Depositions already exceeding critical thresholds in some parks
By Summit Voice
FRISCO — From the high country of the Pacific Northwest to the hardwood forests of New England, airborne deposition of ammonia and nitrogen are already taking a toll on national parks.
In Great Smoky Mountain National Park, for example, the amount of nitrogen being deposited per hectare already far exceeds the threshold that causes damage, according to a study led by Harvard University scientists.
Hardwood trees start to suffer when nitrogen deposition reaches approximately 3 to 8 kilograms per hectare, per year. According to the new study, the actual rate of deposition is about 13.6 kilograms per hectare, per year. In the forests of Mount Rainier National Park, it’s the lichens that suffer first as the critical limit is passed.
Altogether, 38 national parks in the U.S. are experiencing “accidental fertilization” at or above a critical threshold for ecological damage. Unless significant controls on ammonia emissions are introduced at a national level, little improvement is likely between now and 2050.
Along with Harvard researchers, the study included scientists with theNational Park Service, the USDA Forest Service, the U.S. Environmental Protection Agency, and the University of California, Irvine. The findings were published in the the journal Atmospheric Chemistry and Physics.
The environmental scientists, experts in air quality, atmospheric chemistry, and ecology, have been studying the fate of nitrogen-based compounds that are blown into natural areas from power plants, automobile exhaust, and — increasingly — industrial agriculture. Nitrogen that finds its way into natural ecosystems can disrupt the cycling of nutrients in soil, promote algal overgrowth and lower the pH of water in aquatic environments, and ultimately decrease the number of species that can survive.
“The vast majority, 85 percent, of nitrogen deposition originates with human activities,” said Harvard scientist Daniel J. Jacob. “It is fully within our power as a nation to reduce our impact.”
Existing air quality regulations and trends in clean energy technology are expected to reduce the amount of harmful nitrogen oxides emitted by coal plants and cars over time. However, no government regulations currently limit the amount of ammonia that enters the atmosphere through agricultural fertilization or manure from animal husbandry, which are now responsible for one-third of the anthropogenic nitrogen carried on air currents and deposited on land.
“Ammonia’s pretty volatile,” Jacob said. “When we apply fertilizer in the United States, only about 10 percent of the nitrogen makes it into the food. All the rest escapes, and most of it escapes through the atmosphere.”
In many previous studies, environmental scientists have identified the nitrogen levels that would be ecologically harmful in various settings. The new Harvard-led study uses a high-resolution atmospheric model called GEOS-Chem to calculate nitrogen deposition rates across the contiguous United States, and compares those rates to the critical loads.
“The lichens might not be noticed or particularly valued by someone walking around a national park, but they’re integral for everything else that’s dependent on them,” said lead author Raluca A. Ellis, who conducted the research as a postdoctoral fellow at Harvard SEAS. She now directs the Climate and Urban Systems Partnership at the Franklin Institute.
Jacob, Ellis, and their collaborators predict that NOx emissions from the United States will decrease significantly by 2050 (globally, those decreases may be offset to some extent by increases in industrialization overseas).
But for ammonia, the story is different. The team predicts significant increases in the amount and density of agricultural land in the Midwest and the West — to feed a growing population and to meet an anticipated demand for biofuels — requiring more and more fertilizer.
“Even if anthropogenic NOx emissions were globally zero, avoiding exceedance at all national parks would require a 55 percent reduction of anthropogenic NH3 emissions,” according to the report.
“Air quality regulations in the United States have always focused on public health, because air pollution leads to premature deaths, and that’s something you can quantify very well. When you try to write regulations to protect ecosystems, however, the damage is much harder to quantify,” Jacob said. “At least in the national parks you can say, ‘There’s a legal obligation here.’”
“The point is, in the decades ahead, the problem in our national parks is not going to be solved by the reduction of NOx emissions alone,” Ellis explained. “It will require a targeted effort to control ammonia.”
“It’s a national issue, and I think that’s why having the national perspective was so important,” Jacob adds. “We’ve shown that most of the nitrogen deposition to parks in the United States is coming from domestic sources. It’s not coming from China; it’s not coming from Canada—it’s something we can deal with, but we need to deal with it at the national level.”