A quantitative analysis of causes for increasing ozone pollution in Shanghai during the 2022 lockdown and implications for control policy

Ground-level ozone (O3) pollution is a major air quality issue in densely populated urban areas. Despite a significant decline in human activities in the megacity Shanghai from 28 March to 31 May 2022, ground-level measurements indicate a rise in maximum daily average 8-h (MDA8) O3 concentrations in comparison to the corresponding period in 2021. There is a need for quantitative analysis to identify the reasons behind this increasing O3 concentration. We analyzed ground measurements of O3 and its precursors and meteorological parameters made in Shanghai, using random-forest (RF) model and chemical box model to elucidate the roles of meteorological and chemical factors in influencing O3 concentrations. Across urban, suburban, semi-rural, and coastal sites, the urban center of Shanghai experienced the largest decreases in the concentrations of nitrogen oxides (NOx; 53%) and volatile organic compounds (VOCs; 52%), with the most notable rise in MDA8 O3 concentrations (16%). RF modeling indicates that meteorological factors reduced MDA8 O3 concentrations by a marginal 3%, whereas a decline in anthropogenic emissions resulted in a 17% increase in MDA8 O3 concentrations. Chemical box modeling at the Pudong urban site indicates that while the decline in VOCs reduced O3 production by 42%, this was negated by a reduction in NOx from traffic emissions, which enhanced O3 production by 51%, resulting in an increase in O3 production overall. Despite a halving in precursor levels, Shanghai's urban centre remains predominately under VOC-limited conditions throughout the study period, with high NO2 levels from the petrochemical industry and traffic emissions. Joint control of anthropogenic VOCs (AVOCs) and NOx, with a ratio greater than 1.29, could help avoid exacerbation of O3 pollution and reduce NO2 pollution. Our findings emphasize the necessity of reducing industrial emissions along with ongoing green transportation strategies for alleviating O3 pollution in megacities like Shanghai.