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12- Coupled multifractal methods to reveal changes in nitrogen dioxide and tropospheric ozone concentrations during the COVID-19 lockdown [Pavón-Domínguez and Plocoste (2021)]

Due to the COVID-19 pandemic, the effects of confinement on the level of air pollution are undeniable. In this study, the city of Seville (Spain) is used as an example of a high traffic location where nitrogen dioxide (NO2) is significantly reduced (41%) while tropospheric ozone (O3) does not show changes. significant. To assess the existence of differences in O3 behavior that are not detected by statistical procedures, a multifractal approach was used to assess the coupled scaling relationship between NO2 and O3 during the 2020 lockdown compared to a reference period (2017-2019). Therefore, two main coupled multifractal methods were used: trendless multifractal cross-correlation and joint multifractal analysis. Although the cross-correlation analysis does not detect a difference between the cross-fluctuations of NO2 and O3, the joint multifractal analysis, based on the partition function and the method of moments, detects a loss of variability of O3 during the confinement. This leads to a loss of the multifractal characteristic of O3 time series. The drastic reduction of primary pollutants during confinement would be responsible for the tendency to monofractality of O3. These differences were observed for a time scale of 80 min to ~28 days.