  As the decrease in human activities resulting from the COVID-19 control
measures had a significant impact on air quality, the epidemic provided an
opportunity to investigate the extent to which air pollution is influenced by
human activities and review existing measures. However, the corresponding
diffusion pattern on a city scale is seldom mentioned at present stage,
therefore, we chose the Cheng-Yu urban agglomeration, which is the largest city
cluster in Southwest China, as our study area during the COVID-19 period, and
attempted to investigate the process of PM2.5 diffusion using a complex network
method. The results displayed that there was an evident external spillover
effect of PM2.5 across all regions, and the PM2.5 spillovers were concentrated
in several cities in the Cheng-Yu urban agglomeration during the lockdown
period, whereas they are more dispersed during the recovery period. The overall
decline in the impact of PM2.5 pollution source areas on receptor areas from a
normal year to the pandemic year, and the intensity of PM2.5 spillover
decreases gradually as the distance from the center increases. The
implementation of the lockdown measures had an impact on both the input and
output patterns of PM2.5 pollution in the region, the input pattern of PM2.5
pollution exhibited higher vulnerability, while the output pattern showed
higher resilience. Additionally, the spillover relationship of PM2.5 pollution
varies between different blocks, with relatively simple spillover relationships
observed during the lockdown period and more complex dynamics during the
recovery period. These findings have highlighted the importance of joint
controls in combating regional air pollution.
