Multidimensional Observation Analysis of the Air Quality Changes in Wuhan during the Coronavirus Disease-2019 Pandemic

Based on the state-controlled sites and local-controlled sites, using the hourly monitoring data of six conventional pollutants and PM2.5 component data from 1 January to 13 February 2020, the temporal and spatial variation characteristics of air pollutants in the Wuhan area before and after the coronavirus disease 2019 (COVID-19) controls and the impact of control measures at different types of sites were evaluated. The results showed that after implementing epidemic control measures, the particulate matter concentration in Wuhan decreased significantly. Based on the calculation of two observation networks (89 stations), the change rates of PM2.5 and PM10 concentrations were −23.44% and −32.95%, respectively, but the O₃ concentration increased significantly, at a rate of 55.22%, 10.6% higher than that of the state-controlled sites. In terms of the spatial distribution, the particulate matter concentration is higher in the north and lower in the south but decreases more at stations in south Wuhan, which is related to the increased frequency of southerly wind, resulting in more control measures affecting the downwind area. The decrease in NO₂ concentration at the stations in the south is greater, while the O₃ concentration increases. The reason is that as the NO concentration decreases greatly, the titration reaction weakens, and the meteorological conditions are also conducive to continuously accumulating O₃ and maintaining a high concentration of O₃. From the difference in concentration change among different types of stations, the epidemic control measures have the greatest impact on secondary pollutants at traffic and industrial stations. The concentration change rates calculated based on the two observation networks exceed those of state-controlled stations. The change rates of PM2.5 and O₃ are 6% and 18% lower at state-controlled stations compared to traffic stations, respectively. The primary pollutants, SO₂ and CO, show small concentration changes at rates of −6.10% and −5.61%, respectively, possibly because key emission sources were not shut down during the epidemic. Among the six conventional pollutants, the NO₂ concentration changed the most, decreasing by −55.26%, which is directly related to traffic control. Comparing and analyzing the aerosol component concentrations in the same periods in 2019 and 2020 reveals that during the epidemic control period in 2020, the \mathrmN\mathrmO_3^- / \mathrmS\mathrmO_4^2- ratio decreased closer to 1, and the OC/EC ratio increased to 6.07, indicating a decrease in the proportion of mobile sources, such as automobile exhaust, and an increase in the impact of coal combustion.