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traffic is a Source of black carbon emission (photo: mripp / flickr / creative commons)

traffic is a Source of black carbon emission (photo: mripp / flickr / creative commons)

AC/BC

In search of airborne soot, and how it affects the climate

Soot, or black carbon, is among the emissions from human activities that most significantly affects the climate – but how strong is its impact really? The answer depends on where soot is transported after it is emitted. How far does it travel? At what altitudes? Where does it land, and how does it change en route? These are the key questions for the AC/BC project, which combines new measurements of soot concentrations with detailed climate model calculations.

Black carbon is emitted through incomplete combustion. Sources range from cars, industry and forest fires to wood-fuelled cooking stoves and single-use park grills. After emission, the tiny soot particles stay suspended in the air for a few days, and are transported by the prevailing winds.

Unfortunately, we don’t know how much soot is emitted each year, how long it stays airborne, or how high it is transported. All of these factors are significant for the level of impact that soot has on the climate.

Measurements of soot concentration taken by instruments aboard aircraft have very recently become available. These allow us to take snapshots of the geographical distribution of soot at different times. Several measurement campaigns have already flown, and more are planned for the coming years. A main goal of the AC/BC project is to collect information from such campaigns, and combine them to get a complete picture of global soot concentrations.

These measurements will then allow us to perform improved calculations of the climate impact of soot. Once in the air, soot absorbs solar radiation, thus heating the climate in much the same way as a greenhouse gas. In addition, it can influence the formation of clouds, how white they become and how long they last. Soot can land on snow and accelerate melting, and it can change how easily ice crystals form in cold air (see the NetBC project).

Results from the AC/BC project will enable us to give consistent estimates of global soot emissions, their lifetime in the atmosphere, and their concentrations at high altitudes. Combined with climate model calculations, we will achieve a better understanding of how soot affects the climate today – and how important it has been in the past.

The project’s results will help us to answer a key question posed by policymakers: To what degree can reductions in black carbon emissions help to limit the harmful impacts of global warming?

The AC/BC project is a Young Researcher Talents project, funded by the Research Council of Norway. Research partners include met.no, the University of Leeds, NASA GISS, the UK Met Office and the University of Manchester.