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 (NO (2)) 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 NO (2) 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 NO (2) 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 ~28 days.