The impact of COVID-19 lockdown on ozone budget and radical chemistry in a typical industrial city of China

Abstract

Ozone (O) levels in East China suffered from rapid increases during the COVID-19 period. To clarify the reason for the O increase, a continuous campaign was performed in a industrial city in North China Plain (NCP). Meanwhile, the machine-learning technique and the box model were employed to reveal the mechanisms of O increase from the perspective of meteorology and photochemical process, respectively. The result suggested that the ambient O level in Tangshan increased from 18.7 ± 4.63 to 45.6 ± 8.52 μg/m (143%) after COVID-19 lockdown, and the emission reduction and meteorology contributed to 77% and 66% of this increment, respectively. The higher Wind speed (WS) coupled with regional transport played a significant role on O increase (30.8 kg/s). The O sensitivity verified that O production was highly volatile organic compounds (VOC)-sensitive (Relative incremental reactivity (RIR): 0.75), while the NO showed the negative impact on O production in Tangshan (RIR: -0.59). It suggested that the control of VOCs rather than NO might be more effective in reducing O 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 HO and RO radicals. Moreover, the concentrations of these species did not experience marked decreases after COVID-19 lockdown, which were major contributors to O increase during this period.