Elucidating the mechanisms of rapid O₃ increase in North China Plain during COVID-19 lockdown period

Ozone (O₃) levels in North China Plain (NCP) suffered from rapid increases during the COVID-19 period. Many previous studies have confirmed more rapid NO_x reduction compared with VOCs might be responsible for the O₃ increase during this period, while the comprehensive impacts of each VOC species and NO_x on ambient O₃ and their interactions with meteorology were not revealed clearly. To clarify the detailed reasons for the O₃ increase, a continuous campaign was performed in a typical industrial city of NCP. Meanwhile, the machine-learning technique and the box model were employed to reveal the mechanisms of O₃ increase from the perspective of meteorology and photochemical process, respectively. The result suggested that the ambient O₃ level in Tangshan increased from 18.7 ± 4.63 to 45.6 ± 8.52 μg/m³ (143%) during COVID-19 lockdown, and the emission reduction and meteorology contributed to 54 % and 46 % of this increment, respectively. The lower wind speed (WS) coupled with regional transport played a significant role on O₃ increase (30.8 kg/s). The O₃ sensitivity verified that O₃ production was highly volatile organic compounds (VOC)-sensitive (Relative incremental reactivity (RIR): 0.75), while the NO_x showed the negative impact on O₃ production in Tangshan (RIR: -0.59). It suggested that the control of VOCs rather than NO_x might be more effective in reducing O₃ level in Tangshan because it was located on the VOC-limited regime. Besides, both of ozone formation potential (OFP) analysis and observation-based model (OBM) demonstrated that the alkenes (36.3 ppb) and anthropogenic oxygenated volatile organic compounds (OVOCs) (15.2 ppb) showed the higher OFP compared with other species, and their reactions released a large number of HO₂ and RO₂ radicals. Moreover, the concentrations of these species did not experience marked decreases during COVID-19 lockdown, which were major contributors to O₃ increase during this period. This study also underlined the necessity of priority controlling alkenes and OVOCs across the NCP.