Source apportionment of PM2.5 and sulfate formation during the COVID-19 lockdown in a coastal city of southeast China.

Revealing the changes in chemical compositions and sources of PM2.5 is important for understanding aerosol chemistry and emission control strategies. High time-resolved characterization of water-soluble inorganic ions, elements, organic carbon (OC), and elemental carbon (EC) in PM2.5 was conducted in a coastal city of southeast China during the COVID-19 pandemic. The results showed that the average concentration of PM2.5 during the city lockdown (CLD) decreased from 46.2 µg m-3 to 24.4 µg m-3, lower than the same period in 2019 (PM2.5: 37.1 µg m-3). Concentrations of other air pollutants, such as SO2, NO2, PM10, OC, EC, and BC, were also decreased by 27.3%-67.8% during the CLD, whereas O3 increased by 28.1%. Although SO2 decreased from 4.94 µg m-3to 1.59 µg m-3 during the CLD, the concentration of SO42- (6.63 µg m-3) was comparable to that (5.47 µg m-3) during the non-lockdown period, which were attributed to the increase (16.0%) of sulfate oxidation rate (SOR). Ox (O3+NO2) was positively correlated with SO42-, suggesting the impacts of photochemical oxidation. A good correlation (R2 = 0.557) of SO42- and Fe and Mn was found, indicating the transition-metal ion catalyzed oxidation. Based on positive matrix factorization (PMF) analysis, the contribution of secondary formation to PM2.5 increased during the epidemic period, consisting with the increase of secondary organic carbon (SOC), while other primary sources including traffic, dust, and industry significantly decreased by 9%, 8.5%, and 8%, respectively. This study highlighted the comprehensive and nonlinear response of chemical compositions and formation mechanisms of PM2.5 to anthropogenic emissions control under relatively clean conditions.

The change of atmospheric ozone formation sensitivity in Fujian Province based on OMI satellite data during the period of COVID-19

Based on the OMI satellite data, the characteristics of atmospheric ozone sensitivity in Fujian province and its nice municipalities during the period of COVID-19 epidemic were assessed with HCHO and NO2 vertical column densities as proxies for ozone sensitivity. The results showed that Fujian Province was dominated by VOCs-limited regime before the pandemic with the controlled area proportion of 46.5%. The other two regimes: NOx-VOCs-limited regime and NOx-limited regime, controlled 25.0% and 28.5% of the area, respectively. The area proportion controlled by VOXs-limited regime was highest in Xiamen and lowest in Nanping. During the period with strict pandemic control policies, VOXs-limited regime, NOx-VOCs-limited regime and NOx-limited regime controlled 29.5%, 21.1%, and 49.4% of the area, respectively. The area proportion controlled by NOx-limited regime was highest in Ningde and lowest in Putian. During the stable period, VOXs-limited regime, NOx-VOCs-limited regime and NOx-limited regime controlled 23.1%, 29.1%, and 47.8% of the area, respectively. NOx-limited regime was the dominant regime with highest area proportion controlled in Nanping and lowest area proportion in Xiamen. Compared with the before pandemic period, the area proportion of Xiamen controlled by VOCs-limited regime was obviously reduced（38.1% less） during the period with strict control policies, and the lowest reduction was in Sanming with a moderate decrease of 7.9%. According to the conversion results, Putian, Quanzhou, and Xiamen were categorized into the first city group where changes in ozone sensitivity were jointly influenced by its precursors including formaldehyde（HCHO） and nitrogen dioxide（NO2）, while other cities could be categorized into the second group where ozone sensitivities were mainly affected by NO2 column concentrations. Therefore, effective strategies for ozone reduction would be more complex in the first group of cities.