Unusual conditions lead to murky air around Bay Area
In the wake of several recent rounds of heavy rain, and with no major wildfires burning anywhere near the Bay Area, a widespread decline in air quality levels over the weekend and into Monday left some in the region perplexed.
The National Weather Service and Bay Area Air Quality Management District both fielded inquiries Monday on Twitter by people surprised by soupy skies and elevated pollution levels showing up on air quality maps.
Meteorologists said a combination of factors was to blame.
The dense, acrid fog that has pervaded parts of the Bay Area over the past several days originated in the Central Valley, where car exhaust and other pollutants got trapped in a thick atmospheric cloud and were blown in by northeasterly winds at night, experts said.
The tule fog “is a combination of fog and whatever gets emitted day in and day out,” National Weather Service meteorologist Rick Canepa told The Chronicle on Monday, noting that this weather pattern typically occurs in winter after a period of torrential rain.
The fog usually stays in the San Joaquin and Sacramento valleys, hemmed in by surrounding mountains. But this year, an unusual spate of atmospheric rivers and light winds allowed the fog to migrate and settle over the Bay Area.
Bursts of rain in October and November gave way to a stable period, and a layer of fuzzy, contaminated fog several hundred feet deep developed, starting just above the ground. A lack of wind and a seasonally low sun angle kept it in place, pushing the air quality to “very unhealthy” purple levels on some handheld sensors, though readings oscillated from “good” to “moderate” on the official monitors of the Bay Area Air Quality Management District.
Maps from the popular PurpleAir website, which crowdsources its readings from sensors people buy and install at their homes, showed air quality in the 100-150 “unhealthy for sensitive groups” category in many places, compared with “moderate” levels around the 70s on the official air district site.
While the lower-cost sensors are capable of detecting and measuring air pollution in real-time — faster than the hours-long delay for the official monitors — they don’t distinguish as well between humidity and harmful particulate matter from wildfire smoke, fireplaces or factories, air district spokesperson Aaron Richardson said.
Northwest winds were expected to start dissipating the fog Monday evening, Canepa said, noting that barely perceptible gusts were blowing at about 3 p.m.
“We’re right on the leading edge of it,” he said.
Likewise, Richardson said he expected low to moderate levels of pollution for the rest of the week.
In some senses, he added, the harsh air quality is not that surprising.
Normally, fall and winter mark a shift in sources of contamination, from ozone and automobile exhaust that cook during the long hot days of summer, to wood smoke throughout the winter. Winter also brings an offshore flow that pushes fog up from the Central Valley, as well as a temperature inversion, with cold air near the ground and high air far above, forming a lid.
The stagnant weather allows wood smoke to build up, Richardson said. If it gets significantly worse, the air district will issue a Spare the Air alert , making it illegal to burn wood in fireplaces or stoves throughout the Bay Area.