Quantifying the Vertical Confinement Boundary of the Asian Summer Monsoon Anticyclone in UTLS Atmospheric Composition

Based on long-term observation from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer during the summers of 2004–2020, this study systematically reveals the confinement effect and the vertical confinement boundary of the Asian Summer Monsoon Anticyclone (ASMA) on the atmospheric composition in the upper troposphere and lower stratosphere (UTLS) . The observations show that six tropospheric tracers (CO, HCN, C₂H₆, C₂H₂, CH₃Cl, and OCS) with different photochemical lifetimes exhibit continuous enhancements between 12 and 20 km (with peak values at 13–15 km) in the ASMA UTLS region. Short-lived species (e.g., C₂H₂ and CH₃Cl) have display enhanced altitudes (~14 km) that are directly associated with deep convective transport, indicating that deep convective outflow is mainly concentrated near 14 km. However, medium- to long-lived species (e.g., CO, HCN, C₂H₆, and OCS) still show residual enhancements at 16 km, indicating the persistent trapping effect of the ASMA circulation on pollutants. In addition, the stratospheric tracers O₃ and HCl decay synchronously in the ASMA region, showing tropical-like characteristics. Further analysis of the coefficient-of-variation profiles across multiple species quantitatively confirms that the vertical confinement boundary of the ASMA is~20 km. The dynamic structure of the AMSA suppresses horizontal diffusion while enhancing its upward motion, creating a “chimney effect” that extends from the lower troposphere to the stratosphere. These results provide key constraints for understanding cross-tropopause transport of pollutants in the ASMA region.