Comment on egusphere-2025-6493

Abstract. Refractory black carbon (rBC) aerosols strongly influence Arctic atmospheric radiative transfer, making it essential to understand their microphysical properties and mixing state. However, in-situ studies on microphysical properties and mixing state of rBC over the central Arctic marine boundary layer are scarce. To address this gap, we carried out a comprehensive investigation of rBC in the central Arctic onboard the RV Polarstern during the ATWAICE cruise. Our results revealed pronounced spatial and temporal variability in microphysical properties rBC in the Arctic marine boundary layer, governed by transport pathways and removal mechanisms. Under pristine background conditions, rBC mass concentrations were at their lowest (median ~0.4–0.6 ng m-³). Warm airmass intrusions into the Arctic atmosphere were found to bring polluted anthropogenic aerosols into this pristine environment with an eightfold increase in rBC mass concentrations (median ~3.4 ng m-³, maximum ~74 ng m-³). A dominant influence of biomass-burning emissions from Eurasia during the warm airmass intrusion, which coincided with a shift toward larger rBC cores (~264 nm) and moderate coating thickness. The light absorption enhancement of rBC remained low during warm-air-mass intrusions (~1–1.2) than under background conditions (~1.1–1.6), underscoring a strong dependence of rBC radiative effects in the central Arctic on source regions and aging/processing during long-range transport. This study highlights the complexity of rBC aging and mixing state in the central Arctic and will help to increase the accuracy in representing rBC in climate models.