Vertical Characterization and Transport Dynamics of UTLS Aerosols over Hubei: A Multiyear Integrated Analysis using CALIOP and MERRA-2

Abstract. The upper troposphere and lower stratosphere (UTLS) plays a vital role in the global climate system. Central China, particularly Hubei Province, is located directly downstream of the Tibetan Plateau, making it a key region for observing long-range aerosol transport. In this study, the vertical structure and transport mechanisms of UTLS aerosols over Hubei are investigated using a multi-year (2016-2018) satellite dataset. We applied rigorous Cloud-Aerosol Discrimination (CAD) scores and quality control procedures to nighttime CALIOP profiles to minimize cirrus cloud contamination. Our results show that the UTLS background over this region exhibits low aerosol loading during non-monsoon seasons. However, a substantial aerosol enhancement occurs in summer. The monthly mean extinction coefficient at 14-16 km reaches a peak of 8.05 × 10-3 km-1 in August, with a particulate depolarization ratio of ~0.25, indicating the presence of non-spherical particles related to the Asian Tropopause Aerosol Layer (ATAL). To investigate the physical drivers of this seasonal variation, we integrated MERRA-2 meteorological fields and HYSPLIT backward trajectories. The analysis reveals a dual transport mechanism: First, intense local deep convection driven by the East Asian Summer Monsoon (EASM) vertically pumps boundary-layer pollutants into the upper atmosphere. Second, the Asian Summer Monsoon Anticyclone (ASMA) and the westerly jet stream advect aged aerosols horizontally from the Tibetan Plateau to Central China. These findings provide direct observational evidence of how regional monsoon systems synergistically modulate stratospheric aerosol loading.