You may not be able to smell cow poop in Yellowstone, Glacier, and Rocky Mountain National Parks, but the air there has become increasingly contaminated with nitrogen compound ammonium, says a recent report from the National Park Service. Possibly originating in concentrated animal feeding operations, ammonium in the three parks — as well as six other parks in Arizona, Idaho, South Dakota, and Utah — was most likely borne in from the east by snow and rain, says the report. Ammonium can subtly alter ecosystems; for example, scientists are noticing Rocky Mountain’s iconic wildflowers giving way to grasses. Says John Vimont of the Park Service, “I think we should be watching it, from the standpoint that we don’t really know what’s going on.”