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Abstract Frequency and intensity of warm and moist air-mass intrusions into the Arctic have increased over the past decades and have been related to sea ice melt. During our year-long expedition in the remote central Arctic Ocean, a record-breaking increase in temperature, moisture and downwelling-longwave radiation was observed in mid-April 2020, during an air-mass intrusion carrying air pollutants from northern Eurasia. The two-day intrusion, caused drastic changes in the aerosol size distribution, chemical composition and particle hygroscopicity. Here we show how the intrusion transformed the Arctic from a remote low-particle environment to an area comparable to a central-European urban setting. Additionally, the intrusion resulted in an explosive increase in cloud condensation nuclei, which can have direct effects on Arctic clouds’ radiation, their precipitation patterns, and their lifetime. Thus, unless prompt actions to significantly reduce emissions in the source regions are taken, such intrusion events are expected to continue to affect the Arctic climate. 

Abstract. The Asian monsoon anticyclone (AMA) represents one of thewettest regions in the lower stratosphere (LS) and is a key contributor tothe global annual maximum in LS water vapour. While the AMA wet pool islinked with persistent convection in the region and horizontal confinementof the anticyclone, there remain ambiguities regarding the role oftropopause-overshooting convection in maintaining the regional LS watervapour maximum. This study tackles this issue using a unique set ofobservations from aboard the high-altitude M55-Geophysica aircraft deployedin Nepal in summer 2017 within the EU StratoClim project. We use acombination of airborne measurements (water vapour, ice water, waterisotopes, cloud backscatter) together with ensemble trajectory modellingcoupled with satellite observations to characterize the processescontrolling water vapour and clouds in the confined lower stratosphere (CLS)of the AMA. Our analysis puts in evidence the dual role of overshootingconvection, which may lead to hydration or dehydration depending on thesynoptic-scale tropopause temperatures in the AMA. We show that all of theobserved CLS water vapour enhancements are traceable to convective eventswithin the AMA and furthermore bear an isotopic signature of the overshootingprocess. A surprising result is that the plumes of moist air with mixingratios nearly twice the background level can persist for weeks whilstrecirculating within the anticyclone, without being subject to irreversibledehydration through ice settling. Our findings highlight the importance ofconvection and recirculation within the AMA for the transport of water into thestratosphere. 

Abstract Unlike bromine, the effect of iodine chemistry on the Arctic surface ozone budget is poorly constrained. We present ship-based measurements of halogen oxides in the high Arctic boundary layer from the sunlit period of March to October 2020 and show that iodine enhances springtime tropospheric ozone depletion. We find that chemical reactions between iodine and ozone are the second highest contributor to ozone loss over the study period, after ozone photolysis-initiated loss and ahead of bromine. 

With the Arctic rapidly changing, the needs to observe, understand, and model the changes are essential. To support these needs, an annual cycle of observations of atmospheric properties, processes, and interactions were made while drifting with the sea ice across the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from October 2019 to September 2020. An international team designed and implemented the comprehensive program to document and characterize all aspects of the Arctic atmospheric system in unprecedented detail, using a variety of approaches, and across multiple scales. These measurements were coordinated with other observational teams to explore cross-cutting and coupled interactions with the Arctic Ocean, sea ice, and ecosystem through a variety of physical and biogeochemical processes. This overview outlines the breadth and complexity of the atmospheric research program, which was organized into 4 subgroups: atmospheric state, clouds and precipitation, gases and aerosols, and energy budgets. Atmospheric variability over the annual cycle revealed important influences from a persistent large-scale winter circulation pattern, leading to some storms with pressure and winds that were outside the interquartile range of past conditions suggested by long-term reanalysis. Similarly, the MOSAiC location was warmer and wetter in summer than the reanalysis climatology, in part due to its close proximity to the sea ice edge. The comprehensiveness of the observational program for characterizing and analyzing atmospheric phenomena is demonstrated via a winter case study examining air mass transitions and a summer case study examining vertical atmospheric evolution. Overall, the MOSAiC atmospheric program successfully met its objectives and was the most comprehensive atmospheric measurement program to date conducted over the Arctic sea ice. The obtained data will support a broad range of coupled-system scientific research and provide an important foundation for advancing multiscale modeling capabilities in the Arctic.