Tiny particles floating in the atmosphere have a much bigger impact on the planet than you might think, and human activity plays a role.
BY ALEJANDRA BORUNDA for National Geographic
THE MOST VIBRANT sunsets, cloud-choked skies, and cough-inducing days all have something in common: They happen because of aerosols, tiny particles that float in the air. Aerosols can be tiny droplets, dust particles, bits of fine black carbon, and other things, and as they float through the atmosphere they change the whole energy balance of the planet.
Aerosols have an outsized effect on the planet’s climate. Some of them, like black and brown carbon, warm the Earth’s atmosphere, while others, like sulfate droplets, cool it. Scientists think that on balance, the whole budget of aerosols ends up cooling the planet slightly. But exactly how much, and how much that effect can shift over days, years, or centuries is still not totally clear.
What are aerosols?
The term aerosol is a catch-all for many kinds of little bits of stuff that end up suspended in the atmosphere, from the surface of the planet all the way to the edges of space. They can be solid or liquid, infinitesimally small or big enough to see with the naked eye.
“Primary” aerosols, like dust, soot, or sea salt, come directly from the planet’s surface. They get lifted into the atmosphere by gusty winds, shot high into the air by exploding volcanoes, or they waft away from smokestacks or flames. “Secondary” aerosols form when different things floating in the atmosphere—like organic compounds released by plants, liquid acid droplets, or other materials—crash together, culminating in a chemical or physical reaction. Secondary aerosols, for example—make the haze that gives the U.S.’s Great Smoky Mountains their name.
Aerosols come from both natural and human sources—and sometimes both at once. Dust, for example, is scoured from deserts, the dried-out edges of rivers, dry lakebeds, and more. Its concentrations in the atmosphere rise and fall with climate; in cold, dry, periods in the planet’s history like the last ice age, more dust filled the atmosphere than during warmer stretches of Earth’s history. But humans have affected that natural cycle, making some places dustier than they otherwise would be and keeping other areas damp.
Sea salts provide another natural source of aerosols. They’re whipped out of the ocean by wind and sea spray and tend to fill the lower parts of the atmosphere. In contrast, some types of very explosive volcanic eruptions can shoot particles and droplets high into the upper atmosphere, where they can float for months or even years, suspended miles above Earth’s surface.
Human activity produces many different types of aerosols. Fossil-fuel burning produces particles, as well as the well-known greenhouse gases like carbon dioxide—so cars, airplanes, power plants, and industrial processes all produce particles that can collect in the atmosphere. Agriculture produces dust, as well as other things like aerosolized nitrogen products, both of which affect air quality near and far.