Black carbon's role in climate change may be overstated

Black carbon - the main ingredient of airborne soot - has long believed to be a key contributor to climate change with its warming impact second only to carbon dioxide. In fact, some scientists and policy makers have suggested targeting black carbon emissions, which live only one to two weeks in the atmosphere, as a possible way to fight global warming.
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But a new study in smoggy California by Boston College and other researchers suggests that black carbon absorbs significantly less sunlight than previously thought. Published in the journal Science Aug. 31, the study doesn't change the fact that the world is warming, but it could mean climate models may be overstating the role of black carbon.

Boston College chemistry professor Paul Davidovits, an expert on airborne particles, and Timothy B. Onasch, principal scientist at Aerodyne Research Inc. in Billerica and an associate research professor of chemistry at BC, aided in the study led by University of California, Davis with Prof. Christopher D. Cappa as lead author of the Science publication.

Below is a condensed and edited version of an interview with Davidovits.

Green Blog: What is black carbon and why is it so important to manmade climate change?

Paul Davidovits: Some ways we produce black carbon particles include when we run gasoline-powered lawnmowers, drive cars or operate heavy machinery. Natural events, like forest fires, also produce black carbon. These are all examples of incomplete combustion processes that produce soot, which is largely black carbon. Once airborne, particles containing black carbon absorb light and heat the local atmosphere. These particles play an important role in climate change studies because they come largely from manmade sources, they live relatively short lifetimes in the atmosphere, and they possess strong atmospheric warming abilities.

GB: How does black carbon absorb heat and do all black carbon absorb heat the same way?
PD: The type of black carbon produced depends mostly on the fuel–to-oxygen ratio and to some extent also on the type of fuel. But the warming ability of black carbon is not highly sensitive to the source. Perhaps more importantly, black carbon particles during their lifetime are coated with airborne chemicals. The type of coating that accumulates on the particle is strongly dependent on local conditions and may affect its warming ability.

GB: How was it measured in the past and what did your team do differently?
PD: In the past, air samples were collected for several hours or even days on filters for analysis. Such a process did not allow the real-time measurement of the chemical and optical properties of airborne particles, which are central to understanding the way they absorb sunlight and heat the atmosphere. Our team used recently developed instruments that allowed us to directly measure the warming ability of airborne black carbon particles. Additionally, we were able to observe how the warming ability varied as chemicals in the atmosphere condensed onto black carbon particles.

GB:What are the leading explanations for the difference between the warming expectations for black carbon and your field result?
PD: Our observation of less absorption enhancement than predicted by modeling and previous laboratory studies is very likely due to the location of the black carbon component inside each individual particle. The standard models assume that the black carbon component is located in the core of the particle, with other chemical components creating a shell, or even coating, around the black carbon core. Laboratory results for a few specific compounds appeared to support this assumption. However, previous modeling studies exist that look at the effects of the black carbon component being located off-center or even on the surface of each particle. These studies do indicate that less absorption enhancement is expected under these conditions than the core-shell configuration. We believe that this is what explains our observations.

BG:How dramatically do these findings change warming scenarios in the future?
PD: Future global climate change modeling studies will be necessary to determine the magnitude of our observations. At this time, and from our observations, the extent of the implications is not clear. But these findings do require us to reduce our projections about the amount of heating soot produces, at least under some experimental conditions. But the findings don’t point to soot as being harmless in climate change. Soot remains an important climate heating agent, as well as a health problem that has been well documented.

BG: What questions do you still want to answer on black carbon and what will you research next?
PD: One the most important questions that comes directly from our field observations is what type of coating material or atmospheric conditions yield the low absorption enhancements? We are currently conducting laboratory studies of black carbon containing particles with various coating materials to investigate this issue. In addition, we plan on making similar measurements in other parts of the world, focusing on the measurement of black carbon particles that are emitted into the atmosphere by other combustion sources, in addition to the urban environments in northern-southern CA sampled in this study.