Wintertime ozone surges: The critical role of alkene ozonolysis

Ozone (O₃) pollution is usually linked to warm weather and strong solar radiation, making it uncommon in cold winters. However, an unusual occurrence of four high O₃ episode days (with maximum hourly concentrations exceeding 100 ppbv and peaking at 121 ppbv) was recorded in January 2018 in Lanzhou city, China. During these episodes, the average daytime concentration of total non-methane volatile organic compounds (TVOCs) reached 153.4 ± 19.0 ppbv, with alkenes—largely emitted from the local petrochemical industry—comprising 82.3 ± 13.1 ppbv. Here we show a photochemical box model coupled with a Master Chemical Mechanism to elucidate the mechanisms behind this unusual wintertime O₃ pollution. We find that the typically low temperatures (−1.7 ± 1.3 °C) and weak solar radiation (263.6 ± 60.7 W m^-2) of those winter episode days had a minimal effect on the reactivity of VOCs with OH radicals. Instead, the ozonolysis of alkenes generated Criegee intermediates, which rapidly decomposed into substantial RO_x radicals (OH, HO₂, and RO₂) without sunlight. This radical production led to the oxidation of VOCs, with alkene ozonolysis ultimately contributing to 89.6 ± 8.7% of the O₃ formation during these episodes. This mechanism did not activate at night due to the depletion of O₃ by the NO titration effect. Furthermore, the findings indicate that a reduction of alkenes by 28.6% or NO_x by 27.7% in the early afternoon could significantly mitigate wintertime O₃ pollution. Overall, this study unravels the unique mechanism of alkene-induced winter O₃ pollution and offers a reference for winter O₃ reduction strategies in the petrochemical industrial regions.