Soot, unsuitable for the climate

Soot, emitted by burning diesel, is far more responsible for climate change than believed earlier

Last Updated: Saturday 04 July 2015

soot or black carbon particles combine with other aerosols to influence climate. The magnitude of the climate forcing caused directly or indirectly by black carbon exceeds that caused by methane, suggesting that black carbon may be the second most important component of global warming after carbon dioxide.

Mark Z Jacobson, researcher at the department of civil and environmental engineering at the Stanford University, California, usa, has simulated the combination of soot particles with other aerosols present in the atmosphere to find out how soot affects climate change. Black carbon particles are generated by burning diesel and coal as well as by waste incinerators and charcoal burners.

The study shows that large positive component of the climate forcing from aerosols is due to black carbon -- soot -- but the exact forcing is affected by black carbon mixing with other aerosols in the atmosphere. Theoretically, black carbon can exist in one of the three possible states. One, distinct from other aerosol particles in an 'external mixture'. Two, incorporated within the aerosol particles as an 'internal mixture' or else as a black carbon core surrounded by a shell of soot and black carbon mixed well. But so far it has been assumed that aerosols exist predominantly as an external mixture. The study simulated the formation of the aerosols in the atmosphere. It found that the mixing state and direct forcing of the black carbon component approach those of an internal mixture. This is largely due to coagulation and growth of aerosol particles (Nature, Vol 409, p695).

The study was based on the fact the degree of climate forcing changes when different mixing states are assumed. Because black carbon is solid and cannot physically be well mixed in a particle, the third theoretical option was discarded as unrealistic and it was concluded that the real forcing fell between that from an external mixture and that from a coated core.

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