Why did ozone concentrations remain high during Shanghai's static management? A statistical and radical-chemistry perspective

Abstract

During the period of April and May 2022, Shanghai implemented city-wide static management measures to control the spread of the Omicron variant. Compared to the lockdown in early 2020, the static management in 2022 occurred during the high-ozone season and lasted for a longer duration. It can be considered a “large-scale field experiment” to study the response of ambient ozone levels to emission reductions. During this period, we conducted comprehensive observations at Fudan University, Jiangwan Campus, in the northeast corner of Shanghai. Similar experiments were also conducted during the same period in 2020 and 2021. Despite the significant reduction of approximately 30 % in VOCs and around 50 % in NO2 due to static management in 2022, the average ozone level increased by nearly 23 % compared to 2020 and 2021. This suggests that the reduction in ozone precursors and other pollutants did not lead to a corresponding decrease in ozone concentrations as expected. Cluster analysis of diurnal patterns of ozone concentration revealed four distinct types of diurnal ozone variations. Cluster 3 and cluster 4, with high ozone levels, experienced significant increases in their shares during static management, ultimately leading to an overall increase in average ozone levels in 2022. According to the observation-based model (OBM) simulation analysis, the average peak concentrations of OH, HO2, and RO2 in 2022 were estimated to be 5.1×106, 4.7×108, and 2.6×108 molec.cm-3, respectively, representing an increase of about 30 % compared to the levels in 2020 and 2021. Although HONO photolysis was the main contributor to the primary source of ROx radicals, the radical-cycling process remained dominant for the overall production of ROx radicals. Due to a significant decrease in NO2 concentration relative to VOCs, the average VOC / NO2 ratio increased from 1.6 in 2020 to 3.0 in 2022, which is also reflected in the radical cycling. The ratio of OH radical propagation (OH + VOCs) to termination (OH + NO2) was 2.37, higher than 1.18 in 2020 and 1.78 in 2021, indicating that the different reduction proportions of precursors led to a higher VOC / NO2 ratio, strengthening the radical cycling. The differential reduction in precursor VOCs and NO2 levels due to static management is the underlying cause of the increase in ozone concentration in Shanghai.