Double core of ozone valley over the Tibetan Plateau and its possible mechanisms

In this study, the three-dimensional structure of the ozone valley (e.g. ozone depletion) over the Tibetan Plateau (TP) in summer (June-August) has been investigated using Microwave Limb Sounder (MLS) ozone (O3) data from 2005 to 2013. A double core structure of the ozone valley over the TP is found with one depletion center in upper stratosphere (10-2 hPa) named the upper core, and another depletion center, named the lower core, is observed in the upper troposphere/lower stratosphere (200-50 hPa). The analysis indicates that the zonal deviation of ozone O3* (O3*=O3-[O3], here [O3] is the zonal mean of O3) at the upper core is nearly −1 DU while its counterpart's deviation at the lower core is nearly -15 DU. Large scale atmospheric circulation and terrain effects play an important role in the ozone valley at the lower core. In contrast, photochemistry reactions of odd chlorine including chlorine atoms (Cl) and chlorine monoxide (ClO) dominate the ozone valley at the upper core. Based on the MLS products, support for a chemical driver at the upper core is based on: (1) the significant diurnal variability of ozone suggests possible photochemistry reactions impacting the ozone valley; (2) the positive center of the ClO zonal deviation (CLO*) and hydrogen chloride (HCl) zonal deviation (HCl*) are found at the bottom of the upper core, which means the higher odd chlorine may accelerate the ozone loss.