New insights in atmospheric methane variability in the Arctic by ship-borne measurements during MOSAiC

The sparse network of Arctic land-based stations results in significant data gaps for atmospheric methane (CH4), particularly in sea-ice covered regions. Ship-based measurements can complement these data, improving understanding of regional and seasonal CH4 variability. This study presents continuous atmospheric ship-borne recordings of CH4 concentration and isotopic composition above the open ocean and sea-ice surface during Leg 4 (June-July 2020) and Leg 5 (August-September 2020) of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in the central Arctic. Our measurements aim to enhance process understanding by identifying local emission sources and refining transport pathway and atmospheric mixing analysis. We compared three contamination-filtering methods and applied the Pollution Detection Algorithm to the raw data. Comparison with nearby land-based stations and their seasonal cycles suggests ship-borne data capture dynamic changes in CH4 sources, sinks, and transport processes, beyond seasonality. To unravel the underlying processes, we identified air mass transport pathways within the atmospheric boundary layer above the Arctic Ocean and their source areas using five-day backward trajectories from the LAGRANTO tool, based on ERA5 wind field data. Our analysis reveals that CH4 variability is driven by air masses predominantly influenced by open ocean and sea-ice-covered regions, with sea-ice dynamics imparting specific modifications along transport pathways. These findings underscore the importance of air mass transport and origin in shaping central Arctic CH4 variability. The study highlights the value of integrating ship-borne CH4 measurements with trajectory analysis to improve process-level understanding and support enhanced regional modelling.