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Deep convection in the Asian summer monsoon is a significant transport process for lifting pollutants from the planetary boundary layer to the tropopause level. This process enables efficient injection into the stratosphere of reactive species such as chlorinated very short-lived substances (Cl-VSLSs) that deplete ozone. Past studies of convective transport associated with the Asian summer monsoon have focused mostly on the south Asian summer monsoon. Airborne observations reported in this work identify the East Asian summer monsoon convection as an effective transport pathway that carried record-breaking levels of ozone-depleting Cl-VSLSs (mean organic chlorine from these VSLSs ~500 ppt) to the base of the stratosphere. These unique observations show total organic chlorine from VSLSs in the lower stratosphere over the Asian monsoon tropopause to be more than twice that previously reported over the tropical tropopause. Considering the recently observed increase in Cl-VSLS emissions and the ongoing strengthening of the East Asian summer monsoon under global warming, our results highlight that a reevaluation of the contribution of Cl-VSLS injection via the Asian monsoon to the total stratospheric chlorine budget is warranted. 

Abstract Rapid growing emissions of dichloromethane (CH₂Cl₂), a chlorinated very‐short‐lived substance (Cl‐VSLS) and an ozone depleting substance (ODS), has raised concerns as this increase offset a part of the stratospheric chlorine (Cl) reduction due to decreasing long‐lived ODSs. We have combined simulations of the two most abundant Cl‐VSLSs, CH₂Cl₂and chloroform (CHCl₃) using the NASA GEOS Chemistry Climate Model (GEOSCCM) with Asian Summer Monsoon Chemical and CLimate Impact Project aircraft observations to examine transport of Cl‐VSLSs to the stratosphere and to assess their contribution to total stratospheric Cl. With ongoing large emissions (total ∼1,500 Gg yr⁻¹), Cl‐VSLSs add about 100 ppt Cl to the stratosphere between 2020 and 2022. The Asian Summer Monsoon plays a primary role in the troposphere‐to‐stratosphere transport of Cl‐VSLSs and delivers double the amount to the stratosphere, about 200 ppt Cl in August 2022. The overall Cl‐VSLSs impact on stratospheric chlorine (∼3.3%) and ozone (∼1 DU) remain small. 

Abstract Chemistry Climate Models (CCMs) are essential tools for characterizing and predicting the role of atmospheric composition and chemistry in Earth's climate system. This study demonstrates the use of airborne in situ observations to diagnose the representation of chemical composition and transport by CCMs. Process‐based diagnostics using dynamical and chemical coordinates are presented which minimize the spatial and temporal sampling differences between airborne in situ measurements and CCM grid points. The chosen process is the chemical impact of the Asian summer monsoon (ASM), where deep convection serves as a rapid transport pathway for surface emissions to reach the upper troposphere and lower stratosphere (UTLS). We examine two CCM configurations for their representation of the ASM UTLS using a set of airborne observations from south Asia. The diagnostics reveal good model performance at representing tropospheric tracer distribution throughout the troposphere and lower stratosphere, and excellent representation of chemical aging in the lower stratosphere when chemical loss is dominated by photolysis. Identified model limitations include the use of zonally averaged mole fraction boundary conditions for species with sufficiently short tropospheric lifetimes, which may obscure enhanced regional emissions sources. Overall, the diagnostics underscore the skill of current‐generation models at representing pollution transport from the boundary layer to the stratosphere via the ASM mechanism, and demonstrate the strength of airborne in situ observations toward characterizing this representation.