Why Bay Area air quality often worsens in the middle of winter
Californians are used to thinking about bad air coming in the summer, when ozone peaks and wildfires send particulate-heavy air to the Bay Area. But some of the Bay’s worst air quality days happen in the dead of winter.
“In the summertime, we always get that strong bay breeze,” Eugene Cordero, a climatologist at San José State University, explained. The breeze is fueled by cold ocean waters sucking off heat from California’s warm inland areas. “That sea breeze is moving pollutants all the time, but in the winter we don’t have that.”
The Earth’s surface is always radiating warmth into space, but in the winter less sunlight hits the ground. The lack of sun means the air closest to the ground is cool, sometimes so much that it’s colder than the layer of air above it.
Think how the air is cooler and thinner at the top of a mountain than at sea level — this sometimes flips in the winter, creating what meteorologists call an “inversion layer.”
The warm air thousands of feet above is more energetic and higher pressure, so it holds the colder pocket of air down in place like a lid. This turns the bay into a kind of temporary snow globe, with the same air swirling day after day. That’s fine if the air is clean, but trapped pollutants can easily end up in people’s lungs.
“Auto exhaust and factories produce small aerosol particles that float around in the atmosphere,” Cordero said. Exhaust levels can be especially high in the colder months, when people are frequently running furnaces and heat pumps. Without a steady bay breeze to flush
away these aerosols, they fill the snow globe quickly. “On Saturday I did a bike ride in the hills and it was just beautiful, visibility in the bay was great,” says Cordero, but the next day he couldn’t see nearly as far. “Even one day without strong winds, and you can tell the difference.”
It can take days before fresh winds come along and burst the bubble. In the meantime, sparking a fire in a wood stove or fireplace adds to the problem.
Burning wood
“Wood burning is the worst,” says Clifton Mass, a meteorologist at the University of Washington. “It puts out carcinogens and particles — the toxicity is terrible.”
It may be surprising to learn that in a state plagued by seasonal wildfires, those cozy nights huddled around the hearth can pollute the entire Bay Area.
Wildfires produce much more smoke, but these plumes typically drift into our region far overhead, Mass explains. Small fires burning in fireplaces can cause worse air pollution because they’re so close to the ground.
Chimneys have a flue to keep carcinogens from billowing
into the house, but there’s no flue over the Bay on these snow globe days.
“In fact, sometimes when there’s bad Spare the Air days, I can go outside and smell it — it smells like a fireplace,” says Cordero.
When particulate levels trigger a Spare the Air day in the Bay Area, burning wood is illegal.
Ozone vs. particles
Spare the Air days in the summer are usually caused by an entirely different pollutant: ozone.
“You need pollutants from cars and factories, and you need sunlight and heat. That’s the recipe for ozone,” says Cordero.
While this chemical reaction is needed to produce summer ozone, on winter Spare the Air days, Californians breathe in particulate directly, explained Cesunica Ivey, an environmental engineering professor at UC Berkeley.
“Both of them can trigger asthma attacks in high quantities. I don’t think there’s a lesser of the two evils,” Ivey said.