Ozone pollution: a persistent challenge in large cities like Mexico City, Los Angeles and Beijing

General characteristics

With more than 21 million inhabitants, the MCMA is the most populous metropolitan area in North America. It expands over 7866 km², occupying the entire area of Mexico City and 60 neighbouring municipalities of two surrounding states. However, five metropolitan areas, also located in the central region of Mexico, have experienced a rapid expansion, producing a contiguous urban complex known as the Mexico Megalopolis, which covers nearly 100,000 km² with a population of 41.5 million inhabitants [46].

The MCMA is located within a semi-closed basin on the central Mexican Plateau at an elevation of 2240 m above sea level in the tropics (20° N). It has a highland subtropical climate with a well-defined rainy season and two dry seasons, one warm and one cool. The warm dry season, which lasts from March until the end of May, is characterised by high-pressure systems with clear skies and strong solar radiation, which boost photochemistry and result in an increase in O₃ production [10,11]. O₃ pollution is less intense during the other two seasons, but high O₃ concentrations cannot be ruled out, especially on days with low cloud cover and weak winds. The high elevation and the basin–mountain circulation ventilate the city effectively, while the heat absorbed by the built-up surface creates unstable atmospheric conditions and a relatively high boundary layer, so there is relatively little daily accumulation of pollutants; however, due to its subtropical latitude and high altitude, shortwave solar radiation is intense and enhances photochemistry, and severe O₃ episodes can occur throughout the year.

Ozone trends

In the early 1990s, Mexico City acquired the infamous title of the world’s most polluted city [47], when the newly established air quality monitoring network revealed O₃ concentrations above 300 ppb for 40–50 days annually. Over the next 15 years, annual average concentrations decreased by about 65%. There was a steady decline in O₃ peak concentrations, with most days falling below the current hourly threshold of 155 ppb before an air quality contingency is activated (see Fig. 3). Nonetheless, nowadays 60–70% of the days still experience peaks above the 1-h average concentration (90 ppb), and 50–55% exceed the 8-h moving average concentration (65 ppb) deemed unhealthy by Mexican legislation (i.e., air quality standards).

Figure 3

Daily 8-h average maximum O₃ concentrations. (a) Fraction of days exceeding the current local 8-h air quality standard of 65 ppb and the 70 ppb standard that was in effect from July 2014 to October 2021. (b) Time series of daily 8-h average maximum concentrations. Green markers indicate the maximum concentration reported by any air quality monitoring station across the metropolitan area. The blue line is the monthly median of the daily maximum concentrations. Data were obtained from the Mexico City’s Air Quality Monitoring System (SIMAT, Spanish acronym; https://www.aire.cdmx.gob.mx/).

No improvement in O₃ concentrations has been observed since around 2010, and the evidence suggests a possible rebound back to elevated concentrations. Molina et al. [11] determined a mean increase of 0.39 ppb per year during the period 2010–2022. This implies an increase of ∼5 ppb in 12 years. Albeit with periodic reductions in the contingency threshold to adjust for management progress, the O₃ alarm has rung at least once every year since 2016.

The ambient concentrations of other key pollutants, such as SO₂ and CO, have decreased by more than 90% since 1990 and now are below current air quality standards. Concentrations of NO_X and PM₁₀ have also decreased substantially by around 50% and 70%, respectively, but remain above respective air quality standards. Similar to O₃, concentrations of particles smaller than 2.5 μm (PM_2.5) have not decreased in 15 years.

Environmental authorities attribute the growing trend in O₃ concentration to changes in atmospheric chemistry produced by the differences in the extent to which emissions of NO_X and VOCs have been reduced [48]. Indeed, changes in the mix of precursor pollutants responding to improved emission control regulations, technological advances and emerging emission sources may enhance the production of O₃ rather than slow it [49]. It is also necessary to consider the expansion of the urban sprawl and changes in the urban morphology (e.g., increasingly taller buildings), which alter the production of turbulence and wind circulation within the basin, with a direct impact on the city’s ventilation and pollutants’ dispersion. Regional air pollution is becoming increasingly important. Emissions from the other five major cities within the Mexico Megalopolis may now have a greater impact on the MCMA’s air quality and vice versa [50]. Finally, changes in regional meteorology favouring clearer skies [51], and meteorological anomalies associated with climate change, such as heat waves and upper tropospheric troughs triggered by jet streams reaching lower latitudes, can produce high O₃ pollution episodes [52].

Health and economic impacts

In terms of public health, the progress that was achieved up to 15 years ago is undeniable. For example, Rojas-Bracho et al. [53] determined that O₃ reductions in Mexico City between 1990 and 2015 prevented approximately 4100 premature deaths. Similarly, by reducing 10% of the 1-h O₃ peak concentrations, authorities estimated that 124 annual premature deaths could be prevented [54].

The 14 measures outlined in the current Air Quality Improvement Program (Programa Para Mejorar la Calidad del Aire, ProAire ZMVM 2021–2030) seek to cut VOC and NO_X emissions by at least 26% and 40%, respectively, and reduce the annual O₃ concentrations by 2% (∼3 ppb) and the maximum hourly concentration by 7% (∼31 ppb) [48]. Considering the total burden of pollutants, these measures are expected to prevent 6000 annual premature deaths by 2030, which is equivalent to an estimated economic value of US$5880 million or about 2.3% of the current gross domestic product (GDP) in the MCMA. By then, these economic benefits would have offset the implementation costs of the entire programme by more than 40% [48].

Main interventions

Efforts to reduce air pollution began in the 1970s, but it was not until 1993 that the Mexican Government enacted the first air quality standards to protect public health. Since then, air quality has been the subject of political action. At the time, the federal government enacted the General Law of Ecological Equilibrium and Environmental Protection, which set clear jurisdictional responsibilities for addressing air pollution at the federal, state and local levels. At the same time, a metropolitan environmental commission that years later evolved into the Environmental Commission of the Megalopolis (Comisión Ambiental de la Megalópolis, CAMe) was established to coordinate the environmental actions between the Federal Government, and the local governments of the states that make up the MCMA [55].

Plans to reduce air pollution are periodically revised and updated. The Air Quality Improvement Program (ProAire) acts as a preventive and corrective instrument to improve air quality and protect public health, as well as to comply with the applicable legal framework [48]. The first air quality management programme was implemented in 1990 and has been updated five times. The strategies shaped in these programmes respond to the integration of air quality information obtained by authorities and academia to understand the origin, transformation and fate of air pollution, as well as to evaluate its impact on public health and economic growth. A series of studies conducted as part of three intensive air quality field campaigns between 1997 and 2006 were fundamental in integrating the air quality management programmes and succeeded in reducing pollution levels 15–20 years ago [56–58].

Despite notable differences in institutional capacity, financial resources and technical expertise, the MCMA drew lessons from the LA Basin, which began its air cleanup efforts two decades earlier. Similar strategies and emission control technologies were initially implemented [55].

The policies proposed in the current ProAire centre on improving vehicle technology and fuel quality, lowering emissions in industries and the commercial sector, increasing public transportation, improving urban planning and encouraging environmental education and research [48]. They are backed by an integrated financial strategy aiming at achieving sustainability by developing mechanisms that produce economic benefits, offset programme investment expenses, and become self-sustaining through the benefits of breathing clean air. It combines investments, employment and air quality in an effort to direct human and financial resources into initiatives involving the private sector, academia and society.

Challenges ahead

Despite significant progress in improving air quality from the late 1990s to around 2010, the MCMA is on the verge of returning to previous O₃ pollution levels due to several contributing factors, as described. The most prominent of these factors appears to be urban expansion. The surrounding municipalities are of the greatest concern, as urban sprawl has grown with little or no planning across them. Except for the MCMA, there is limited air quality monitoring and air pollution studies in the entities that comprise the Mexico Megalopolis. This makes it difficult to assess the regional air quality and its impact on the MCMA. The different administrative and legislative jurisdictions of these entities, and the limited available financial resources, have created an ongoing challenge that does not contribute to solving the air pollution problem.

New financial schemes and political will are needed to ensure robust air quality management across the region to enable timely and informed action. Local governments and federal agencies must foster a spirit of openness and collaboration and be receptive to independent science-policy advice in order to find long-term sustained solutions. In this context, a vigorous and active academic community is needed. Scientists must fill gaps in knowledge and influence policymakers. Policies must be in line with the scientific knowledge and evidence collected locally. The current challenge posed by persistently high O₃ concentrations demands stakeholders who are fully committed to society and specialists with knowledge in atmospheric sciences.

General characteristics

The LA Basin covers five counties (Los Angeles, Orange, Riverside, San Bernardino and Ventura), being the second largest urban area in the United States and California’s largest metropolitan region. It is home to approximately 18 million people, over 40% of California’s population. It is also home to over 12 million passenger vehicles that, together with the commercial vehicles on the roads, travel over 230,000 million km per year. Emissions from these vehicles, along with those from ships, ports, rail yards and airports, combined with stationary sources such as refineries and power plants, and emissions from households, all contribute to O₃ air pollution [13,61]. It is situated in the South Coast Air Basin, bordered by the Pacific Ocean on the west and mountains on the other three sides. The transport of air pollutants into and out of the basin is determined by factors such as sea breezes and topography-driven winds. The interplay of these factors frequently causes air masses to circulate back and forth daily [62]. Consequently, air masses can become trapped within the mountainous coastal region for several days, allowing for extensive photochemical processing and formation of O₃. Ozone concentrations are highest in the spring through early fall.

Ozone trends

The LA Basin is an example of an urban region with long-standing O₃ pollution. Substantial reductions in O₃ concentrations were achieved during the last three decades of the 20th century thanks to extensive pollution control measures [63]. In the early 1980s, daily 8-h average peak concentrations over 200 ppb were recorded for 20–30 days each year; the last day exceeding this threshold was in 1994. Peak 8-h average concentrations in the late 1990s revolved around 120–150 ppb; in 2000, only two days exceeded 120 ppb. By 2010, the number of days above 100 ppb had dropped to a single digit, albeit 2023 and 2024 experienced over 10 days with higher peak concentrations. No further reductions have been achieved during the last two decades, and even small increases have started to be observed, despite important emission reductions of precursor pollutants, NO_X and VOCs, of anthropogenic origin [13]. Nowadays, 25–30% of the days each year, the majority during the summer, exceed the 8-h air quality standard of 70 ppb for health protection (see Fig. 4).

Figure 4

Daily 8-h average maximum O₃ concentrations. (a) Fraction of days exceeding the current local 8-h air quality standard of 70 ppb. (b) Time series of daily 8-h average maximum concentrations. Green markers indicate the maximum concentration reported by any air quality monitoring station across the LA Basin. The blue line is the monthly median of the daily maximum concentrations. Data were obtained from the United States’ Environmental Protection Agency (EPA, https://www.epa.gov/outdoor-air-quality-data).

Continuous efforts to decrease emissions from vehicular traffic and stationary sources have enabled NO_X emission reductions of more than 50% since the late 1980s. Similarly, total VOC emissions have decreased by 60–70% [13]. Control measures have successfully reduced traffic emissions by a factor of 4–5. The contributions of so-called VCPs, associated with household and commercial activities previously overlooked, have gained relevance, despite a two-fold decline [13]. Studies suggest that evaporative emissions of these products and other non-traditional anthropogenic sources, such as cooking, are currently the largest source of anthropogenic VOCs with significant impacts on O₃ production [64,65]. As anthropogenic emissions have dropped, biogenic contributions have increased. According to recent studies, vegetation is likely responsible for 50–60% of current summer O₃ concentrations [66]. Biogenic emissions are expected to increase further due to a clear trend of rising temperatures.

Reducing the emissions of one or both O₃ precursors (i.e., VOCs and NO_X) would not necessarily inhibit O₃ production. Historically, the LA Basin has been under a VOC-limited regime due to an overabundance of NO_X, but about 20 years ago, it entered into a transitional state, moving towards an NO_X-limited regime as VOC reductions have been outpaced by NO_X reductions, and no further O₃ reduction has been achieved [67]. Recent studies have suggested that drastic reductions in NO_X emissions, larger than those experienced during the COVID-19 pandemic (20–30%), are needed to reduce O₃ concentrations; otherwise, temporal O₃ increases could be expected [67–69]. However, meteorological variability triggered by climate change may obscure the effectiveness of such reductions. Heat waves and wildfires enhance O₃ pollution [36,60], while droughts may reduce O₃ production by decreasing biogenic emissions [59].

In addition to locally produced O₃, polluted plumes from East Asia crossing the Pacific Ocean and entering California complicate efforts to reduce O₃ concentrations in the LA Basin. These plumes add on average 45 ppb, but can occasionally add up to 60 ppb, which is comparable to concentrations resulting from local production [70].

Health and economic impacts

Malashock et al. [4] projected 5.4 premature deaths per 100,000 inhabitants, or around 810 total deaths, from O₃ exposure in the LA Basin for 2019. The Institute for Health Metrics and Evaluation (IHME) estimated that O₃ pollution caused 79 years of life lost from premature death and living in poor health (DALYs) for the entire state of California in 2024 [43]. According to IHME, the number of premature deaths has not changed since the 1990s, while the number of DALYs has decreased 15–20%.

Stewart et al. [71] evaluated the public health and financial benefits associated with less O₃ pollution in the LA Basin by reducing NO_X and VOC emissions by 61% and 32%, respectively, using 2008 as a reference year. They found that 204 premature deaths, 660 emergency hospital admissions and 236,000 cases of acute respiratory symptoms could be avoided, with an estimated economic value of US$2645 million, or about 0.22% of the local GDP adjusted to current purchasing power.

Main interventions

The first recognised air quality episode in Los Angeles occurred in the summer of 1943 [72]. However, it was not until 1967 that the California Air Resources Board (CARB) was established. CARB’s mission is to promote and protect public health, welfare and ecological resources through the effective and efficient reduction of air pollutants while recognising and considering the economic effects [72]. All air quality programmes are founded on scientific research, seeking out the pollutants’ origin and their consequences on human health and the environment. For such an endeavour, CARB counts on a robust air quality monitoring network with stations strategically located to assess the pollutants spatial distribution within and outside metropolitan areas [72]. Air quality data is made available to the public in real time and used in combination with emission inventories and air quality modelling forecasts to provide accurate and timely advice to people. CARB is in charge of developing and amending statewide regulations, as well as monitoring regulatory action. CARB develops and adopts regulations through a process that allows for public input by encouraging the involvement of stakeholders and citizens through board hearings and workshops. Parrish et al. [61] provides a summary of CARB’s evolution and main tasks to overview air quality throughout California.

A wide array of emission control technologies and regulations have been developed through years of scientific and engineering research. Efforts are focused on reducing anthropogenic emissions from all major sources by implementing strict regulations, introducing cleaner fuels and promoting the use of advanced technology. Some of the most significant actions have been the following:

1. Mobile sources: rigorous emission regulations in passenger cars, motorcycles, commercial trucks, public buses and off-road equipment such as lawn mowers have been implemented. Catalytic converters were introduced for the first time in 1975, and the installation of diesel exhaust filters in heavy-duty trucks became mandatory in 2008. Every 5 years, CARB has to review and update strategies on mobile emissions to meet California’s clean air and climate change targets. For such an endeavour, CARB uses scenario planning tools to model levels of clean technologies needed and identifies actions to reduce vehicle distance travelled and high-level concepts for each mobile source sector that could be further developed into regulations and other programmes to achieve these levels of cleaner technology [73]. The next revision was scheduled to be published in late 2025, but at the time of writing, its development was on hold due to staff cuts and administrative issues at the federal level.

2. Goods movement sources: railroads, oceangoing vessels, commercial harbour craft, cargo handling equipment, drayage trucks and transport refrigeration have gone through various regulations to reduce their emissions. For example, through the continuously revised Advanced Clean Fleets Regulation [72], CARB has attained significant emission reductions from medium- and heavy-duty vehicles [74]. The use of clean fuels by ships approaching California’s coast was adopted in 2008, and it has been effective in reducing emissions from the shipping sector [75].

3. Cleaner fuels: various diesel and gasoline quality standards have been adopted over the years [72], including low-carbon standards to reduce carbon intensity [76].

4. Area sources: regulations have been imposed on emission sources that individually emit small quantities of pollutants but collectively are significant. Over 100 categories of consumer products have been standardised, resulting in nearly a 50% reduction in emissions since 1990 [72].

5. Industrial sources, including power plants, refineries, manufacturing facilities and small stationary sources such as gasoline service stations, dry cleaners, bakeries, workshops, etc., have been equipped with the best available control technology [72].

Simultaneously, CARB and the South Coast Air Quality Management District (SCAQMD) have used strategies that foster technological progress and self-regulation of emissions to implement such control measures. Three key strategies have been the following:

1. The Regional Clean Air Incentives Markets (RECLAIM) Program reduces industrial NO_X and SO_X emissions by using market-based mechanisms [77].

2. The Carl Moyer Memorial Air Quality Standards Attainment Program provides financial incentives for low-emission technology adoption. It provides grant funding for cleaner-than-required engines, equipment and other air pollution sources [72].

3. The Consumer Products Regulatory Program seeks to lower VOC emissions from household and personal care products by working with industry and other stakeholders to achieve feasible VOC emission reductions without eliminating such products from the market [72].

Challenges ahead

California is taking bold steps toward near-zero carbon emissions under a circular economy model that will undoubtedly contribute to reducing emissions of both greenhouse gases and O₃ precursors, such as initiatives to phase out internal combustion engines in light-duty and heavy-duty vehicles [72]. However, at the time of writing this article in September 2025, the shifting political landscape at the federal level was holding back the implementation of environmental initiatives like this. A number of federal policies had already been proposed to roll back some environmental regulations and reduce research funding, notably related to climate change initiatives and environmental justice.

The federal waiver granted to the State of California by the Environmental Protection Agency (EPA) under the Clean Air Act to request the ability to enact more stringent emissions standards for new motor vehicles compared to federal regulations had not been revoked [78]. However, multiple challenges were underway regarding California’s waivers, including US Supreme Court cases and Congressional Review Act resolutions, creating uncertainty in the regulatory environment. The state of California will need to enforce its rights to protect its environmental laws and preserve its air quality management capabilities.

The reduction in federal support for environmental policies is creating a new reality in which states and cities will have to be the primary drivers of action. Cases such as the US Supreme Court’s decision to block the enforcement of EPA’s ‘Good Neighbor Plan’ to reduce NO_X emissions from power plants and industrial facilities, which might not have an impact on local O₃ concentrations but would impact those of neighbouring states [79], could become more common.

Another challenge ahead is the increasingly frequent fires on the outskirts of the city, where development and wildland areas overlap. Aside from the destruction and loss they cause, they release massive amounts of pollutants, including O₃ precursors, as well as reactive nitrogen and chlorine species, that accelerate O₃ formation [80]. These fires are triggered by urbanisation trends under a changing climate and a lack of prevention and updated strategies to manage fires. The blazes that devastated the Palisades and Eaton districts in January 2025 are a wake-up call to take immediate action against this threat, which endangers not only the lives of those who live where they spark, but also the lives of all Angelenos due to the significant deterioration in air quality that they cause.

General characteristics

Beijing, China’s capital, together with the municipality of Tianjin and the province of Hebei, makes up the BTH, commonly referred to as Jing–Jin–Ji, an urban agglomeration of 110 million inhabitants that covers 212,000 km² in northeast China.

Beijing is the political, economic, and cultural centre of China. Tianjin is the gateway to northern China, as well as the shipping and logistics centre, and the manufacturing base of the region. Hebei is a major manufacturing province, famous for its steel industry. The Jing–Jin–Ji region is the largest and most dynamic economy in northern China; its economy has grown 1.8 times in 10 years, reaching a GDP of US$1477 billion (CN¥10,444 billion), equivalent to 8.3% of the national GDP in 2023. However, rapid population growth and massive urban expansion have accompanied this economic growth, altering the landscape, increasing energy consumption and exacerbating traffic congestion and air pollution.

Ozone trends

China’s air quality nationwide monitoring network was launched in 2013 and has seen no significant change in O₃ concentrations in the Jing–Jin–Ji region, as well as many other highly urbanised areas of the country. Since 2016, the annual 90th percentile of the 8-h average O₃ concentration has exceeded 85 ppb, reaching 92 ppb in 2023 [81]. Figure 5 shows that O₃ episodes exceeding the 8-h average threshold of 80 ppb set as air quality standard are unfortunately common; about a third of the days throughout the year exceed this limit. Nonetheless, monthly maximum median concentrations have recently started to decrease, presumably due to improved efforts to control both NO_X and VOC emissions [82].

Figure 5

Daily 8-h average maximum O₃ concentrations. (a) Fraction of days exceeding the current local 8-h air quality standard of 80 ppb. (b) Time series of daily 8-h average maximum concentrations. Green markers indicate the maximum concentration reported by any air quality monitoring station in Beijing and Tianjin. The blue line is the monthly median of the daily maximum concentrations. Data were obtained from the TOAR Database (https://doi.org/10.5281/zenodo.10911196).

O₃ concentrations are highest in the summer, driven by fast photochemical production of hydrogen oxide radicals (HO_X) that can overcome the radical titration caused by high emissions of NO_X. Under a VOC-limited or NO_X-saturated regime, as in the urban cores of the Jing–Jin–Ji region, the decreasing NO_X would increase O₃ concentrations [83], although in some suburban areas, O₃ concentrations have started to decrease in response to NO_X reductions [84]. Nevertheless, the increased O₃ trend cannot be simply explained by changes in anthropogenic emissions of NO_X and VOCs. Increased urban greenery and changes in particle pollution apparently have a significant impact on O₃ production.

Along with urban expansion, green spaces have increased 120% in the last two decades, accounting for almost half of the urban area nowadays. This massive expansion of urban vegetation has increased the contribution of biogenic VOCs, enhancing O₃ formation, especially in the summer. According to numerical simulations, biogenic VOC emissions account for 5–8 ppb of Beijing’s maximum daily 8-h average O₃ concentration [85]. Similarly, reductions in PM_2.5 have increased solar radiation reaching the surface, which has increased NO₂ photolysis rates and slowed down the sink of HO_X, thus stimulating O₃ production [83]. Furthermore, the reduction in particle pollution has resulted in changes in aerosol optical properties. Reductions in black carbon have altered the aerosol optical depth and the single-scattering albedo, also accelerating O₃ production [86]. This suggests that control measures must take into account the combined effect of NO_X and VOC emissions in the formation of PM_2.5 and O₃.

Regarding the impact of climate change, warmer and drier summers are worsening O₃ pollution. Longer summers and heat waves have been increasingly common in recent years, bringing with them adverse weather conditions that favour the formation and accumulation of O₃. Higher temperatures, along with clear skies and stagnation conditions, foster O₃ production by increasing evaporative and biogenic VOC emissions, as well as VOC reactivity with the hydroxyl radical [87]. Heat wave duration has been found to determine the severity of O₃ episodes [88].

Zhang et al. [89] found that nitrous acid (HONO) and nitryl chloride (ClNO₂), two important chemical species in atmospheric chemistry, boost 12–14% O₃ production in summer. HONO is directly emitted into the air by vehicular traffic and biological activity in agricultural fields, particularly after fertilisation, as well as being formed through heterogeneous chemical processes (i.e., chemical reactions on particles), as also happens with ClNO₂. The excess of HONO in concert with the high emissions of VOCs seems to be the cause of higher O₃ concentrations observed in winters in recent years, suggesting a switch to an O₃ production regime, in which decreasing VOC emissions would avoid further spreading of high O₃ episodes into the winter and spring [90].

Regional transport also has a role. Studies suggest that polluted plumes from neighbouring provinces, including Shandong, Henan, Jiangsu and Anhui, might account for 35% of the O₃ concentrations in the Jing–Jin–Ji region [91]. The O₃ production in upper layers within the boundary layer appears to also make a significant contribution. The accumulation of oxygenated VOCs (OVOCs) higher in the atmosphere, supports vigorous O₃ production, while shifting towards an NO_X control regime, as NO_X concentrations decline faster than those of OVOCs with increasing height [92]. This indicates that regional O₃ control strategies must account for vertical variations in O₃ production and assess the formation regime throughout the entire boundary layer.

Health and economic impacts

Malashock et al. [4] estimated 2910 premature deaths due to O₃ exposure for 2019 only for Beijing, while Hu et al. [44] calculated a total toll of 6717 premature deaths for the entire Jing–Jin–Ji region for the same year. These authors found a peak of 7391 premature deaths in 2018 when annual concentrations were higher. In 2021 the number of premature deaths fell to 4282, but a slight increase could be expected since then, as annual average concentrations have increased in the last 3 years. Between 2018 and 2021, 3,200,000 to 5,400,000 people each year experienced short-term respiratory and cardiovascular problems as a result of O₃ exposure. The economic cost imposed by O₃ pollution in the region fluctuated between US$0.93 and 1.37 billion (CN¥5.96 and 9.06 billion) during that period. According to Wang et al. [93], meeting the long-term targets of current air quality improvement programmes might reduce O₃-related premature deaths by 89%, preventing an economic loss of US$9.45 billion (CN¥68.4 billion) in 2035 compared to O₃ levels in 2019.

Main interventions

Air quality governance in the Jing–Jin–Ji region started in the 1990s and focused on pollutants emission control, especially SO₂ and NO_X from coal-burning industries. However, air quality did not improve. Even as emission limits tightened, poor accountability for emissions reporting, as well as a lack of incentives to adopt cleaner technologies, prevented significant improvements [94]. Local administrations were primarily responsible for implementation and supervision, but no standardised methodology and verification mechanisms were in place.

By 2005, the Central Government recognised that improving air quality required more than just reducing emissions from industry, and it expanded control efforts to include other critical pollutants, as well as economic sectors, including energy, transportation and agriculture. The regional nature of air pollution also became clear, and control measures started to be implemented in neighbouring cities and provinces. Beijing’s Olympic Games in 2008 acted as a catalyst for the implementation of regional air quality strategies. Temporal control measures such as halting industrial production, suspending construction, cutting vehicular traffic by more than 50% and significantly reducing other economic activity that could contribute to pollutant emissions proved effective. During the Olympic Games, the local Air Pollution Index was 36% lower than the previous years’ average [95].

In subsequent years, a number of institutional reforms were adopted, as well as surveillance measures implemented to improve air quality management. Air quality targets became mandatory and were integrated into provincial and municipal economic and social development plans. However, such efforts were not sufficient to avoid the air quality crisis of 2013 caused by persistently high, sometimes record-breaking, concentrations of PM_2.5. The Central Government, in response, enacted the Action Plan on Prevention and Control of Air Pollution the same year, laying down actions to reduce PM_2.5 concentration by 25% over a 5-year period [96]. Air quality standards were revised, and the air quality topic became relevant in both political discourse and public opinion. Subsequently, the Three-Year Action Plan to Win the Blue-Sky Defense War 2018–2020 was enacted in 2018 [97].

The air pollution control measures included strengthening emission standards for power plants and equipping them with continuous emission monitoring systems, tightening industrial emission standards, phasing out old and inefficient factories, adopting cleaner technologies and installing end-of-pipe pollution control devices in small factories, controlling VOC emissions in the petrochemical industry, eliminating small coal-fired boilers, switching to electricity and gas-powered heating from coal in households, strengthening vehicle emission standards, retiring old vehicles and improving fuel quality [97].

As a result of such control measures, from 2013 to 2017, China reduced its anthropogenic emissions by 59% for SO₂, 21% for NO_X, 23% for CO, 36% for PM₁₀ and 33% for PM_2.5. However, VOC emissions increased 2% [98], pointing to a lack of effective measures for VOCs and explaining in part why O₃ concentrations did not decrease [99].

The current plan underlined in the 14th Five-Year Plan for Ecological Environmental Protection (2021–2025) is the first national plan to mention ground-level O₃ pollution. It recognises that NO_X and VOCs emissions from transportation and industry remain high, and their control is critical to reducing O₃ pollution. This plan aims at the synergistic control of PM_2.5 and O₃, while addressing climate change targets by simultaneously reducing CO₂ emissions. This plan is based on four principles: (1) science-based pollution control, (2) strengthening legislation, (3) strict standards and (4) increased investment [100].

Importantly, the Jing–Jin–Ji region operates under an innovative regional framework designed to coordinate actions between the Central Government and the local governments to prevent and control air pollution [100]. This framework encompasses integrated planning, standardised monitoring, coordinated emergency responses and real-time data sharing across jurisdictions. For such an endeavour, air quality monitoring has been strengthened, and significant investments have been made to build detailed emission inventories and develop air quality forecasting models. Similarly, new mechanisms have been implemented to ensure governance and the enforcement of environmental regulations.

Air quality in the Jing–Jin–Ji region has improved. Ambient concentrations of PM_2.5, as well as other key pollutants, have substantially decreased, proving the effectiveness of the regional framework that has been implemented. However, the region still experiences O₃ concentrations that exceed recommended guidelines, and high pollution days continue to pose significant health risks. Changes in emission sources of O₃ precursors and the rapid decrease of PM_2.5 concentrations, driven by past clean air actions, have influenced O₃ production [22]. The next phase of air pollution control strategies must simultaneously address air quality standards and climate change targets, specifically focusing on the dual impacts of ambient O₃ and PM_2.5 pollution on climate and air quality.

Challenges ahead

Curbing O₃ pollution in the Jing–Jin–Ji region will require improved VOC emission controls as NO_X emissions continue to decrease under current regulations across the three entities. This will require even more stringent environmental policies efficiently coordinated by local, regional and federal agencies. However, some policies in place may appear draconian, at least by Western standards, and the question arises whether implementing even stricter measures could cause social unrest. For example, since 2013, over 3000 manufacturing and polluting enterprises have been closed in the region [101], bringing unemployment and social inequity, and forcing workers to relocate to places far from their hometowns [102].

Similarly, one of the most common wishes among Beijing residents nowadays is to obtain a permit to purchase a car, even though they do not have a pressing need for one [103]. Since 2011, a licence plate lottery system has been used to limit the number of cars that locals can acquire each year in order to reduce traffic congestion and pollutants emission. At the start of this decade, the odds of obtaining a plate were 1 in 2900, and it could take up to 40 years [104]. Initially, people began purchasing cars in neighbouring cities and then bringing them home to evade the lottery system [105]. This loophole was closed by imposing strict driving restrictions on those cars [106]. The lottery system also created a black market for licence plates, where issued plates are resold at higher prices, allowing wealthy residents to circumvent the lottery system [107]. In very recent years, there has been a surge in electric vehicles (EV) that has somewhat alleviated these unforeseen consequences. The price of an EV has dropped since 2023, even though the government ended the subsidy for their purchase that year, at the same time that it launched a subsidy to replace fossil-fuel vehicles with electric ones, and increased their annual quota by making the licence plate lottery system more flexible [108]. Furthermore, EVs are not subject to the same driving restrictions as gasoline cars, allowing drivers to use their vehicles every day. However, this surge of EVs has led to an increase in traffic, extending the hours when Beijing’s roads are gridlocked [109]. This is without taking into account the proliferation of large and heavy EVs that undermines the environmental and economic benefits of the transition towards electric mobility [110].

Air pollution control measures in Beijing must be sustainable, as they should be in any other metropolitan area. The social costs associated with decisive actions following a strict top-down enforcement, as implemented so far in the Jing–Jin–Ji region, may prove unsustainable in the long term [111]. Stakeholders must address unforeseen social repercussions and establish information exchange platforms for decision-making and implementation that guarantee social fairness.

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