Coronavirus Disease 2019 (COVID-19) lockdown was related to a significant reduction in air pollutant emissions. However, severe air pollution events still occurred frequently in Taiyuan, north China. To better understand this unusual phenomenon, O₃ and other pollutants have been investigated before, during and after the COVID-19 lockdown in the early 2020. Results show that PM₁₀, PM_2.5, SO₂, NO₂ and CO underwent a more marked reduction while O₃ pollution was amplified during lockdown and the average concentrations O₃-8hr during AL stage (101.92 μg m^-3) exceeded its Grade I limiting value (100 μg m^-3). O₃ were negatively correlated with the other pollutants and positively correlated with temperature, visibility and the average solar irradiance during the daytime. The increase in daylight hours and average solar irradiance might be the key factors leading to the increase in O₃ concentrations. The reduction of PM_2.5 and NO₂ promoted the rise in O₃ during the lockdown. The majority of the O₃ was likely formed by photooxidation in the shadow of Taiyuan and linked with air masses from the northern part of Shanxi Province before the lockdown and Henan, Hebei and Shandong during and after the lockdown.