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In 1991, the eruption of Mount Pinatubo in the Philippines released some 20 million tonnes of sulphur dioxide into the upper atmosphere. Sulphur dioxide reacts with other substances to produce airborne nanoparticles called sulphate aerosols, which tend to reflect sunlight.

Consequently, in the two years after the eruption, the global temperature dropped by about half a degree. 

The effect of aerosols on the Earth’s climate is hugely important but mind-bogglingly complex. In addition to cooling the Earth, some aerosols, such as soot, tend to absorb sunlight and so heat up the atmosphere. 

A huge outstanding question in climate science is how these processes of heat absorption and reflection balance out.  

Part of the problem is that nobody understands how nanoparticles absorb and emit heat. In theory, this process is governed by Planck’s law, which describes the amount of electromagnetic radiation emitted by a perfect black body at a given temperature.  

But in practice, real objects don’t emit heat perfectly so physicists have to apply a correction factor called the spectral emissivity. This depends on the properties of the object’s surface–it’s material and roughness, for example. 

In recent years, however, tantalising evidence has emerged indicating that an object’s shape and volume may also play a role in this process.

Today, Christian Wuttke and Arno Rauschenbeutel at The Vienna University of Technology in Austria show for the first time how and why this true. 

They point out that when an object is large compared to the wavelength of the radiation it is emitting, then surface effects dominate. But when an object is small compared to the wavelength, then radiation can be emitted from any point within its volume. In that case the geometry of the particle must play a role.

To prove the point, they measured the heat radiated by a silicon nanofibre with a diameter of 500nm, which is much smaller than the wavelength of thermal radiation.   

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