As wildfire smoke plumes travel thousands of miles, sunlight and atmospheric chemistry bleach their color, strip away their campfire aroma and add them to the mix of urban smog.

Smoky skies in summer 2026, as wildfires hundreds of miles away forced people to and run air conditioners around the clock.

Living in the western U.S., I am no stranger to this scene. Wildfires and the thick smoke they produce have become a fixture of summer life here. I also , in particular how the smoke evolves as it moves downwind from the flames to the communities where people breathe it.

To understand what’s in the smoke you might be breathing, you have to look at where it’s been.

Wildfire smoke is often too high in the atmosphere to be noticed, but when atmospheric conditions bring it close to the ground, people are left breathing unhealthy particles.

When a forest burns, the wildfire emits large amounts of , along with and . The compounds and particles emitted by wildfires can , including the lungs, heart and organs, and include known carcinogens.

How this potent mix of pollutants transforms over time and distance is a complex puzzle. At the , our team tracks this behavior, from studying in the lab to . Understanding how wildfire smoke ages is essential for predicting its impacts on human health.

It all starts at the flame

The story of smoke begins with how the wildfire burns. High temperature wildfires behave very differently from cool, smoldering wildfires.

Hot, intense flames produce , which can harm a person’s respiratory system and contribute to the formation of secondary pollutants and reactive carbon compounds like , which can . They also emit smaller amounts of , , all of which can be toxic to humans.

Smoke from a wildfire lofts high into the air. As it moves, its chemical composition changes. Steven S. Brown

In contrast, lower-temperature smoldering wildfires release a different mix of compounds, but it is still enriched with toxic and , which can irritate the respiratory system. This isn’t to say that these wildfire emissions are safer for human health, but rather that burning conditions dictate the chemical makeup of the smoke and its subsequent chemical fate.

Wildfires are rarely just one or the other; they are dynamic mixtures that shift throughout the day as air temperatures rise, humidity drops or evening thunderstorms roll in.

The Twin Otter research planes that National Oceanic and Atmospheric Administration scientists use to study wildfire smoke are packed with instruments. Steven S. Brown

Once smoke leaves the flames, three main factors govern its journey: wind speed, atmospheric temperature and sunlight.

Think of a campfire. If you are sitting in the wrong spot, the prevailing wind blows smoke directly in your face. This horizontal movement is called advection. Small fires often don’t generate enough heat to loft their smoke high, meaning you can smell your neighbor’s campfire.

However, massive wildfires generate immense heat and . This buoyancy acts like an elevator, lifting the smoke plume out of the and injecting it into the free troposphere, roughly 1.2 miles (2 kilometers) above the ground. Up there, high-altitude winds take over, across the continent.

A large smoke plume rises high into the atmosphere. Michael A. Robinson

On occasion, wildfire plumes can generate their own weather, making smoke-laden , which can inject large amounts of particles into the stratosphere.

How sunlight ‘ages’ the smoke

As smoke travels, it undergoes rapid physical and chemical changes.

First, the plume dilutes as cleaner background air mixes into it. Close to the fire, the smoke is dense and opaque. As it moves downwind, it spreads out and grows more diffuse.

Second, intense sunlight acts as a chemical engine. Solar ultraviolet photons break apart bonds in molecules, . Nitrogen oxides play a .

Within just , this mixture reacts to form , which can irritate the lungs and is a key component of smog.

When the Sun shines through thick wildfire smoke, it turns the sky a dirty orange. Steven S. Brown

If smoke is lofted high, into cold free tropospheric air, the chemical aging process can temporarily freeze. The smoke can remain chemically fresh until the air parcel sinks closer to the surface, below about 1.25 miles (2 kilometers), where warmer temperatures can restart the chemical reactions.

When an aging smoke plume passes over a major city, it can mix with urban pollution, such as car exhaust. This interaction can rekindle chemical reactions, on top of the fine particulate matter emitted from the fire.

Why distant smoke smells and looks different

Have you ever noticed that long-distance wildfire smoke doesn’t smell like a campfire? There is a chemical reason for that.

The specific compounds responsible for the classic smoky aroma of a campfire – and – are highly reactive. Sunlight and the associated chemistry destroy them in a matter of hours. By the time the smoke has traveled across several states, the smoky smell is completely gone.

Sunlight also alters the color of the smoke through of aerosols. Fresh smoke contains dark brown and black carbon particles that absorb light. Over several days of exposure to sunlight and oxidants, chemical reactions break down these dark compounds.

A sailboat passing a lifeguard stand along Chicago’s Lake Michigan is obscured by Canadian wildfire smoke that blanketed the city on July 16, 2026.

The result?

Dark, light-absorbing particles turn into light-scattering particles, transforming dark, dense plumes into the milky white haze seen drifting across the eastern skies. These particles efficiently scatter the blue and green light while letting red and orange wavelengths pass through, creating the .

The wildfire smoke inhaled hundreds of miles downwind is chemically distinct from the smoke that left the flames, but it remains a and an area of active research. Consequently, smoke transport and its associated atmospheric chemistry represent a major North American air quality issue, one projected to intensify in the coming years.

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