We use a global chemical transport model (GEOS-Chem CTM) to interpret observations of black carbon (BC) and organic aerosol (OA) from the NASA ARCTAS aircraft campaign over the North American Arctic in April 2008, together with longer-term records in surface air and in snow. We find that Russian open fires were the dominant source of OA in the troposphere during ARCTAS but that BC was more of anthropogenic origin, particularly in surface air. This source attribution is confirmed by correlation of BC and OA with acetonitrile and sulfate in the model and in the observations. Asian emissions are the main anthropogenic source of BC in the free troposphere but European, Russian and North American sources are also important in surface air. Russian anthropogenic emissions appear to dominate the Arctic source of BC in surface air in winter. Open fire influences on Arctic surface BC in spring are much higher in the Eurasian than in the North American sector. Most of the BC transported to the Arctic in the lower troposphere is deposited within the Arctic, in contrast to the BC transported at higher altitudes. Pan-Arctic 2007-2009 observations of BC concentrations in snow are well reproduced by the model, with maximum values in the Russian Arctic and much lower values in the North American Arctic. We find that anthropogenic sources contribute 90% of BC deposited to Arctic snow in January-March and 57% in April-May 2007-2009. The mean decrease in Arctic snow albedo from BC deposition is estimated to be 0.6% in spring 2007-2009, resulting in a regional surface radiative forcing consistent with previous estimates.