A heavy curtain of toxic air hung over California’s Yosemite National Park on Sunday as the Washburn Fire continued to burn out of control across parched terrain amid hot, dry weather.
“Visitors to Yosemite should reduce or avoid physical activity outdoors,” the national park advises.
Air quality in and around the park reached “very unhealthy” levels, and smoke from the wildfire spread north along the Sierra Nevada toward the Tahoe Basin and into the foothills.
Smoke from the #WashburnFire will affect the Sierra today. For more on air quality, please visit #CAwx pic.twitter.com/T5uULe0yXE
— NWS Sacramento (@NWSSacramento) July 10, 2022
“Smoke is forecast to continue moving north overnight with southerly winds,” said Robert Barufaldi, a forecaster with the National Weather Service’s Sacramento office.
Smoke is not expected to reach the San Francisco Bay Area on Sunday as offshore winds blow from the Pacific Ocean into the Central Valley, preventing smoke from reaching the region. The choked air could start moving into the Bay Area on Monday if the winds change.
“Plume of smoke moving north but may move into the Bay Area on Monday,” the weather service’s Monterey office tweeted.
Satellite images of the San Francisco-Monterey area show a stratus layer just off the coast and the smoke plume from the #WashburnFire (It’s the gray blob west of Lake June.)
The plume of smoke is heading north, but could head into the Bay Area on Monday. pic.twitter.com/Im1nL7AMhV
— NWS Bay Area 🌉 (@NWSBayArea) July 11, 2022
The concentration of pollution in the air is measured using the air quality index, which works on a scale of 0 to 500. The higher the AQI value, the higher the level of air pollution and the greater the health concern. An AQI value of 50 or lower represents good air quality, 51 to 100 is moderate, 101 to 150 is unhealthy for sensitive groups, 151 to 200 is unhealthy, above 201 to 300 is very unhealthy, and above 301 signals hazardous conditions .
At 6 p.m., the widely used website PurpleAir measured AQI levels above 200 in several locations in the park and above 150 in several others. Air quality levels in Bear Valley and Kirkwood were between 100 and 110, while locations around Lake Tahoe were generally between 50 and 75, with several locations above 100. The federal website AirNow.gov showed unhealthy levels throughout Yosemite National Park.
PurpleAir numbers are measured in real time (averaged over the last 10 minutes). AirNow’s numbers, which are based on EPA standards, are calculated using a complex algorithm that “uses longer averages during periods of stable air quality and shorter averages when air quality the air is changing fast’. Results are updated every hour, but with a delay compared to PurpleAir.
PurpleAir sells its own sensors to citizens and then uses the data from the monitors to track particulate pollution on a global scale. People can place them indoors or outdoors at their discretion.
The sensors use a laser particle counter to measure the number of particles in the air and then use an algorithm to calculate a mass concentration based on the number. These differ from the sensors used by AirNow, which measure particulate matter by drawing air through a filter and then weighing the filter.
The Washburn fire was 1,591 acres as of Sunday afternoon and was burning at the south end of the park near Mariposa Grove, threatening its 500 giant sequoias.
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