Black Carbon: Is It Worth the Fuss?

by Ogochukwu Yvonne Enekwizu, Graduate Research Assistant, New Jersey Institute of Technology

Unless you’ve driven behind or walked past a diesel engine truck spouting plumes of smoke, you may be completely oblivious to black carbon pollution. One of the many particulate aerosols that currently resides in our atmosphere, black carbon is generated from the incomplete combustion of fossil fuels and other fuel sources such as biomass or biofuel. Black carbon is more colloquially known as soot. Internal combustion engines, industrial power plants, residential heating, and open-air cooking are all major sources. This article explores the role of black carbon in our environment and why this man-made pollutant presents an intriguing problem to researchers.

The many ‘gifts’ of black carbon pollution

Black carbon is an atmospheric pollutant that plays multiple nefarious roles in the environment. Perhaps the most recognizable effect is the reduction of atmospheric transparency and visibility, by so much in China and India that human activities have been temporarily halted several times in a bid to reduce air pollution levels.

Black carbon particles are like tiny sponges that soak up toxic organic material and can penetrate deeply into human tissue when inhaled, causing many lung and heart-related diseases. As an efficient light absorber, black carbon is estimated to be second to carbon dioxide as a climate warming agent and its high concentrations in developing countries give rise to regional hotspots. By warming the atmosphere and cooling the earth’s surface, black carbon creates a temperature inversion that reduces vertical air mixing and exacerbates pollution.

Long-range transport in the Northern Hemisphere also delivers soot to polar regions where it accelerates ice cap melting and contributes to rising sea levels. Additionally, it is aesthetically displeasing and soils landmarks such as India’s iconic Taj Mahal.

Curbing carbon

Since black carbon is anthropogenic, lowering its concentrations to zero, the only known safe level, is not only impractical but rather impossible. Rapid urbanization coupled with increased vehicle and energy use in developing countries keep black carbon levels on the rise along with its environmental effects. For instance, living in Delhi, India, in November 2017 was likened to smoking 50 cigarettes a day. However, unlike carbon dioxide which remains in the atmosphere for much longer, black carbon is rather short-lived. Hence, sustained reductions in emissions could have immediate effects on air quality and human health. On the other hand, the role of black carbon on climate is not solely dependent on its concentration levels.

What makes black carbon unique?

Our atmosphere is a dynamically changing system. As a result, the extent of black carbon’s environmental impacts is strongly dependent on its mixing with other atmospheric pollutants and its particle structure, which varies with source and combustion conditions. Black carbon particle morphology is complex, consisting of branched aggregates of small carbon spheres that are arranged in an open configuration. When these black carbon particles mix with other aerosols, they undergo structural transformation to a near spherical geometry. Because it is just one component of this mixed aerosol, black carbon cannot be studied in isolation. Mixed aerosols differ from pure black carbon in their atmospheric lifetimes, interactions with solar radiation, and interactions with clouds.

Some modeling studies have shown that these alterations in particle composition and structure can increase the light absorption of black carbon by as much as 50%. However, this conclusion is inaccurate as field studies have shown that the black carbon particles can assume structures of varying complexity within the mixed aerosol giving rise to different levels of light absorption enhancement and in some cases, suppression.

This constantly evolving structure and composition has confounded atmospheric research scientists and resulted in differences in observed measurements and modeled predictions of black carbon impacts on climate. Such uncertainties in global warming by black carbon will remain until the underlying processes governing the transformations of these enigmatic particles are understood. Given that we may have to wait for that impact to be better understood, efforts to counter climate change may have to remain more focused on carbon dioxide than black carbon.

As to whether black carbon is worth the fuss, residents in Beijing and Delhi seem to think so, regardless of whether it affects global warming. If you ever get stuck in traffic behind a diesel truck, you may be inclined to agree as well.