Wintertime Pollution Particles in China’s Skies: A Study of Their Formation and Size

Researchers from the School of Earth and Atmospheric Sciences have discovered the formation of hazardous sulfates, with particles increasing in size, within the pollution plumes emitted from coal-fired power plants.

APA 7: TWs Editor. (2023, November 20). Wintertime Pollution Particles in China’s Skies: A Study of Their Formation and Size. PerEXP Teamworks. [News Link]

Earlier investigations have identified sulfate particles in the atmospheric haze over Beijing, a significant contributor to outdoor air pollution known to harm respiratory health and exacerbate pre-existing asthma symptoms, as noted by the California Air Resources Board.

Typically, sulfates are generated through atmospheric oxidation, a process accelerated by ample sunlight, particularly during the summer. This oxidation transforms sulfur dioxide into hazardous aerosol particles. The question arises: How does China exhibit heightened pollution, rich in sulfates, during the winter when sunlight is less abundant, and atmospheric oxidation proceeds at a slower pace?

In a study led by Yuhang Wang, a professor in the School of Earth and Atmospheric Sciences at Georgia Tech, along with his research team, an investigation was conducted to unravel this puzzle. The study suggests that all the essential chemical reactions responsible for the conversion of sulfur dioxide into sulfur trioxide, and subsequently into sulfate, predominantly occur within the smoke plumes that generate pollution. This mechanism not only explains the winter production of sulfates in China but also highlights an accelerated and intensified formation of larger sulfate particles in the atmosphere.

Wang introduces the term “in-source formation” to describe the identified phenomenon. Unlike the conventional process where oxidants disperse throughout the atmosphere, gradually transforming sulfur dioxide into sulfur trioxide and eventually sulfate, this concentrated production occurs within the exhaust plumes. Within these plumes, sulfuric acid is rapidly converted into substantial sulfate particles. This concentrated in-source formation clarifies the presence of large sulfate particles observed in China.

The discovery of the in-source formation of larger sulfate particles during the winter in China has significant implications. It aids scientists in precisely evaluating the influence of aerosols on radiative forcing, elucidating how climate change and global warming affect the Earth’s energy and heat balances. Moreover, in the realm of health, the presence of larger aerosols implies more substantial deposits in human lungs, underscoring the importance of understanding and addressing these particles for public health concerns.

The study titled “Wintertime Formation of Large Sulfate Particles in China and Implications for Human Health” has been published in Environmental Science & Technology. Among the co-authors are Qianru Zhang from Peking University and Mingming Zheng from Wuhan Polytechnic University. Notably, both Zhang and Zheng, who were former students of Wang, conducted the research while affiliated with Georgia Tech.

Origins of a pervasive historical smog

China continues to heavily rely on coal in power plants due to its lower costs in comparison to natural gas, as explained by Wang. This situation offers a convenient basis for drawing parallels between China’s hazy winters and a significant historical event that brought attention to perilous environmental hazards—the Great London Smog in the United Kingdom.

Portrayed in the Netflix series “The Crown,” the incident unfolded in December 1952 when London was engulfed by a severe smog. The onset of unusually cold weather contributed to bringing the haze, produced largely from coal, down to ground level. Official statements from UK authorities attributed 4,000 deaths and 100,000 illnesses to the Great London Smog (also referred to as the Great London Fog). However, subsequent studies suggested a higher death toll ranging from 10,000 to 20,000.

Wang expresses the historical challenge of elucidating the winter production of sulfate, spanning from the era of the London Fog to the contemporary issue of extreme winter pollution in China. This underscores the difficulty researchers have faced in comprehending the mechanisms behind sulfate formation during winter months.

Motivated to address this challenge, Wang and his research team embarked on an endeavor to unravel the complexities of sulfate production during winter.

Aerosol size and heavy metal influence?

The elevated sulfate levels observed in China, particularly in January 2013, challenge traditional explanations that depended on standard photochemical oxidation. The prevailing notion suggested that nitrogen dioxide or similar mild oxidants within alkaline or neutral particles in the atmosphere were responsible. Contrary to these expectations, measurements indicated that the resultant sulfate particles were highly acidic.

During Zheng’s tenure at Georgia Tech, Wang describes her as a former student actively seeking engaging projects. In guiding her research, he proposed exploring aerosol size distributions, specifically focusing on the dimensions of aerosols to delve into intriguing aspects of the study.

Observing the study results, Zheng and Wang made a notable observation: sulfate particles generated during China’s winter were significantly larger compared to those resulting from photochemically-produced aerosols. Typically falling within the range of 0.3 to 0.5 microns, these sulfate particles approached a size of 1 micron. To provide context, a human hair is approximately 70 microns in diameter. This deviation from the norm, where aerosols dispersed over a broader area are typically smaller, marked a distinctive characteristic of the winter sulfate particles in China.

Wang interprets the micron-sized aerosol observations as indicative of substantial growth in an environment rich in sulfur trioxide. Larger aerosol particles pose increased health risks, as they result in more significant deposition in the front part of the respiratory system, while depositing less in the end part, such as the alveoli. Considering the larger particle size, the estimated total aerosol deposition in the human respiratory system is projected to increase by 10 to 30 percent.

Nevertheless, an additional component is required to contribute to the chemical combination, allowing sulfur dioxide to transform into sulfur trioxide and facilitating the enlargement of resulting sulfate particles. Wang proposes a potential pathway involving the catalytic oxidation of sulfur dioxide to sulfuric acid, facilitated by “Transition metals.”

Wang notes that elevated temperatures, acidity, and water content in the exhaust contribute significantly to catalytic sulfur dioxide oxidation, particularly in comparison to ambient atmospheric conditions. He suggests the likelihood of similar heterogeneous processes taking place on the hot surface of a smokestack coated with transition metals, potentially elucidating the substantial presence of sulfur trioxide observed in the exhaust of coal-fired power plants. This production of sulfur trioxide is attributed to either combustion processes or metal-catalyzed oxidation at higher temperatures.

A chance to implement more environmentally friendly coal power facilities

The implications of in-source sulfate formation highlight the potential effectiveness of implementing measures to cool and eliminate sulfur trioxide, sulfuric acid, and particulates from emissions produced by coal-combustion facilities. This approach could serve as a viable strategy to mitigate pollution, ultimately reducing the risk of severe health problems associated with such pollutants.

Wang emphasizes that the advancement and application of such technology hold the potential to provide global benefits, especially for nations heavily dependent on coal as their primary energy source. This underscores the broader positive impact that the development and adoption of these measures could have on a global scale.


  1. NEWSPAPER Miguel, R. S. & Georgia Institute of Technology. (2023, November 17). Study reveals wintertime formation of large pollution particles in China’s skies. []
  2. JOURNAL Zhang, Q., Wang, Y., Liu, M., Zheng, M., Yuan, L., Liu, J., Tao, S., & Wang, X. (2023). Wintertime formation of large sulfate particles in China and implications for human health. Environmental Science & Technology. [Environmental Science & Technology]

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