Chimney Sweep

California wildfire smoke rising increased, makes air high quality worse

Colossal wildfire plumes that can be spotted from space have erupted on several California wildfires in the past months.

The Mosquito Fire burning in Placer and El Dorado counties produced torrents of smoke that soared tens of thousands of feet into the air. The cloud of soot and debris could be seen 60 miles away. Smoke from the blaze blanketed large swaths of Northern California and western Nevada, resulting in hazardous-level air quality.

Wildfire plumes in the western US are reaching greater heights than ever, a recent study reports — especially in California’s Sierra Nevada.

“The higher the plume reaches, the more likely it is to be transported rapidly over large distances,” said David Peterson, a meteorologist with the US Naval Research Laboratory-Monterey who was not part of the new study.

That means smoke, carrying a hodgepodge of chemical compounds, wafts farther distances and impacts more people. Wildfire smoke can irritate lungs and even cause wider health issues, especially for vulnerable populations like older adults, children and those with underlying health conditions.

A pyrocumulonimbus cloud from mosquito fire as seen from the air of a commercial airline flight over Sacramento, September 8, 2022. Video: Courtesy Gregory Van Acker

“Once these particles enter our bloodstream they can pretty much affect our entire bodies,” said Rosana Aguilera Becker, an environmental health scientist at Scripps Institution of Oceanography at UC San Diego.

The study, out of the University of Utah, used mathematical models to simulate plume heights for about 4.6 million wildfires. The analyzes examined burned areas detected by satellite in the western US and Canada from 2003 through 2020, during August and September. The researchers found that over those years, plume top heights increased hundreds of feet across much of the mountainous western US

But not all areas saw the same amount of growth.

“Sierra Nevada definitely stands out,” said study author John Lin, an atmospheric scientist at the University of Utah. In the Sierra, the researchers found that plumes grew about an additional 750 feet per year. On wildfires like the Mosquito, that’s meant intense plumes have soared to some 40,000 feet.

Firefighters watch a smoke column from a distance during the Mosquito Fire in unincorporated Placer County.

Stephen Lam/The Chronicle

Other regions, like the Southern Rockies and Eastern Cascades, increased by over 300 feet per year.

This upward trend increased even more after 2015, although the results weren’t statistically significant.

This uptick in plume top height was accompanied by increases in wildfire emissions that cause poor air quality, especially in the Sierra Nevada region. As the plumes explode in height, smoke can surge above the planetary boundary layer, the layer of the atmosphere closest to the ground.

You’ve encountered this boundary any time you’ve been on a plane descending to land.

“If you’re going into SFO, you probably are familiar with the times when it suddenly gets really rocky,” Lin said, “That’s a pretty good indication of where the (planetary boundary layer) starts.”

Smoke that makes it to these altitudes — about 3,000 feet above the ground — disperses more readily due to strong winds.

The researchers propose that the increase in wildfire plume height is due in part to climate change: Drier conditions and warmer temperatures enable fires at higher elevations. This vertical shift gives wildfire plumes a head start toward sending smoke particles higher into the atmosphere, above the planetary boundary layer.

Some plumes make it even farther, past the boundary between the troposphere and stratosphere. These are pyrocumulonimbus clouds, also known as pyroCbs.

“A pyroCb generally reaches the typical cruising altitudes of jet aircraft and beyond,” Peterson said. “So we’re talking 30,000 feet or higher.”

PyroCbs are similar in appearance to thunderstorm-producing cumulonimbus clouds: puffy and towering. A recent example is the immense plume produced by the Mosquito Fire, which soared to heights observed from airspace.

These clouds act like chimneys, funneling smoke up into the stratosphere.

“It’s a thunderstorm that’s ingesting smoke at the cloud base,” Peterson said. “And then it gets accelerated through that thunderstorm cloud and ejected through the top of it.”

The researchers identified increasing pyroCb activity within the Colorado Plateau over the study period. They also found a slight uptick in pyroCbs in the Sierra Nevada in recent years. Additional research is needed to get a clear picture of what’s happening with these extreme plumes, which are still a developing research area.

What is known, however, is that smoke that makes it to these altitudes can linger for months and spread over vast distances, potentially causing health issues for many.

“There is a great concern that wildfires will be — and are already — a major source of air pollution,” Aguilera Becker said.

Jack Lee (he/him) is a San Francisco Chronicle staff writer. Email:

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