Air pollution helps wildfires create their own lightning

Two titans of nature, a forest fire and a thunderstorm, collide northeast of Phoenix, Arizona. The Charley Fire featured here was started two days prior via a lightning strike. It has since neared 1,000 acres in size as it burns through the Mazatzal Mountains east of Bartlett Lake.

Most people know lightning can spark wildfires. But wildfires can also spark lightning—a phenomenon that has puzzled scientists for decades. Now, researchers have revealed a key cause of this phenomenon: air pollution. Dirtier air means more lightning above wildfires—and more rain, too.

The finding “demonstrates that we should be thinking about the impact air pollution has on weather systems, including the impact on storms,” says Joel Thornton, an atmospheric scientist at the University of Washington, Seattle, who was not involved in the research.

Many factors are believed to cause lightning during a wildfire. Scientists have blamed everything from air currents to urbanization. These elements have been hard to disentangle in the air above land because the rough topography, changing land use, and varying heat create a complex set of variables that are hard to tease out.

Enter Australia’s devastating 2019–20 fire season. Known as “Black Summer,” massive blazes charred more than 186,000 square kilometers of land, and large amounts of smoke billowed southeast over the Tasman Sea. But the vast, flat, cool surface of the sea provided an opportunity to study the fires over a relatively blank canvas.

Yakun Liu, a meteorologist at the Massachusetts Institute of Technology (MIT), and his colleagues used satellite data and land-based lightning detection systems to track the movement of the aerosols—tiny airborne particles—generated by Black Summer wildfires and measure lightning activity in the region. During the fires, the air above a broad swath of the ocean became exceptionally polluted; it would probably take a volcanic eruption to do worse, Liu says.

Amid this seafaring air pollution, lightning activity increased by as much as 270% (from about 12 strokes up to 43 strokes, or lightning flash segments, per square kilometer per year) compared with the same time period the year before, the researchers report this month in Geophysical Research Letters. “That is much higher than I would have expected,” says Graciela Raga, an atmospheric scientist at the National Autonomous University of Mexico, Mexico City, who was not involved in the study.

In thunderclouds, upward-lofting ice crystals collide with descending bits of hail, and these collisions produce an electrical charge that gives rise to lightning. Liu and his colleagues detected higher concentrations of smaller ice crystals in aerosol-polluted clouds over the Tasman Sea. They believe the large influx of aerosols into the skies above the sea provided more surfaces around which cloud ice crystals could form.

The researchers also found that air pollution did more than enhance lightning; wildfire smoke more than tripled the intensity of thundershowers. Over the Tasman Sea, rainfall in lightning storms increased by 240% (from 12.2 to 41.8 millimeters per day).

There are two competing feedback loops, says Earle Williams, a meteorologist at MIT and co-author on the study. Lightning sparks fires and fires enhance lightning, he explains, whereas enhanced rainfall cleanses the atmosphere and helps fight fires.

This is a story that goes beyond wildfires, however,  Thornton says. The work is another example of how human beings—through our cities, traffic, and industrial activities—can change the weather, he says. “When you think about the size of the particles and the scale of the impacts they’re having on the nature of storms, it’s remarkable.”

Air pollution helps wildfires create their own lightning | Science | AAAS

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