Air-pollutant mass concentration changes during COVID-19 pandemic in Shanghai, China

To curb the spread of the coronavirus, China implemented lockdown policies on January 23, 2020. The resulting extreme changes in human behavior may have influenced the air pollutants concentration. However, despite these changes, hazy weather persisted in Shanghai and became a public issue. This study aims to investigate air pollutant mass concentration changes during the lockdown in Shanghai. Air pollutant mass concentration data and meteorological data during the pre-lockdown period and the level I response lockdown period were analyzed by statistical analysis and a Lagrangian particle diffusion model. The data was classified in three periods: P1 (pre-lockdown: 10 days before the Spring Festival), P2 (the first 10 days after lockdown: during the Spring Festival celebration), and P3 (the second 10 days after lockdown: after the Spring Festival). Data for the same period in 2019 were used as a reference. The results indicate that the Spring Festival holiday in 2019 resulted in a reduction in energy consumption, which led to a decrease in PM_2.5 (26.4%) and NO₂ (43.41%) mass concentration, but an increase in ozone mass concentration (31.39%) in P2 compared with P1. The integrated effect of the Spring Festival holiday and lockdown in 2020 resulted in a decrease in PM_2.5 (36.5%) and NO₂ (51.9%) mass concentrations, but an increase in ozone mass concentration (43.8%) in P2 compared with P1. After the Spring Festival, the mass concentrations of PM_2.5, SO₂, and NO₂ increased by 74.41%, 5.52%, and 53.28%, respectively in P3 compared with P2 in 2019. However, PM_2.5 and SO₂ concentrations in 2020 continued to decrease, by 14.74% and 4.61%, respectively, while NO₂ mass concentration increased by 7.82% in P3 compared with P2. We also found that PM_2.5 mass concentration is susceptible to regional transmission from the surrounding cities. PM_2.5 and other gaseous pollutants show different correlations in different periods, while NO₂ and O₃ always show a strong negative correlation. The principal components before the Spring Festival in 2019 were O₃ and NO₂, and after the Spring Festival, they were PM_2.5 and CO, while the principal components before the lockdown in 2020 were PM_2.5 and CO, and during lockdown they were O₃ and NO₂.