Why 4 California National Parks Made the 'Worst Ozone Pollution' List

Smog obscures the view from Keys View in Joshua Tree National Park | Photo: Laura Hamilton/Flickr/Creative Commons License

California's national parks are the pride of the nation, but this week four of them were awarded a distinction their supporters would just as soon have missed: a place on the "worst ozone pollution" list.

The list, compiled by the Associated Press from data provided by the Environmental Protection Agency and the National Park Service, included the seven National Parks with the highest concentrations of ground-level ozone, a serious pollutant. At the top of the list, with the worst ozone pollution of any National Park, was Sequoia National Park in the southern Sierra Nevada. Federal clean air standards consider concentrations of ozone above 75 parts per billion (ppb) unhealthy: Sequoia had ozone levels higher than 75 ppb on 87 days during 2011, with ozone concentrations reaching 98 ppb.

Ozone concentrations between 76 and 95 ppb are considered unsafe for sensitive people, including children and people with asthma. Above 95 ppb even healthy adults can suffer ill effects.

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Joshua Tree National Park had the second-worst ozone levels on the list, with 56 days above 75 ppb and a high concentration reaching 91 ppb. Sequoia and Joshua Tree have the worst ozone pollution by far of any parks listed. Number three on the list, the Great Smoky Mountains National Park in Tennessee and North Carolina, exceeded 75 ppb on just 12 days in 2011: certainly bad enough, but only a fifth of the days Joshua Tree spent above the limit, and one seventh that of Sequoia's.

The remaining four parks on the list each violated standards for a total of about a week in 2011, with peak concentrations at 80 ppb or lower. California's Yosemite National Park and Mojave National Preserve are among the four, as are Big Bend National Park in Texas and Rocky Mountain National Park in Colorado.

Sequoia and Joshua Tree have been jockeying for Last Place in the clean air sweepstakes for some time. According to the National Park Service's figures, Joshua Tree had a worse ozone problem in 2010 than did Sequoia, while in 2009 Joshua Tree spent more days above the limit, but Sequoia reached a higher peak ozone concentration. 2008 was a really bad year for both parks. The chart below, compiled from data gathered from 2003-2005, shows Sequoia and Joshua Tree at the top of the ozone-polluted park pile even then. (The chart refers to an 85 ppb standard for ozone; this standard was knocked down to 75 ppb in 2008.)

Ozone readings in National Parks, 2003-2005. Click chart for larger version | NPS

Why do Sequoia and Joshua Tree fare so poorly in the ozone pollution category? Location, location, location.

Ozone is oxygen in a highly reactive form. The oxygen we depend on to breathe has two oxygen atoms per molecule. Ozone has three. In the upper atmosphere ozone protects us from ultraviolet radiation, but when it's at ground level it offers no such services. Instead, it's a chemical component of smog, a result of our petroleum-based economy, and it poses serious health risks. We don't generally emit ozone directly as a pollutant, but indirectly. Burning fossil fuels releases Nitrogen oxides (NOx) into the atmosphere, and when NOx mixes with volatile organic chemicals -- gasoline fumes, solvents, pesticides and other complex hydrocarbons -- in the presence of sunlight, the two react to form ozone.

Sequoia and Joshua Tree have the dubious fortune to be located downwind of two of the biggest producers of NOx and volatile organic chemicals in the US. Joshua Tree sits downwind of the famously smoggy Los Angeles basin and Inland Empire. Sequoia is just up the hill from the San Joaquin Valley, a region that routinely has the worst air pollution in the country. Industrial effluent and transportation exhaust fill the airsheds of both regions, with the San Joaquin adding fugitive emissions from massive agriculture and oil drilling to the mix. (Yosemite, in its spot farther north in the Sierra Nevada, has the benefit of arguably cleaner air coming in off the ocean from the Golden Gate to dilute things a bit, while the Mojave Preserve's ozone would seem to come mainly from Las Vegas.)

As ozone is oxygen looking to give up an extra atom, it readily oxidizes most substances in comes into contact with. This includes your delicate lung tissues. Not only can ozone damage them and make it harder for you to breathe, but there's some thought that ozone in the lungs can lead to an increased likelihood of atherosclerosis. It can also cause eye damage when concentrations are high enough.

Plants don't fare much better in ozone poisoning, an important consideration for any National Park, let alone Sequoia and Joshua Tree, which are (after all) named for their vegetation.

When taken into plants' leaves, ozone can interfere with photosynthesis, decreasing plant vigor and increasing susceptibility to diseases: not something generally thought of as desirable in National Park vegetation management plans. In this respect, Joshua Tree's plants may fare slightly better than Sequoia's: plants only take in ozone when their stomata -- the leaf pores through which they breathe -- are open, and in drought conditions plants keep their stomata closed as much as they can. Sequoia is a lot wetter than Joshua Tree, suggesting that its plants may run a greater average risk of damage. Of course, Joshua Tree's beloved cottonwood trees generally grow where there's soil moisture, and cottonwoods are especially susceptible to ozone damage.

Sadly, the single most effective remedy to the National Parks' ozone problem was postponed last September. In an announcement that infuriated environmentalists and public health advocates alike, President Obama ordered the EPA to back off on lowering the 8-hour ozone standard to somewhere between 60 and 70 parts per billion. (The World Health Organization recommends 51 ppb.) His action, widely seen as a response to pressure from conservatives and the fossil fuel industry, delays any EPA revision of the standard until late next year.

Park visitors may want to hold their breath until then.

Chris Clarke is an environmental writer of two decades standing. Director of Desert Biodiversity, he writes from Palm Springs regularly at his acclaimed blog Coyote Crossing and comments on desert issues on KCET weekly. Read his recent posts here.

For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.

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