[Response of Air Quality to COVID-19 Lockdown in Xiamen Bay]

Air pollutant concentrations in the Xiamen Bay cities during the period before and after COVID-19 lockdown(from January 11 to February 21, 2020) were studied to determine the influence of human activities on air quality in this region. During the Chinese Spring Festival holiday and the lockdown period, the concentrations of SO₂, NO₂, CO, PM₁₀, and PM_2.5 decreased by 6%-22%, 53%-70%, 34%-48%, 47%-64%, and 53%-60%, respectively. However, the changes in O₃ concentrations were not consistent with the variations of human activities. The reduction rates for PM_2.5, PM₁₀, CO, and NO₂ during the Spring Festival were greater than in previous years(2018 and 2019), but the reduction rates for SO₂ were comparable. The concentrations of NO₂ increased sharply(38%-138%), and much higher those of SO₂(2%-42%), after the resumption of socioeconomic activities, indicating the importance of traffic reductions due to the lockdown measures on NO₂. Higher wind speeds and rainfall after the Spring Festival were also favorable for the decline of SO₂, NO₂, and PM. The spatio-temporal distributions of the six criterial pollutants in the Xiamen Bay city cluster were obtained based on the Inverse Distance Weight method. The variability in regions with high NO₂ concentrations was strongly linked to traffic emissions, while spatial patterns for CO and SO₂ changed little over the six-week study period. The concentrations of PM_2.5 and PM₁₀ increased notably in the region, linked to more construction activity, but changed comparatively little in regions with dense populations and traffic networks. O₃ remained relatively stable but low-value regions corresponded to those regions with high NO₂ concentrations, indicating the significant titration effect of NO₂ on O₃. These results provide valuable information that can inform O₃ pollution reduction measures.