Influence of Meteorological Conditions on the Mass Concentration of Pollutants in Sichuan Basin Cities during the “COVID-19” Pandemic in the First Half of 2020

The mass concentration changes of air pollutants are mainly controlled through changes in meteorological conditions and anthropogenic emissions. In the first half of 2020, under the context of the impact of the “COVID-19” pandemic, the mass concentrations of the six major pollutants in the Sichuan Basin have changed compared with those in previous years. To distinguish these changes from the impact of meteorological conditions and anthropogenic emission changes, the influence of meteorological condition changes on the mass concentration of pollutants in the Sichuan Basin is analyzed. The results show that the overall air quality from January to June 2020 was better than that from January to June 2019. Among the meteorological parameters that mainly affect the concentration of pollutants, the average wind speed in the Chengdu Plain and northeastern Sichuan was the same as in previous years, whereas the average wind speed in southern Sichuan was lower than that in previous years. Starting from March 2020, the total precipitation in various cities in the basin has been significantly lower. The lowest precipitation in May was the most serious. Correspondingly, the relative humidity was lowest in May. The average temperature in 2020 was higher than that in 2019; in particular, the average temperature in May and June was approximately 5°C higher. Starting from March or April, the total monthly hours of sunshine, which reflect the amount of solar radiation, were significantly longer than those in previous years. The numerical simulation results obtained using fixed pollution sources and only changing the initial meteorological conditions show that PM2.5 and PM10 were similar. Moreover, the concentrations in January and February were lower than those in 2019, the concentrations in March and April were higher than those in previous years, and the concentrations in May and June were lower than those in previous years. The main deviation area was in southern Sichuan. SO₂ is likely to spread in January and February but tends not to spread in March in northeastern Sichuan. The meteorological condition of NO₂ was relatively unfavorable in February, and the meteorological condition of CO was relatively unfavorable in April. Furthermore, the meteorological conditions in the basin in April involved ozone diffusion, and the meteorological conditions in the basin in May and June played a strong role in promoting the O₃ pollution process. Quantitative analysis of the contribution rate of pollutant concentration in each city determined that, because of the impact of the “COVID-19” pandemic, the contribution of particulate matter and gaseous pollutants other than ozone was negative in January to April, and the resumption of work and production in May and June increased the concentration of pollutants in the basin. Because of the considerably increased anthropogenic emissions and unfavorable weather conditions, serious O₃ pollution is observed in the basin.