Smog in Beijing originates outside of the city

Severe pollution meets high population
Air pollution is responsible for several million deaths worldwide every year.  The most critical regions are those where high pollution meets a high population. Beijing, a megacity known for its notorious haze, continues to grapple with severe air pollution despite rigorous mitigation strategies. With over 22 million people living in Beijing, it is a location of critical importance for studying and mitigating air pollution. As a team of dedicated researchers, we embarked on a journey to unravel the mysteries of secondary organic aerosol, a significant component of this pollution, and its seasonal variability in sources and formation pathways. Secondary organic aerosols forms through the atmospheric oxidation of volatile organic compounds  from both natural and anthropogenic sources. Its formation involves intricate chemical reactions, making it difficult to trace its origins and design effective control measures. 
Multitude and complexity of particulate pollution sources
Our measurements took place between November 2019 and July 2020, at a typical urban site in the heart of Beijing with the photos below showing the magnitude and complexity of the different emission sources. The site is surrounded by residential buildings, restaurants and a major highway. However, our study reveals that the sources of particulate air pollution in Beijing cannot all be captured in these photos. A lot of these pollutants travel long distances exceeding hundreds of kilometers and transform physically and chemically before they arrive to our measurement location. Such a transformation complicates the identification of their emission sources and geographical origins. This is where our unique measurements of the molecular fingerprints of the aerosol particles came in handy. For conducting our study , we utilized advanced mass spectrometry techniques to achieve substantial molecular speciation of organic aerosols which facilitates the pinpointing of pollutant sources and regions, thereby advancing our comprehension of smog dynamics. 
Using our unique measurements and techniques, we find that Beijing is subject to a multitude of particulate pollution sources which vary between the seasons. During winter, we found that primary solid-fuel emissions and secondary organic aerosols from these emissions dominate, with significant contributions transported from the Beijing-Tianjin-Hebei Plain and  mountainous areas west of Beijing. Conversely, during summer, the secondary organic aerosol fraction dominates, but with a different composition. Aromatic emissions, likely from the Xi’an-Shanghai-Beijing region, become the main contributors, while biogenic secondary organic aerosol remains relatively minor.
    

Our measuring instruments and their inlets on the roof of the university building.

Major highway runs adjacent to our measurement site. 

Impact of the COVID-19 Lockdown
The COVID-19 lockdown in early 2020 provided a unique opportunity to examine how reduced human activity affected air quality. With significant reductions in traffic, coal consumption, and overall economic activity, one might have anticipated a sharp decline in pollution levels. However, our study revealed that while particulate pollution levels did decrease, they remained sufficiently high to raise concerns. This period underscored the resilience of secondary pollution sources and the complexities of atmospheric chemistry.
The pandemic not only posed a health threat but also disrupted our ability to consistently conduct our measurements. During the lockdown, some members of our research team were unable to physically access our measurement site. Despite these challenges, through collaborative efforts with local scientists, we successfully achieved our study’s objectives. Such a collaboration also ensured that our research was firmly rooted in the local context, providing data that accurately reflected the environmental conditions experienced by Beijing’s residents. 
The future of particulate air pollution in Beijing
Our research highlights the critical need to address particulate pollution sources beyond Beijing’s urban boundaries. Local measures alone are insufficient, given the long-range transport of pollutants, necessitating regional collaboration and comprehensive mitigation strategies. This includes working with neighboring regions to control pollution, focusing on major sources of secondary organic aerosol precursors, and integrating policies for both local and regional pollution. In addition, effectively managing long-range transport requires sustained regional efforts and continuous monitoring to evaluate and adjust strategies as needed. We hope that our results provide insights for mitigation strategies that result in more frequent occurrences of clear, smog-free days in Beijing, even in the midst of cold winter periods.

A smog-free day during in Beijing.

Publication: Substantial contribution of transported emissions to organic aerosol in Beijing
Authors of the paper: Kaspar R. Daellenbach, Jing Cai, Simo Hakala, Lubna Dada, Chao Yan, Wei Du, Lei Yao, Feixue Zheng, Jialiang Ma, Florian Ungeheuer, Alexander L. Vogel, Dominik Stolzenburg, Yufang Hao, Yongchun Liu, Federico Bianchi, Gaëlle Uzu, Jean-Luc Jaffrezo, Douglas R. Worsnop, Neil M. Donahue & Markku Kulmala
Paper: https://doi.org/10.1038/s41561-024-01493-3
Contributors: Lubna Dada, PhD and Kaspar R. Daellenbach, PhD