A Primer on Aerosols

Aerosols, explained
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.

Overall, humans have increased the total amount of particles floating around in the atmosphere, which is about twice as dusty now as it was in the 19th century. The amount of very fine material—generally referred to as “PM2.5”—particulate matter less than 2.5 microns across—has increased by something like 60 percent since before the Industrial Revolution. Other aerosols, like ozone, have also increased—with dramatic health impacts around the world.

Air pollution has been linked to increased risk of heart disease, stroke, lung disease, asthma, and more. By some recent estimates fine particles in the air contributed to over four million premature deaths globally in 2016, hitting children and the elderly the hardest. The health risks from fine-particle exposure are highest in China and India, particularly in urban areas.

What do aerosols do to climate?
Aerosols influence climate in two primary ways: by changing the amount of heat that gets in or out of the atmosphere, or by affecting the way clouds form.

Some aerosols, like many kinds of dust from ground-up rocks, are light-colored and even a little bit reflective. When the sun’s rays beam down on them, they bounce the rays back out of the atmosphere, preventing that heat from ever reaching Earth’s surface. The effect can be dramatic: The Mt. Pinatubo volcanic eruption in 1991, in the Philippines, spewed the equivalent of 1.2 square miles of tiny, reflective rock particles into the high stratosphere—cooling the planet for two full years afterward. The 1815 Tambora eruption, similarly, spawned an epic, globe-spanning “Year without a Summer” so cold and bleak it inspired Mary Shelley’s dark horror novel, Frankenstein.

But other aerosols, like little flecks of black carbon from burned coal or wood, do the opposite, absorbing heat from the sun as it beats down. That ends up warming the atmosphere, though it cools the surface of the Earth by preventing the heat from escaping. Overall, that effect is probably smaller than the cooling most aerosols induce—but it’s far from nonexistent, and the more carbon-based material that collects in the atmosphere, the more warming the atmosphere experiences.

Aerosols also influence how clouds form and grow. Water droplets coalesce readily around particles, so a particle-rich atmosphere promotes cloud formation. White clouds reflect incoming sun, preventing it from getting to the surface and warming land or water—but they also absorb the heat that the planet is constantly emitting back outward, trapping it in the lower atmosphere. Depending on the cloud type and location, they can either warm their surroundings or cool them.

Aerosols have a complicated suite of different effects on the planet, but humans have directly impacted their presence, abundance, and distribution. And while the climate effects are complex, the health impacts are clear: More fine material in the air hurts human health.