Hazy skies trigger debate over San Francisco’s air quality

‘I can see and feel the smog. It’s bad.’

Dense fog and hazy gray skies blanketed the Bay Area over the weekend into Monday, causing Air Quality Index readings to spike and triggering debates on the quality of The City’s atmosphere and how it’s measured.

The smoggy air was caused by a temperature inversion, where warm air is held above cooler air, trapping wood smoke and vehicle exhaust, and other particulate matter close to the ground.

“It’s kind of like a lid that’s trapping pollution at ground level, which is why we’re seeing some mid to high, moderate air quality readings,” said Air District spokesperson Erin DeMerritt. Offshore winds also drove smog in from the Central Valley, the Air District said.

But just how much pollution was fouling the air became an open question. On Monday afternoon, a map from Purple Air, a network of personal air sensors that has become an essential tool for many during wildfire season, showed much of the Bay Area glowing orange, well above 100 AQI, which is considered hazardous for sensitive groups.

But the Air District’s readings told a different story. Although its monitors also showed elevated pollution levels, the Air District’s AQI readings hovered in the high 70s on Monday.

“Low-cost sensors do tend to overreport AQI values,” the Air District tweeted. “Many variables may impact readings that may not be related to air quality such as fog or humidity.”

But some who have come to rely on monitors like Purple Air expressed a growing distrust with the Air District’s data. “I can see and feel the smog. It’s bad. I trust @ThePurpleAir more than other authorities on air quality these days,”one user, Tom Schmidt, responded on Twitter (@enshun).

“One benefit of technology is that curious people have a lot of access to data and can keep the government on its toes! Good job citizens!” another user, Joanna Warrens tweeted (@j_jwarrens).

DeMerritt said the increasing number of at-home air sensors fills an important void in density and real-time air monitoring.

“It’s great that a lot of people are taking monitoring into their own hands and getting these low-cost sensors. They do provide information about real-time changes in air quality, but you have to view their readings cautiously, and we recommend viewing them in conjunction with the Air District’s data,” she said.

Currently, the Air District has one air monitoring station in San Francisco, at Arkansas Street near Potrero Hill, which takes readings hourly, versus sensors like Purple Air which uses laser counters to measure particulate matter in alternating five-second readings averaged over 120 seconds.

But DeMerritt cautioned that accuracy from at-home sensors can vary from manufacturer to manufacturer, sensor to sensor.

Purple Air monitors, for example, she said, don’t have driers or heaters, which means they can’t differentiate between road dust, sea salt or the fine particulates in wood smoke, which typically spike in the wintertime as temperatures drop.

“The Air District’s air monitoring network is the most accurate data available because these are very high-tech sensors. They are constantly maintained. They’re validated according to quality control and quality assurance requirements from the EPA to ensure that the data is consistent and accurate,” she said.

The Air District expects the inversion to dissipate as a weaker weather front passes through the area.


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