In recent decades, China has experienced serious air pollution problems, mainly caused by intense emissions from human activities. To improve air quality, particularly in reducing PM2.5 (tiny particles smaller than 2.5 micrometers), the Chinese government implemented Clean Air Action (CAA) plans from the year 2013. These efforts have led to a significant decrease in deaths linked to PM2.5, saving over 370,000 lives every year. While many studies have focused on the health benefits of these plans, the effects of reduced air pollution on land ecosystems have not been widely noticed.

Previous studies have shown that aerosols and ozone have complex effects on how land ecosystems absorb carbon. Aerosols can help plants use sunlight more efficiently by scattering it, which spreads the light in multiple directions. This means that even leaves in shaded areas receive some light, improving the overall light availability for photosynthesis. However, aerosols can also change weather conditions near surface, like lowering temperatures and reducing rainfall, which can either help or hurt plant photosynthesis depending on the type of aerosols and the background environment. On the other hand, surface ozone consistently harms plant photosynthesis because it enters through the plant’s stomata (tiny openings) and damages the cells due to its strong oxidative properties. Since China’s land ecosystems play a critical role in absorbing carbon, it is crucial to understand if the air quality improvements from the CAA plans can help restore regional carbon absorption, measured as net primary productivity (NPP).

In our recent study, “Recovery of ecosystem productivity in China by the Clean Air Action plan”, we explored whether changes in aerosols and ozone due to the CAA could enhance carbon absorption in China. Initially, we thought that the significant reduction in aerosols caused by the CAA would lower the amount of diffuse radiation, potentially reducing carbon absorption by plants. However, our findings showed that the decrease in absorbing aerosols, such as black carbon, actually increased both direct and diffuse sunlight, which compensated for the reduced scattering of light. This led to an overall increase in terrestrial NPP in China (Figure 1). Therefore, the radiative effects of aerosols from the CAA had a positive contribution to ecosystem productivity in China, despite the mixed effects from changes in direct and diffuse radiation. 

Our study also looked at how aerosols affect climate conditions, like temperature and rainfall. To do this, we used data from two climate modes to predict how aerosols influence these factors, and then applied the simulated perturbations to drive a dynamic vegetation model. Normally, high levels of aerosols lower surface temperatures, which benefits plant growth in southern China by improving photosynthesis and reducing plant respiration. However, aerosols also decrease rainfall, which limits the ability of plants to absorb carbon, leading to an overall negative effect on NPP, even with the cooling benefits. After the CAA plants were implemented, the reduction in aerosols allowed for more rainfall, which boosted plant photosynthesis. As a result, the decrease in aerosol-induced climate effects from the CAA plans also helped increase terrestrial NPP (Figure 1).

In addition to the effects of aerosols, we also examined how changes in ozone levels, due to the CAA plans, influenced land carbon uptake. We discovered that NPP recovered more strongly during 2018-2020 (CAA Plan Phase II) compared to 2014-2017 (CAA Plan Phase I) (Figure 1). This difference may be explained by the varying ozone responses across the two phases: ozone concentrations increased during 2014-2017 but decreased during 2018-2020. These changes in ozone had negative effects on NPP during Phase I but positive effects during Phase II, contributing to the stronger recovery of NPP in China. 

 
Figure 1 Diagram showing how net primary productivity (NPP) responded to changes in aerosol and ozone during different phases of the CAA plans. This diagram illustrates the various processes affecting NPP during (a) Phase I (2014-2017) and (b) Phase II (2018-2020). It shows how aerosols influenced NPP through changes in sunlight and climate, as well as the damage caused by ozone to NPP in China during 2014-2020. Red lines indicate processes that increase NPP, while blue lines represent processes that decrease NPP.

Our study found that the CAA plans led to a national NPP recovery, mainly due to reductions in aerosols. This was driven by increased sunlight from reduced black carbon and more rainfall due to weaker aerosol effects on the climate. Over time, the positive impact of lower ozone levels became even more significant, surpassing the benefits of aerosol reduction by 2020, and is expected to continue driving NPP recovery in the future. Our research highlights the substantial carbon gains from reducing both aerosols and surface ozone, emphasizing the dual benefits of air pollution control for improving public health and achieving carbon neutrality in China.