↵* Present address: Department of Earth Sciences, Vrije Universiteit Amsterdam, 1081 HV Amsterdam, Netherlands.

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Black carbon (BC) contributes to Arctic climate warming, yet source attributions are inaccurate due to lacking observational constraints and uncertainties in emission inventories. Year-round, isotope-constrained observations reveal strong seasonal variations in BC sources with a consistent and synchronous pattern at all Arctic sites. These sources were dominated by emissions from fossil fuel combustion in the winter and by biomass burning in the summer. The annual mean source of BC to the circum-Arctic was 39 ± 10% from biomass burning. Comparison of transport-model predictions with the observations showed good agreement for BC concentrations, with larger discrepancies for (fossil/biomass burning) sources. The accuracy of simulated BC concentration, but not of origin, points to misallocations of emissions in the emission inventories. The consistency in seasonal source contributions of BC throughout the Arctic provides strong justification for targeted emission reductions to limit the impact of BC on climate warming in the Arctic and beyond. Black carbon (BC) aerosols, originating from incomplete combustion of fossil fuels and biomass, contribute to the increased rates of warming of the Arctic (1–3). Policy-focused research suggests that collaboration and alliances of even small groups of countries could achieve urgently needed, efficient, rapid, and substantial BC mitigation (4). Atmospheric transport models—fundamental for validation of inventories used in climate policy discussions—have difficulties in accurately reproducing Arctic BC concentrations (5–7). Comparison of model predictions with source-diagnostic observations offers a means to better understand the emissions of BC reaching the Arctic (8–10). Source attributions are challenged both by a lack of observational constraints and by large uncertainties in emission inventories, the latter being a key element for modeling transport and climate effects of BC, specifically in the Arctic (8, 11, 12). Observation-based Arctic BC studies are scarce and rarely extend over more than 1 year (13–15), especially with regard to data on source-diagnostic dual-isotopic composition (δ13C and Δ14C). Hereafter, BC is used when referring to model results or the aerosol in general, and elemental carbon [EC; the mass-based BC analog (16)] is used when referring specifically to observational data. The present study provides new year-round δ13C/Δ14C-based source apportionment of EC from the Arctic sites Alert (Canadian High Arctic; n = 9), Zeppelin (Svalbard; n = 11), and Barrow (north Alaska; n = 10), covering a period of ~3 years. To provide a comprehensive circum-Arctic perspective (Fig. 1), these three records are combined with our recently published studies of EC aerosol concentrations and isotopic signatures from two long-term campaigns from Abisko (northern Scandinavia; n = 17) (7) and Tiksi (northeast Siberia; n = 17) (17) and a winter study (n = 6) from Barrow (18). The 14C/12C isotope ratio of an EC sample allows determination of the biomass burning fraction (fbb; containing contemporary 14C) relative to the fossil fuel combustion fraction (ffossil; devoid in 14C) (19). The 13C/12C ratio helps to further distinguish between various types of fossil fuel sources [e.g., natural gas, coal, or oil (17)]. Last, these observations of atmospheric BC are compared with results from an atmospheric transport model, which includes both anthropogenic and natural-fire BC emissions, and has shown great potential to accurately simulate observational data (5, 7).