Multifractal property change of NO_x and O₃ variations in port area in responding to COVID-19 lockdown

The air quality in the urban area has been verified to undergo significant changes during COVID-19. However, the inner dynamics of air quality in port area during COVID-19 lockdown period are still not well understood. Besides, owing to the non-stationary and nonlinear features of air pollutants, simple statistical methods are difficult to accurately describe the cross-correlation between NO_x and O₃ in responding to COVID-19. Fortunately, multifractality theory is widely used to quantitatively describe the nonlinear evolution of a complicated system and the multiscale characteristics of physical quantities. Given the COVID-19 scenario, assessing the multifractal characteristics of pollutants before and during the pandemic in the Hong Kong port can indicate the effectiveness of lockdown policy and provide guidance for reducing port pollution. In this study, the multifractal detrended fluctuation analysis (MF-DFA) and multifractal detrended cross-correlation analysis (MF-DCCA) were used to investigate the multifractal properties of NO_x and O₃ before and during COVID-19 in port. Furthermore, the impact of lockdown measures on the emission reduction efficiency of NO_x and O₃ was investigated. The results shown that the different multifractal characteristics was determined during the pre-lockdown and lockdown period. In contrast to pre-lockdown, the cross-correlation behaviors between NO_x and O₃ during lockdown period were stronger in port station. It is deduced that on port station, the reduction of human activities presents the notable impact on the decrease of NO_x emission and then results in the direct impact on the production of O₃ during the photochemical reactions.