In partnership with mainland universities, researchers from Hong Kong Polytechnic University have discovered the startling phenomenon of winter ozone pollution, with extreme ozone (O₃) levels observed in Lanzhou, China, during the winter. High hourly O₃ levels surpassing 100 ppbv were recorded during January 2018, peaking at 121 ppbv, in contrast to the usual ozone pollution associated with warm weather and intense solar radiation. The local petrochemical industry’s pollutants were blamed for this extraordinary event.

The study identified alkene ozonolysis, which takes place in the absence of sunlight, as the main driver using a sophisticated photochemical box model. Ozone synthesis is accelerated by the Criegee intermediates produced by this process, which quickly create reactive radicals (OH, HO₂, and RO₂). Alkene species like propene and trans/cis-2-butene were major contributors to the event, accounting for around 90% of the ozone formation during the occurrences.

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Petrochemical Emissions as Dominant Contributors

According to the study, a major contributor to winter ozone pollution in Lanzhou was the city’s industrial emissions, which were mostly alkenes. Alkenes accounted for the bulk of the total volatile organic compounds (VOCs) during the events, with an average daytime concentration of 153.4 ppbv. Low temperatures and moderate solar radiation did not impact the high rates at which alkenes reacted with ozone, highlighting the significance of regional industrial activity.

Lanzhou’s distinct basin topography and the emissions from its petrochemical sector made it possible for pollutants to build up and for severe ozone to emerge. Alkene ozonolysis became the main process generating ozone despite the cold and low light levels.

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Proposed Mitigation Strategies and Broader Implications

The report makes recommendations for practical mitigation techniques that might drastically reduce ozone levels, such as cutting nitrogen oxides (NOx) by 27.7% or alkene emissions by 28.6% in the early afternoon. By showing that cold low-light settings can also cause serious pollution through dark reactions, these findings cast doubt on the accepted theory of ozone generation.

In order to address this rising environmental problem, the research offers important insights into photochemical reactions during the winter and emphasizes the urgent need for targeted emissions management in petrochemical industrial regions.

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