AsianScientist (May 16, 2016) – One major component of the smog is black carbon, fine particles produced from incomplete fossil fuel combustion and biomass burning. The question is, could these black carbon particles also contribute to global warming? A Chinese study published in Proceedings of the National Academy of Sciences presents a more reliable way of measuring if they do.
Scientists are divided over the warming effects of black carbon. Some say its effect is insignificant, while others say it is large. The estimated direct radiative forcing (DRF) of black carbon, an index of how severely black carbon warms the world, ranges from about 0.2-1 W.m-2, a poorly-constrained estimate.
Now, Professor Hu Min and colleagues from Peking University have designed a method to quantify black carbon’s contribution to global warming. Interestingly, their model may also explain why scientists have so many conflicting answers about black carbon’s DRF.
The team developed a system, called QUALITY chamber, which can detect the temporal variation of black carbon particle properties. Using this system, researchers have found that black carbon particles will go through two typical stages: from fractal to spherical morphology, and from spherical morphology to fully compact particles.
In the first stage, black carbon particles show low light absorption (low DRF); while in the second stage, black carbon particles rapidly enhance light absorption (high DRF). The more light it absorbs, the more it contributes to global warming. This also explains why scientists get different results: some of them measure DRF of fresh black carbon; some of them measure DRF of aged black carbon.
The speed of black carbon particle aging is also influenced by environment. In a more polluted environment, black carbon particles transform more quickly. The team recorded a 2.3-4.6 hour timescale for black carbon particles to change from fresh to fully aged particles in Beijing; in Houston, it takes 9-18 hours.
The article can be found at: Peng et al. (2016) Markedly Enhanced Absorption and Direct Radiative Forcing of Black Carbon under Polluted Urban Environments.
Source: Peking University.
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