Solving Beijing's smog: study reveals over 80% of key pollutants drift in from surrounding regions

A new modeling study by researchers at Beijing Normal University provides a detailed account of the origins of the dense winter haze that frequently affects Beijing. By simulating the formation of a major air pollutant, the study found that the vast majority of these harmful particles in urban Beijing—over 80%—are formed from precursor chemicals transported from surrounding areas. The findings point to regional sources, particularly residential emissions, as the primary target for effective air quality improvement.

The Invisible Pollutant

During severe haze events, a large fraction of the harmful PM2.5 fine particulate matter is composed of secondary organic aerosols, or SOAs. Unlike soot which is emitted directly, SOAs are not released from a tailpipe or smokestack. Instead, they form in the atmosphere through complex chemical reactions involving precursor gases like volatile organic compounds VOCs. Understanding where these precursors come from and how they transform into SOAs is essential for developing effective pollution control strategies.

Advancing the Simulation

To investigate this complex process, the research team, led by Yuepeng Zhang and Jing Chen, employed a sophisticated air quality model called WRF-CAMx. They compared a traditional simulation method with a more advanced approach known as the volatility basis set VBS. The VBS scheme, which better accounts for the chemical aging and volatility of organic compounds, produced SOA concentrations that more closely matched real-world measurements taken at urban and suburban sites in Beijing during a winter pollution episode.

Key Ingredients for Haze

The study performed a series of sensitivity tests to determine which factors most influenced the amount of SOA produced. The results showed that the model was most sensitive to the emissions of VOCs and intermediate volatility organic compounds IVOCs—gaseous chemicals that act as the building blocks for SOAs. The concentration of atmospheric oxidants, which drive the chemical reactions, was also a determining factor. This shows that controlling the emission of these precursor gases is a direct way to reduce SOA formation.

An Out-of-Town Problem

Using their optimized model, the scientists traced the SOAs in urban Beijing back to their origins. The results were clear: local emissions within Beijing contributed only 12.4% of the city’s SOA pollution during the haze period. The remaining 87.6% was the result of regional transport, where pollutants and already-formed SOAs drifted into the city from the wider North China Plain. The largest single contributor was Beijing’s own suburban areas 32.7%, followed closely by the surrounding Hebei province 21.4%.

Identifying the Regional Source

When examining the sources across the entire region, the model identified residential emissions as the dominant contributor to Beijing's urban SOA problem, accounting for 74.6% of the total. These emissions likely come from the widespread use of coal and biomass for heating and cooking in areas surrounding the city. Industrial emissions were the second-largest regional source at 15.6%, while transportation and power generation had smaller contributions.

A Regional Solution for a Regional Problem

This research indicates that policies aimed at improving Beijing’s air quality must adopt a regional perspective. While reducing urban emissions is helpful, making substantial progress against severe winter haze requires controlling residential and industrial emissions throughout the North China Plain. The authors suggest that focusing on emission sources in the suburban areas of Beijing and in Hebei province would effectively reduce the SOA pollution that plagues the urban center.

Corresponding Author:

Jing Chen

Original Source:

https://doi.org/10.1007/s44246-023-00040-w

Contributions:

Yuepeng Zhang: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Visualization. Huiying Huang: Methodology, Formal analysis, Visualization. Weihua Qin: Methodology, Formal analysis, Visualization. Qing Yu: Methodology, Formal analysis. Yuewei Sun: Methodology, Formal analysis. Siming Cheng: Methodology. Mushtaq Ahmad: Investigation. Wei Ouyang: Methodology. Tseren-Ochir Soyol-Erdene: Methodology, Investigation. Jing Chen: Supervision, Conceptualization, Data Curation, Resources, Writing - review & editing, Project administration, Funding acquisition. All authors have read and approved the manuscript.