Aerosol composition shifts in the High Arctic during the sea ice melt onset (ARTofMELT2023)

During the ARTofMELT 2023 expedition (8 May–14 June) aboard the icebreaker Oden, we investigated the High Arctic aerosol system across the transition from late spring into sea-ice melt onset. Submicron aerosol properties were characterized using high-resolution soot particle aerosol mass spectrometry (SP-AMS; chemical composition, organic fingerprints and elemental ratios), complemented by measurements of equivalent black carbon (eBC), particle size distributions, back-trajectory analysis, and size-segregated offline analyses by means of ion chromatography and total carbon analysis. We identified nine regimes, spanning Arctic-confined conditions, warm-air intrusions, and fog sampling.

Campaign-average submicron particulate matter (PM1) was sulfate-dominated (66% sulfate, 29% organic aerosol OA, minor nitrate, ammonium and chloride; eBC ~1%), but approaching the melt onset variability shifted from accumulation-mode, sulfate-rich background conditions to Aitken-mode dominated aerosols with higher relative contribution from OA. The analysis of the OA fragmentation patterns showed a persistently oxidized background, repeatedly perturbed by transport and fog, with coherent shifts toward fresher material (lower f44 and O:C, higher H:C) coinciding with Aitken-mode dominance. Size-resolved total carbon (TC) measurements (<4 µm) indicate that TC was concentrated at diameters below ~0.15 µm, and its loading increased in air masses that travelled over open water. Toward melt onset, we observed increases in methanesulfonic acid (MSA) concentration and non-sea-salt sulfate, consistent with a stronger relative contribution from marine/organosulfur compounds. Overall, the depletion of accumulation mode particles during the early melt season favors CCN-relevant Aitken-mode carbon in the High Arctic.