U.S. Exports Nitrogen Pollution Beyond Its Borders, Europe’s Nitrogen Deposited Close to Sources

The United States exports nitrogen pollution beyond its borders, and some of this nitrogen may end up in Western Europe, according to a recent data analysis by scientists at the National Center for Atmospheric Research (NCAR) and the University of New Hampshire. Most of the nitrogen pollution produced in Western Europe is deposited within its own boundaries, the authors found. The findings are an important step in quantifying total U.S. pollution export for policy makers. The study was published in the February issue of the journal Ecological Applications.

Nitrogen pollution degrades air and water quality and alters terrestrial and aquatic ecosystems worldwide, with profound consequences for human health and agriculture.

Atmospheric nitrogen emission and deposition are out of balance for the continental United States but more closely balanced in Europe, the authors found when they analyzed data gathered between 1978 and 1994. Only 40% of U.S. nitrogen released into the atmosphere as trace gases, including nitrogen oxides (NOx) and ammonia (NH3), was found deposited within U.S. boundaries during the study period. Between 5.30 and 7.81 million metric tons (5.83 and 8.59 million U.S. tons) of total NOx and NH3 were unaccounted for and possibly transported elsewhere each year, the authors estimate.

“According to our study, the United States is exporting nitrogen, ”says lead author Elisabeth Holland of NCAR, “with the majority of it coming from fossil fuel combustion in factories, cars, and power plants. And there’s some evidence that a portion of the U.S. nitrogen pollution may end up in Western Europe.” More research is needed before scientists can say how much nitrogen reaches Europe, she adds.

Trace gases can be blown downwind over long distances until the gases and their reaction products are washed out by rain or snow or deposited in a dry state on Earth’s surface. Nitrogen emitted in the United States is often blown eastward and deposited on the surface of the Atlantic Ocean. Some nitrogen compounds, traveling at higher elevations, could reach Western Europe.

The authors used the limited number of network observations available over land together with geostatistical mapping techniques and computer models of chemical removal to map nitrogen emission and deposition in the United States and Western Europe from 1978 to 1994.

They found that nitrogen deposited in the United States was mainly in the form of compounds produced by fossil-fuel burning. However, the amount of nitrogen found deposited here was only 40% of the total amount emitted into the atmosphere in trace gases. The difference suggests nitrogen-containing gases were being transported beyond U.S. boundaries.

In contrast, Western Europe received five times more nitrogen in precipitation than the lower 48 states during the study period, with nearly that much deposited on the surface there. Most of the nitrogen found in deposits in Western Europe was in the form of compounds produced by farming and animal husbandry.

Human population growth, fossil fuel consumption, deforestation, and intensification of agriculture have accelerated nitrogen emission and deposition over the past century and a half, especially in the Northern Hemisphere. The result is that higher levels of nitrogen enter the atmosphere in trace gases, which can act as pollutants. Nitrogen pollution is most known for its role in damaging ecosystems through acid rain and contaminating air by forming ozone, which harms living tissue and decreases plant production.

The authors caution that their results are only estimates. The data were gathered from an observation network originally set up to measure the effects of acid rain on rural and remote regions. The sparsely scattered U.S. sites, situated far from cities, may be under-measuring nitrogen deposition from urban emissions, they say.

The study was funded by the National Science Foundation, the Department of Energy, NASA, and Germany’s Max Planck Institute for Biogeochemistry. NCAR’s primary sponsor is the National Science Foundation.