Since China's State Council approved its first national environmental standard for limiting PM_2.5 (particulate matter with aerodynamic diameter ≤ 2.5 µm) concentration in 2012, the changes in concentrations of PM_2.5 and aerosol components (e.g., nitrate, sulphate) in China have recently received considerable interest across the science-policy interface, with some suggesting that the annual mean PM_2.5 concentrations in China have declined by approximately 30% from 2013 to 2017 (e.g., Refs. 1‒4). However, the changes in emission sources that induce aerosol reductions remain unclear.

China has implemented a series of clean air policies since 2013, including technological and policy controls and emission reductions for industries (power plants, cement, steel), residential heating and the burning of crop straw in fields, resulting in a certain degree of reduction in PM_2.5 concentrations. Setting a cap for provincial coal consumption may affect residential and commercial fuel burning to switch to replace fuel, thus impacting the source contribution. China's megacities have also enacted a series of policies and regulations to restrict and control the use of fossil-fuelled motor vehicles on urban roads. For example, cities like Beijing and Shanghai have implemented policies for time-limited driving restrictions, as well as restrictions on non-local licensed vehicles entering the urban centre areas during specific times and dates. They have also implemented vehicle purchase restrictions, controlling the number of licensed vehicles through auction or lottery systems. In addition, some other cities have implemented policies restricting vehicles on the road based on their odd-even license numbers, limiting vehicle usage during specific dates or times. Furthermore, these megacities have gradually relocated large industrial enterprises with high emissions and reduced agricultural and rural residents.

From the annual average concentration reduction perspective, policies in China have effectively reduced aerosol pollution, benefiting people's health. Nevertheless, severe aerosol pollution still exists in some regions of China, especially in wintertime in China's megacities. It affects hundreds of millions in central and eastern China while confounding city dwellers and policymakers about its source. The highly uncertain sources and geographical emissions after China’s clean air action from 2013 to 2017 suggest that we need to scientifically understand changes in sources of carbonaceous aerosols (e.g., black carbon (BC) and water-soluble organic carbon (WSOC)) and their effects in the recent atmosphere for accurate modelling of impacts and the creation of science-baked mitigation policies.

Our observations in Shanghai and Chongming Island at the mouth of the Yangtze River in eastern China have found that wintertime haze still occurs frequently, but PM_2.5 concentrations in summer are much lower. However, there are no significant differences in urban transportation and industrial emissions between winter and summer. We must uncover why frequent haze events occur in winter but rarely in summer. Therefore, a further understanding of haze in megacities like Shanghai requires a better understanding of haze components' emission sources and regional contributions.

Our study first conducted source apportionment of carbonaceous aerosols in suburban Chongming Island and urban Shanghai from December 2018 to November 2019 based on dual-carbon isotopes (δ¹³C/∆¹⁴C) and the Monte Carlo technique, revealing how much these sources (liquid fossil, coal and biomass burning (BB) for BC; fossil and biomass for TC or WSOC) contributed. We found that the relative contribution of BB to BC and WSOC during haze events has increased significantly (15‒30%) compared to the current clean summer and previous wintertime observations (before the implementation of clean air actions). In common conception, East Asian megacities like Shanghai have previously been treated as fossil-fuel (traffic and industry) emission hotspots affecting their surrounding areas. It prompts us to understand why BB increases significantly and where it comes from during winter haze events, as the contribution of BB in large urban agglomerations like this is generally considered very low (or even negligible). Such a high BB emission may be transported from outside of Shanghai.

Therefore, we must understand BC's regional contributions, such as each province's contributions to BC arriving at the Yangtze River Estuary (YRE). We used the FLEXPART transport model to simulate the geographical BC sources to Chongming Island, providing insights into where the sources (i.e., emission sectors and BB) originate. The model coupling estimates that in winter haze events in the YRE, residential BB from the central and eastern corridors significantly contribute to BC at YRE, and the large scattered residential emissions in these regions may be one of the critical reasons for the frequent occurrence of winter haze in the YRE. This simulated result is consistent with the observed significant increase in the contribution of BB to BC during winter haze events in the YRE. For further reducing PM_2.5 concentrations, replacing solid biofuels (and coal) with cleaner energies concerning particle emissions, such as natural gas (NG), liquid petroleum gas (LPG) and electricity, and filtration of burning exalts are potential mitigation options in the residential and small commercial/industrial sectors. However, it needs to consider trade-offs between air quality and affordability of residents since gas and electricity are expensive for heating and cooking for low-income residents. The understanding of the emission sources and regional contributions of BC in Shanghai in a changing atmosphere suggests more work is needed to balance the demands for residential/commercial fuel use and urban vehicle transportation with the need for air quality improvement.

 References

1. Zhang, Q. et al. Drivers of improved PM₅ air quality in China from 2013 to 2017. Proc. Natl Acad. Sci. USA 116, 24463–24469 (2019).
2. Geng, G. et al. Drivers of PM₅ air pollution deaths in China 2002–2017. Nat. Geosci. 14, 645–650 (2021).
3. Zhai, S. et al. Control of particulate nitrate air pollution in China. Nat. Geosci. 14, 389–395 (2021).
4. The surface PM₅ observational data from the Chinese Ministry of Ecology and Environment can be obtained from https://quotsoft.net/air/.