A wavelet analysis of multiday extreme ozone and its precursors in Mexico city during 2015-2016

During the past decades, many authors have studied extreme ozone (O3) events on successive days in several cities around the world, where extreme pollution concentrations are considered as values exceeding air quality standards. These multiday episodes are caused by different variables: weather conditions, pollution precursors life times and air pollution transport. However, a complete characterization of the temporal behavior of multiday extreme O3 episodes is still lacking. In the present paper, we used the Haar wavelet transform to study the period (T in days) of multiday extreme O3 episodes in Mexico city during 2015-2016, when 10 ozone contingencies occurred and changes in driving restrictions were implemented. In addition, we studied the temporal correlations between extreme O3 and extreme: nitrogen dioxide (NO2), carbon monoxide (CO) and ultraviolet B radiation (UVB) for a broad range of time scales by means of the Haar Wavelet cross-correlation method. The results show that multiday O3 episodes mainly exhibit periods of T>4 days, while NO2 and CO show multiday episodes comprising principally periods of T>2 days. The cross correlation analysis reveals that CO and NO2 are temporal anti-correlated with O3 for daily variations T<1. However, NO2 and CO are strongly and moderately correlated with O3 for T>4, respectively, indicating that NO2, CO and O3 are correlated in a multi-temporal clustered form.