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Halogen chemistry is a central element of tropospheric ozone depletion events (ODEs) during polar spring. Key processes such as source mechanisms that produce reactive halogen species, their transport, and interhalogen interactions as well as the influence of the quickly changing climate, however, remain in the centre of Arctic research.We deployed a Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument in Utqiagvik (formerly Barrow), Alaska, in December 2023. First results from measurements performed between March and May 2024 show that this period exhibits active halogen chemistry with many episodes of enhanced bromine monoxide coinciding with strongly reduced ozone concentrations. Further, analysis results of chlorine monoxide are presented. Additional Multi-AXis (MAX-) DOAS observations have been conducted since the beginning of April 2024.Comparison to data from the instrument’s previous deployment at the German research station Neumayer, Antarctica (Nasse, 2019), indicates differences in the prevailing atmospheric conditions and trace gas amounts between both hemispheres which will be discussed in detail. 

Abstract. The science guiding the EUREC4A campaign and its measurements is presented. EUREC4A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement.