Up in the air

Aerosol particles collected at UCI help scientists characterize the atmospheric seasonality of Southern California

As an oceanographer, Kate Mackey usually spends her time investigating biogeochemical cycles involving nutrients and the aquatic species that rely on them. But when she came to UCI in 2014, she met two researchers who inspired her to also look to the skies.

Claudia Czimczik, UCI professor of Earth system science, and Kathleen Treseder, UCI professor of ecology and evolutionary biology, had been collecting air samples from the rooftops of campus buildings. They asked Mackey, the Clare Boothe Luce Associate Professor of Earth System Science, if she might want to study what was in the samples to understand their sources and to determine how the chemicals they contained might affect organisms on land and in the sea.

Kate Mackey
Meet the Expert: Kate Mackey, Clare Boothe Luce Associate Professor of Earth System Science

For a paper that was published in the journal Atmospheric Environment in January, Mackey’s team – including Linh Anh Cat, at the time a Ph.D. candidate in ecology and evolutionary biology, and undergraduates Stephanie Stragier and Laura Robledo – examined the carbon and nitrogen isotopes in the residue to see what kinds of aerosols were in the atmosphere when the specimens were gathered.

“The research helped us to understand that there is a distinct seasonal cycle that has to do with Southern California’s climatological features,” Mackey says. “The marine layer that’s present in the summer traps all the exhaust from the cars and from industry – and so the aerosol signature looks very anthropogenic from fossil fuel combustion. In winter, the signature looks more like what you would expect from land plants being burned.”

She knows firsthand what it means to live under California’s current wildfire conditions. During the Silverado Fire in Orange County in 2020, she had to evacuate with her family and a few prized possessions to her father’s house in Arizona to escape the smoke.

Kate Mackey (top), Clare Boothe Luce Associate Professor of Earth System Science, consults with doctoral researcher Raisha Lovindeer.
Kate Mackey (top), Clare Boothe Luce Associate Professor of Earth System Science, consults with doctoral researcher Raisha Lovindeer. Steve Zylius / UCI

“I’ve been here seven years, and I can honestly say I’m struck by how much more frequent and intense the fires are now compared to when I arrived,” Mackey says. “That’s not long enough to draw a complete conclusion about climate, but there is definitely a trend that I’m noticing.”

She says her work traditionally has revolved around atmospheric deposition, meaning aerosols that drop into the ocean and change aquatic chemistry, so this angle of research represented an extension of her interests. Still, she says, her eyes were opened to the unique ecosystem in Southern California.

“We have a very stable climate. It’s sunny and warm most of the time, so most people don’t think there’s a vast difference between summer and winter here,” Mackey says. “But with aerosols, you can see this very distinct pattern in the isotopes that confirms there is seasonality here. Winds change direction; the aerosol sources change; the aerosol contents change. It’s very dynamic.”

– Brian Bell, UCI