“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 fire spread north along the Sierra Nevada toward the Tahoe Basin and foothills. Smoke from the #WashburnFire will affect the Sierra today. For more information on air quality 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 Baruffaldi, a meteorologist at 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 area. Sooty air could begin to move into the Bay Area on Monday if the winds change. “The plume of smoke is heading north but could drift into the Bay Area on Monday,” the weather service’s Monterey office tweeted. Satellite imagery of the San Francisco-Monterey area shows a plume just offshore and the plume of smoke from the #WashburnFire (It’s the grayish patch west of June Lake.) The smoke is heading north but could drift into the Bay Area on Monday. pic.twitter.com/Im1nL7AMhV — NWS Bay Area 🌉 (@NWSBayArea) July 11, 2022 The concentration of air pollution is measured using the Air Quality Index which operates on a scale of 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. An AQI value of 50 or below 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. As of 6 p.m., the widely used website PurpleAir measured AQI levels above 200 in some locations around 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 some spots 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 figures — which are based on Environmental Protection Agency standards — are calculated using a complex algorithm that “uses longer averages during periods of stable air quality and shorter averages when air quality changes rapidly.” Results are updated hourly but delayed compared to PurpleAir. PurpleAir sells its proprietary sensors to citizens and then uses the data from the monitors to monitor particulate pollution on a global scale. People can place them indoors or outdoors as they see fit. The sensors use a laser particle counter to count the number of suspended particles and then use an algorithm to calculate the mass concentration based on the measurement. They 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.
