"Ozone levels here are comparable to urban settings such as L.A.," said Emily Schrepf of the nonprofit advocacy group the National Park Conservation Association as she beheld the diminished view. "It's just not right."
This is not the place to take in a whiff of fresh mountain air. Smog is so bad that signs in visitor centers caution guests when it's not safe to hike. The government employment website warns job applicants that the workplace is unhealthy. And park workers are schooled every year on the lung and heart damage the pollution can cause.
Ozone also is to blame for weakening many stands of the park's Jeffrey and ponderosa pines, leaving telltale yellowing of their long needles. Instead of absorbing carbon dioxide, they soak up ozone through the stoma in their needles, which inhibits photosynthesis. Ozone also stresses young redwood seedlings, which already face challenges to survival.
Although weakened trees are more susceptible to drought and pests, the long-term impact on the pines and on the giant redwoods that have been around for 3,000 years and more is unclear.
"It's not a great story to tell, but it's an important story to tell because you can look at us as being the proverbial canary in a coal mine," said Annie Esperanza, a park scientist who has studied air quality there for 30 years. "If this is happening in a national park that isn't even close to an urban area, what do you think is happening in your backyard?"
It's a problem in a handful of the nation's 52 parks that are monitored constantly for ozone, including Joshua Tree National Park in California's Mojave Desert and North Carolina's Great Smoky Mountains National Park, which is ringed by power plants and several major highways including I-40, a major tractor-trailer shipping route. But none is in the ballpark with Sequoia and its neighbor, Kings Canyon.
Smog is created when the sun's rays hit pollutants such as oxides of nitrogen and volatile organic compounds that are in motor-vehicle exhaust, solvents, pesticides, gasoline vapors, and decaying dairy manure.
"There is no simple answer to ozone pollution," said Thomas Cahill, a researcher at the University of California, Davis who studies air problem in Sequoia and across California.
Already this year, the level of ozone in Sequoia park has exceeded federal health standards, even though it's early in the summer ozone season. During the June-to-September summer season last year, the park violated the National Ambient Air Quality standard at least 87 times, compared with 56 at Joshua Tree and 12 at Great Smoky Mountains.
"It's tragic that the National Park Service is known for clean air, and then you see a sign saying it's unhealthy to breathe," Esperanza said. "It's so contrary to the national-parks idea."