Soot and smoke are the secret to predicting how forest fires might behave

It's been a devastating summer for residents of British Columbia as hundreds of wildfires burned through almost 13,000 square kilometers of the province, setting a new record.

Scientists think this could be a new normal in a warmer world. And if so, we need to get better at preventing and controlling fires.

Alexander Josephson is a graduate research assistant in the wildfire modeling research group at Los Alamos National Laboratory in New Mexico. He has been studying soot, the black powdery substance produced by fires, to understand how it contributes to fires, so as to make prescribed burning safer.

• Research paper in the journal Combustion and Flame

Prescribed burns are creating smaller, intentionally set fires, which are used under controlled conditions to pre-empt large, uncontrolled wildfires. Forest scientists are typically very cautious with them, though, since they don't want their intentional fire to turn into a wildfire.

In the hope of doing prescribed burning more safely, researchers are trying to understand every aspect of a fire —  down to the smallest particles of soot, so as to better predict how they will behave. 

Do you really know what soot is?

Soot is the product of incomplete combustion of fuel. As wood burns it emits volatile gases. The gases are very hot, and coalesce together into particles, which grow in size.  A hot enough fire will burn soot particles, but soot can escape into the surrounding air if the fire is not able to consume them.

"These particles. when they get into the atmosphere have a number of consequences when inhaled," said Josephson, who's also interested in soot's effect on health and the environment. "They can have health impacts on animals and people, and when they settle on the terrain they have a environmental impact."

Soot's contribution to fire

Soot also influences the temperature and spread of a fire. For example, if no soot is emitted in a flame, that means all the fuel is burning, and all that energy is translated into heat. Conversely, if a lot of soot or smoke is coming out of a flame, that means some of the energy went into making those particles instead.

Soot can also increase the spread of a fire if there's an abundance of it inside the flame. They are highly thermally radiative, which means when you have soot particles, the temperature of the flame is actually lower because it's losing heat that is thermally radiating outwards. This helps to dry out fuels around it and increases fire spread.

A delicate balance

"Typically what we want is a low intensity burn [in a prescribed fire]," said Josephson. "Often, these low intensity burns can be very heavily sooted and that's part of what keeps them at a low intensity in order to successfully predict their behaviour and their spread."

Soot is clearly a double-edged sword. Josephson hopes to better understand it in order to strike a balance between creating safe controlled fires and releasing pollutants into the air.

Safer and more effective prescribed fires

Ultimately, understanding soot is a weapon in our arsenal to fight wildfires.

Prescribed burning is still the only way we know how to prevent wildfires, according to Josephson. "Knowing the soot and the emissions of a fire will give us confidence in order to prescribe good fires. There's always been the story of that prescribed fire that went out of control and did more damage than good. When we have these predictive capabilities those stories will become less and less frequent and as we'll be able to control our environment and help our environment too that these wildfires become less and less frequent."

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