Sources of uncertainties in modelling Black Carbon at the global scale
Our understanding of the global black carbon (BC) cycle is essentially\nqualitative due to uncertainties in our knowledge of its properties.\nThis work investigates two source of uncertainties in modelling black\ncarbon: those due to the use of different schemes for BC ageing and its\nremoval rate in the global Transport-Chemistry model TM5 and those due\nto the uncertainties in the definition and quantification of the\nobservations, which propagate through to both the emission inventories,\nand the measurements used for the model evaluation.\nThe schemes for the atmospheric processing of black carbon that have\nbeen tested with the model are (i) a simple approach considering BC as\nbulk aerosol and a simple treatment of the removal with fixed 70% of\nin-cloud black carbon concentrations scavenged by clouds and removed\nwhen rain is present and (ii) a more complete description of\nmicrophysical ageing within an aerosol dynamics model, where removal is\ncoupled to the microphysical properties of the aerosol, which results in\na global average of 40% in-cloud black carbon that is scavenged in\nclouds and subsequently removed by rain, thus resulting in a longer\natmospheric lifetime. This difference is reflected in comparisons\nbetween both sets of modelled results and the measurements. Close to the\nsources, both anthropogenic and vegetation fire source regions, the\nmodel results do not differ significantly, indicating that the emissions\nare the prevailing mechanism determining the concentrations and the\nchoice of the aerosol scheme does not influence the levels. In more\nremote areas such as oceanic and polar regions the differences can be\norders of magnitude, due to the differences between the two schemes. The\nmore complete description reproduces the seasonal trend of the black\ncarbon observations in those areas, although not always the magnitude of\nthe signal, while the more simplified approach underestimates black\ncarbon concentrations by orders of magnitude.\nThe sensitivity to wet scavenging has been tested by varying in-cloud\nand below-cloud removal. BC lifetime increases by 10% when large scale\nand convective scale precipitation removal efficiency are reduced by\n30%, while the variation is very small when below-cloud scavenging is\nzero.\nSince the emission inventories are representative of elemental\ncarbon-like substance, the model output should be compared to elemental\ncarbon measurements and if known, the ratio of black carbon to elemental\ncarbon mass should be taken into account when the model is compared with\nblack carbon observations.