Fossil fuel combustion and biomass burning sources of global black carbon from GEOS-Chem simulation and carbon isotope measurements

We identify sources (fossil fuel combustion versus biomass burning) of black carbon (BC) in the atmosphere and in deposition using a global 3-D chemical transport model GEOS-Chem. We validate the simulated sources against carbon isotope measurements of BC around the globe and find that the model reproduces mean biomass burning contribution (f(bb); %) in various regions within a factor of 2 (except in Europe, where f(bb) is underestimated by 63 %). GEOS-Chem shows that contribution from biomass burning in the Northern Hemisphere (f(bb): 35 +/- 14 %) is much less than that in the Southern Hemisphere (50 +/- 11 %). The largest atmospheric f(bb) is in Africa (64 +/- 20 %). Comparable contributions from biomass burning and fossil fuel combustion are found in southern (S) Asia (53 +/- 10 %), southeastern (SE) Asia (53 +/- 11 %), S America (47 +/- 14 %), the S Pacific (47 +/- 7 %), Australia (53 +/- 14 %) and the Antarctic (51 +/- 2 %). f(bb) is relatively small in eastern Asia (40 +/- 13 %), Siberia (35 +/- 8 %), the Arctic (33 +/- 6 %), Canada (31 +/- 7 %), the US (25 +/- 4 %) and Europe (19 +/- 7 %). Both observations and model results suggest that atmospheric f(bb) is higher in summer (59 %-78 %, varying with sub-regions) than in winter (28 %-32 %) in the Arctic, while it is higher in winter (42 %-58 %) and lower in summer (16 %-42 %) over the Himalayan-Tibetan Plateau. The seasonal variations of Atmospheric f(bb) are relatively flat in North America, Europe and Asia. We conducted four experiments to investigate the uncertainties associated with biofuel emissions, hygroscopicity of BC in fresh emissions, the aging rate and size-resolved wet scavenging. We find that doubling biofuel emissions for domestic heating north of 45 degrees N increases f(bb )values in Europe in winter by similar to 30 %, reducing the discrepancy between observed and modeled atmospheric f(bb) from -63 % to -54 %. The remaining large negative discrepancy between model and observations suggests that the biofuel emissions are probably still underestimated at high latitudes. Increasing the fraction of thickly coated hydrophilic BC from 20 % to 70 % in fresh biomass burning plumes increases the fraction of hydrophilic BC in biomass burning plumes by 0 %-20 % (varying with seasons and regions) and thereby reduces atmospheric f(bb) by up to 11 %. Faster aging (4 h e-folding time versus 1.15 d e-folding time) of BC in biomass burning plumes reduces atmospheric f(bb) by 7 % (1 %-14 %, varying with seasons and regions), with the largest reduction in remote regions, such as the Arctic, the Antarctic and the S Pacific. Using size-resolved scavenging accelerates scavenging of BC particles in both fossil fuel and biomass burning plumes, with a faster scavenging of BC in fossil fuel plumes. Thus, atmospheric f(bb) increases in most regions by 1 %-14 %. Overall, atmospheric f(bb) is determined mainly by f(bb) in emissions and, to a lesser extent, by atmospheric processes, such as aging and scavenging. This confirms the assumption that f(bb) in local emissions determines atmospheric f(bb) in previous studies, which compared measured atmospheric f(bb) directly with local f(bb) in bottom-up emission inventories.