Why did chemists watch light disintegrate aerosols from within?

When you look out the window or above you, you may notice huge clouds or a huge amount of air. Scholars note this as well; Large masses such as air and clouds are the focus of the models they use to observe Earth’s atmosphere.

It’s easy to miss the tiny liquid droplets or solid particles floating near the blue sky, also known as aerosols. Although they are often invisible, they are essential to the way our atmosphere works. Every parcel of air around you is filled with hundreds of aerosols. They may serve as seeds that sprout clouds. Or they may come together as a smog suffocating the city.

Aerosols are still some of the most misunderstood aspects of Earth’s atmosphere, but it’s clear that they don’t just drift through the atmosphere without consequences. When the sun drives the winds that carry the aerosols, its light can also break them up. Moreover, light can bend through the aerosol as if passing through a lens, speeding up the destructive process.

Scientists have observed this latter effect in detail for the first time, as they report in a research paper published in Science On April 15th. These processes – how sunlight affects and breaks up aerosols – are essential to understanding how pollution works.

This process can have significant effects on the chemistry of the aerosol, “and reactions can occur faster in some parts of the particle than in others,” Paul Coral Arroyoa chemist at ETH Zürich in Switzerland and one of the Science The authors of the paper.

You might recognize aerosols as vectors for the transmission of the coronavirus – but that’s just one subtype of them. 90 percent of aerosols are natural, such as sea salt and volcanic ash, from processes that existed long before humans. But others are our fault: vehicle emissions, soot from burning plant material, and dust thrown into the air by machines.

The study of aerosols is not entirely new. In particular, scientists have known that sunlight can spoil aerosols by cracking and shrinking them. Light – especially ultraviolet sunlight – erodes the chemical bonds that hold these molecules together. This may cause the aerosol to become smaller or its contents to degrade into other substances.

[Related: Tiny air pollutants may come from different sources, but they all show a similar biased trend]

But only now are scientists beginning to realize that aerosols can act in subtle ways with significant effects. “We have to be careful how we deal with small things floating in the air. They cannot be treated as analogous to bulk water, to liquids,” says Christian Georges, an atmospheric chemist at Claude Bernard Lyon 1 University in France, who was not a member of the research team. bulk”.

For example, the thing scientists only understand now is that particles act like lenses, magnifying and amplifying the light that passes through them. This decay is also accelerated if the aerosol is made of certain substances. The scientists knew this happened: In previous experiments, they trapped tiny particles containing a pigment in light and watched them degrade. They found that as the particles shrink, the dye degrades more quickly.

But trying to observe how this effect actually works, and trying to look inside the droplet and watch an accelerating reaction, is difficult. The particle had to be just the right size to see inside: too big and it would be too big for instruments to see—even when using an X-ray microscope, as these researchers did. Too small, and the differences in chemical composition would be too subtle to be seen.

To look at something visible, these researchers used a chemical called iron (III) citrate. It is present in the atmosphere, particularly near the Earth. But the researchers chose it primarily because when it interacts with sunlight, it degrades into another chemical called iron(II) citrate in a reaction that’s easy to see, but only if you can look closely enough.

Coral-Arroyo and colleagues blasted iron(III) particles with ultraviolet light for hours on end. Meanwhile, they carefully monitored the particles with an X-ray microscope. X-rays allowed them to see which parts of the individual particle – less than a hundredth the width of a human hair – were interacting and when.

“This is what really allowed us to track the chemical composition of different parts of the particle,” Coral Arroyo says.

Now that they have seen how light breaks down particles from within, chemists may try to classify the behavior of light in different types of aerosols. Not all particles and droplets are created equal when it comes to insolation and disintegration in sunlight. Carbon black soot from burning coal and other dark particles tends to absorb light, rather than letting it travel indoors.

On the other hand, sea salt and many aerosols of organic origin will experience accelerated reactions. Knowing that this happens in molecules has a huge impact on the models that scientists use to understand pollution behaviour.

“If you really want to get accurate models, you need to consider these effects,” Corral Arroyo says. “Otherwise, your model is not working properly.”

In fact, most current atmospheric models focus primarily on large masses of air or water. “What this paper really shows is that we cannot move forward as we currently do,” says George. If impacts like these are significant — and the study’s authors say they are — it’s a sign that these models, crucial to everything from weather forecasting to understanding climate change, are incomplete.

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