Unveiling the Sources of Severe Wintertime Ozone Pollution in Lanzhou

Researchers from Hong Kong Polytechnic University and mainland partners discovered an unexpected phenomenon: severe wintertime ozone (O₃) pollution in Lanzhou, China, driven primarily by alkene emissions from nearby petrochemical industries.

In January 2018, during cold winter days, hourly O₃ concentrations exceeded 100 ppbv, reaching a maximum of 121 ppbv. This level of ozone is uncommon in cold conditions, as it is generally associated with warm weather and strong solar radiation.

Using a photochemical box model, the study found that alkene ozonolysis was the main contributor to O₃ formation, rather than the typical radical sources initiated by photolysis. This reaction occurs without sunlight, producing Criegee intermediates that generate reactive radicals (OH, HO₂, and RO₂), which then enhance O₃ production. Alkenes accounted for approximately 90 % of the O₃ during these episodes.

The study identified trans/cis-2-butene and propene as significant alkene contributors to the pollution. It suggests mitigation strategies, such as reducing alkene emissions by 28.6 % or nitrogen oxides by 27.7 % during early afternoon hours, to decrease O₃ levels effectively.

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

Journal Reference:

Yang, J., et al. (2024) Wintertime ozone surges: The critical role of alkene ozonolysis. Environmental Science and Ecotechnology. doi.org/10.1016/j.ese.2024.100477