Air pollution long has been a threat to national parks, diminishing the viewsheds in places such as Sequoia, Great Smoky Mountains, Grand Canyon, and Mesa Verde national parks.
    In an effort to keep up monitoring efforts, the National Park Service has extended a contract with RTI International, which has been doing air monitoring in national parks since 1985. The five-year, $2.3 million contract extension calls for the company to sample the air in more than 150 national parks and wilderness areas for chloride, sulfur and nitrogen compounds. The scientists also will check ozone levels.