In the upper layers of the atmosphere, ozone protects life from harmful ultraviolet radiation. However, closer to the Earth’s surface, ozone contributes to global warming and is detrimental to human health and vegetation. Cleaning the air can be achieved by reducing emissions at the source for pollutants directly emitted into the atmosphere, but ozone is not directly emitted. Near-surface ozone is formed by complex chemical reactions of precursor gases, mainly volatile organic compounds (VOCs) and nitrogen oxides (NOx = NO + NO2), in the presence of sunlight.

In ozone mitigation efforts, these reactions are typically considered within two regimes called “NOx-limited” and “VOC-limited”. The NOx-limited chemical regime describes conditions in which NOx emissions reductions are most beneficial to reduce ozone, while the VOC-limited regime represents situations where lowering emissions of organic compounds would be most effective. Writing in Nature Geoscience, Ivatt et al.1 show a third chemical regime, named “aerosol-inhibited”, under which the reduction of particulate pollution can lead to an increase of ozone if the emissions of the two main ozone precursors, NOx and VOCs, are not sufficiently reduced simultaneously.

The production of ground-level ozone involves hundreds of non-linear photochemical reactions between NOx and VOCs and intermediate chemical compounds. In mid-latitude regions, surface ozone is at a maximum in spring/summer when sunlight is prominent2. One of the key reactions to produce ozone occurs between hydroperoxyl radicals and nitric oxide. Hydroperoxyl radicals, however, can be removed from the atmosphere at the same time by particulate matter with a diameter equal to or smaller than 2.5 μm (PM2.5). Therefore, a decrease in PM2.5 causes less hydroperoxyl radicals to be removed and thus can lead to an increase in ozone.

This trade-off between O3 and PM2.5 has recently been found to be an unintended consequence of air pollution control in China after the government implemented the Clean Air Action plan3,4,5. Since the implementation of the policy, the ozone level has increased in summer over the North China Plain, in association with a decrease in NOx emissions, constant or an increase in VOC emissions, and a large decrease in PM2.5 levels. The lessons learned are that ozone mitigation is not as simple as considering NOx-limited and VOC-limited regimes and the role of PM2.5 must also be considered.

Ivatt et al. incorporate the role of PM2.5 by defining the uptake of hydroperoxyl radicals onto the surfaces of particulate pollutants as the key component of a third chemical regime for ozone formation at a global scale. Using a global chemical model, Ivatt and colleagues compared the rate of the typical chemical reactions of the three regimes in the Northern Hemisphere for July 1750, 1970 and 2014. They find that regions where this aerosol-inhibited regime dominates have evolved with time. From 1970 to 2014, the percentage of the population in the Northern Hemisphere living in the aerosol-inhibited regime increased from 2% to 21%.

The authors find that in 1750, biomass-burning regions in North America and Northeast Asia were the most affected by the aerosol-inhibited regime. In 1970, the highly industrialized regions were most affected, including North America, Europe and parts of Asia. In 2014, South and East Asia had become the regions where the aerosol-inhibited regime dominated. South Asia (Fig. 1) is particularly affected by this aerosol-involved regime at present. It is estimated that a 40% reduction in NOx emissions is required to offset the increase in surface ozone resulting from a 50% reduction in emissions of PM2.5 precursors.

Fig. 1: South Asia is a region where the aerosol-inhibited regime prevails.
figure 1

Kolkata, where PM2.5 and NOx decreased by 41% and 26%, respectively, experienced a 12% increase in ozone in the lockdown months of 2020 compared to the period 2015–2019. The air might appear clearer but it does not always mean that it is cleaner. Figure reproduced with permission from ref. 1, Springer Nature Limited.

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The findings of Ivatt et al. are supported by observed changes in atmospheric chemistry during the COVID-19 lockdown in spring 2020. A drastic drop in economic activity led to a big decrease in emissions of air pollutants globally, especially NOx, and also in PM2.5 levels. But changes of ozone varied by region, depending on local atmospheric chemistry conditions6,7. Two cities in India represent a good example of this phenomenon: New Delhi saw a 24% decrease in ozone associated with a 48% decrease in PM2.5 and a 55% decrease in NOx. In contrast, Kolkata, where PM2.5 and NOx decreased by 41% and 26%, respectively, experienced a 12% increase in ozone7.

In 2019, PM2.5 and ozone pollution are estimated to together cause 4.5 million global premature deaths8, of which PM2.5 accounts for more than 90%. Ozone also negatively impacts crop productivity as it oxidizes the leaves of the plants after being absorbed via the open stomatal pores, reducing photosynthesis that plants need in order to grow. For example, ozone pollution in South Asia during 2010–2012 led to crop yield losses, including 7% for maize, 5% for rice, 11% for wheat and 16% for soy beans9. Therefore, to maximize the benefits of air pollution control efforts, co-control of PM2.5 and ozone is important, especially for regions where the aerosol-inhibited regime dominates.

Ivatt et al. show that it is now possible to detect regions, such as South Asia, where the aerosol-inhibited regime prevails. The third chemical regime is bringing new understandings that are crucial to reduce ozone pollution. The reduction of emissions of O3 precursors in tandem will avoid the trade-off between O3 and PM2.5 and the resulting consequences for both vegetation and human health. If policymakers would like to control O3 then they will have to consider the chemical aerosol-inhibited regime.