Tropospheric ozone changes and ozone sensitivity from the present day to the future under shared socio-economic pathways

Tropospheric ozone is important to future air quality and climate. We investigate ozone changes and ozone sensitivity to changing emissions in the context of climate change from the present day (2004–2014) to the future (2045–2055) under a range of shared socio-economic pathways (SSPs). We apply the United Kingdom Earth System Model, UKESM1, with an extended chemistry scheme including more reactive volatile organic compounds (VOCs) to quantify ozone burdens as well as ozone sensitivities globally and regionally based on nitrogen oxide (NO_x) and VOC mixing ratios. We show that the tropospheric ozone burden increases by 4 % under a development pathway with higher NO_x and VOC emissions (SSP3-7.0) but decreases by 7 % under the same pathway if NO_x and VOC emissions are reduced (SSP3-7.0-lowNTCF) and by 5 % if atmospheric methane (CH₄) mixing ratios are reduced (SSP3-7.0-lowCH4). Global mean surface ozone mixing ratios are reduced by 3–5 ppb under SSP3-7.0-lowNTCF and by 2–3 ppb under SSP3-7.0-lowCH4. However, surface ozone changes vary substantially by season in high-emission regions under future pathways, with decreased ozone mixing ratios in summer and increased ozone mixing ratios in winter when NO_x emissions are reduced. VOC-limited areas are more extensive in winter (7 %) than in summer (3 %) across the globe. North America, Europe, and East Asia are the dominant VOC-limited regions in the present day, but North America and Europe become more NO_x-limited in the future mainly due to reductions in NO_x emissions. The impacts of VOC emissions on ozone sensitivity are limited in North America and Europe because reduced anthropogenic VOC emissions are partly offset by higher biogenic VOC emissions. Ozone sensitivity is not greatly influenced by changing CH₄ mixing ratios. South Asia becomes the dominant VOC-limited region under future pathways. We highlight that reductions in NO_x emissions are required to transform ozone production from VOC to NO_x limitation, but that these lead to increased ozone mixing ratios in high-emission regions, and hence emission controls on VOC and CH₄ are also necessary.

1 Introduction

Ozone (O₃) is a chemically reactive component in the atmosphere that is produced from natural and anthropogenic sources. Emissions of O₃ precursors including nitrogen oxides (NO_x), volatile organic compounds (VOCs), methane (CH₄), and carbon monoxide (CO) lead to the formation of O₃ by a series of photochemical reactions in the presence of sunlight. O₃ has important impacts on human health, ecosystems, and climate change (Lefohn et al., 2018; Zhang et al., 2019; Agathokleous et al., 2020). O₃ mixing ratios are largely governed by the magnitudes of O₃ precursor emissions, transport, deposition, and transport from the stratosphere. O₃ exerts a positive radiative forcing (Stevenson et al., 2013; O'Connor et al., 2021; Thornhill et al., 2021a), and changes in climate in turn influence ozone (Fiore et al., 2012; Doherty et al., 2013). Climate change can alter natural emissions of biogenic VOCs (BVOCs), lightning NO_x, and CH₄, along with temperature, humidity, convection, and clouds, which further influence O₃ mixing ratios (Thornhill et al., 2021b). The interactions between air quality and climate play an important role in the coupled Earth system, and we focus on the impacts of future emissions in the context of climate change on tropospheric O₃ in this study.

The tropospheric O₃ burden is controlled by the amount of O₃ production, O₃ destruction, O₃ deposition, and the O₃ transport from the stratosphere (Lelieveld and Dentener, 2000; Wild, 2007). From pre-industrial times to the present day, the tropospheric O₃ burden has increased from approximately 240 to 350 Tg mainly due to substantial increases in anthropogenic O₃ precursor emissions (Lamarque et al., 2010; Young et al., 2013; Griffiths et al., 2021). However, regional surface O₃ changes between the pre-industrial and present day vary substantially due to different regional emission changes (Turnock et al., 2020) and to differences in O₃ sensitivity to NO_x and VOC emissions. In recent decades, there has been a decrease in surface O₃ mixing ratios in North America and Europe due to emission controls (Simon et al., 2015; Colette et al., 2016; Tarasick et al., 2019). In contrast, increases in surface O₃ levels are observed in South Asia and East Asia due to industrialisation, urbanisation, and social development (Hakim et al., 2019; Lu et al., 2020). Furthermore, while emission controls have been implemented across industrial regions of China in recent years, these have focused on emissions of NO_x and particulate matter and have led to increased O₃ pollution in some places (Wang et al., 2017; Silver et al., 2018).

It is important to investigate O₃ sensitivity to understand how O₃ chemical regimes might change in different parts of the world and to guide suitable emission control strategies. O₃ sensitivity is typically characterised by NO_x- or VOC-limited regimes for O₃ production, and this determines the effectiveness of different emission control strategies. It is dependent on the relative abundances of NO_x and VOC (Sillman, 1999) or of their oxidation products, nitric acid (HNO₃) and hydrogen peroxide (H₂O₂) (Kleinman, 1994; Sillman, 1995). VOC-limited regimes typically occur in highly urbanised regions with high NO_x mixing ratios in which decreases in NO_x emissions increase O₃ mixing ratios, and O₃ production increases with higher VOC emissions. In contrast, changes in NO_x mixing ratios dominate O₃ changes in NO_x-limited regimes such that decreases in NO_x emissions decrease O₃ mixing ratios, and O₃ mixing ratios are less sensitive to VOC emissions. O₃ sensitivity indicators such as the ratios of ${\mathrm{NO}}_x/\mathrm{VOCs}$ and ${\mathrm{HNO}}_{\mathrm{3}}/{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ allow us to identify O₃ sensitivity regimes relatively easily. However, most studies focus on O₃ sensitivity in specific regions and for short time periods (Dunker et al., 2002; Sillman and West, 2009; Ye et al., 2016), leading to inconsistency in the critical indicator values that distinguish O₃ sensitivity regimes. To address this, we generalise the approach by quantifying O₃ sensitivity using a consistent indicator across the globe. This is the first time that the full range of surface chemical environments across the globe has been explored with a global chemistry–climate model, as far as we are aware. We quantify O₃ sensitivity based on the ratio of NO_x and VOC mixing ratios and investigate how regional O₃ sensitivity might change in the future.

The shared socio-economic pathways (SSPs) are future emission and climate scenarios accounting for future social, economic, and environmental developments (O'Neill et al., 2014; van Vuuren et al., 2014). The SSPs represent a range of levels of policy strength (weak, medium, and strong) to control emissions of near-term climate forcers (NTCFs) that include tropospheric O₃, O₃ precursors, and aerosols (Rao et al., 2017). Our study is based on simulations using historical and future SSP emissions and climate undertaken as part of the Aerosol Chemistry Model Intercomparison Project (AerChemMIP; Collins et al., 2017) and the wider Coupled-Model Intercomparison Project Phase 6 (CMIP6; Eyring et al., 2016). The aim of AerChemMIP is to quantify the effects of chemistry and aerosols on air quality and climate in CMIP6 by conducting historical and future experiments using chemistry–climate models with specified climate and emission trajectories.

We examine tropospheric O₃ and surface O₃ sensitivity under present-day (2004–2014) and future conditions (2045–2055). Model development and application are described in Sect. 2 along with descriptions of the emission and climate scenarios used. We compare and evaluate the present-day tropospheric O₃ burden and surface O₃ mixing ratios with two different chemistry schemes in Sect. 3. We then investigate the seasonal, daytime, and nighttime differences in O₃ changes in the future compared to the present day for different regions in Sect. 4. Analysis of O₃ mixing ratios and production is used to quantify O₃ sensitivity and to explain contrasting regional O₃ changes in Sect. 5. We then show the changes in O₃ sensitivity between different seasons and scenarios in Sect. 6 and present our conclusions in Sect. 7.

2 Materials and methods

2.1 Model description, development, and application

We use version 1 of the United Kingdom Earth System Model, UKESM1 (Sellar et al., 2019), to reproduce present-day (2004–2014) O₃ mixing ratios and to predict O₃ responses to emissions and climate in the future (2045–2055). UKESM1 consists of a physical climate model, the Hadley Centre Global Environment Model version 3 (HadGEM-GC3.1), configured with the Global Atmosphere 7.1 and Global Land 7.0 (GA7.1/GL7.0) components (Walters et al., 2019), to which Earth system (ES) processes have been coupled (Sellar et al., 2019). Atmospheric composition is modelled using a state-of-the-art chemistry and aerosol module, the United Kingdom Chemistry and Aerosol model (UKCA; Morgenstern et al., 2009; O'Connor et al., 2014). UKCA includes a stratosphere–troposphere gas-phase chemistry scheme (StratTrop; Archibald et al., 2020a) coupled to the aerosol scheme GLOMAP-mode (Mann et al., 2010; Mulcahy et al., 2020). The model resolution is N96L85 in the atmosphere, with 1.875^∘ in longitude by 1.25^∘ in latitude, 85 terrain-following hybrid height layers, and a model top at 85 km.

While the UKESM1 configuration for CMIP6 used the UKCA StratTrop mechanism, this study also uses an extended gas-phase chemistry scheme that incorporates more reactive VOC species to permit a more realistic representation of O₃ production in polluted environments. The extended chemistry scheme (denoted as Ext_StratTrop hereafter) is based on the StratTrop scheme and includes oxidation of the additional chemical components propene (C₃H₆), butane (C₄H₁₀), and toluene (C₇H₈) to represent alkenes, alkanes, and aromatic VOC classes, as described in Liu et al. (2021). The extended chemistry scheme includes 101 species, 244 bimolecular reactions, 26 uni- and termolecular reactions, 70 photolytic reactions, 5 heterogeneous reactions, and 3 aqueous-phase reactions for the sulfur cycle.

The atmosphere-only configuration of UKESM1 is used with prescribed sea surface temperatures and sea ice to show the transient impacts of emissions under present-day and future climates. These are prescribed using monthly mean time-evolving fields from the fully coupled UKESM1. Greenhouse gas concentrations are prescribed as in historical and future simulations conducted by UKESM1 as part of CMIP6 (Meinshausen et al., 2017, 2020).

2.2 Emissions and experiments

Present-day CMIP6 anthropogenic and biomass burning emissions are taken from Hoesly et al. (2018) and van Marle et al. (2017), respectively. Biogenic VOC emissions are calculated interactively within the iBVOC emissions scheme (Pacifico et al., 2011) in the Joint UK Land Environmental Simulator (JULES) land-surface scheme, which is coupled to UKCA. Other aspects of the emissions used here are the same as described in Turnock et al. (2020). Anthropogenic emissions are categorised into five sectors (industry, power plants, transport, residences, and agriculture) as inputs to the model, with independent diurnal and vertical emission profiles applied for each sector (Bieser et al., 2011; Mailler et al., 2013; Liu et al., 2021).

Three CMIP6 SSP scenarios are used for future simulations: SSP3-7.0, SSP3-7.0-lowNTCF, and SSP3-7.0-lowCH4. The SSP3-7.0 pathway has a large anthropogenic climate forcing signal (a radiative forcing of 7.0 W m⁻² at 2100) and weak emission controls on O₃ precursors and aerosols and rapidly increasing CH₄ mixing ratios (Fujimori et al., 2017; Rao et al., 2017). SSP3-7.0-lowNTCF and SSP3-7.0-lowCH4 are additional pathways which use the same underlying climate policies as SSP3-7.0. SSP3-7.0-lowNTCF has strong controls on all NTCF emissions. SSP3-7.0-lowCH4 follows SSP3-7.0 but assumes strong mitigation of CH₄ emissions in the future, with 24 % decreases in surface CH₄ mixing ratios from 1802 to 1364 ppb. The last time that historical surface CH₄ mixing ratios were this low was more than 50 years ago, in the late 1960s (Prather et al., 2014). BVOC emissions increase under all these pathways due to a warmer climate. We perform four model experiments in this study to investigate tropospheric O₃ for the present day (2004–2014) and three future pathways (SSP3-7.0, SSP3-7.0-lowNTCF, and SSP3-7.0-lowCH4; 2045–2055). Table 1 shows the model configuration for the four simulations. Table 2 lists the CMIP6 global mean NTCF total surface emissions and surface CH₄ mixing ratios for the four scenarios.

Table 1 Model configurations for present-day and future simulations. “Emissions” refers to emissions of O₃ precursors and aerosols. “CH₄” refers to prescribed surface CH₄ mixing ratios. “$\mathrm{SST}/\mathrm{SI}$” refers to prescribed sea surface temperature and sea ice concentrations. “Historical” means that the emissions, CH₄ mixing ratios or $\mathrm{SST}/\mathrm{SI}$ evolve as for the CMIP6 historical simulations, and “Reference” means that they evolve as for SSP3-7.0. “Low” emissions or CH₄ mixing ratios evolve following SSP3-7.0 but with lower emissions or CH₄ mixing ratios.

Table 2 Overview of global annual mean time-varying surface emissions of NO_x, VOCs, CO, sulfur dioxide (SO₂), black carbon (BC), and organic carbon (OC) from anthropogenic (ANT), biomass burning (BB), and biogenic (BIO) sources for the present day (2004–2014) and future (2045–2055) SSP3-7.0, SSP3-7.0-lowNTCF, and SSP3-7.0-lowCH4. Annual mean surface CH₄ mixing ratios (ppb) are also shown.

2.3 O₃ sensitivity indicators

A number of different indicators have been used to distinguish O₃ sensitivity regimes, and typical indicators are the ratios of ${\mathrm{NO}}_x/\mathrm{VOC}$ mixing ratios or emissions and the ratio of ${\mathrm{HNO}}_{\mathrm{3}}/{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ mixing ratios (Kleinman, 1994; Sillman, 1999). For the ${\mathrm{NO}}_x/\mathrm{VOC}$ ratio, it is often more appropriate to use mixing ratios than emissions because this accounts for emissions, transport, chemical reactions, and deposition. Indicators based on ${\mathrm{HNO}}_{\mathrm{3}}/{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ ratios also account for differences in photochemical conditions and VOC reactivity. In a previous study we found that O₃ sensitivity regimes diagnosed with ${\mathrm{HNO}}_{\mathrm{3}}/{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ and ${\mathrm{NO}}_x/\mathrm{VOC}$ ratios were similar (Liu et al., 2021). However, the ${\mathrm{HNO}}_{\mathrm{3}}/{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ indicator is more sensitive to uncertainties in the chemical mechanism, and studies have shown that there are errors in the simulation of short-lived radicals in polluted areas (Whalley et al., 2021). The ${\mathrm{HNO}}_{\mathrm{3}}/{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ ratio also does not account for gas–aerosol conversion as a termination route for NO_x, a mechanism that is included in many chemistry–climate models. We hence choose the ratio between NO_x and VOC mixing ratios as a simple indicator of O₃ sensitivity in this study.

We quantify the sensitivity of O₃ to NO_x and VOC mixing ratios by examining monthly mean O₃ mixing ratios and O₃ net production in each UKESM1 surface grid cell in each of the scenarios in turn. This provides a global overview of the dependence of O₃ and its production on NO_x and VOC across different environments. It also allows us to determine a globally averaged critical threshold value distinguishing NO_x-limited and VOC-limited regimes.

We note that dilution of short-lived NO_x over coarse-resolution model grid cells may lead to the underestimation of local NO_x mixing ratios in high-emission regions. This results in underestimation of ${\mathrm{NO}}_x/\mathrm{VOC}$ ratios in these conditions and the regimes may thus be more VOC-limited in reality than we are able to simulate in a global model.

3 Model evaluation of tropospheric and surface O₃

3.1 Comparison of StratTrop and extended chemistry schemes

We first compare averaged tropospheric O₃ burdens, chemical lifetime, chemical production, chemical loss, and deposition during 2004–2014 from the extended chemistry scheme (Ext_StratTrop) with those from the StratTrop chemistry scheme used in AerChemMIP simulations (Table 3). We define the O₃ production rate as the sum of reactions fluxes through ${\mathrm{HO}}_{\mathrm{2}}/{\mathrm{RO}}_{\mathrm{2}}+\mathrm{NO}$ and the O₃ loss rate as the sum of O(¹D)+H₂O and ${\mathrm{O}}_{\mathrm{3}}+{\mathrm{HO}}_{\mathrm{2}}/\mathrm{OH}/\mathrm{alkenes}$. The O₃ burden with the extended chemistry scheme (376 Tg) lies at the upper end of the uncertainty range for the observed burden: 340±34 Tg for 2000 (Archibald et al., 2020b). The magnitude of the O₃ burden is also consistent with the CMIP6 multi-model mean burden of 356±31 Tg for 2005–2014 (Griffiths et al., 2021). The extended chemistry scheme produces a 5 % higher tropospheric O₃ burden than that of StratTrop (358 Tg), demonstrating a more reactive environment for net O₃ formation throughout the troposphere due to reactive VOCs. This is also reflected in the higher rates of chemical O₃ production (11 %), loss (6 %), and deposition (6 %) with the extended chemistry scheme. However, the higher O₃ production is offset by greater O₃ destruction and by faster O₃ deposition, and hence the mean O₃ chemical lifetime remains very similar at about 22 d, which is consistent with previous multi-model estimates of the mean lifetime of 22.2±2.2 d (Stevenson et al., 2006).

Table 3 Comparison of tropospheric O₃ burden and budget terms during 2004–2014 simulated with StratTrop and Ext_StratTrop chemistry schemes in UKCA. One standard deviation in the annual terms over the 2004–2014 period is shown. We define the tropopause based on the highest layer with an O₃ mixing ratio of less than 150 ppb.

Simulated surface O₃ mixing ratios during 2004–2014 with the two chemistry schemes are compared in Fig. 1. Using the extended chemistry scheme, the spatial distribution of surface O₃ is similar to that using StratTrop in both winter and summer but shows a general increase in global O₃ levels of about 2 ppb (Fig. 1a–d) due to the inclusion of the additional reactive VOCs. The O₃ increases (Fig. 1e–f) are most notable for South Asia and East Asia due to the relatively high VOC emissions in these regions (Janssens-Maenhout et al., 2015; Huang et al., 2017; Feng et al., 2020). There is a much larger O₃ increase in South Asia in winter than in summer (Fig. 1e), mainly due to greater transport of O₃ precursors during the summer monsoon in South Asia (Lu et al., 2018). This leads to higher O₃ mixing ratios in winter than in summer, consistent with Gao et al. (2020), and demonstrates a larger seasonal variation in O₃ mixing ratios with the Ext_StratTrop chemistry scheme than with the StratTrop chemistry scheme. In addition, there are substantial O₃ increases in East Asia and in other polluted continents in summer (Fig. 1f) when using the extended chemistry scheme. This is not seen in winter (Fig. 1e) because titration of O₃ by nitric oxide (NO) remains strong, despite high VOC emissions. The relatively small influence of the additional VOCs on O₃ in winter is also seen in other heavily populated regions that have high NO_x emissions such as North America and Europe.

Figure 1 Comparison of present-day seasonal mean surface O₃ mixing ratios (2004–2014) between the Ext_StratTrop chemistry scheme (a, b) and the StratTrop chemistry scheme (c, d). Winter time (December–January–February, DJF) and summer time (June–July–August, JJA) global mean O₃ mixing ratios are shown. Seasonal differences in surface O₃ mixing ratios between the two chemistry schemes are shown in (e) and (f).

3.2 Evaluation of surface O₃ mixing ratios

We now evaluate surface O₃ mixing ratios simulated with the Ext_StratTrop chemistry scheme against gridded monthly mean rural observations from the TOAR dataset over the 2004–2014 period (Schultz et al., 2017a, b). Surface O₃ mixing ratios for winter (DJF) and summer (JJA) during 2004–2014 are shown in Fig. 2. We find that global mean surface O₃ mixing ratios are underestimated in winter (−7.3 ppb) and overestimated in summer (+13.5 ppb), with relatively small biases in spring and autumn. The positive biases in summer and negative biases in winter for O₃ mixing ratios are also seen in results using the StratTrop chemistry scheme (Archibald et al., 2020a; Turnock et al., 2020). The model seasonality with both schemes (19.4–45.5 ppb; DJF-JJA) is rather stronger than that observed (26.7–32 ppb; DJF-JJA), but the Ext_StratTrop chemistry scheme improves the model performance slightly in DJF for surface O₃ (Fig. 2b) despite larger biases in summer (Fig. 2d). Numerical diffusion of O₃ precursor emissions due to coarse model horizontal resolution may explain the biases (Wild and Prather, 2006; Stock et al., 2014; Fenech et al., 2018), and we note that these seasonal O₃ biases are also evident in other chemistry–climate models (Young et al., 2018; Turnock et al., 2020). Insufficient turbulent mixing in the planetary boundary layer may also contribute to the bias (O'Connor et al., 2014), as accumulation of NO_x at the surface leads to greater O₃ production in summer and greater titration by NO in winter. However, we note that a much more comprehensive chemistry scheme applied in UKCA, the Common Representative Intermediates Mechanism (CRI-Strat), shows similar systematic biases in surface O₃ mixing ratios: −4.6 ppb in DJF and +12.0 ppb in JJA for the 2010–2018 period (Archer-Nicholls et al., 2020). This suggests that other biases in the model are primarily responsible for the biases in surface O₃. We choose to apply the extended chemistry scheme as it permits representation of a more appropriate chemical environment for O₃ production in high-emission areas and is thus more suitable to investigate O₃ sensitivity.

Figure 2 Modelled surface O₃ biases with the Ext_StratTrop chemistry scheme for winter (DJF) and summer (JJA) over 2004–2014 (a, c). Probability distribution function (PDF) of seasonal mean O₃ mixing ratios between observations, StratTrop, and Ext_StratTrop chemistry schemes (b, d). Vertical dashed lines indicate seasonal mean surface O₃ mixing ratios. Observations from the Tropospheric Ozone Assessment Report (TOAR) dataset (Schultz et al., 2017a) are used for comparison.

4 O₃ changes under future scenarios

4.1 Emission changes

The SSP3-7.0 pathway is characterised by relatively strong emission controls in some parts of the world such as North America and Europe but weaker controls or emission increases elsewhere. Increases in NO_x and VOC emissions from anthropogenic and biomass burning sources are seen in Central and South America, North Africa, the Middle East, and South and East Asia (Fig. 3a, c). SSP3-7.0-lowCH4 has the same NTCF emissions as SSP3-7.0 but lower CH₄ emissions that lead to lower CH₄ mixing ratios (Table 2). SSP3-7.0-lowNTCF represents strong emission controls across the globe, with reductions in emissions in most major high-emission regions except for South Asia (Fig. 3b, d). Total BVOC emission changes are driven by changes in land use, vegetation, and temperature. Figure 3e and f show general increases in total BVOC emissions in most parts of the Northern Hemisphere except for South Asia in the future, which partly offset the decreased anthropogenic and biomass burning VOC emissions in North America and Europe. Similar BVOC emission changes are found under SSP3-7.0 and SSP3-7.0-lowNTCF relative to the present day as the climate change signal and carbon dioxide concentrations under future pathways are the same.

Figure 3 Differences in annual mean surface emissions of NO_x (a, b), anthropogenic and biomass burning VOCs (c, d) and total biogenic VOCs (e, f) between the present day (PD; 2004–2014) and SSP3-7.0 and SSP3-7.0-lowNTCF (2045–2055).

4.2 Tropospheric O₃ changes

Changes in tropospheric O₃ from the present day to the future are shown in Table 4. Changes in NTCF emissions control O₃ burden changes with a 4 % increase in the O₃ burden under SSP3-7.0 and a 7 % decrease under SSP3-7.0-lowNTCF relative to the present day. Changes in O₃ production rates are also controlled by changes in NTCF emissions, and there is higher O₃ production under SSP3-7.0 and lower O₃ production under SSP3-7.0-lowNTCF. The decrease in the O₃ burden (5 %) under SSP3-7.0-lowCH4 is slightly less than that under SSP3-7.0-lowNTCF (7 %) and shows that reductions in CH₄ mixing ratios effectively reduce the tropospheric O₃ burden despite high NTCF emissions under SSP3-7.0-lowCH4. O₃ production rates under SSP3-7.0-lowCH4 are slightly higher than in the present day partly due to higher hydroxyl radical (OH) concentrations that promote O₃ production. However, these higher O₃ production rates are offset by higher O₃ loss rates and result in lower O₃ net production under SSP3-7.0 and SSP3-7.0-lowCH4. We find that the O₃ chemical lifetime decreases slightly by 0.4–1.6 d under future pathways partly due to decreased O₃ net production and partly due to increased O₃ loss associated with higher temperature and humidity in a warmer climate (Young et al., 2018). Changes in O₃ dry-deposition rates principally reflect changes in surface O₃ mixing ratios, although high temperatures under a warmer climate may reduce O₃ deposition rates due to vegetation stress (Lin et al., 2020).

Table 4 Comparison of the tropospheric O₃ budget under present-day (2004–2014) and future conditions (2045–2055).

4.3 Surface seasonal O₃ changes

Seasonal differences in simulated surface O₃ mixing ratios between the present day (2004–2014) and future pathways (2045–2055) are shown in Fig. 4a–f, along with a comparison between SSP3-7.0 and SSP3-7.0-lowCH4 (Fig. 4g, h). SSP3-7.0 represents less stringent emission control policies and has slightly higher global mean O₃ mixing ratios (0.7–0.9 ppb; Fig. 4a, b) than the present day. In contrast, tightened emission controls under SSP3-7.0-lowNTCF reduce surface O₃ mixing ratios substantially across many parts of the world (3.3–5.2 ppb; Fig. 4c, d). The reduction in CH₄ mixing ratios from 1803 to 1364 ppb under SSP3-7.0-lowCH4 relative to the present day successfully reduces O₃ mixing ratios (2.7–3.5 ppb; Fig. 4e, f) for regions that show O₃ increases under SSP3-7.0. SSP3-7.0-lowCH4 with reduced CH₄ mixing ratios alone shows uniform O₃ decreases across the globe (3.4–4.4 ppb; Fig. 4g, h) compared with SSP3-7.0, where mean CH₄ mixing ratios are 2472 ppb, and this offsets high O₃ levels in regions with high NTCF emissions. This demonstrates the importance of CH₄ in governing surface O₃ mixing ratios and the need to account for CH₄ in mitigating O₃ pollution in future (Fiore et al., 2008; Allen et al., 2021). We highlight that O₃ changes vary by season. From Fig. 4a and c we can see that O₃ mixing ratios generally increase in winter in continental areas such as North America and Europe under SSP3-7.0 and SSP3-7.0-lowNTCF. These regions have large reductions in NTCF emissions under future pathways, and thus there is less O₃ titration due to lower NO_x emissions in these regions. Since NO_x emissions decrease in East Asia under SSP3-7.0-lowNTCF, we also see O₃ increases in winter in this region. This highlights that NO_x emission reductions are not beneficial for reducing surface O₃ mixing ratios in winter. Conversely, NO_x emission increases under SSP3-7.0 lead to O₃ decreases in winter in South and East Asia for the same reason. The situation is different in summer, with O₃ decreases in North America and Europe but O₃ increases in South and East Asia under SSP3-7.0 (Fig. 4b). This demonstrates a shift in O₃ sensitivity from VOC limitation in winter to NO_x limitation in summer. O₃ changes in summer and winter are generally consistent in South America and Africa and reflect NO_x limitation in these regions throughout the year. For regions that are projected to have lower NO_x emissions in SSP3-7.0-lowNTCF, such as eastern China, we note that O₃ mixing ratios increase in both winter and summer. The industrial regions of China are in VOC-limited regimes throughout the year, and thus decreased NO_x emissions increase O₃ mixing ratios (Jin and Holloway, 2015; Wang et al., 2021; Liu et al., 2021). This suggests that reductions in both NO_x and VOC emissions may be needed to reduce O₃ in these regions.

Figure 4 Differences in seasonal mean surface O₃ mixing ratios between the present day (PD; 2004–2014) and the future SSP3-7.0 (a, b), SSP3-7.0-lowNTCF (c, d), and SSP3-7.0-lowCH4 (e, f) (2045–2055) in winter (DJF) and summer (JJA). Differences between SSP3-7.0 and SSP3-7.0-lowCH4 are shown (g, f) to isolate the impacts of reduced CH₄ mixing ratios. Absolute global mean O₃ changes (ppb) are shown at the top right of each panel.

Surface O₃ mixing ratios are also influenced by climate change, reflecting changing natural emissions, O₃ production and destruction rates, and O₃ deposition rates (Doherty et al., 2013, 2017). Global annual mean surface O₃ mixing ratios decrease by 1 ppb with a 1.5 ^∘C temperature rise but show little change in continental areas (Naik et al., 2021). This is principally due to increased humidity and greater O₃ destruction in oceanic areas, but in continental areas these effects may be offset by O₃ increases due to higher soil NO_x (Romer et al., 2018) and BVOC emissions and by decreased O₃ deposition rates (Lin et al., 2020). O₃ mixing ratios can also be impacted by more frequent and intense heat waves under a warmer climate (Schnell and Prather, 2017; Ma et al., 2019). We find that the resulting changes in surface O₃ mixing ratios in continental regions due to climate change are relatively small, and a reduction in anthropogenic emissions is the dominant factor governing surface O₃ mixing ratios in the near future.

We also examine daytime and nighttime O₃ changes for different regions under future pathways because daytime O₃ mixing ratios are typically more relevant for human health. For daytime O₃ we consider the maximum daily average 8 h O₃ mixing ratio (MDA8), an important metric used to evaluate O₃ impacts on human health. Nighttime O₃ mixing ratios are correspondingly given by the minimum daily average 8 h O₃ mixing ratio (MIN8). We show a comparison of the changes in these metrics in Fig. 5.

In general, differences in global mean O₃ changes are relatively small in both daytime and nighttime under all pathways. However, in South and East Asia with increased NO_x emissions under SSP3-7.0 and SSP3-7.0-lowCH4, O₃ mixing ratios tend to increase in daytime but show smaller increases or reduction at night. This demonstrates the impact of O₃ titration by NO at nighttime in high-NO_x environments. In North America and Europe, which have lower NO_x emissions in the future, daytime O₃ mixing ratios decrease greatly in summer, but daytime and nighttime changes are similar in winter, demonstrating the large influence of NO_x emissions on summer daytime O₃ mixing ratios. The substantial differences between daytime and nighttime O₃ changes suggest that the underlying impacts of O₃ changes on human health are likely to be larger than those estimated using seasonal mean O₃ mixing ratios, and this is particularly important for high-emission areas with increased NO_x emissions in the future.

Figure 5 Seasonal mean maximum daily average 8 h (MDA8) and minimum daily average 8 h (MIN8) surface O₃ changes for North America, Europe, South Asia, East Asia, and the globe from the present day (PD; 2004–2014) to SSP3-7.0 (a), SSP3-7.0-lowNTCF (b), and SSP3-7.0-lowCH4 (c) (2045–2055). DJF and JJA situations are shown with blue and red bars, respectively.

5 O₃ sensitivity in the present day and the future

Non-linearity in chemical O₃ formation can result in differences in the effectiveness of emission control strategies regionally and may aggravate O₃ pollution issues. We therefore investigate O₃ sensitivity for the present day and the future, as it is important to understand how O₃ mixing ratios will respond to changing emissions. Ratios of NO_x and VOC mixing ratios provide a useful indicator of regional O₃ sensitivity regimes. Here we quantify the critical ${\mathrm{NO}}_x/\mathrm{VOC}$ ratio that distinguishes VOC-limited and NO_x-limited regimes by examining monthly mean surface O₃ mixing ratios and net chemical production rates as a function of monthly mean NO_x and VOC mixing ratios; see Fig. 6. Monthly mean O₃ mixing ratios and net production rates in the lowest model layer from all months and all scenarios are used to plot the figure. For NO_x we use the sum of NO and NO₂ mixing ratios, and for total VOC we use the sum of the mixing ratios of primary emitted VOC species. CO and CH₄ are not included due to their relatively low reactivity. We classify NO_x and VOC mixing ratios in each model grid cell into 150 bins on a logarithmic scale ranging from 0.01 ppb to 100 ppb and calculate mean O₃ mixing ratios and mean O₃ net chemical production rates in each NO_x–VOC bin. Approximate thresholds of monthly mean ${\mathrm{NO}}_x/\mathrm{VOC}$ ratios for O₃ sensitivity are shown in Fig. 6, ranging from 0.6 to 1 with a central value of 0.8. We hence apply a threshold value of 0.8 to distinguish O₃ sensitivity regimes hereafter. Areas above the threshold represent VOC-limited regimes in which increased NO_x emissions reduce O₃ mixing ratios and O₃ production rates, and areas below the threshold represent NO_x-limited regimes. We find the highest O₃ mixing ratios and O₃ production close to the threshold line, demonstrating that the threshold values are consistent and that the approach is robust.

We further investigate O₃ sensitivity for different regions. The regions considered here are those defined for the Task Force on Hemispheric Transport of Air Pollutants Phase 2 (TF HTAP2; Janssens-Maenhout et al., 2015); see Fig. 7a. The regions dominating each part of the NO_x$/$VOC mixing ratio space are shown in Fig. 7b and c. We determine the dominant region contributing to each bin in NO_x–VOC space based on the region contributing the greatest number of model grid cells to that bin. This approach reveals differences in regional O₃ sensitivity. We also show the shift in O₃ sensitivity in different regions between the present day (Fig. 7b) and the future (SSP3-7.0-lowNTCF; Fig. 7c) to demonstrate the impacts of decreased NTCF emissions on the evolution of O₃ sensitivity on a regional basis.

Figure 6 Surface O₃ mixing ratios (a) and O₃ net chemical production (ncp) rates (b) as a function of monthly mean NO_x and VOC mixing ratios. Monthly mean data for all months and all scenarios are used. The straight lines show the approximate thresholds of ${\mathrm{NO}}_x/\mathrm{VOC}$ to distinguish VOC-limited (above the line) and NO_x-limited (below the line) regimes.

Figure 7 Geographical regions defined in the Task Force on Hemispheric Transport of Air Pollutants Phase 2 (TF HTAP2) (a). Regions dominating each part of NO_x$/$VOC mixing ratio space in the present day (b) and under SSP3-7.0-lowNTCF (c). The straight line shows the ${\mathrm{NO}}_x/\mathrm{VOC}$ threshold of 0.8. Monthly mean data for all months under each pathway are used to plot (b) and (c).

Figure 7b clearly shows that low-NO_x and low-VOC environments are most common in oceanic and polar regions where surface O₃ levels are typically low and where O₃ production is NO_x-limited. In contrast, Europe, North America, and East Asia dominate the high-NO_x and high-VOC environments in the present day where O₃ levels are high and O₃ production is VOC-limited. Europe and North America have similar VOC mixing ratios, but NO_x mixing ratios are generally higher in Europe than in North America, which results in Europe lying further above the ${\mathrm{NO}}_x/\mathrm{VOC}$ threshold. This demonstrates that stricter controls on NO_x emissions are required for Europe to shift from VOC-limited to NO_x-limited regimes than for North America. East Asia dominates VOC-limited regimes due to much higher NO_x and VOC mixing ratios than other regions. Parts of South Asia and the Middle East are also VOC-limited. Major biogenic emission source regions such as South America, South Africa, and South-East Asia have high VOC mixing ratios but moderate levels of NO_x, and the chemical environment is therefore NO_x-limited.

The impacts of reductions in NTCF emissions are shown in Fig. 7c. We find that Europe and North America are no longer the most dominant VOC-limited regimes due to decreased NO_x mixing ratios. East Asia is still a dominant VOC-limited region under SSP3-7.0-lowNTCF, but reduced NO_x emissions shift parts of East Asia into NO_x limitation. South Asia becomes the main VOC-limited region with relatively high NO_x mixing ratios. South America, South Africa, and South-East Asia are still NO_x-limited because there are no large NO_x increases in these regions.

O₃ sensitivity in major present-day VOC-limited regions under different scenarios is shown in Fig. 8. Reductions in NO_x emissions are important and effective in transforming VOC limitation to NO_x limitation, reflected in large NO_x-limited areas in Europe and North America under all scenarios. In contrast, most parts of East Asia are VOC-limited under SSP3-7.0 and SSP3-7.0-lowCH4 due to increased NO_x emissions. Since changes in VOC mixing ratios are relatively small for all scenarios, they have little substantial influence on O₃ sensitivity. Under future climate, increased biogenic VOC emissions in Europe and North America partly offset decreased anthropogenic and biomass VOC emissions and are hence beneficial to maintain an NO_x-limited environment. We note that South Asia is the only region that is substantially VOC-limited in the future due to increased NO_x emissions. Reductions in CH₄ mixing ratios have relatively little impact on O₃ sensitivity, although they greatly reduce surface O₃ mixing ratios (Sect. 4.3).

Figure 8 O₃ sensitivity across Europe, North America, East Asia, and South Asia in the present day (a) and under SSP3-7.0 (b), SSP3-7.0-lowNTCF (c), and SSP3-7.0-lowCH4 (d). The ${\mathrm{NO}}_x/\mathrm{VOC}$ threshold of 0.8 is shown. Horizontal and vertical lines indicate regional mean NO_x and VOC mixing ratios in the present day.

6 Spatial distributions of O₃ sensitivity

Global spatial distributions of annual O₃ sensitivity in the present day and the future are shown in Fig. 9. VOC-limited regimes are represented by high ${\mathrm{NO}}_x/\mathrm{VOC}$ ratios, reflecting relatively high NO_x or low VOC mixing ratios. In the present day, high NO_x emissions contribute to VOC limitation in large areas of North America, western and central Europe, and East Asia. While North America and Europe have lower NO_x mixing ratios than East Asia, lower VOC mixing ratios still lead to VOC limitation. Only south-west parts of India are VOC-limited. In the future, O₃ production in more areas of North America and Europe becomes NO_x-limited. However, VOC-limited regimes in East Asia, particularly China, are persistent due to projected increases in NO_x emissions until 2055 under SSP3-7.0 and SSP3-7.0-lowCH4. We find that reductions in CH₄ mixing ratios have relatively little influence on O₃ sensitivity over continental regions (Fig. 9b vs. d). We note that SSP3-7.0-lowNTCF shows the smallest VOC-limited areas across the globe. East Asia is still partly VOC-limited under this scenario, particularly in northern China (Fig. 9c), which indicates that further reductions in NO_x emissions are required in addition to those expected in this region in the future (Fig. 3b).

Figure 9 Spatial distributions of annual mean ${\mathrm{NO}}_x/\mathrm{VOC}$ in the present day (2004–2014; a) and under SSP3-7.0 (b), SSP3-7.0-lowNTCF (c), and SSP3-7.0-lowCH4 (d) (2045–2055). An ${\mathrm{NO}}_x/\mathrm{VOC}$ threshold ratio of 0.8 is used here to distinguish O₃ sensitivity regimes.

We contrast regional O₃ sensitivities for winter and summer seasons in Fig. 10, as we note that O₃ responses in different seasons can be substantially different in high-emission regions. This suggests that static emission control strategies throughout the year may not be the best way to lower annual mean O₃ pollution, and adjustments may be needed according to seasonal O₃ sensitivities. More extensive VOC-limited areas are found in winter than in summer under both present-day and future conditions, but these account for less than 7 % of the total area of the world in the present day (Table 5). Over 50 % of North America, Europe, and East Asia are VOC-limited in winter in the present day. This explains why North America and Europe show O₃ increases in winter (Fig. 4) despite reduced NO_x emissions. However, there are fewer VOC-limited regions across the globe under all future pathways (Fig. 10). About 1 % of North America is VOC-limited in summer and less than 7 % of Europe. In contrast, over 48 % (winter) and 39 % (summer) of South Asia are VOC-limited in future. Slightly more areas are VOC-limited under SSP3-7.0-lowCH4 than under SSP3-7.0 (Fig. 10c, d). Overall, reductions in NO_x emissions are important to reduce O₃ production in high-emission regions and shift VOC-limited areas to NO_x limitation, but this may lead to higher O₃ mixing ratios in winter without further emission controls on VOC and CH₄.

Figure 10 Seasonal differences in O₃ sensitivity regimes for the present day (2004–2014; a) and under SSP3-7.0 (b), SSP3-7.0-lowNTCF (c), and SSP3-7.0-lowCH4 (d) (2045–2055). An ${\mathrm{NO}}_x/\mathrm{VOC}$ threshold ratio of 0.8 is used here to distinguish O₃ sensitivity regimes.

Table 5 The percentage of VOC-limited areas (%) in different regions under different pathways in winter (DJF) and in summer (JJA).

7 Conclusions

We use a global chemistry–climate model, UKESM1, to assess the impacts of changing near-term climate forcer (NTCF) emissions and CH₄ mixing ratios in the context of climate change on tropospheric O₃ in the present day (2004–2014) and the near future (2045–2055). CMIP6 future scenarios including SSP3-7.0, SSP3-7.0-lowNTCF, and SSP3-7.0-lowCH4 are used from the AerChemMIP project. We have examined O₃ changes from the present day to the future and investigated regional O₃ sensitivities to explain contrasting O₃ changes in different seasons.

An extended chemistry scheme incorporating more reactive VOC species is used to permit the representation of more active photochemical environments for O₃ production. This shows higher surface O₃ mixing ratios in high-emission regions and a 5 % higher tropospheric O₃ burden. While simulated surface O₃ mixing ratios are biased low in winter and high in summer, these systematic model biases are similar to those using the original chemistry scheme as well as a more comprehensive chemistry scheme. This indicates that other factors in the model are likely to be responsible for the biases, but the extended chemistry scheme permits the representation of a more appropriate chemical environment for O₃ production in high-emission areas.

From the present day to the future, the tropospheric O₃ burden increases by 4 % under SSP3-7.0 and decreases by 7 % and 5 % under SSP3-7.0-lowNTCF and SSP3-7.0-lowCH4. The tropospheric O₃ chemical lifetime remains similar (21–22 d) under all scenarios, and this is similar to previous estimates. Seasonal global mean surface O₃ mixing ratios increase by 0.7–0.9 ppb under SSP3-7.0 and decrease by 3.3–5.2 ppb under SSP3-7.0-lowNTCF and by 2.7–3.5 ppb under SSP3-7.0-lowCH4. We find that reductions in NTCF emissions are effective in reducing surface O₃ mixing ratios, and reductions in CH₄ mixing ratios are also important. We also find that both the magnitude and direction of seasonal, daytime, and nighttime O₃ changes relative to the present day can vary greatly across different regions especially South and East Asia.

O₃ sensitivity is quantified using monthly mean ${\mathrm{NO}}_x/\mathrm{VOC}$ ratios to give a broad assessment of regional O₃ sensitivity. The estimated monthly mean ${\mathrm{NO}}_x/\mathrm{VOC}$ thresholds range from 0.6 to 1.0, and 0.8 is used to distinguish O₃ sensitivity regimes. Most VOC-limited regimes occur in high-emission regions across the Northern Hemisphere, such as North America, Europe, the Middle East, South Asia, and East Asia. More areas in North America and Europe become NO_x-limited under all future pathways due to the projected decrease in NO_x emissions. There are more VOC-limited areas in East Asia under SSP3-7.0 and SSP3-7.0-lowCH4 due to the projected increase in NO_x emissions, although there are fewer VOC-limited areas in East Asia under SSP3-7.0-lowNTCF. South Asia becomes the dominant region for VOC-limited O₃ production in the future. Projections of regional O₃ sensitivity demonstrate that reductions in NO_x emissions are the most important factor to shift VOC-limited regimes to NO_x limitation.

We highlight that O₃ sensitivity varies by season. There are more VOC-limited regimes in winter (7 %) than in summer (3 %) across the globe. Reductions in NO_x emissions increase surface O₃ mixing ratios in high-emission areas particularly in winter, and reductions in VOC emissions should be targeted. In the future, reductions in NO_x and VOC emissions should both be effective in mitigating O₃ pollution in most areas of North America and Europe in summer because there are only 1 % and 7 % VOC-limited areas in these two regions. However, further reductions in NO_x emissions are needed for parts of East Asia and South Asia to convert most VOC-limited areas to NO_x-limited ones. While anthropogenic and biomass emissions may be controlled in the future, more biogenic emissions under a warmer climate would hinder the impacts of VOCs on O₃ mitigation. Reductions in CH₄ mixing ratios are also important to reduce surface O₃ pollution. NO_x decreases are important to reduce surface O₃ mixing ratios from a global perspective but will lead to increased O₃ mixing ratios in some regions, and hence emission controls on VOC and CH₄ are necessary to mitigate regional O₃ pollution during the transition from VOC to NO_x limitation.