Introduction

Fine particulate matter (PM2.5) and ground-level ozone (O3) adversely impact human health, leading to premature mortalities, especially in highly polluted regions [1, 2]. High levels of O3 and PM2.5 also have detrimental impacts on agriculture thereby incurring major economic losses [3,4,5]. Besides impacting human health and crop yields, PM2.5 and O3 affect the Earth’s radiation budget thereby altering the climate. For example, particulate matter reflects (directly or via cloud formation) or absorbs incoming solar radiation depending on its chemical composition and physical properties [6], whereas ozone is an effective greenhouse gas (GHG) that traps outgoing terrestrial radiation and thereby increases surface temperatures [7,8,9]. Considering the impacts on air quality, health, economy, and climate, the coordinated control of both PM2.5 and O3 is highly desirable. Notably, achieving several of the sustainable development goals (SDGs) set by the United Nations [10] is not possible without mitigating air pollution including both PM2.5 and O3. Several regions of the world have already been experiencing pollution levels exceeding the air quality standards set by the World Health Organization (WHO). Recently revised standards [11, 12] for PM2.5 (24-h mean: 15 µg m−3) and O3 (8-h mean: 100 µg m−3) require even more stringent interventions to mitigate air pollution.

Anthropogenically sourced PM2.5, besides having primary emissions from the combustion of fossil and biomass-based fuels, is also formed through reactions of inorganics (e.g. SO2, NOx, NH3, etc.) and volatile-organic precursors. Ozone (O3) on the other hand does not have direct emission and is formed in the atmosphere through a complex set of chemistry involving the oxidation of carbon monoxide (CO), methane (CH4), and volatile organic compounds (VOCs) and photolytic dissociation of NO2 to NO. In presence of O3, higher concentrations of NOx or SO2 can produce nitrate or sulphate and thereby enhance the PM2.5 concentrations in the atmosphere [13, 14].

The dependence of O3 on its precursors is highly non-linear and reductions in some precursors (which are also precursors for particulate matter) may enhance O3 pollution [15, 16]. PM2.5, with an atmospheric lifetime of about a week, can readily be transported over local to regional scales while surface O3 (with a lifetime of more than a month) can be transported over longer, inter-continental, and hemispheric scales depending on the prevailing meteorological conditions [17]. Historically, the problems of elevated levels of PM2.5 and O3 have been confined to specific seasons [18]. In general, wintertime stagnation and meteorological inversions cause severe haze contributed by PM2.5 [19,20,21], whereas, in contrast, during hot summer conditions, the intense solar radiation favours the photochemical formation of ozone. However, in addition to seasonal trends, several major cities around the globe experience higher levels of both PM2.5 and O3 in spring as well as summer [22]. Additionally, due to the aforementioned roles of chemistry and atmospheric dynamics, air pollution is no longer confined to highly urbanised regions but is being experienced in rural and remote locations as well [23, 24]. Higher PM2.5 levels are seen in parts of South Asia, East Asia, the Middle East, Southeast Asia, and Africa regions throughout the year [25]. However, the widespread enhancements are most pronounced typically during winter over the Indo-Gangetic Plain (IGP) and the North-China Plain (NCP) [21] The studies show higher NO2 levels (≥ 16 ppbv) generally near the source regions, i.e. over the IGP, eastern China and North America, whereas O3-rich airmasses are seen to be distributed more uniformly also over remote mountains and oceanic regions [17, 24, 26, 27].

Dramatic reductions in aerosol loadings can allow more sunlight to reach near the Earth’s surface resulting in more ozone production [28]. Heterogeneous losses of O3 and precursors are also lower due to less aerosol surface area under lower PM2.5 conditions, which can also contribute to the enhanced O3 pollution [29, 30]. Such enhancements in ground-level O3 were profound during the recent socio-economic slowdown caused by the COVID-19 (coronavirus disease-2019) lockdown restrictions [31]. Furthermore, from an air quality perspective, both local/regional and inter-continental sources impose additional burden on pollution episodes during unfavourable meteorological conditions [21, 32, 33]. The winter-time stagnant conditions substantially increase the severity of haze episodes, whereas the heatwaves during summer lead to excess biogenic emissions of VOCs with escalated chemistry of tropospheric O3 production [34].

Figure 1 highlights the key connections of PM2.5 and O3 concentrations with emissions and meteorology. An increase in PM2.5 reduces incoming solar radiation and photolysis rates thereby reducing O3 production. This decrease in incoming solar radiation also reduces surface temperature leading to a general decrease in planetary boundary layer height (PBLH). Reduced PBLH means decreased turbulent mixing of all pollutants in the boundary layer which leads to increased surface concentrations of both PM2.5 and O3. Daily, seasonal, and long-term variations in the PBLH, as well as levels of pollutants, are strongly governed by synoptic and seasonal weather patterns. For example, substantial build-up in PM2.5 levels corresponding to reduced BLH due to synoptic forcing is seen in several megacities in NCP, Sichuan Basin, and Central China [35]. On a seasonal basis, severe PM2.5 pollution is evident during winters when the average PBLH is lowest [25, 36].

Fig. 1
figure 1

A summary of key processes governing the variations in O3 and PM2.5 in the atmosphere. Red lines denote decreasing effects with an increase in PM2.5 while green lines denote increasing effects

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Designing science-driven strategies to simultaneously reduce PM2.5 and O3 is a win–win solution toward clean air and climate change mitigation. Nevertheless, highly complex and non-linear chemical processes and atmospheric dynamics make it very challenging to pinpoint the driving sources/mechanisms over local-to-regional scales. In recent years, the availability of the state-of-the-art ground-based instrumentation [37, 38], more reliable satellite retrievals [39, 40], and models accounting for the detailed atmospheric chemistry [47].

A recent synthesis from the State of Global Air [48] suggests an explicit geographical heterogeneity has been observed in the impacts of PM2.5 on health and mortality patterns showing the highest burden over Asia and Africa. The combination of dense population and poor air quality has led to higher pre-mature mortalities in China and India. The episodic nature of air pollution in some regions of China additionally elevates the health risks associated with PM2.5 [49]. Steep enhancement in PM2.5-induced mortality is also predicted for sub-Saharan, North Africa, and the Middle East [48]. The burden of diseases has high economic costs associated with hospitalisation, loss of working days, treatment costs, and premature mortalities. The potential increase in PM2.5 and pollutants like tropospheric ozone is projected to raise the global healthcare costs from USD 21 billion in 2015 to USD 176 billion in 2060 [50].

The PM2.5 composition is chiefly constituted by inorganic ions, carbonaceous compounds, and some fractions of mineral dust. Besides direct emissions through, PM2.5 is also formed in the atmosphere via chemical reactions involving precursors through gas-to-particle conversion [51]. New particle formation (NPF) via gas-to-particle conversion can occur under diverse environmental conditions such as urban locations, forest areas, marine/coastal regions, and the remote or free troposphere [52]. A recent study has suggested a major role of NPF (~ 65% of the number concentration of haze particles from NPF) in causing present-day haze events in Bei**g [53]. Among various sources, solid-fuel combustion (mostly coal) constitutes nearly 27% of the total global PM2.5 burden, eliminating which can help in avoiding 1.05 million deaths annually [54]. In the last decade, about 55% of the global population was exposed to higher levels of PM2.5 with varying spatial concentrations and impacts across the world. A decline in the population-weighted PM2.5 concentration from 12.4 to 9.8 μgm−3 was observed over North America and Europe but a sharp increase from 54.8 to 61.5 μgm−3 was noticed over Central and Southern Asia [55, 56]. Among major megacities in the world, Toronto (Canada), Miami and New York (USA), and Madrid (Spain) were identified among the least polluted with concentrations ranging between 7 and 10 μg m−3, while Delhi (India), Cairo (Egypt), ** countries [58]. Such patterns in the develo** world have many climatic implications, particularly over the tropical and Northern Hemisphere mid-latitude regions due to a significant drop in the wet deposition associated with less large-scale precipitation over land [62]. Although with increased awareness and stringent policy implementations, steep declines in PM2.5 are being recognised in some of the emission hotspots in Asia. An emerging dipole has been observed in the columnar aerosol loading with decreasing patterns over China while increasing over India [63, 64].

To tackle the impacts of PM2.5 on the climate, health, and socio-economic sustainability, worldwide efforts have been observed toward the reduction of ambient concentration as well as minimising human exposure. The U.S. Clean Air Act, the European Union Clean Air Policy, the Action Plan on Prevention and Control of Air Pollution (APPCAP) in China [15], and the Indian National Clean Air Program (NCAP) are some examples of major policy-oriented efforts. Even with the unequivocal global consensus on reducing PM2.5 reductions, the spatiotemporal heterogeneity in their sources and complex formation chemistry makes it a cumbersome task. The total mass concentration and chemical composition of PM2.5 depend on the local meteorological conditions and the nature and strength of emission sources. Moreover, air pollution is a transboundary phenomenon and does not adhere to geopolitical boundaries. Develo** a precise understanding of the spatio-temporal trends of PM2.5 and its drivers and chemical and meteorological interactions of its precursors is hence of utmost importance before aiming to design effective mitigation measures.

Ozone: Importance, Processes, and Control Pathways

Higher concentrations of O3 pose adverse impacts on human health including respiratory illnesses such as asthma exacerbation, chronic obstructive pulmonary disease (COPD), lung and cardiovascular diseases, etc [65]. Furthermore, O3 damages terrestrial vegetation by reducing photosynthesis, altering carbon allocation, and impairing stomatal function causing visible foliar injury and disturbing whole-plant level responses [66, 67]. Besides tropospheric ozone’s role as a criteria pollutant, it is also a potent greenhouse gas which significantly contributes to global warming [11]. The photochemical reactions involving VOCs, CO, and methane (CH4) in presence of NOx result in the production of O3 [6]. The efficiency of O3 production depends on the reactivity of VOC with hydroxyl radical (OH) and the ratio of VOC/NOx concentration [6]. O3 is lost through chemical pathways involving its reactions with water vapour (after photolysis), radicals (HO, HO2, NO), and through dry deposition at the surface. Besides gas-phase sinks, heterogeneous reactions of O3 with aerosols contribute to O3-sink processes [68]. The global and regional scale meteorology (e.g. temperature, relative humidity (RH), wind speed and direction, and precipitation) affects the chemical conditions for O3 production.

Recent investigations revealed a positive correlation between O3 and temperature, driven by faster chemistry and enhanced natural emissions, e.g. isoprene [69, 70]. Such deterioration of air quality due to global warming—even without an increase in anthropogenic emissions—is considered the “climate penalty” [71]. Therefore, efforts to reduce anthropogenic emissions for controlling O3 pollution could be offset by the projected rise in global surface temperatures in the future. In addition, enhanced biogenic VOCs due to higher temperatures can also increase the production of secondary organic aerosols [72]. This increase in aerosols may increase cloud condensation nuclei (CCN) and affect cloud albedo [72]. Such feedback, starting with climate warming (higher temperatures) either intensifies or reduces the warming depending on various atmospheric factors such as meteorology, location, topography, properties of aerosols, and concentration of reactive trace gases in the region. Stronger NOx emissions through soil microbes have been observed to contribute nearly half of the O3 increase in a rural site in the USA with increasing temperature [73]. Additionally, meteorological variability could affect O3 production by modulating NOx emissions by lightning [74], wildfire emissions [75], and methane emissions from wetlands [76] and shale gas [77].

The multiphase processes associated with the emissions of reactive halogen species also alter the ground-level O3 chemistry [65]. Over the oceans, the halogen species (mainly involving iodine and bromine) catalytically react with O3 resulting in lower O3 levels [78, 79]. On the contrary, in rural-urban environments of as northern China and India, chlorine-related species are reported to accelerate the oxidation of several VOCs resulting in the enhancement of O3 [80, 81]. Additionally, the oxidation of VOCs by chlorine radicals could be a potent source of secondary organic aerosol formation [82]. Therefore, it is suggested that reducing chlorine emissions along with reducing VOCs and NOx could be beneficial in reducing O3 as well as SOA formation and thus reducing PM2.5.

Challenges and Mitigation Pathways for Co-controlling PM2.5 and O3

The effects of particulate-precursor chemistry must be carefully considered for designing an effective policy for PM2.5 and O3 control. The relative reduction of fine particles and the reduced rate of hydroperoxyl radical removal help in the escalation of ozone formation [83]. Further, attempts to reduce oxides of nitrogen (NOx) can exacerbate the ambient O3 pollution in urban regions, while equally controlling NOx and volatile organic compounds (VOCs) may have minimal effect on air quality improvement.

In view of highly complex atmospheric chemistry, ** megacities are needed. Tropospheric Ozone Assessment Report (TOAR; https://igacproject.org/activities/TOAR) and Monitoring, Analysis, and Prediction of Air Quality (MAP-AQ; https://igacproject.org/activities/map-aq) are few important initiatives toward assessment and mitigation of air pollution. Further works also aiming at energy optimisation and application of advanced emission-reducing technologies globally are needed. Since PM2.5 and O3 have some common precursors and O3 itself affects chemistry leading to PM2.5 formation, a multi-pollutant multi-effect approach should be applied for co-control of O3 and PM2.5. Akimoto et al. [85] recommended a stepwise approach in which first the anthropogenic emissions of NOx and SO2 are to be controlled simultaneously. This step will help reduce nitrate (NO3−) and sulphate (SO42−), the key constituents of inorganic PM2.5. As a precursor of O3, reducing NOx will also reduce O3 in NOx-limited environments. Additionally, VOCs may get reduced when common sources of NOx and SO2 (e.g. fossil fuel burning) are controlled. In the next step, NOx and VOC emissions should be reduced simultaneously to directly reduce O3 significantly. This will also reduce secondary PM2.5 since both NOx and VOCs are important sources of secondary aerosols. This strategy can reduce PM2.5 and O3 simultaneously as it focuses on preferential control of the secondary pollutants. Few studies reported that NOx reductions in the first step may exacerbate the O3 pollution in polluted urban environments. For example, **ang et al. [84] targeted the precursors contributing to both O3 and PM2.5 precursors, i.e. NOx and VOCs, over the Bei**g-Tian**-Hebei region and observed that reducing NOx enhances O3 due to VOC-limited chemistry [86, 87], especially where O3 pollution is severe. However, reducing NOx and VOC emissions together led to small improvements in air quality. In this view, they proposed a “first VOC-focused, then NOx-focused” strategy. In the first step, reducing anthropogenic VOC emissions by 60% and NOx emissions by 20% and in the next step reducing more VOCs and NOx. This policy was suggested to control both PM2.5 and O3 pollution to a greater extent.

An analysis over Delhi [42] showed that local traffic emissions from within the city and transport of pollution from the National Capital Region (NCR) surrounding Delhi were the dominant factors influencing PM2.5 and O3. Reducing traffic emissions by 50% in Delhi alone reduces PM2.5 by 15–20% but increases O3 by 20–25%. Reducing domestic emissions can decrease PM2.5, although lesser than the reduction achieved by reducing traffic emissions but without increasing O3. This could be possible since domestic emissions might not be a major source of NOx as traffic emissions. Moreover, VOCs are reduced more than NOx when controlling domestic emissions i.e. VOC/NOx ratio was 1.8 in contrast to a ratio of 0.4 for traffic emissions. Greater reductions of VOCs suppress the increase in O3 in Delhi, which is a VOC-limited environment [88,89,90]. However, the O3 formation regime may vary for different sites within Delhi depending on VOCs/NOx [28, 91]. Reducing NCR regional emissions at the same time reducing traffic emissions in Delhi by 25–30% would further reduce the PM2.5 in Delhi by 5–10% and avoid an O3 increase [42].

Therefore, reducing VOC and NOx simultaneously with a larger reduction in VOCs compared to NOx can be an effective step to co-mitigate the PM2.5 and O3. Conclusively, reducing common sources of PM2.5 and O3 will help reduce both together. Therefore, studies with more detailed information on specific emission sectors and their contribution to the polluted regions are required for effective mitigation policies. Since modelling of sector-specific and species-specific source contributions is computationally expensive, statistically trained reduced-form models as well as machine learning-based models can be used for exploring potentials of multi-sector and multi-species emission reductions [92, 93].

A holistic model evaluation approach is required to develop a correct understanding of the co-control of PM2.5 and O3. Model simulations with reduced NOx and VOC emissions for O3 control should evaluate not only the modelled O3 but also the modelled NOx and VOC against observations. This highlights the need for continuous monitoring especially covering the periods when emissions were also reduced in reality. Since reducing NOx and VOCs will also reduce secondary PM2.5, therefore, in addition to validating the simulated PM2.5 against the observations, several individual SOA should also be validated—this requires chemically-resolved monitoring of aerosol components. Additionally, quantification of the primary and secondary fractions of the PM2.5 burden for different regions is highly recommended. The reduction of primary aerosols is relatively straightforward and can ensure rapid improvement of air quality depending on its contribution to the total PM2.5 mass.

COVID-19 Lockdown: a Reality Check

Despite large emission reductions during the COVID-19 lockdown, unexpected changes in O3 and PM2.5 were experienced in different regions of the world (Table 1). Mean PM2.5 levels generally showed reductions by about 30–40% but dust transport, biomass burning, and secondary PM formation caused enhancements in some parts of Europe and Asia [94, 95]. O3 exhibited diverse trends with insignificant change or slight increase over Europe but 25–30% higher levels over East Asia and South America [94]. Enhanced relative humidity by 30–50% and reduced wind speed under lower PBL height during winter promoted the multiphase chemistry which enhanced SOA levels during lockdown in China [96]. Additionally, the reduced NOx contributed to the enhancement in O3 levels due to non-linear chemistry. The higher O3 levels increased the atmospheric oxidising capacity and facilitated enhanced SOA and SO4 formation and further contributed to PM2.5 loading [96]. While there was a significant reduction in PM2.5 (13–29%) as well as NO2 (39–53%), O3 levels were found to be higher by a factor of 1.6–2 in northern China [97, 98].In the USA, changes in NO2 from 5 to 49% are mainly due to lower transportation and utility demands [99]. Significant reductions (up to 45%) in PM2.5 were seen in Northeast and California/Nevada metropolises, where NO2 declined strongly. Minor changes (within ± 20%) in O3 concentration were seen. The Sao Paulo region in Brazil experienced extreme reductions in NOx (~54–77%), and PM2.5 (~29.8%) but ~30% increase in the O3 concentrations, compared to the 5-year monthly mean [100]. Here, lesser changes in the levels of pollutants were observed in the industrial areas, as industries were not restricted to shut down, but they were partially affected by the decreased demand.

Satellite-based observations revealed up to 50% reductions in boundary layer NO2 over urban areas of Europe [101]. The aircraft-based measurements under the BLUESKY campaign showed large variabilities in reactive nitrogen and aerosols (20–70%) over German cities during the lockdown as compared to multi-year average data [101]. In situ measurements across the UK also showed reductions in NO2 (up to 48%) but a slight increase in O3 by ~11% [102]. In some other European sites, the NOx and PM2.5 concentrations were observed to be reduced by 50–71% and 3–50%, respectively, while O3 increased by ~ 2.4–50% [103, 104]. Such reductions were also attributed to the restrictions on population mobility leading to reduced road traffic, and industrial operations.

The reductions in some of the key air pollutants were also pronounced over India during the COVID-19 lockdown exhibiting a sharp decline in PM2.5 (~25–60%) and mixed changes in O3 across different regions [105, 106]. In Delhi, O3 concentrations showed more or less site-specific trends with a mean increase of ~ 20%. This increase could be attributed to the significant reductions (~40–60%) in NO2, PM10, and PM2.5 levels [106], which enhanced photochemical O3 production via non-linear chemistry and more solar radiation in conditions of lower aerosol burden. Ahmedabad, an urban hotspot in western India experienced ~41% enhancement in O3 levels in contrast to the reduction in NOx (~43–55%), which was attributed to chemistry (25%) and meteorology (16%) [107]. Here, enhancement in NO2/NO ratio during the lockdown (3.3) in comparison to the pre-lockdown period (2.6) results in the lesser titration of O3 as during the lockdown period [107]. Kolkata over the eastern region of the Indo-Gangetic Plain showed a decrease in PM2.5 levels by 20–66%, but an increase in O3 levels by 11–91% during five different phases of lockdown [108]. In contrast, Thiruvananthapuram, a tropical coastal site in India experienced reduced daytime O3 (~36%) accompanied by reduced daytime NO2 (~40%) [109]. Similar reductions in the O3 were also seen in the south and central regions of India [106]. Both PM and NO2 show a strong reduction due to less fuel consumption in various economic sectors i.e. transport (50–60%), aviation (90%), industries (40%), and construction activities (70%), even with a 12% increase in household fuel consumption, while O3 was increased in the populated areas of India [111].

Heterogeneities in the levels of pollutants were attributed to the complex air chemistry, meteorology, and episodic events such as dust, biomass burning, and crop fertilising [94, 95]. Such insights from the societal slowdown and reduced anthropogenic activities during the COVID-19 restrictions, albeit not intended for co-mitigation of PM2.5 and O3 but for containing the pandemic, suggest the achievable improvements in the air quality by curbing major emissions, which were not economically and socially viable otherwise.

Table 1 A list of some studies from different regions of the world describing the changes in the levels of various pollutants during the COVID-19 lockdown period
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Conclusion

The challenges to reducing air pollution greatly depend on the sources of specific pollutants, the effects of atmospheric dynamics and complex chemical interactions under various meteorological conditions. Exposure to both PM2.5 and ground-level O3 has been reported to have deleterious impacts on human and plant health and the built environment. Hence, a strong scientific consensus is building globally for co-controlling both PM2.5 and ground-level O3 for achieving several overarching air quality benefits. The complex interplay between the reduction in the hydroperoxyl radical removal helps in the escalation of ozone formation while efforts to reduce NOx exacerbate ambient O3 (in several polluted urban environments). Further, equally controlling NOx and VOCs results in the marginal improvement of air quality. PM2.5 and O3 have some common precursors; therefore, a multi-pollutant multi-effect approach to co-controlling them is urgently needed including a stepwise approach for controlling NOx and SO2 emissions simultaneously from the anthropogenic sources first, which will lower down NO3− and SO42− while as a precursor of O3, reducing NOx will further reduce the levels of O3. The societal slowdown during COVID-19 with consequent lowered energy consumption provided a natural experiment and evidence for orienting future research on co-control of both pollutants. Further, it highlighted a strong prerequisite for further scientific investigation to understand the PM2.5-O3 interactions over the polluted regions.