A new study from the University of Utah has shown that wildfire smoke can significantly raise ozone (O₃) levels, even in remote areas far from urban pollution sources.
The massive wildfires that swept through the western U.S. in August 2020 didn’t just fill the skies with thick smoke, they also changed the chemistry of the air over huge areas. 
The new research used an advanced computer model to show that wildfire smoke can produce large amounts of ozone (O₃), even with minimal contribution from human-made emissions.
Ozone isn’t released directly by fires, it forms when sunlight drives chemical reactions between nitrogen oxides (NOₓ) and volatile organic compounds (VOCs). While NOₓ often comes from human activities, VOCs are abundant in wildfire smoke, meaning fires alone can fuel significant ozone production.
Normally, ozone forms when sunlight triggers chemical reactions between gases from cars, factories and other sources. But wildfires also release those gases, known as ozone precursors, along with the particulate matter that makes up most of the visible smoke.
The team used a model that, in addition to tracking smoke, also accounts for how fires interact with weather and sunlight. The study compared the effects of wildfire smoke with emissions from human activities and also examined a lesser-known factor: smoke shading, where thick smoke blocks sunlight from reaching the ground.
The researchers found that smoke increased ozone concentrations to levels around 20–30% higher than normal. Within the smoke plume, wildfire emissions raised ozone levels by about 21 parts per billion (ppb), while human-made emissions added around 11 ppb.
That means wildfires alone could push ozone above the U.S. EPA’s 8-hour safety limit of 70 ppb in many places downwind, even if local human emissions were removed from the equation.
The World Health Organization’s recommended 8-hour safety limit is just 47 ppb.
Lead author Derek Mallia, a research assistant professor of atmospheric sciences said: ‘The question I wanted to ask was, if we don’t have urban emissions, let’s say that we zero out all emissions, will we still have an ozone problem? This study suggests that we could remove all of the regional emissions from anthropogenic sources of NOx, but fires can still produce a large amount of ozone.’
Smoke shading had a major influence on the weather and air chemistry. In the thickest plumes, sunlight reaching the ground dropped by about 400 watts per square metre, with surface temperatures falling by roughly 4°C.
This cooling slowed the upward mixing of air, which kept the smoke more concentrated and less dispersed. At the same time, with less sunlight available to drive chemical reactions, ozone levels inside the plume dropped by 5–10 ppb compared to sunny conditions.
Malia added: ‘You also have a lot of particulate matter, which is a pollutant, too, but it can block sunlight and therefore that will reduce the amount of sunlight available for ozone photochemistry. It can be substantial in some cases. If you’re right over the fire, there’s usually enough smoke shading where it limits the amount of ozone. But if you get far enough away and the plume becomes relatively diffuse, it’s usually not thick enough to really limit ozone.’
The full research can be read here.
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