• Light amplification accelerates chemical

    From ScienceDaily@1:317/3 to All on Thu Apr 14 22:30:46 2022
    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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