Microphysical properties and light absorption enhancement of refractory black carbon aerosols in the central Arctic marine boundary layer: role of warm airmass intrusions on mixing state

Abstract. Refractory black carbon (rBC) aerosol particles strongly influence Arctic atmospheric radiative transfer, making it essential to understand their microphysical properties and mixing state. However, in-situ investigations on microphysical properties and mixing state of rBC particles over the central Arctic marine boundary layer are scarce. To address this gap, we carried out a comprehensive investigation of rBC particles in the central Arctic onboard the RV Polarstern during the ATWAICE cruise. Our results revealed pronounced spatial and temporal variability in microphysical properties of 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). The mass median diameter of rBC cores was found to increase with latitude, from the lowest value (∼ 156 nm) in lower-latitude regions influenced by higher anthropogenic emissions to ∼ 220 nm in the high Arctic, consistent with the persistence of aged aerosols under background conditions. 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, rBCmax⁡ ∼ 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 estimated using core-shell Mie theory remained low during warm airmass 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, driven by variable source characteristics and summertime processing conditions and will help to increase the accuracy in representing rBC in climate models.