Light amplification accelerates chemical reactions in aerosols

Date:
April 14, 2022
Source:
ETH Zurich
Summary:
Aerosols in the atmosphere react to incident sunlight. This
light is amplified in the interior of the aerosol droplets and
particles, accelerating reactions. Researchers have now been able
to demonstrate and quantify this effect and recommend factoring
it into future climate models.



FULL STORY ========================================================================== Aerosols in the atmosphere react to incident sunlight. This light
is amplified in the interior of the aerosol droplets and particles, accelerating reactions.

ETH researchers have now been able to demonstrate and quantify this
effect and recommend factoring it into future climate models.


========================================================================== Liquid droplets and very fine particles can trap light -- similar to how
light can be caught between two mirrors. As a result, the intensity of
the 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 at ETH Zurich and the Paul Scherrer
Institute (PSI) have now investigated how light amplification affects photochemical processes that take place in the aerosols. They were able
to demonstrate that light amplification causes these chemical processes
to be two to three times faster on average than they would be without
this effect.

Using the Swiss Light Source at the 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 makes it
possible to distinguish areas within the aerosol particles composed
of iron(III) citrate from those made up of iron(II) citrate down to
a precision of 25 nanometres. In this way, the scientists were able
to observe and map in high resolution the temporal sequence of this photochemical reaction in individual aerosol particles.

Decay upon exposure to light "For us, iron(III) citrate was a
representative compound that was easy to study with our method," says
Pablo Corral Arroyo, a postdoc in the group headed by ETH Professor
Ruth Signorell and a lead author of the study. Iron(III) citrate stands
in for a whole range of other chemical compounds that can occur in the
aerosols of the atmosphere. Many organic and inorganic compounds are
light- sensitive, and when exposed to light, they can break down into
smaller molecules, which can be gaseous and therefore escape. "The aerosol particles lose mass in this way, changing their properties," Signorell explains. 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 don't yet take this light amplification effect into account," ETH Professor Signorell says. The researchers suggest incorporating the effect into these models in future.

Non-uniform reaction times in the particles Now precisely mapped and quantified, the light amplification in the particles comes about through resonance effects. The light intensity is greatest on the side of the
particle opposite the one the light is shining on. "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 above-mentioned factor
of two to three. Photochemical reactions in the atmosphere usually 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 photochemical reactions of typical aerosols in the atmosphere. It turned
out that the effect does not pertain just to iron(III) citrate particles,
but all aerosols - - particles or droplets -- made of compounds that
can react with light. And these reactions are also two to three times
faster on average.


========================================================================== Story Source: Materials provided by ETH_Zurich. Original written by
Fabio Bergamin. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Pablo Corral Arroyo, Gre'gory David, Peter A. Alpert, Evelyne A.

Parmentier, Markus Ammann, Ruth Signorell. Amplification
of light within aerosol particles accelerates in-particle
photochemistry. Science, 2022; 376 (6590): 293 DOI:
10.1126/science.abm7915 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220414143955.htm

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