Black carbon (BC) is an important short-lived climate forcer that contributes to Arctic warming, yet observations of BC in the Arctic marine boundary layer remain sparse. Here we report shipborne measurements of refractory BC (rBC) mass and particle mixing state during a cruise from southeast to west Greenland in May–June 2022. Using a single-particle soot photometer (SP2XR) and transmission electron microscopy (TEM), we characterize both rBC concentrations and individual-particle morphology, representing one of the first ship-based SP2XR datasets in the Arctic marine boundary layer. rBC concentrations were highest in the Greenland coastal region (2.4 ng m⁻³), compared with the open ocean (1.7 ng m⁻³) and the marginal ice zone (0.6 ng m⁻³). Mixing-state analysis shows that 54% of soot particles in coastal air masses were externally mixed, consistent with relatively fresh emissions, whereas most soot particles over the open sea and sea-ice regions were internally mixed, indicative of aged, transported aerosol. FLEXPAER modelling suggests BC enhancements along the Greenland coast. Our data indicate that Greenland acts as a regional source of BC to the Arctic marine boundary layer in late spring and early summer, when mid-latitude transport is weak.

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