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Black carbon (BC) concentrations observed in 22 snowpits sampled in the northwest sector of the Greenland ice sheet in April 2014 have allowed us to identify a strong and widespread BC aerosol deposition event, which was dated to have accumulated in the pits from two snow storms between 27 July and 2 August 2013. This event comprises a significant portion (57% on average across all pits) of total BC deposition over 10 months (July 2013 to April 2014). Here we link this deposition event to forest fires burning in Canada during summer 2013 using modeling and remote sensing tools. Aerosols were detected by both the Cloud‐Aerosol Lidar with Orthogonal Polarization (on board CALIPSO) and Moderate Resolution Imaging Spectroradiometer (Aqua) instruments during transport between Canada and Greenland. We use high‐resolution regional chemical transport modeling (WRF‐Chem) combined with high‐resolution fire emissions (FINNv1.5) to study aerosol emissions, transport, and deposition during this event. The model captures the timing of the BC deposition event and shows that fires in Canada were the main source of deposited BC. However, the model underpredicts BC deposition compared to measurements at all sites by a factor of 2–100. Underprediction of modeled BC deposition originates from uncertainties in fire emissions and model treatment of wet removal of aerosols. Improvements in model descriptions of precipitation scavenging and emissions from wildfires are needed to correctly predict deposition, which is critical for determining the climate impacts of aerosols that originate from fires. An event in late July/early August deposited 57% of black carbon that accumulated in northwest Greenland from July 2013 to April 2014Satellite observations and modeling indicate that the origin of this event is emissions from forest fires burning in CanadaChemical transport modeling predicts the event at the right time but underpredicts black carbon deposition compared to observations

Reanalysis datasets from atmospheric models and satellite products are often used for Arctic surface shortwave (SW) and longwave (LW) radiative budget analyses, but they suffer from limitations and require validation against local‐scale observations. These are rare in the high Arctic, especially for longer periods that include seasonal transitions. In this study, radiation and meteorological observations acquired during the Norwegian Young Sea Ice Cruise (N‐ICE2015) campaign over sea ice north of Svalbard (80–83°N, 5–25°E) from January–June 2015, cloud lidar observations from the Ice‐Atmosphere‐Ocean Observing System and the Cloud and Aerosol Lidar with Orthogonal Polarization are compared to daily and monthly satellite retrievals from the Clouds and the Earth's Radiant Energy System (CERES) and ERA‐Interim and ERA5 reanalysis. Results indicate that surface temperature is a significant driver for winter LW radiation biases in both satellite and reanalysis data, along with cloud optical depth in CERES. In May, the SW and LW downwelling irradiances are close to observations and cloud properties are well captured (except for ERA‐Interim), while SW upward irradiances are biased low due to surface albedo biases in all datasets. Net SW and LW radiation biases are comparable (∼20–30 Wm−2) but opposite in sign for ERA‐Interim and CERES in May, which allows for error compensation. Biases reduce to ±10 Wm−2in ERA5. In June downward LW remains biased low (8–10 Wm−2) in all datasets suggesting unsettled cloud representation issues. Surface albedo always differs by more than 0.1 between datasets, leading to significant SW and total flux differences. Surface albedo, temperature, and cloud properties contribute to bias Clouds and the Earth's Radiant Energy System (CERES) and ERA‐Interim surface irradiances, while ERA5 performs betterIn spring ERA‐Interim and CERES SW and LW biases compensate allowing estimates of total surface radiation to agree with surface observationsDifferences up to 0.1 in gridded surface albedo remain between the datasets and affect shortwave and total surface radiation budgets Surface albedo, temperature, and cloud properties contribute to bias Clouds and the Earth's Radiant Energy System (CERES) and ERA‐Interim surface irradiances, while ERA5 performs better In spring ERA‐Interim and CERES SW and LW biases compensate allowing estimates of total surface radiation to agree with surface observations Differences up to 0.1 in gridded surface albedo remain between the datasets and affect shortwave and total surface radiation budgets

The Arctic is influenced by air pollution transported from lower latitudes, and increasingly by local sources such as shipping and resource extraction. Local Arctic emissions could increase significantly in the future due to industrialization in a warming Arctic and further influence Arctic climate. We use the regional model Weather Research and Forecasting, including chemistry, to investigate current (2012) and future (2050) sources of Arctic aerosol and ozone pollution and their radiative impacts, focusing on spring and summer emissions from midlatitude anthropogenic sources, biomass burning, Arctic shipping, and Arctic gas flaring. Results show that remote anthropogenic and biomass burning emissions are likely to remain the main source of Arctic pollution burdens and of black carbon (BC) deposition over snow, and the main contributors to direct aerosol and ozone radiative effects in the Arctic. However, local Arctic flaring emissions are already a major source of BC in northwestern Russia, with a direct radiative effect of ∼25 mW/m2, and Arctic shipping is a strong current source of aerosols and ozone during summer in the Nordic Seas. We find that the direct effect of ozone and aerosols from summertime Arctic shipping is respectively negative (due to frequent temperature inversions) and positive (because of the high surface albedo) in our simulations, two new results. With the development of diversion shipping through the Arctic Ocean in summer 2050, Arctic shipping emissions could become the main source of surface aerosol and ozone pollution at the surface, with strong associated indirect effects of −0.8 W/m2, while flaring would remain an important BC source. Arctic shipping and gas flaring significantly contribute to present‐day and future surface Arctic pollutionOzone from shipping cools the atmosphere due to the near‐surface temperature inversions in the ArcticFuture Arctic diversion shipping causes significant negative summertime aerosol indirect effects (‐0.8 W/m2)