Warren P. Smith

The level of deep convective transport (LCT) is a critical quantity for determining the impact of deep convection on the composition of the upper troposphere and lower stratosphere (UTLS) within the Asian summer monsoon anticyclone (ASMA). Here, the LCT is estimated using high resolution balloon-borne profile measurements of water vapor obtained over Lhasa and Kunming during eleven ASM seasons. The signature of deep convection is revealed by a modal profile of near 100% saturation over ice (RHi) derived from frost-point temperature and air temperature. Analysis shows that the LCT is ~16 km (359–376 K) over Lhasa and reaches the tropopause (~17 km, 372–391 K) over Kunming. Deep convection frequently reaches above the center of the ASM anticyclone (150 hPa or 14 km), allowing upward transport into the stratosphere, whereas earlier satellite studies suggested a much lower top altitude for deep ASM convection.

The Asian Summer Monsoon (ASM) lofts air from polluted boundary layer regions to the upper-troposphere and lower-stratosphere. High concentrations of aerosols have been observed in ASM outflow, which influence climate by interacting with radiation and clouds and affecting atmospheric composition. Prior observations indicate that small particles form in ASM convective outflow. Laboratory and modelling studies hypothesise the importance of lofted ammonia in this process and growth of the particles to sizes where they can interact with radiation and clouds. Here we combine in-situ observation in ASM convective outflow with trajectory and Earth-system modelling to show the likely role of pollution in enhancing ASM outflow new particle formation. We show that the particles grow to sizes where they can interact with clouds and radiation, and are transported over large areas within the troposphere and stratosphere.

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.

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.

Anthropogenic emissions over China have recently declined due to environmental actions. This work estimates the sensitivity of sulfate aerosol () concentration to the amount of emissions reduction over China from 2010 to 2020 using an Earth system model with two different aerosol representations. We find that a larger rate of emissions decline over 2010–2020 from an updated Chinese emission inventory leads to improvement in modeled and concentrations when evaluated with targeted airborne observations from the Asian summer monsoon region from the 2022 Asian summer monsoon Chemical and Climate Impact Project. Updated Chinese emissions reduce concentration by >20% at 200 hPa over the North Pacific, and by >7% at 100 hPa throughout the tropics. These reductions result in an increase to global net instantaneous radiative forcing of 0.10–0.15 W by 2020, with regional effects up to 6 times greater.

The Asian summer monsoon Chemical and Climate Impact Project is centered on a large airborne field campaign investigating the role the Asian Summer Monsoon (ASM) plays in changing the atmospheric composition in the upper troposphere and lower stratosphere. This campaign, conducted in Summer 2022 using two research aircraft, has obtained the most comprehensive data set for chemical composition, including hundreds of trace gas species as well as the physical and chemical properties of aerosols in the ASM convective outflow over East Asia and the Western Pacific. These data show that the ASM deep convection is creating a chemically distinct layer near the tropopause that has large anthropogenic influences. This overview provides a summary of the project, including the scientific objectives, dynamical, and meteorological conditions of the campaign season, key elements of the campaign operations, selected scientific highlights, and ongoing research.

The Asian Summer Monsoon (ASM) acts as a key mechanism for 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 17 years (2007–2023) 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 dynamical 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 -14.5% per decade in the UT, consistent with reduced anthropogenic surface emissions in Asia over this period. CO patterns from 2007 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 towards 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 transport.

Anthropogenic emissions over China have recently declined due to environmental actions. This work quantifies the sensitivity of sulfate aerosol (SO4) concentration in the upper troposphere and lower stratosphere (UTLS) to the reduction of SO2 emissions over China from 2010-2020 using an Earth system model (ESM) with two different aerosol representations. We find that a larger rate of SO2 emissions decline over from an updated Chinese emission inventory leads to improvement in modeled SO2 and SO4 concentrations when evaluated with airborne observations. The updated Chinese SO2 emissions reduce SO4 concentration by >20% at 200 hPa over the northern Pacific Ocean, and by >7% at 100 hPa throughout the global tropics. These SO4 reductions result in an estimated increase to global net radiative forcing of ~0.10-0.15 W m-2 by 2020, with local effects ~5-6 times greater over the North Pacific Ocean where the SO4 reductions are greatest.

New particle formation (NPF) in the upper troposphere and lower stratosphere (UTLS) has been suggested to contribute to cloud condensation nuclei in the troposphere, as well as for maintaining the stratospheric aerosol layer over the Asian summer monsoon (ASM). However, the fate of newly formed particles and their potential impact on climate are largely unknown. Here we use in situ aircraft measurements of aerosol size distributions collected in the UTLS over the eastern ASM region, combined with Lagrangian trajectory modeling and satellite‐derived convection, to show that NPF events primarily occur downstream of convection in the upper troposphere (below the tropopause) between 350 and 360 K potential temperature levels. While the majority of air parcels containing NPF events descend into the lower troposphere, approximately 20% of NPF event parcels ascend into the stratosphere and thereby may contribute as a source of material to the stratospheric aerosol layer.