The compaction, which is greater for icy than supercooled cloud droplets, should be taken into account in future climate models, the researchers say.

"A change in the scattering [resulting from compaction] can have quite a large effect on how much soot would warm the atmosphere," said Claudio Mazzoleni of Michigan Technological University in the US. "Therefore, knowing how soot compaction might affect the scattering is important to better understand future climate."

Unlike most atmospheric particles, soot is black and absorbs the Sun’s radiation. It is therefore a major contributor to global warming – perhaps only second in strength to carbon dioxide.

Quite how much soot absorbs radiation is unclear, however. The ratio of radiation absorption to scattering, known as the single scattering albedo, depends on the structural properties of the soot particles. Generally speaking they have a fractal structure like random bunches of grapes, albeit with sub-micron dimensions.

But soot particles assist in the formation of clouds, and in doing so are thought to be deformed, possibly changing their optical properties. It is this process that Mazzoleni – together with colleagues from other US institutions – has now studied experimentally.

The researchers generated soot particles with a diesel engine and injected them into an ice chamber. By controlling the temperature and humidity of the chamber, they could make the soot particles seed either ice crystals or supercooled water droplets – both of the type found in clouds. The scientists then extracted the crystals or droplets, dried them, and studied the resultant soot particles in an electron microscope.

The microscope showed that the soot particles had been compacted in their formation of supercooled water droplets, and had been compacted further in their formation of ice crystals. To understand how this compaction affected the optical properties, Mazzoleni and colleagues built virtual versions of the particles on a computer, and applied a mathematical model that represented the amount of radiation scattering and absorption.

The model showed that the compacted soot particles had a single scattering albedo as much as 40% greater than soot particles that had not been through a cloud-formation process. That raised albedo equates to a reduction in the particles’ capacity to warm the top of the atmosphere – their so-called radiative forcing – by some 60%.

The researchers cannot yet say how important this reduction will be for climate predictions. But, said Mazzoleni, "our findings can indeed be used by modellers to better estimate the warming by soot particles, and we do hope that modellers will use our results to this goal."

Mazzoleni and colleagues now want to see whether the compaction is the same in different atmospheric conditions, and also to gather different types of soot morphology, to find out how widely applicable their results are. "This will be important to assess the global impact of the changes in the soot optical properties that we found on a single particle basis in this paper," he said.

The study is published in Environmental Research Letters (ERL).

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