Coupled multifractal methods to reveal changes in nitrogen dioxide and tropospheric ozone concentrations during the COVID-19 lockdown

Due to COVID-19 pandemic, the lockdown effects on air pollution level are undeniable. Several studies around the world have detected an uneven behaviour in tropospheric ozone (O-3) concentrations. In this work, Seville (Spain) is used as example of faced to traffic place in which the nitrogen dioxide (NO2) is drastically reduced (41%) while O-3 has no significant changes. In order to evaluate the existence of differences in O-3 behaviour that is not detected by statistical procedures, a multifractal approach was used to assess the coupled scale relationship between NO2 and O-3 during the 2020 lockdown against a period reference (2017-2019). For this purpose, the two main coupled multifractal method were employed: multifractal detrended cross-correlation and joint multifractal analysis. While cross-correlation analysis did not detect differences between the cross-correlated fluctuations of NO2 and O-3 in the periods analysed, the joint multifractal analysis, based on the partition function and the method of moments, found a loss of variability in O-3 during the lockdown. This leads to a loss of multifractal characteristic of O-3 time series. The drastically reduction of primary pollutants during the lockdown might be the responsible of the tendency to monofractality in O-3 time series. These differences were found for a wide temporal extent ranging from 80 min to similar to 28 days.