Measurement report: Shipborne observations of black carbon aerosols in the western Arctic Ocean during summer and autumn 2016–2020: boreal fire impacts

Abstract. Black carbon (BC) aerosol is considered one of the important contributors to the fast climate warming and snow and sea ice melting in the Arctic. Yet the observations of BC in the Arctic Ocean have been limited due to infrastructural and logistical difficulties. We observed BC mass concentrations (m_BC) using light absorption methods on board the icebreaker R/V Araon in the Arctic Ocean (166° E–156° W and <80° N) as well as the North Pacific Ocean in summer and early Autumn of 2016 to 2020. The levels, interannual variations and pollution episodes of m_BC in the Arctic were examined, and the emission sources responsible for the high-BC episodes were analyzed with global chemistry-transport model simulations. The average m_BC in the surface air over the Arctic Ocean (72–80° N) observed in 2019 was over 70 ng m^–3, which was substantially higher than in other years (approximately 10 ng m^–3). The much higher m_BC observed in 2019 was perhaps due to more frequent wildfires occurred in the Arctic region than in other years. The model suggested that biomass burning composed the largest contribution to the observed BC in the western Arctic Ocean and the marginal seas. For these five years, we identified 10 elevated-BC episodes, including one in 2018 that was associated with co-enhancements of CO and CH₄ but not CO₂ and O₃. The model analysis indicated that most episodes were attributed to the airmasses transported from boreal fires to the Arctic Ocean, with some near-surface and others in the mid-troposphere. This study provides crucial datasets on BC mass concentrations and the mixing ratios of O₃, CH₄, CO, and CO₂ in the western Arctic Ocean regions and highlights the significant impact of boreal fires on the observed Arctic BC during the summer and early autumn months.