Trend worsening for pollutant in 16 national parks

A pollutant that can slowly trigger changes in the lives of plants and animals is increasingly found in 16 National Park Service sites, mostly in the Western United States.

Air quality data obtained by The Associated Press shows significant worsening trends for ammonium in several flagship parks, including Yellowstone, Mount Rainier and Utah's Canyonlands.

Ammonium is often linked with fertilizers, agricultural operations and urban pollution. Rich in nitrogen, the pollutant can act like a fertilizer and cause changes to soil and water chemistry. In large enough doses, it can also affect the diversity of species and give fuel for exotic grasses that, in turn, facilitate the spread of wildfires.

Scientists say they're unable to pinpoint the sources of the ammonium, which is carried in rain and snow.