Deep Convective Trace Gas Transport to the UTLS: Highlighting Remote Sensing and Modeling Challenges With ACCLIP Campaign In Situ CO Observations

Abstract

China is a major source of anthropogenic emissions, with important implications for chemical composition and aerosol processes in the upper troposphere and lower stratosphere (UTLS), particularly within the Asian Summer Monsoon (ASM) anticyclone. Carbon monoxide (CO) is a robust tracer of anthropogenic influence and is routinely observed by remote platforms. In the ASM region, the GEOS‐FP forecast system predicts frequent, rapid convective transport of boundary‐layer CO into the UTLS. During the 2022 Asian Summer Monsoon Chemical and Climate Impact Project (ACCLIP), five in‐situ spectrometers aboard two coordinated research aircraft provided some of the first vertically resolved UTLS CO measurements in the ASM region, providing generally good agreement with forecast abundances. On 19 August 2022, exceptionally enhanced UTLS CO, exceeding 325 ppb, was recorded by both ACCLIP research aircraft over the Yellow Sea west of Korea, exceeding GEOS‐FP predictions and producing a pronounced “C‐shaped” vertical profile indicative of strong convective outflow. None of the satellite profile products examined (MLS, MOPITT, AIRS, CrIS, or IASI) captured the magnitude or vertical structure of this enhancement, highlighting limitations in remote‐sensing sensitivity to sharp UTLS gradients. Simulations from global models with sophisticated chemistry schemes, GEOS‐GOCART and CESM2‐WACCM, attribute the sampled plume to a local convective outbreak approximately 12 hr before in‐situ sampling. GEOS‐GOCART better reproduced the observed profile, while CESM2‐WACCM simulated weaker lofting; however, both underestimated the magnitude of CO in the UTLS. This case underscores challenges in validating localized deep‐convective transport and demonstrates the continued need for high‐resolution (spatial and temporal) in‐situ UTLS observations.