Ozone downward flux revealed by high-resolution differential absorption lidar over Tibet during stratosphere-troposphere exchange

Abstract

This study characterizes a prominent ozone intrusion event driven by stratosphere-to-troposphere exchange (STE) that occurred on 18–19 October 2017. The analysis is enabled by high spatiotemporal resolution ozone profile observations from the USTC Ozone Lidar deployed at Yangbajing, Tibet (29° N, 99° E). The high-spatiotemporal-resolution lidar observations resolve a series of short-lived, rapid ozone descent episodes during the STE event. Temperature profiles independently retrieved from the simultaneously measured 387 nm N 2 Raman signal further reveal a close temporal correspondence between these ozone descent episodes and temperature variations in the tropopause region, with temperature gradients exceeding 8 K km −1 for six of the seven identified episodes. Based on Wei's flux diagnostic and a sensitivity test of PV-based dynamical tropopause thresholds, 3 PVU is adopted as the representative threshold for this event. ECMWF Reanalysis v5 (ERA5) is then used to characterize the spatiotemporal evolution of cross-tropopause mass fluxes over 25–28° N, 95–99° E, showing that the ozone variations observed by the lidar were modulated by gravity waves associated with the tropopause fold. In addition, lidar-measured ozone profiles are incorporated into a cross-tropopause ozone flux calculation framework, yielding an instantaneous peak STE ozone flux of about 3–4×10-10 kgs-1m-2, slightly higher than the corresponding ERA5 value, while maintaining strong agreement in overall flux magnitude and temporal evolution throughout the event. These results show that high-resolution vertical ozone observations and Raman-retrieved temperature profiles from the USTC Ozone Lidar, combined with wind field data, enable accurate quantification of STE-related ozone fluxes. This approach facilitates in-depth investigation of coupled atmospheric composition and dynamical processes.