A heavy curtain of toxic air hung over California’s Yosemite National Park on Sunday, as the Washburn Fire remained uncontrollable through parched terrain amid hot, dry weather.
“Visitors to Yosemite should reduce or avoid outdoor physical activity,” the national park advised.
Air quality in and around the park reached “very unhealthy” levels and smoke from the wildfires spread northwards along the Sierra Nevada toward the Tahoe Basin and into the foothills.
“The smoke is predicted to continue moving north at night with southerly winds,” said Robert Baruffaldi, a forecaster with the National Weather Service’s office in Sacramento.
The 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 the smoke from reaching the region. If the wind changes, sooty air could pour into the Bay Area on Monday.
“The plume of smoke is moving north, but could drift to the Bay Area on Monday,” the Monterey Weather Service’s office wrote. Twitterâ€
The concentration of pollution in the air is measured using the Air Quality Index which works on a scale from 0 to 500. The higher the AQI value, the greater the degree of air pollution and the greater the health risk. An AQI value of 50 or lower indicates good air quality, 51 to 100 is moderate, 101 to 150 is unhealthy for sensitive groups, 151 to 200 is unhealthy, more than 201 to 300 is very unhealthy and more than 301 indicates dangerous circumstances.
At 6pm, the widely used PurpleAir website measured AQI levels above 200 at a few locations in the park and above 150 at several others. Air quality levels in Bear Valley and Kirkwood were between 100 and 110, while sites around Lake Tahoe were generally between 50 and 75, with a few spots above 100. The federal AirNow.gov site showed unhealthy levels throughout Yosemite National Park.
PurpleAir’s numbers are measured in real time (averaged over the past 10 minutes). AirNow’s numbers — 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 own sensors to citizens and then uses the data from the monitors to monitor particulate matter pollution on a global scale. People can place them inside or outside as they see fit.
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 count. They differ from the sensors that AirNow uses, which measure particulate matter by drawing air through a filter and then weighing the filter.
The Washburn fire covered 1,591 acres on Sunday afternoon, burning on the south side of the park near Mariposa Grove, threatening the 500 giant redwoods.