Seasonal Variations of the Impacts of Low-Level Jets on Surface Ozone Photochemistry in Beijing

Low-Level Jet (LLJ) is a vital meteorological phenomenon driving vertical air mass transport in the troposphere, yet its effects on near-surface ozone (O₃) chemistry remain poorly understood. Based on comprehensive observations, this study investigates the seasonal influence of LLJs on surface O₃ concentrations and associated photochemical mechanisms in Beijing. We find that LLJs consistently elevate nighttime O₃ levels across all seasons by transporting O₃-rich air from aloft. During daytime, however, their net effect on O₃ varies seasonally, e.g., LLJs enhance O₃ concentrations in spring and winter but suppress them in summer and autumn, as a result of a combination of both photochemical and physical processes. In summer, daytime surface O₃ concentrations decrease on LLJ days due to the strong dilution/transportation effect, which outweighs enhanced photochemistry, where production rates of HO₂, RO₂, and O₃ rise by approximately 47%, 71%, and 47%, respectively, due to lower NO_x, higher J_NO2, and elevated temperature. In winter, daytime O₃ concentrations increase, driven by a higher O₃ production rate (10 ± 36%) associated with elevated HO₂ and RO₂ production rates (38% and 32%, respectively), alongside high O₃ concentration before sunrise and a comparable dilution effect on LLJ days compared to Non-LLJ days.