Source Attribution of Arctic Black Carbon Constrained by Surface and Aircraft measurements

The Arctic has warmed rapidly over the last decades with a substantial contribution from black carbon (BC), however sources of Arctic BC and their contributions are highly uncertain. We use the GEOS-Chem global chemical transport model to interpret measurements of Arctic BC from the ground at Alert (2009-2014), Barrow (2009-2015) and Zeppelin (2009-2014), and from aircraft performed by the NETCARE campaign in 2015 and the PAMARCMiP campaigns in 2009 and 2011. Our simulated BC concentrations are consistent with the ground-based measurements to within 40% for Alert and Barrow, and with the aircraft measurements to within 18%. Excluding flaring emissions from the model would increase the bias versus both the ground and aircraft measurements by up to 25% except at Zeppelin where the bias would decrease. Our simulations reveal spatial and seasonal variations in sources of Arctic BC. In winter and early spring, Europe, northern Asia and eastern Asia are comparable sources of BC at Alert and Barrow, whereas at Zeppelin, Europe is the predominant contributor (52%). In summer, biomass burning is the major source of BC at all stations. North America has a minor influence (< 8%) on surface BC throughout the year. In the free troposphere, springtime Arctic BC largely originates from eastern Asia, followed by Europe. Our adjoint simulations indicate pronounced spatial and seasonal heterogeneity in the sensitivity of Arctic BC column concentrations to emissions with noteworthy sensitivities to gas flaring emissions in West Siberia in January and to industrial emissions in eastern China in April.