Reducing global air pollution: the scope for further policy interventions

Over the last decades, energy and pollution control policies combined with structural changes in the economy decoupled emission trends from economic growth, increasingly also in the developing world. It is found that effective implementation of the presently decided national pollution control regulations should allow further economic growth without major deterioration of ambient air quality, but will not be enough to reduce pollution levels in many world regions. A combination of ambitious policies focusing on pollution controls, energy and climate, agricultural production systems and addressing human consumption habits could drastically improve air quality throughout the world. By 2040, mean population exposure to PM2.5 from anthropogenic sources could be reduced by about 75% relative to 2015 and brought well below the WHO guideline in large areas of the world. While the implementation of the proposed technical measures is likely to be technically feasible in the future, the transformative changes of current practices will require strong political will, supported by a full appreciation of the multiple benefits. Improved air quality would avoid a large share of the current 3–9 million cases of premature deaths annually. At the same time, the measures that deliver clean air would also significantly reduce emissions of greenhouse gases and contribute to multiple UN sustainable development goals. This article is part of a discussion meeting issue ‘Air quality, past present and future’.

Over the last decades, energy and pollution control policies combined with structural changes in the economy decoupled emission trends from economic growth, increasingly also in the developing world. It is found that effective implementation of the presently decided national pollution control regulations should allow further economic growth without major deterioration of ambient air quality, but will not be enough to reduce pollution levels in many world regions. A combination of ambitious policies focusing on pollution controls, energy and climate, agricultural production systems and addressing human consumption habits could drastically improve air quality throughout the world. By 2040, mean population exposure to PM2.5 from anthropogenic sources could be reduced by about 75% relative to 2015 and brought well below the WHO guideline in large areas of the world. While the implementation of the proposed technical measures is likely to be technically feasible in the future, the transformative changes of current practices will require strong political will, supported by a full appreciation of the multiple benefits. Improved air quality would avoid a large share of the current 3-9 million cases of premature deaths annually. At the same time, the measures that deliver clean air would also significantly reduce emissions of greenhouse gases and contribute to multiple UN sustainable development goals.
This article is part of a discussion meeting issue 'Air quality, past present and future'.

Background (a) Context
The current exposure to air pollution in ambient air has been identified as the worldwide largest environmental risk factor for human health [1]. Anthropogenic activities emerge as the main drivers for emissions of air pollutants and add to pre-existing sources of natural emissions (soil dust, sea salt, vegetation, etc.) [2]. Human development affects emissions along multiple pathways: increasing pressure from population growth, industrialization and modern lifestyles is counteracted by technological progress, structural changes in the economy and targeted pollution control efforts. The interplay of these factors is changing over time; while pollution levels in industrialized countries have decreased, the developing world witnesses unprecedented levels of pollution today. The World Health Organization (WHO) estimates that currently about 90% of the people living in cities are exposed to PM2.5 levels above the WHO guideline value of 10 µg m − ³, and globally between 3 and 9 million cases of premature deaths annually have been attributed to exposure to ambient air pollution [1,3,4]. Given the dynamics of these factors and their complex interplay, what could be expected for future air quality around the world, and which determinants will be dominating? To answer this question, this paper identifies key factors that contributed to historic air pollution trends in different world regions, outlines conceivable ranges of their future development and examines their interplay on global air quality in the next decades. In particular, the paper provides a fresh perspective on how ambitious policy interventions could achieve clean air worldwide.

(b) Connecting air pollution and development (i) Past trends
The observed increase and subsequent decline in SO 2 emissions in many industrialized countries during the second half of the twentieth century inspired the 'environmental Kuznets curve' hypothesis, suggesting that environmental degradation tends to get worse as modern economic growth occurs until average income reaches a certain point [5,6]. However, the explanatory power and general validity of this hypothesis has been strongly contested [7][8][9][10][11][12], inter alia, because similar turning points have not yet been observed for other substances, including agricultural ammonia (NH 3 ) and greenhouse gas emissions. Also, the role of (environmental) policy interventions is not explicitly recognized but implicitly subsumed as an autonomous concomitant of economic growth.
Rafaj et al. [13] identified three key factors that contributed to the observed decoupling between economic activity and air pollution in Europe between 1960 and 2010: (a) economic structural change, i.e. conversion to less energy-intensive economic activities, (b) energy policy, i.e. phaseout of oil and coal, and (c) dedicated air pollution control policies requiring efficient end-of-pipe cleaning devices (e.g. flue gas desulfurization, catalytic converters). As a result, during a period in which GDP quadrupled, SO 2 emissions in Europe declined by 90% and NO x emissions returned to their 1960 levels after an increase by a factor of three. Similar findings are reported by [14] royalsocietypublishing.org/journal/rsta Phil. Trans for Europe and by [15] for North America. Most recently, decoupling of economic development trends and SO 2 and NO x emissions and the critical role of policy interventions have been observed in China [16][17][18], but not yet in India [19].

(ii) Future projections of air pollutant emissions
A range of studies in the scientific literature explored the implications of these findings on future emissions and air quality. For a long time, future global air pollutant trends were mainly modelled in the context of long-term greenhouse gas emission scenarios [20]. The early global studies on air pollutant emissions, notably the scenarios developed for the 'Special Report on Emissions Scenarios' [21] and the 'Representative Concentration Pathways' [22,23] that have been prepared for the Intergovernmental Panel on Climate Change (IPCC) proposed declining trends of (energyrelated) air pollutants, due to autonomous technological progress and assumed pollution control policies along the environmental Kuznets hypothesis. Later, the improved understanding of the importance of targeted air quality policy interventions motivated a more differentiated approach to projections of air pollutant emissions, resulting in a wider range of air pollutant trajectories than in previous global scenarios [24][25][26]. At the same time, the climate community addressed the interactions between decarbonization strategies and air pollutant emissions, both with the interest to reveal health benefits from low carbon policies [24,[27][28][29][30][31]) and to explore the combined impacts of long-lived greenhouse gases and short-lived air pollutants (e.g. SO 2 and black carbon) on radiative forcing and temperature increase [32][33][34]. In general, the literature reveals strong impacts of ambitious decarbonization strategies on energy-related air pollutants SO 2 , NO x and PM, due to the phase-out of fossil fuels and the containment of all flue gases connected with carbon capture and storage. However, enhanced use of biomass as a greenhouse gas policy measure may lead to higher PM emissions [35][36][37][38].
Compared to the climate-focused analyses that deal mainly with energy-related emissions and the role of climate policy interventions, only a few studies addressed the longer-term prospects for air pollution from a health-and ecosystems perspective [4,39,40]. These studies take full account of other sources that also contribute substantially to population exposure to harmful air pollution, such as agricultural activities, waste management and materials handling. Also, they developed a more holistic approach towards the understanding of future trends in nitrogen emissions and their health and environmental impacts.
Not so many studies address air pollution at the global scale [26,41,42]. This is not surprising as, due to its physical features, air pollution is often considered as a local/regional and short-term problem, even if it is of universal nature, i.e. occurring as a concomitant of development in most industrialized areas throughout the world. However, a global perspective is of interest due to the intercontinental transport of pollution [43,44], the serious global health burden of air pollution [1] and the intimate interactions between clean air policies and efforts to achieve the UN global sustainable development goals [45][46][47].
For the second half of the twenty-first century, most long-term studies are in general agreement about declining energy-related air pollutant emissions; greenhouse gas mitigation strategies would accelerate the decline. However, projections differ for the next few decades, and particularly for the timing of global emission peaks.

Method
To explore the conceivable range of future air quality and, in particular, of population exposure to PM2.5 which has been associated with the most harmful health impacts [48], this paper develops a series of alternative emission scenarios up to 2040. Building on a widely accepted economic growth path with its structural economic changes, these scenarios combine different assumptions on the key policy areas that have been identified as critical for air pollution trends in the past, i.e. The analysis employs the greenhouse gas-air pollution interactions and synergies (GAINS) model [49]-see electronic supplementary material. For exogenous projections of future emissiongenerating human activities (i.e. energy, transport, industrial production, agricultural activities and waste volumes), GAINS computes emissions of 10 air pollutants and six greenhouse gases (see electronic supplementary material) considering the current emission characteristics in 180 regions. For the future, GAINS takes account of the penetration of already decided control measures and estimates the potential for additional emission reductions that is offered by several hundreds of control measures. The resulting emissions of all precursor emissions of PM2.5 in ambient air, i.e. primary PM2.5, SO 2 , NO x , NH 3 and volatile organic compounds (VOCs), are then fed into an atmospheric dispersion model to compute annual mean concentrations of PM2.5 across the globe. GAINS employs reduced-form source-receptor relationships that have been derived from the EMEP atmospheric chemistry-transport model [50] with a spatial resolution of 0.125°× 0.0625°(approx. 7 km × 7 km) in Europe [51] and 0.1°× 0.1°(approx. 5-10 km × 5-10 km, depending on latitude) outside Europe, distinguishing about 6000 individual cities with more than 100 000 inhabitants. Resulting concentration fields are then compared with air quality standards, and corresponding population exposure is computed for the population distribution assumed in the socio-economic projection. More detail is given in the electronic supplementary material.

(a) Historic emission trends
Over the last 25 years, SO 2 emissions from anthropogenic activities declined by 40% at the global level. Primary emissions of PM2.5 and of VOC stabilized, NO x grew by about 10% and NH 3 by one-third (figure 1). Obviously, these global trends emerge as an aggregate of sometimes diverging regional trends (see §3c(iii)).
During the same period, GDP doubled, and world population grew by more than a third, i.e. faster than most of the air pollutant emissions. Thus, except for NH 3 , emissions on a per capita basis declined globally in these 25 years, in line with the earlier observations that led to the formulation of the environmental Kuznets hypothesis [5,6] (figure 2). It is noteworthy that after 1990 clear peaks in per capita emissions could only be observed for SO 2 and NO x in China, while all other world regions exhibit constant or declining trends following the increase in per capita income. Also, per capita emissions of the six world regions that showed striking differences in the past reduced over time despite large disparities in income levels.
For energy-related emissions, a global level decomposition analysis apportions about half of the past divergences of economic and pollution trends to environmental policies, while the remainder occurred from economic structural changes and energy policies (figure 3). By contrast, for global agricultural NH 3 emissions no impacts from structural changes nor from pollution control policies can be identified. However, trends differ across world regions. For 2015, calculations confirm large differences in PM2.5 concentrations across the world (figure 4), with patterns rather similar to other studies that have been derived from a limited set of ground-level monitoring data [1] or from interpretation of remote sensing products [52,53]. A comparison of ground-based monitoring data [54,55] with model calculations is presented in figure 5.
The main sources of PM2.5 in ambient air vary over world regions [56]. While in many densely populated or highly industrialized areas the major share originates from anthropogenic sources, natural sources (soil dust, sea salt and vegetation) are important in other areas, with soil dust dominating in arid regions in Asia, Africa, Latin America and Australia. Human activities might contribute only indirectly to these natural emissions, e.g. through desertification, land use royalsocietypublishing.org/journal/rsta Phil. Trans. R. Soc. A 378:  While there is little evidence to suggest a threshold below which no adverse health effects would be anticipated, the WHO has issued a global air quality guideline value for PM2.5 in ambient air of 10 µg m − ³ as annual mean, together with a series of interim targets of 15, 25 and 35 µg m − ³, respectively [57]. For 2015, it is estimated that globally more than 90% of the city dwellers [54] and more than 80% of the total population (this study) were exposed to PM2.5 levels above the WHO guideline, especially in Asia, Africa and the Middle East. Focusing on the exposure that can be influenced by policy decisions, more than 60% of the global population

(c) Air quality futures
In the past, changes in air pollutant emissions have been driven by economic development and policy choices [13,19]. There is general agreement in the scientific literature that in the long run, i.e. in the second half of the twenty-first century, continuing growth in economic wealth, technological development and hypothesized further policy interventions should lead to significant declines in energy-and poverty-related emissions [24]. However, the prospects for the first half of the twenty-first century are less clear, especially when and at what level emissions are likely to peak in the various world regions. Although comparably less work has addressed future agricultural emissions [58], there are hardly any studies that would indicate a possible peaking, e.g. of NH 3 emissions, at the global scale [59].

(i) Macro-economic trends
Although there is some uncertainty about future socio-economic development trends, the current scientific literature sees only moderate differences for the next few decades (e.g. until 2040) due to the long time scales of fundamental transformations of the social and economic systems. Thus, this paper adopts the macro-economic development assumptions of the World Energy Outlook 2018 of the International Energy Agency (IEA) [60], which up to 2040 are rather similar to the middle range of the long-term climate scenarios developed for the CMIP6 Coupled Model Intercomparison Project [28,61]. In total, the trends in 25 world regions of the IEA scenario     (i) traditional air pollution policies, (ii) energy and climate policies, (iii) agricultural policies and (iv) food policies. In particular, the scenario assumes full implementation of the best emission control technologies that are currently available on the market and, with a visionary perspective, policies and measures that are discussed in the context of societal transformation towards global long-term sustainability [64]. These include decarbonization strategies to achieve the Paris climate accord and keep global temperature increase well below 2°C, dietary changes to optimize human health and environmental sustainability, modifications of current agricultural practices to minimize alterations of the global nitrogen and phosphorus cycles, and other policies to achieve the UN Sustainable Development Goals, e.g. on waste management, circular economy, etc. The various measures are discussed in the scientific literature and are included in the scenario to the extent that they are conceived as technically feasible in the next few decades. Obviously, at present many of these measures find little political support, and their implementation will strongly depend on appropriate political will. In particular, the Clean Air scenario comprises four sectoral policy packages : increase in nuclear and, where economically viable, use of carbon capture and storage. This leads to a sharp decline in coal use. In transport, a combination of efficiency measures for conventional cars and trucks, electrification and fuel switch (to biofuels, natural gas and hydrogen in the longer term) are taken into account. By 2040 nearly half of the world's car fleet will be electric, which will lead to important reductions of exhaust emissions, while no major impacts are expected for emissions from road abrasion, tyre and break wear. Efficiency, electrification and fuel switching are also adopted in buildings and industrial sectors. (iv) Focusing on the agricultural sector, the Agricultural Policy package considers massive modifications of agricultural practices to reduce NH 3 and greenhouse gas emissions to the atmosphere. Industrial farms would introduce enclosed systems for manure treatment (e.g. anaerobic digestion, composting), whose residual products would be used as soil amendments to replace mineral fertilizers. Nonindustrial/small farms would either deliver manure to central processing facilities or extend grazing of animals. Furthermore, to reduce nitrogen losses, policies would aim to increase nitrogen use efficiency by optimizing nutrient additions to crop requirements and by minimizing excess protein in animal feed, to reach the maximum nitrogen use values supported in the scientific literature [58]. Also, wellmanaged and healthy animal stocks that are genetically well adapted to the local environment could simultaneously enhance productivity, fertility and longevity. This would allow a reduction in the number of non-productive animals in the stock, and thereby minimize emissions per unit of meat or milk produced [65]. (v) The Food Policy package aims to reduce emissions from meat production through modified human diets and lower food waste. Policies would promote the 'Planetary Health Diet' proposed by the EAT-Lancet Commission on Food, Planet and Health to address the simultaneous global problems of malnutrition (under-nutrition) and over-nutrition [66]. The shift from animal-based protein to plant-based protein, together with reduced food waste [67], would decrease meat consumption and allow smaller herd sizes and less mineral fertilizer use for animal feed production. The scenario employs the quantitative implications on future livestock numbers and mineral fertilizer use developed with the GLOBIOM model [68] for the Food and Land Use Coalition [69].

(iii) Future emissions
The different assumptions on policy interventions deliver a wide range of future emission trends for PM2.5 precursors (figure 8; electronic supplementary material). By 2040, without policies and measures introduced in the last decades, energy-related emissions could have been up to 120% higher than in 2015. Current policies, if effectively implemented and enforced, would reduce global SO 2 emissions by about one-third and NO x by about 10%, and stabilize global PM2.5 emissions. By contrast, with adequate political will, global PM2.5 and SO 2 could be cut by about 90% below today's level, and NO x by about 70%. However, current policies will do little to decouple global NH 3 emission trends from population growth, while ambitious interventions have a potential to enable 60% lower emissions. However, as this analysis does not consider potential impacts of climate change on meteorological conditions, changes in emissions from natural sources, e.g. desert dust and biogenic VOCs, are not excluded from the analysis. These could potentially enhance wind-blown PM2.5 and enhance ground-level ozone.
In general, the range of future emissions developed for this paper is consistent with the scenarios of the CMIP6 Coupled Model Intercomparison Project [28,61,70]       Natural sources will continue to make sizeable contributions to population exposure, especially in Africa, Latin America and the Middle East. In these regions, the Clean Air scenario would bring contributions from anthropogenic sources to below 3 µg m − ³ (figure 12b).
The significant exposure reductions of the visionary Clean Air scenario emerge from a range of policies and measures that have been grouped into four policy packages: (i) environmental pollution control policies, (ii) energy and climate policies, (iii) agricultural policies, and (iv) food policies ( figure 13). Further environmental pollution controls offer the largest improvements in population exposure. However, even full implementation according to the assumptions in this paper would still leave about 2.4 billion people exposed to more than 10 µg m − ³ PM2.5 from anthropogenic sources. Beyond this, energy and climate measures could reduce exposure below this level for another 820 million people, changes in agricultural practices for 670 million people and the assumed food policies for an additional 500 million people. Thus, further air pollution control policies would exhaust only 60% of the total potential offered by the full range of policy interventions. Vice versa, the notion that other sectoral policies such as ambitious energy and climate policies would resolve air pollution problems on their own, e.g. [23], is not supported by this analysis, and a clear role for dedicated environmental air quality policies will remain in the future.

(d) Co-control of emissions of greenhouse gases and short-lived climate pollutants
Some of the measures of the Clean Air scenario will not only reduce emissions of PM2.5 precursors but simultaneously also other substances that contribute to temperature increase. As a consequence of the deep energy system restructuring in the Clean Air scenario, CO 2 emissions in this scenario in 2040 are 40% lower than in the 2018 legislation reference case, CH 4 33% and black carbon 90% lower ( figure 14). Quantification of the temperature impact of these cuts in longlived greenhouse gases and short-lived climate pollutants is beyond the scope of this paper. We also did not quantify reductions of N 2 O emissions, which can be expected as a result of reduced application of mineral fertilizer and more efficient use of manure nutrients. As this paper focuses on health impacts from PM2.5, the Clean Air scenario does not target substances that contribute to other air pollution problems. SO 2 , NO x and NH 3 emissions, which affect acidification and eutrophication of ecosystems, are included as PM precursors. For groundlevel ozone, NO x and VOC emissions are considered as PM precursors as well. However, CH 4 and CO emissions are not addressed here, as their contribution to PM levels is considered marginal. A comprehensive clean air policy that would also aim at ground-level ozone would need to include CH 4 and CO mitigation in its portfolio, especially in view of the past increases in hemispheric ozone levels that have been attributed to growing CH 4 emissions [71]. From such a perspective, measures that reduce only CH 4 but would not affect PM precursor emissions could cut global CH 4 by more than half in 2040 [72].

(e) Co-benefits on the sustainable development goals
The Clean Air scenario achieves a substantial reduction in exposure to PM2.5, bringing about significant progress in moving towards the WHO air quality guideline value for PM2.5, which is currently set at 10 µg m − ³ including contributions from natural sources. In large areas of the world, the Clean Air scenario would reduce anthropogenic PM2.5 below 5 µg m − ³ and thereby leave space for contributions from natural sources. These improvements will directly contribute to In addition, the measures assumed in the Clean Air scenario with the focus on health impacts from PM would deliver a host of co-benefits in other policy areas through several pathways.
-The measures would lead to considerably lower emissions of greenhouse gases (CO 2 , CH 4 ) and black carbon, which will reduce temperature increase and thereby contribute to SDG 13 (Climate action). -The policies and measures in the energy sector will enhance energy efficiency and provide access to clean energy (SDG 7-Affordable and clean energy). In addition, enhanced access to clean household fuels will eliminate the health impacts from indoor exposure to PM from household use of traditional solid fuels, currently estimated to cause another three million cases of premature deaths annually [73]. -As an important feature, the Clean Air scenario contains a host of measures that will enhance nitrogen use efficiency. This will secure crop production (SDG 2-Zero hunger; SDG 12-Responsible consumption and production) and deliver positive impacts on the global nitrogen and phosphorus cycles. Nitrogen runoff to water (SDG 14-Life under water) will be reduced and protect biodiversity through reduced ecosystems exposure to excess nitrogen deposition (SDG 15-Life on land). food policies people exposed below threshold even without any policies millions people protected against exposure above the threshold through people exposed above threshold even with all policies atmospheric dispersion and the role of policy interventions with a focus on improving human wellbeing through better global air quality. The wide range of aspects included in the analysis and the pioneering character of this paper leave a number of issues that deserve further attention.
First, the paper focuses on population exposure to PM2.5 in ambient air, as associated health impacts have been found to dominate other negative consequences of air pollution in economic terms [74][75][76]. Thereby, the paper does not explicitly address health impacts from ground-level ozone [77], vegetation damage from ozone including economically important losses in crop production [78,79], biodiversity impacts of excess nitrogen deposition [80] and acid deposition that exceeds the absorption capacity of ecosystems [81].
For the later issues, i.e. excess nitrogen and acid deposition, the Clean Air scenario delivers significant reductions in precursor emissions as these contribute to PM2.5 formation too. By 2040, NO x would be lower by three-quarters, NH 3 by two-thirds and SO 2 by 80% than compared to the 2018 legislation projection. These reductions will alleviate pressure on ecosystems, but the benefits have not been quantified for this paper.
Similarly, the Clean Air scenario, even while aiming at ambient PM2.5, would lead to much lower precursor emissions of ground-level ozone. Global NO x emissions would decline by about three-quarters and global VOC emissions by about two-thirds compared to the 2018 legislation case, because of their role in the formation of secondary particles. In addition, the measures would also reduce the other precursors of ground-level ozone that are often neglected in local and regional pollution control strategies. Global CH 4 emissions, which have an important impact on hemispheric background levels of ozone [82], would decline by 33% as a side-effect of the changes in fossil fuel production, agricultural practices and food demand that are motivated by the interest to reduce PM precursor emissions including NH 3 . More work will be required to quantify the consequences of these emission changes on ozone fields and resulting health and vegetation impacts.
Second, the paper limits the analysis to 2040, in order to explore near-term policy interventions without excessive speculation about long-term technological, socio-economic and climate trends. At the same time, the long-term transformational energy and agricultural policies that are assumed in the Clean Air scenario involve measures that will fully unfold only by 2050, so that the 2040 focus does not reveal their full potential in the long run. Additional work would be required to assess the impacts beyond 2040.
Furthermore, any visionary analysis requires assumptions and methods that are associated with uncertainties. The following aspects have been identified as particularly relevant.
-Political will: The Clean Air scenario deliberately assumes a range of ambitious policy interventions that clearly deviate from current practices. Implementation will require clear political will and public support. It is the purpose of this paper to reveal the multiple benefits from such actions and thereby contribute to enhanced societal push for such policies. -Technical feasibility of measures: Care has been taken to compile a portfolio of technical measures that is likely to be feasible from a technical perspective within the coming decades, given appropriate political will. While some of the measures appear as visionary, they are discussed in other contexts as necessary for the transformational changes that are required for achieving global sustainability. The emphasis in this paper is to reveal the cobenefits of such measures on clean air that are often overlooked, and to explore how they could contribute to clean air policies, in addition to their core policy objectives. Furthermore, the specific conditions in any given year may deviate from these in our base year, leading to higher or lower exposure. 4. Quantification of emissions from natural sources: There is only imperfect understanding of the source strengths of natural emission sources (e.g. soil dust, sea salt, biogenic emissions) and temporal patterns and their distribution at the global scale. The approach chosen for this paper, i.e. to focus on the anthropogenic fraction of PM2.5, attempts to minimize the impact of these uncertainties on the findings. However, uncertainties are particularly relevant when estimating population exposed to specific PM2.5 concentrations above or below a given threshold, e.g. the WHO guideline value which includes natural sources. Especially at low concentrations, differences in the contributions from natural sources of a few µg m −3 could easily double the number of people that are exposed to such levels.
-Other air pollutants: With a focus on human health and following epidemiological evidence, this paper addresses exposure to fine particulate matter (PM2.5) as the pollutant causing the majority of health impacts. Obviously, health impacts have been identified from exposure to other substances as well, notably to ground-level ozone. However, as all quantifications of health impacts from air pollution reveal a dominating role of PM2.5 over all other substances [48], this initial analysis has been restricted to PM2. 5 as precursors of PM2.5 in the atmospheric chemistry calculations of this paper as well (results for VOC are shown in the electronic supplementary material). -Quantification of health benefits: Although a host of literature has produced a large array of estimates of the health impacts of air pollution at the global scale [1,74,84,85], uncertainties remain, inter alia, about the general shape of the exposure-response functions [86], the levels of counterfactual concentrations, and the validity of the epidemiological evidence at the global scale. In particular, current methods applied for global analyses include PM exposure from natural sources in the assessment, while for European assessments WHO Europe recommended a focus on anthropogenic sources [87]. Given new epidemiological evidence, inter alia on the relevance of health impacts at low concentrations [88], the World Health Organization is currently reviewing and possibly revising the current air quality guidelines. In order not to pre-empt the results of that review, this paper refrains from the quantification of health impacts and restricts the assessment to population exposure. -Economic aspects: The paper does not address the cost-effectiveness and economic feasibility of the policies and measures. Also, the Clean Air scenario assumes (ambitious) measures throughout the world, resulting in many world regions in rather low PM2.5 concentrations. To the extent that higher concentrations appear acceptable, a costeffectiveness analysis could reveal a sub-set of measures that deliver benefits at lowest cost.

(b) Summary
Over the last decades, emissions of key air pollutants have decoupled from economic growth at the global level, and increasingly also in the developing world. This trend break was to a large extent a result of policy decisions on pollution control and energy, in addition to structural changes in the economy and in consumption patterns. It is estimated that without dedicated pollution control policies, global SO 2 emissions would have been more than twice as high as in 2015, and PM2.5 and NO x emissions twice as high. Given the lack of large-scale policy interventions, the evolution of agricultural NH 3 emissions closely followed global population growth. The dominating role of policy interventions will prevail in the future. Instead of autonomous improvement of air quality connected with increasing economic wealth as hypothesized in the environmental Kuznets curve, this analysis reveals that future air quality will be mainly determined by policy decisions and their implementation. Timely implementation and full compliance with all air legislation that has been decided by 2018 is likely to decrease global anthropogenic SO 2 emissions by 35% by 2040, and NO x and primary PM2.5 by about 10%. By contrast, for NH 3 emissions a growth of one-quarter is estimated, with no peaking in sight. However, as illustrated by the Clean Air scenario, with adequate political will further policy interventions could cut global SO 2 and PM2.5 by about 90%, NO x by 70% and NH 3 by 60% below today's levels. Decisions in four policy areas will be critical: environmental policies focusing on pollution controls, energy and climate policies aimed at global temperature stabilization in line with the Paris Agreement, policies to transform the agricultural production system, and policies to shift human food consumption patterns towards largely plant-based diets such as the 'Planetary Health Diet' proposed by the EAT-Lancet Commission on Food, Planet and Health. However, emissions from biogenic sources (e.g. VOC) might further increase due to other factors, e.g. climate change.
Obviously, the policy interventions considered in the Clean Air scenario would require fundamental transformations of today's practices in many sectors. These are visionary but considered likely to be technically achievable in the future. As they exceed current policy ambitions, their implementation would require strong political will.
Political will could emerge from a solid understanding of the full range of benefits. Most importantly, the policies and measures of the Clean Air scenario would drastically improve air quality throughout the world. Population exposure to PM 2.5 from anthropogenic sources would decline by about 75% relative to 2015, or by 80% compared to a future without additional policies. This would deliver substantial health benefits and avoid a large portion of the 3-9 million cases of premature deaths from ambient air pollution that are estimated for 2015. In addition, enhanced access to clean household fuels will drastically reduce the health impacts from indoor exposure to PM2.5, currently estimated to cause another 1.6-4 million cases of premature deaths annually [3,73,89].
These reductions in PM2.5 exposure from anthropogenic sources would allow substantial progress in moving towards the WHO air quality guideline value, which is currently set at 10 µg m − ³. In large areas of the world, the Clean Air scenario would reduce anthropogenic PM2.5 concentrations below 5 µg m − ³ and thereby bring total PM2.5 below the current WHO guideline value, which includes contributions from natural sources. However, violations will remain in areas where natural sources alone deliver already more than 10 µg m − ³, as well as at small-scale hot spots which are not captured by the spatial resolution of this analysis.
In addition to improved human health (SDG 3-Improve human health and wellbeing), particularly in urban areas (SDG 11-Sustainable cities and communities), the Clean Air scenario would deliver a host of co-benefits in other policy areas through several pathways.
-For mitigating climate change (SDG 13-climate action), some of the measures will not only reduce emissions of PM2.5 precursors but will simultaneously reduce emissions that contribute to temperature increase. In particular, CO 2 emissions of the Clean Air scenario will be about 40% lower than in the reference case in 2040, CH 4 33%, and black carbon by 90%. -The policies and measures in the energy sector will enhance energy efficiency and provide access to clean energy (SDG 7-Affordable and clean energy). -As an important feature, the Clean Air scenario contains a large number of measures to enhance nitrogen use efficiency, which will have positive impact on the global nitrogen and phosphorus cycles. This will contribute, inter alia, to SDG 2-Zero hunger, SDG 14-Life under water, SDG 15-Life on land, SDG 12-Responsible consumption and production, and SDG 13-Climate action.
Importantly, the Clean Air scenario includes a range of fundamental transformative changes (e.g. in the energy, agricultural and food systems) which are required for global sustainability and whose benefits occur globally and in the long term. The tangible local and near-term health benefits of the Clean Air scenario could enhance social acceptance and political support for such transformative policies.
Despite the importance of a deep involvement of and interactions with numerous other policy areas, the analysis reinforces the continued relevance of environmental air quality policies. Ambitious action in other policy domains such energy, climate, food and agriculture will not resolve the air pollution problem on its own and-if not coordinated within a comprehensive multi-sectoral air quality management approach-could even lead to counterproductive impacts on air quality. areas where natural sources (e.g. soil dust) contribute major shares to or even exceed the guideline value. -Such a portfolio needs to involve (i) environmental policies focusing on pollution controls, (ii) energy and climate policies, (iii) policies to transform the agricultural production system, and (iv) policies to modify human food consumption patterns. None of these policy areas alone can deliver clean air, and interventions need to be coordinated across sectors. -These policy interventions would require fundamental transformations of today's practices in many sectors. They are visionary but considered likely to be technically achievable in the future. As they exceed current policy ambitions, their implementation would require strong political will. -Political will could emerge from a solid understanding of the full range of benefits.
The policy package would avoid annually millions of premature deaths worldwide through drastic improvements in air quality and through healthier diets. It would reduce emissions that contribute to global temperature increase, alleviate distortions of the global nitrogen and phosphorous cycles, and enhance the protection of ecosystems and biodiversity. At the same time, policies would contribute to multiple UN Sustainable Development Goals and trigger transformations that are required for global long-term sustainability. -Lowering emissions from agricultural activities will be critical for achieving clean air worldwide. The scientific understanding of the relevance of nitrogen emission reductions that has emerged over the last decade has not yet penetrated decision-making in many parts of the world, and possible measures often face strong resistance from interest groups. However, a focus on industrialized agriculture would provide competitive advantages to small and subsistence farmers. Also, some of the more advanced measures considered in the agricultural policy package reduce the greenhouse gas footprint of the agricultural sector as well.
Data accessibility. Data are accessible in the online version of IIASA's Greenhouse gas-Air pollution Interactions and Synergies (GAINS) model (http://gains.iiasa.ac.at). Data will be made available after acceptance of the paper. Key data are provided in the electronic supplementary material.