Temporal Variations in the Air Quality Index and the Impact of the COVID-19 Event on Air Quality in Western China

This study investigated the AQI (air quality index) and atmospheric pollutants including PM_(2.5), PM_(10), CO, SO_2, NO_2 and O_3 in Chongqing, Luzhou and Chengdu from 2017 to 2019. In addition, the impacts of the COVID-19 event on the air quality in the three cities in 2020 were compared and discussed. For the combined AQIs for the three cities, in spring, the daily AQIs ranged between 25 and 182 and averaged 72.1. In summer, the daily AQIs ranged between 24 and 206 and averaged 77.5. In autumn, the daily AQIs ranged between 22 and 170 and averaged 61.1, and in winter, the daily AQIs ranged between 28 and 375 and averaged 99.6. The distributions of the six AQI classes in spring were 3%, 94%, 3%, 0%, 0%, and 0%; in summer, they were 11%, 74%, 15%, 0%, 0% and 0%; in autumn, they were 29%, 70%, 1%, 0%, 0%, and 0%, and in winter, they were 1%, 52%, 44%, 3%, 0%, and 0%, respectively. The average AQIs, in order, were Chengdu (85.4) ＞ Chongqing (73.8) ＞ Luzhou (73.2). Both the highest AQIs and PM_(2.5) (as the major indicatory air pollutant) occurred mainly in the low temperature season (January, December, and February), while O_3 was the main air pollutant in June and August when the weather was hot. In February 2020, during the epidemic prevention and control actions taken in response to COVID-19 for the three cities, the combined AQIs for the top five days with the highest AQIs in February 2020 was 79.4, which was 23.6% lower than that from 2017-2019 (AQI = 100.7), and the average concentrations of PM_(2.5), PM_(10), SO_2, CO, and NO_2 were 89.4 μg m^(-3), 106 μg m^(-3), 2.31 ppb, 0.72 ppm, and 12.3 ppb, respectively, and were 17.9%, 30.8%, 83.8%, 19.8%, and 62.1%, lower than those in February 2017-2019. However, the average O_3 concentration (31.8 ppb) in February 2020 rather than decreasing, increased by 6.2%. This is because a lower NO_2 concentration hindered the NO + O_3 reaction and led to increase O_3 concentration in the ambient air.