Wildfire smoke evolves as it drifts from wilderness to city
Wildfire smoke evolves as it drifts from wilderness to city Where wildfire...
Wildfire smoke evolves as it drifts from wilderness to city Where wildfire smoke blows In many parts of North America, wildfire is a natural part of the landscape. And after decades of full suppression policies, fires today have plenty of fuel to burn; a stray spark can quickly turn into an unmanageable inferno.
These large modern fires send billowing plumes of harmful pollutants across thousands of kilometers, imperiling millions of people living far from the flames themselves. As the smoke travels, it changes. With predictions of a smokier future, scientists are racing to understand the health and climate risks of aged smoke.
Before they understand aged smoke, researchers must understand fresh smoke. And to understand fresh smoke, they need to understand fire. Some fire burns hot and fast, producing visible flames that are characteristic of relatively complete combustion.
The resulting plume is rich in hot, invisible gases that loft soot particles high into the air. Other fire burns cool and low, the flames smoldering and releasing a thick cloud. The less efficient combustion sends vast quantities of soot and aerosol particles into the air.
At the large scale of a wildfire, both flaming and smoldering conditions exist. So scientists must follow the fate of gases and particles as a plume ages. As trees and grasses burn, they release a slew of gases.
Carbon dioxide and carbon monoxide are the most abundant combustion products. Large quantities of lung-irritating volatile organic compounds (VOCs) and nitrogen oxides (NO x ) are also released. Scientists have also measured toxic polyaromatic hydrocarbons (PAHs), organic acids, and metals in plumes.
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The exact mix of gases depends on what burns. As the flames chew through fuels, the released particles and gases mix. Specks of solid soot—black carbon—are lofted into the sky along with aerosol droplets full of organic molecules.
When they’re fresh, many of these organic aerosols absorb light and so are called brown carbon. Billowing away from the flames, this mixture of gases and particulates starts to age and transform. For example, the vast quantities of VOCs and NO x from the plume quickly mix with atmospheric oxygen.
As the plume thins, these reactants are bathed in sunlight and quickly react to form ozone. Simultaneously, atmospheric hydroxyl radicals assail the sunbathed organic gases. The resulting reactions transform the molecules, functionalizing them, breaking them down, or otherwise changing them.
With these alterations, some gases condense into aerosol particles known as secondary organic aerosols (SOAs). The particles traveling with the gases are similarly transformed. The black and brown carbon aerosols provide airborne surfaces where the transformed gases accumulate.
And as the particles remain aloft, sunlight bleaches the light-absorbing molecules they contain. Eventually, dark particles lighten. As the smoke travels, it continues to transform.
Its ultimate fate is weather dependent. Wind can send it into urban centers or out to sea. Rain can wash the air clean.
And if the fire is powerful enough, a plume can even punch its way past the troposphere into the stratosphere, where it will linger even longer. Understanding how aged smoke differs from fresh smoke is vital to understanding the climate and health effects borne by a smokier future.
Illustration and scroll animation by Kay Youn The video could not be loaded. Features Recurring stories and special news packages from C&EN. Read More Key insights Decades of fire-suppression policies have left forests rife with fuel to burn; either through wildfires or prescribed burns, our future will likely be smokier than our recent past.
By the time a smoke plume reaches a distant city, its contents have aged and chemically changed. Researchers are working to determine how the composition of aged plumes differs from their fresh counterparts in an effort to understand climate and health risks of our fiery future. For a few days this summer, millions of people living far from the edge of the wilderness were reminded of the combustive power of wildfires when smoke from uncontrolled infernos burning through the forests of Ontario billowed across the border into the US.
Unfortunately, it’s likely that such intermittent hazy days are here to stay. “We’re in a fire deficit,” explains Tyler McIntosh , a PhD student who studies wildfires with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder. “We’ve had a lot less fire on the landscape than we theoretically should have over recent history.” Return to top
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