Officials are chalking up recent discrepancies in the Bay Area's private and government air quality readings to a temperature inversion and offshore coastal winds.
Readings from PurpleAir sensors in parts of the Bay Area exceeded 100 on the Air Quality Index over the weekend and again on Monday, while the Bay Area Air Quality Management District's own readings measured no worse than "moderate" air quality – or, between 50 and 100 on the 500-point scale – throughout the weekend. The agency said Bay Area air quality didn't exceed the federal health standard.
In a Twitter thread on Monday, the Air District said offshore winds originating in the Central Valley brought pollutants to the Bay Area, while the current "strong temperature inversion" can trap those and other pollutants at ground level. Combined with PurpleAir sensors' sensitivity to fog and humidity, the Air District said readings can look much different.
The Air District has said previously its air quality readings provide the most accurate data, whereas the U.S. Environmental Protection Agency's AirNow Fire and Smoke Map is the timeliest. The latter incorporates PurpleAir’s readings, as well as data from the Air District's own monitoring stations.
Those stations are more precise, according to the agency, because PurpleAir sensors and other low-cost alternatives might be placed in a location that isn’t representative of the surrounding area.
Moderate air quality is expected in the Bay Area through the end of the week, according to Spare the Air’s five-day forecast.
In replies to Bay Area residents, the National Weather Service Bay Area tweeted that air quality isn’t likely to improve until later in the week. The agency said it might take a weather front or a storm to undo the current weather pattern, which is a big driver of the discrepancies in air quality readings.
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