Long-term IASI observations of upper tropospheric CO in the Asian Summer Monsoon region: trends and eddy shedding signatures

The Asian Summer Monsoon (ASM) acts as a key mechanism for the vertical transport of surface pollutants into the upper troposphere (UT), which can have far-reaching impacts on air quality and climate at regional to global scales. We analyze 16 years (2008–2023) of carbon monoxide (CO) observations from the Infrared Atmospheric Sounding Interferometer (IASI) to investigate UT variabilities over the ASM region. Seasonal cycles, long-term trends, and dynamic features such as eddy shedding are examined to assess pollution transport within and beyond the ASM anticyclone. IASI observations are sensitive in the 12–15 km range, showing reasonable agreement with in situ aircraft observations from the 2022 Asian Summer Monsoon Chemical and Climate Impact Project (ACCLIP), with a mean bias of −16.1% ± 11.4%. A significant decline in CO concentrations is observed over the ASM region over 2008–2023, with trends of −5.4% per decade in total column and −13.3% per decade in the UT, consistent with reduced anthropogenic surface emissions in Asia over this period. CO patterns from 2008 to 2023 reveal persistent enhancements aligned with the Tibetan Plateau and Western Pacific anticyclone modes, confirming for the first time IASI's ability to capture the ASM dynamical signatures on climatology-relevant time scales. A case study of an eddy shedding event shows long-range transport of ASM-sourced CO across the Pacific toward North America. These results highlight the added-value of IASI observations as a long-term indicator of ASM-driven pollution in the UT and its significance in both regional and intercontinental transports.