of the different source regions to the UK receptor regions for May are presented in Fig. 9. The figure contains 12 nested pie charts, each one associated with a receptor region in the UK that shows the absolute and relative contributions to O3 mixing ratios in the UK from all anthropogenic NOx sources, including ship emissions. Note that the contributions from the Rest_Eu source need to be carefully interpreted since these include emissions from the Republic of Ireland, Iberian Peninsula, southern EU, south-eastern EU, eastern EU, and northern EU and ship emissions from the Atlantic, Baltic Sea, and Mediterranean.

The LB source is the principal contributor to the modelled mean O3 mixing ratios in every receptor region. The contributions peak in May (mean absolute contribution 25 ppbv), reflecting the seasonal cycling in the northern hemispheric background O3 (e.g. Monks, 2000; Pilling et al., 2009). Contributions from this source are more prominent in the regions located in the north, east, and north-west of the UK, e.g. Scotland (30 ppbv), Northern Ireland (28 ppbv), North East (27 ppbv), the North West (26 ppbv), and Wales (26 ppbv). These regions contain about 20 % of the UK population and are primarily impacted by westerly flows and associated hemispheric O3 background due to their geographical position (Pilling et al., 2009). Also, they generally experience less than 10 d with O3 concentrations above the EU limit of 120 µg m-3 (DEFRA, 2020) because of low NOx emissions locally. The contributions from the LB source in the South East, East Anglia, and East Midlands, on the other hand, can be up to 8 ppbv smaller than in the east of the UK, particularly during summertime; see Figs. S10–S11 in the Sect. S2 in the Supplement.

The UK contributions are generally more significant in the east, south-east, and Midlands, showing a maximum value in June and July in every receptor area (Figs. S10 and S11 in the Supplement). The source region provides up to 20 % of the surface O3 mixing ratios in East Anglia, 18 % in the London area and East Midlands, and 16 % in Yorkshire and Humberside and the South East, making it the second-biggest source of O3 in these locations after the LB. This area incorporates about 50 % of the UK population and often experiences more than 10 d with O3 concentrations above the EU and UK O3 threshold (concentration >120 and 100 µg m-3, respectively) (DEFRA, 2020). The Eu super-region, on the other hand, is the second-largest source region in the northern and western UK, with contributions in summertime reaching up to 10 % in Scotland and 16 % in the South West, England, and 14 % in Wales. Regardless, this source region still significantly impacts the South East, East Anglia, and London, where the relative contributions can increase from 13 %, 12 %, and 13 % in May to 16 %, 15 %, and 16 %, respectively, in July.

The contributions from ship emissions from the North Sea and English Channel are significantly lower than those from UK sources and the Eu super-region (3 %–4 % of the total contribution in the South East and East Anglia in May and up to 6 % of the total surface O3 during the summer months). The impact is also less important in the west than in the east and south of the UK due to the proximity with the region. As for the relative contributions from the different Eu sub-regions (inner circle in Fig. 9), these are largely influenced by the geographical situation of the receptors and the predominant wind direction. In every receptor, the principal contributor from the Eu super-region is the Rest_Eu source region, providing between 60 %–70 % of the Eu super-region O3 in May and up to 83 % of the O3 during summertime. The relative contributions of the Rest_Eu region are larger in the northern and western locations, in particular during the summer months when there is a marked difference in the distribution of the contributions across the UK regions. The summer months see an increase in the input from France, Germany, and the Benelux region, in particular during anticyclonic weather conditions and over the receptor regions located in the south and east of the UK (e.g. the South East, East Anglia, the London area, and the East Midlands); see Figs. S10–S11 in the Supplement. This is consistent with results of studies on extreme O3 in the EU and the UK reporting an increase in surface O3 concentrations under anticyclonic conditions (e.g. Pope et al., 2016; Ordóñez et al., 2017; Romero-Alvarez et al., 2022). Romero-Alvarez et al. (2022), in particular, have shown that a wide area of high pressure centred over the Netherlands coast affected most of England during the first days of July 2015. During the same period, regions such as East Anglia reported increases in O3 mixing ratios of up to 16.6 ppbv h-1 that overlapped with wind direction changes from south-south-west to south-south-east.

Depending on the predominance of the wind direction (south-south-east and south-south-west), O3 from anthropogenic sources within France can impact both the west and the east of the UK. The contribution is greater in the southern UK due to the proximity to the source region. The contributions from the Benelux region and Germany are more significant in the east of the UK due to the proximity with the continent and association with easterly flows (east and south-east); about 14 % and 6 % of the Eu super-region in East Anglia during the summer months come from these two source regions, respectively.

Notably, anticyclonic conditions and easterly winds in the UK have been associated with enhanced O3 concentrations, whereas cyclonic conditions and westerly winds have been linked to O3 transport from the UK mainland and cleaner air from the North Atlantic (Jenkin et al., 2002; Pope et al., 2016; Romero-Alvarez et al., 2022). The contribution patterns described above may thus serve as predictors of future O3 source apportionment over the UK regions.

The mean contribution from each source region for the hours when the MDA8 O3 exceeded 50 ppbv at each receptor area from May to August is presented in Fig. 10. The figure shows large contributions from source regions that were not seen as dominant sources. France, for example, becomes a major source, particularly in receptors in densely populated areas such as the south and east of the UK. The absolute mean contributions at the sites sometimes exceed the input from the LB O3 (mean value ranging between 10 and 15 ppbv and maximum reaching up to 35 ppbv in the London area). The impact of UK NOx on O3 varies across the sites, but in general, its share increased from the south-east to the north. In the Midlands, the North East, the North West, Scotland, and Yorkshire and Humberside, O3 from UK sources becomes dominant, surpassing the LB mean input in most receptors. In the remaining locations, the UK source is the third-largest input for surface O3 except for the South West, where most of the O3 comes from France (mean ∼ 18 ppbv), the LB (mean ∼ 14 ppbv), NOS (mean ∼ 6 ppbv), and Rest_Eu (mean ∼ 8 ppbv). The impact from the shipping component (NOS) also becomes important in all receptor regions with an estimated mean of 4–7 ppbv. O3 from central Eu, Germany, the Netherlands, Belgium, and Luxembourg, on the other hand, is almost negligible in the west of the UK (mean less than 1 ppbv). However, its impact increases towards the east and north with mean values ranging about 1–6 ppbv (e.g. in the East Midlands, the North East, Yorkshire and Humberside, and Scotland), reflecting the efficient transport of pollutant-loaded air masses under anticyclonic conditions.

Figure 11 shows the mean contribution at each receptor area to hourly surface O3 when the MDA8 O3 exceeded the 60 ppbv thresholds. There were two occasions when the modelled MDA8 exceeded 60 ppbv, the main occasion being the episode on 1 July. Romero-Alvarez et al. (2022) have shown that MDA8 O3 above 50 ppbv in the South East and East Anglia regions in July 2015 coincided with days when easterly winds prevailed (east-south-east flows). In contrast, MDA8 O3 above 60 ppbv coincided with a shift in the wind direction from east-south-east to south-south-east and south and a sharp rise in the surface temperature.

France was the most significant contributor to the build-up of O3 when the mixing ratios exceeded the EU threshold in the South East (mean ∼ 18 ppbv), East Anglia (mean ∼ 21 ppbv), and the London area (mean ∼ 26 ppbv) because convergence of westerly and south-easterly winds in the west of the UK diverted the contributions of domestic sources from these regions, as reported in Romero-Alvarez et al. (2022). O3 from UK NOx emissions, on the other hand, has a greater impact on the East Midlands (mean ∼ 16 ppbv) and Yorkshire and Humberside (mean ∼ 15 ppbv). In the South East and the London area, the contributions from Rest_Eu equal those from UK O3, while the influence is comparable to that from the west and central Europe in the rest of the regions. As in the contributions to the MDA8 O3 threshold of 50 ppbv above, the lateral-boundary component remained nearly constant in all receptor areas with a mean contribution of about 12 ppbv. This is because most of the UK's weather was dominated by anticyclonic conditions. The impacts from the North Sea and the English Channel are also important in all receptor regions, with a mean between 4–7 ppbv. Results suggest that ship emissions along these routes affect the air quality of the UK, particularly over the east and south-east. However, the current model configuration does not consider the chemical evolution of the different emitted species (chemical loss and production rates) during the dispersion of the ship plume. In fact, once species are emitted, they are instantaneously mixed in each model grid cell (27 km × 27 km). In the case of chemically reactive species such as NOx, this can lead to overestimations of both NOx and O3 concentrations due to the non-linearity of the chemical processes involving NOx and O3 evolution during the dispersion of the ship plume (e.g. Huszar et al., 2010; Van Der Werf et al., 2010).

The average simulated AOT40 for 2015 in the two most relevant arable areas in the UK – East Anglia and the South East – is 3674 and 1833 µg m-3 h-1, respectively (the target value for the EU and UK is ∼ 18 000 µg m-3 h-1). Figure 12 shows the source contributions to the surface O3 when the mixing ratio exceeded 40 ppbv during the daytime hours (08:00 and 20:00) central European time from May to July in two receptor regions in the UK. When exceedances to the hourly surface O3 mixing ratios above 40 ppbv are considered, the LB component becomes the dominant source in both receptor regions (estimated mean concentration between 21–24 ppbv) as its threshold is close to the tropospheric baseline ozone level associated with maritime North Atlantic air masses. The second-largest contributor is the UK with a higher impact in the East Anglia region (estimated mean concentration 10 ppbv) than in the South East (estimated mean concentration 6 ppbv). The third, fourth, and fifth contributions in East Anglia come from the Rest_Eu and Rest_CEu regions and France, while in the South East the contributions come from the North Sea and English Channel, Rest_Eu, and Germany. The contributions from the Netherlands, Belgium, and Luxembourg become almost negligible.

Note that the AOT40 metric assesses the impacts of O3 on the vegetation by considering an O3 threshold (e.g. concentrations above 40 ppbv) during the months when plant growth is most likely to be affected and when daytime O3 concentrations are at their highest. However, research experiments have shown that the response of plants to O3 exposure is non-linear due to a mismatch between the peak daytime O3 concentrations and stomatal opening (Heath et al., 2009). This means that the effective amount of O3 taken up by plants is not always correlated to the ambient O3 concentrations. The AOT40 index does not account for the plant's physiological control of stomatal opening, which limits the potential of the index to accurately assess the impacts of O3 on the vegetation. Future work should consider flux-based metrics, which have proven to be more suitable for ozone risk assessment on plants as they take into account the ambient concentration of O3, the physiological control on stomatal openings, and the efficiency of the leaf antioxidant system (Fares et al., 2010) as well as improved the O3 deposition routines in WRF-Chem so that they can take into account factors such as stomatal opening cycles.