Abstract
Produced from incomplete combustion of biomass and fossil fuel in the absence of oxygen, black carbon (BC) is the collective term for a range of carbonaceous substances encompassing partly charred plant residues to highly graphitized soot. Depending on its form, condition of origin and storage (from the atmosphere to the geosphere), and surrounding environmental conditions, BC can influence the environment at local, regional and global scales in different ways. In this paper, we review and synthesize recent findings and discussions on the nature of these different forms of BC and their impacts, particularly in relation to pollution and climate change. We start by describing the different types of BCs and their mechanisms of formation. To elucidate their pollutant sorption properties, we present some models involving polycyclic aromatic hydrocarbons and organic carbon. Subsequently, we discuss the stability of BC in the environment, summarizing the results of studies that showed a lack of chemical degradation of BC in soil and those that exposed BC to severe oxidative reactions to degrade it. After a brief overview of BC extraction and measurement methods and BC use for source attribution studies, we reflect upon its significance in the environment, first by going over a theory that it could represent parts of what is called the ‘missing sink’ of carbon in global carbon cycle models. Elaborating upon the relationship of BC with polycyclic hydrocarbons, we show its significance for the sorption and transport of pollutants. A description of pulmonary-respiratory health effects of soot BC inhalation is followed by a discussion on its impact on climate and climate change. We explain how soot BC acts as a global warming agent through light (and heat) absorption and how it reduces the snow’s albedo and promotes its uncharacteristic thawing. On a more positive note, we conclude this review by illustrating recent observations and simulations of how pyrolytic processes can stabilize plant carbon stocks in the form of biochar BC that can sequester carbon and can help mitigate climate change, in addition to improving soil fertility.
Highlights
As the atmospheric carbon dioxide (CO 2 ) levels continue to increase from pre-industrial concentrations of 280 ppm to an average value of 385 ppm, as reported in 2008 [1], precise investigation and documentation of different carbon (C) forms, pools and sinks is of utmost significance
Black carbon (BC) in the form of aerosols has received far less consideration, it is a primary air pollutant produced by combustion activities and has a high global warming potential of 680 on a 100-year basis [2]
Combustion of biomass and fossil fuel is the only form of BC production, BC effects vary distinctively depending on its different forms
Summary
As the atmospheric carbon dioxide (CO 2 ) levels continue to increase from pre-industrial concentrations of 280 ppm (parts per million volume) to an average value of 385 ppm, as reported in 2008 [1], precise investigation and documentation of different carbon (C) forms, pools and sinks is of utmost significance. Black carbon (BC) in the form of aerosols has received far less consideration, it is a primary air pollutant produced by combustion activities and has a high global warming potential of 680 on a 100-year basis [2]. When BC is buried and incorporated in the pedosphere, its long residence time and tendency to enhance other soil chemical and physical properties can increase the soil’s potential to sequester carbon. Such burial is preceded by natural, anthropogenic or a mixture of both kinds of events such as wildfires, prescribed burns, biomass-based cooking and fossil fuel combustion. We elaborate upon the above properties of BC, focusing on what they imply for the environment and humans
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