Light amplification accelerates chemical reactions in aerosols

Aerosols in the atmosphere react to incident sunlight. This light is amplified inside the droplets and aerosol particles, accelerating the reactions. The ETH researchers have now been able to demonstrate and quantify this effect and recommend taking it into account in future climate models.

Liquid droplets and very fine particles can trap light, much like light can be caught between two mirrors. As a result, the intensity of light inside them is amplified. This also happens in very fine water droplets and solid particles in our atmosphere, i.e. aerosols. Using modern X-ray microscopy, chemists from ETH Zurich and the Paul Scherrer Institute (PSI) investigated how light amplification affects photochemical processes that take place in aerosols. They were able to demonstrate that the amplification of light makes these chemical processes two to three times faster on average than they would be without this effect.

Using the Swiss light source at PSI, the researchers studied aerosols consisting of tiny particles of iron(III) citrate. Exposure to light reduces this compound to iron(II) citrate. X-ray microscopy can distinguish areas within aerosol particles composed of iron(III) citrate from those consisting of iron(II) citrate to an accuracy of 25 nanometers. In this way, scientists were able to observe and map in high resolution the time sequence of this photochemical reaction in individual aerosol particles.

Decomposition on exposure to light

“For us, iron(III) citrate was a representative compound that was easy to study with our method,” says Pablo Corral Arroyo, postdoctoral fellow in the group led by ETH Professor Ruth Signorell and lead author of the study. Iron (III) citrate represents a whole range of other chemical compounds that can be found in aerosols from the atmosphere. Many organic and inorganic compounds are light sensitive and when exposed to light can break down into smaller molecules, which can be gaseous and therefore escape. “The aerosol particles thus lose mass, changing their properties,” says Signorell. Among other things, they scatter sunlight differently, which affects weather and climate phenomena. In addition, their characteristics as condensation nuclei in cloud formation change.

As such, the results also have an effect on climate research. “Current computer models of global atmospheric chemistry do not yet account for this light-amplifying effect,” says Professor Signorell from ETH. The researchers suggest incorporating the effect into these models in the future.

Non-uniform reaction times in particles

Now precisely mapped and quantified, the amplification of light in particles occurs through resonance effects. The light intensity is greatest on the side of the particle opposite to that on which the light shines. “In this hotspot, photochemical reactions are up to ten times faster than they would be without the resonance effect,” says Corral Arroyo. Averaged over the entire particle, this gives an acceleration by the aforementioned factor of two to three. Photochemical reactions in the atmosphere generally last several hours or even days.

Using the data from their experiment, the researchers were able to create a computer model to estimate the effect on a range of other typical aerosol photochemical reactions in the atmosphere. It turned out that the effect was not just iron (III) citrate particles, but all aerosols – particles or droplets – made up of compounds that can react with light. And these reactions are also two to three times faster on average.

Source of the story:

Material provided by ETH Zürich. Original written by Fabio Bergamin. Note: Content may be edited for style and length.

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