Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols

Aerosols affect the Earth's temperature and climate by altering the radiati ve properties of the atmosphere. A large positive component of this radiati ve forcing from aerosols is due to black carbon-soot-that is released from the burning of fossil fuel and biomass, and, to a lesser extent, natural fi res, but the exact forcing is affected by how black carbon is mixed with ot her aerosol constituents. From studies of aerosol radiative forcing, it is known that black carbon can exist in one of several possible mixing states; distinct from other aerosol particles (externally mixed(1-7)) or incorpora ted within them (internally mixed(1,3,7)), or a black-carbon core could be surrounded by a well mixed shell 7. But so far it has been assumed that aer osols exist predominantly as an external mixture. Here I simulate the evolu tion of the chemical composition of aerosols, finding that the mixing state and direct forcing of the black-carbon component approach those of an inte rnal mixture, largely due to coagulation and growth of aerosol particles. T his finding implies a higher positive forcing from black carbon than previo usly thought, suggesting that the warming effect from black carbon may near ly balance the net cooling effect of other anthropogenic aerosol constituen ts. The magnitude of the direct radiative forcing from black carbon itself exceeds that due to CH4, suggesting that black carbon may be the second mos t important component of global warming after CO2 in terms of direct forcin g.