Effect analysis of meteorological conditions on air quality during the winter COVID-19 lockdown in Beijing

ABSTRACT

The influence of meteorological conditions on the pollution processes was investigated in this study by analyzing the changes of air quality as well as the characteristics of two persistent heavy pollution episodes during the Coronavirus Disea se 2019（COVID-19） prevention（January 24 to February 29） of 2020 winter compared with the same period of 2015～2019. Cold air intensity in 2020 winter was weaker with the cold surges frequency decreased by 50%. Air temperature was 0.73℃ higher, and wind speed and mixed layer height were 17.8% and 32.5% lower, respectively. Relative humidity and dew point temperature increased by 60. 9% and 48.1%, respectively. Northerly wind frequency reduced 7.5% while both of southerly and easterly wind increased 6.0%. As shown above, all meteorological conditions in 2020 winter were significantly more favorable for air pollution than the same historical period. Moreover, two heavy pollution episodes（January 24～29 and February 8～14） lasted for 59 and 75 hours were analyzed. At the cumulative stage, regional transport that can be divided into east and south channel greatly affected PM2.5, with the contribution of 70% and 58% for two episodes. By contrast, the contribution of local pollution was 67% and 48%, respectively, indicating the increased proportion of hygroscopic growth and secondary generation in the maintenance and aggravation stages. Under the meteorological background of "high humidity and high atmospheric stability", the combined effects of atmospheric vertical dynamics and horizontal convergence accumulated PM2.5 and water vapor in Beijing plain and prevented them from spreading beyond the boundary layer. Further bidirectional feedback between increased pollutants and meteorological factor s in stable boundary layer resulting in aggravation of pollution. According to EMI index, meteorological conditions during the epi demic prevention in 2020 winter caused an increase of 70.1% in PM2.5 concentration compared to pre-COVID-19. Emissions reduction caused by emergency measures for COVID-19 lockdown offset 53% of the adverse impact induced by meteorological conditions. As for the two episodes in 2020 winter, EMI was 26.9% and 19.7% larger than the average of other nine episodes in the correspond ing period of 2015～2019, and PM2.5 concentration was basically unchanged or slightly reduced. Overall, if the current social emission level is not changed, emission reduction caused by city blockade under special circumstances can only partially reduce the pollution concentration, however, cannot completely offset the adverse impact of meteorological conditions.