Researchers from the Hong Kong Polytechnic University, alongside mainland collaborators, have uncovered an unexpected phenomenon: severe wintertime ozone (O₃) pollution in Lanzhou, China, driven primarily by alkene emissions from local petrochemical industries.

The work is published in the journal Environmental Science and Ecotechnology.

Traditionally associated with warm weather and strong solar radiation, hourly O₃ levels exceeding 100 ppbv were recorded during cold January days in 2018, peaking at an alarming 121 ppbv.

Using an advanced photochemical box model, the study identified alkene ozonolysis as the dominant driver of O₃ formation, rather than the traditional radical sources initiated by photolysis. This chemical reaction occurs without sunlight and produces Criegee intermediates that rapidly generate reactive radicals (OH, HO₂, and RO₂), which then accelerate O₃ production. Ultimately, alkenes contributed to nearly 90% of the O₃ during the episodes.

The research highlights key alkene species—trans/cis-2-butene and propene—as major contributors to this unusual pollution. Importantly, the study proposes actionable mitigation strategies: reducing alkene levels by 28.6% or nitrogen oxides by 27.7% during early afternoon hours could significantly reduce O₃ levels.

“This study updates how we understand O₃ pollution, proving that intense O₃ formation can occur in cold, low-light conditions,” said the authors Jin Yang and Yangzong Zeren. “Our findings complement conventional views and call for targeted action in industrial regions.”