Residents checking air quality Sunday as smoke from the Boyle Heights cold-storage warehouse fire drifted across Southern California may have found very different answers depending on which website or app they consulted.
One source showed unhealthy air in parts of Pasadena while another rated conditions as fair or moderate, raising questions about what data to trust.
Experts say different readings don’t necessarily mean one source is wrong. Instead, they can reflect differences in where monitors are located, how frequently they update, the methods used to calculate air quality and rapidly shifting smoke carried by changing winds.
As the warehouse fire continued to burn Sunday, the South Coast Air Quality Management District extended its Special Particle Pollution Advisory through Monday at 12:30pm, warning that smoke could continue affecting communities beyond Boyle Heights.
“The atmosphere is very dynamic,”John Crounse, a research scientist at California Institute of Technology who leads the team behind the institute’s air quality monitoring station, said Sunday.
He said the warehouse can be thought of as a single point source of pollution. Rather than spreading evenly across Southern California, the smoke largely follows the direction of the wind, meaning neighborhoods directly downwind may experience much poorer air quality while communities only a few miles away can see much clearer conditions.
That pattern was reflected in Sunday’s weather forecast, said David Gomberg, lead meteorologist at the National Weather Service office in Los Angeles/Oxnard.
An onshore sea breeze was expected to push smoke and pollutants primarily east and northeast of Boyle Heights, with the greatest impacts extending into the San Gabriel Valley and farther east into parts of San Bernardino and Riverside counties, he said.
“The wind today is coming from kind of an onshore sea breeze, so that’s coming more from the southwest to west direction,” Gomberg said. “That means that locations to the east and northeast are going to be most impacted by the smoke and the pollutants.”
Gomberg said some smoke could also drift into portions of the San Fernando Valley and the L.A. basin during the morning because of a brief overnight wind reversal before the stronger daytime sea breeze became dominant.
Where many readers become confused, experts said, is that different services aren’t always measuring or displaying air quality in the same way.
According to Nahal Mogharabi, a spokesperson for the South Coast Air Quality Management District, the agency’s air quality map combines data from regulatory monitors, temporary regulatory monitors deployed specifically for the Boyle Heights fire and a network of calibrated air quality sensors. The agency also uses U.S. Enivronmental Protection Agency-established methods to calculate the Air Quality Index.
Other services, Mogharabi said, may rely on different monitoring networks, different calculation methods or broader computer models that are less suited for neighborhood-level conditions in Southern California’s complex terrain.
Caltech’s monitor is designed for a different purpose altogether. Crounse said the station reports air quality using measurements from the previous 30 minutes to provide near-real-time information for people deciding whether to exercise or spend time outdoors, rather than relying on longer averaging periods that can smooth out rapid changes.
That approach was reflected Sunday morning, when Caltech’s monitor showed air quality in Pasadena deteriorating sharply around 9 am before improving dramatically by mid-morning.
Crounse said the rapid shift was driven by changing winds that carried the smoke plume through Pasadena before moving it elsewhere.
“The wind in Los Angeles will change fairly regularly as a time of day,” Crounse said. “They certainly can explain why you might see smoke at 9 a.m. … and then there’s no smoke at 11 a.m. That’s just an effect due to the winds changing.”
That also means residents shouldn’t assume a reading from one monitor represents conditions across the region.
“I would say, at first take, believe the numbers that you see,” Crounse said, noting that those readings may represent pollution averaged over a period of time and apply to the area immediately surrounding the monitor.
AQMD recommends residents rely on vetted air quality sources that use established scientific methods and to take precautions if they see or smell smoke at ground level.
Crounse offered similar advice.
“If it smells bad and looks bad, but still, the air quality number you read is good,” he said, “ well, trust your senses.”