The Chinese Academy of Sciences recently released the latest findings from its special research group on "Tracing the Causes and Control of Atmospheric Haze." The study concluded that the recent severe haze events were caused by a combination of abnormal weather patterns leading to stable atmospheric conditions in central and eastern China, human pollution emissions, airborne dust, and abundant water vapor—making it an incident driven by both natural and human factors.
Pollutants react with water vapor to trigger haze events
The study believes that human pollutant emissions are the root cause of haze weather, essentially the "main culprit." Chen Liangfu, a researcher at the Institute of Remote Sensing and Digital Earth of the Chinese Academy of Sciences and a member of the task force, said that when soluble components in air pollutants encounter mineral dust condensation nuclei, they rapidly envelop them, forming mixed particles. When exposed to higher relative humidity, these particles quickly undergo hygroscopic growth—their size increases by two to three times, and their light extinction coefficient rises by eight to nine times, meaning visibility drops to one-eighth to one-ninth of its original level. In simpler terms, smaller pollutant particles already present in the air combine with water vapor to form larger particles visible to the naked eye, which then trigger haze events.
Monitoring data from 15 PM2.5 monitoring stations deployed by the Chinese Academy of Sciences across the Beijing-Tianjin-Hebei region showed that in January, five major haze pollution episodes occurred in the region—between January 6–8, 9–15, 17–19, 22–23, and 25–31. Water vapor, as Chen Liangfu described, played the role of an "accomplice" in all five instances.
Dangerous organic compounds detected in haze
Wang Yuesi, leader of the “Atmospheric Haze Source Tracing” project under the task force and a researcher at the Institute of Atmospheric Physics of the Chinese Academy of Sciences, said the chemical composition of the intense haze pollutants sweeping through central and eastern China was a mix of those from the 1952 London smog event and the photochemical smog events in Los Angeles, USA, which began in the 1940s–50s—along with China-specific dust aerosols.
It’s worth mentioning in particular the Los Angeles photochemical smog event, where more than 800 people lost their lives. In a subsequent U.S. government investigation, it was stated that petroleum vapors (hydrocarbons) and nitrogen dioxide, under the intense ultraviolet rays of the sun, produce a kind of irritating organic compound. This process is called a photochemical reaction, and its product is highly toxic photochemical smog.
In the specialized study of haze weather in the Beijing-Tianjin-Hebei region, the task force detected large amounts of nitrogen-containing organic particulate matter. Wang Yuesi regarded this as “the most dangerous signal,” as it is “one of the main components of the photochemical smog in Los Angeles during the last century.”
Through source apportionment technology, these organic substances—including nitrogen-containing organic particulate matter—were identified as four types of organic components: oxidized organic particulate matter, mainly originating from the surrounding areas of Beijing; cooking-related organic matter, primarily from local cooking emissions; nitrogen-enriched organic matter, a type of chemical byproduct; and hydrocarbon-based organic particulate matter, mainly from vehicle exhaust and coal combustion. Among these, oxidized organic particulate matter accounted for the largest proportion of the entire pollution process at 44%, while the other three groups made up 21%, 17%, and 18% respectively.
Suggest focusing control on industry and coal burning
The task force attributed these factors to “the direct consequences of extensive human emissions and the destruction of natural ecosystems.” In the Beijing area, vehicles are the largest source of urban PM2.5, accounting for about one-quarter; followed by coal burning and external transmission, each contributing about one-fifth. For the entire Beijing-Tianjin-Hebei region, the task force believes priority should be given to controlling industrial processes and coal burning, with a focus on desulfurization, denitrification, and dust removal during combustion; at the same time, high attention must be paid to diesel vehicle emissions and fuel quality.