Researchers find effects of NOx emissions in urban areas worse than thought

A team of U.S. scientists has found that the harmful effects of nitrogen oxide (NOx) emissions from fossil fuels could be worse in urban areas than previously thought. They were the first to conduct a large scale, 3-year study based on methods for tracing sources of nitrate in rainfall. Results have been published in the online edition of the journal Environmental Science and Technology. The news is important for the bioenergy community, because some biofuels (ethanol, first generation biodiesel) produce more NOx than conventional fossil fuels, whereas others (notably synthetic biofuels) emit significantly less.

The researchers say nitrogen oxides, the noxious byproduct of burning fossil fuels in cars and power stations that can return to Earth in rain and snow as harmful nitrate, could taint urban water supplies and roadside waterways more than scientists and regulators realize.

The three-year study, led by Emily Elliott, a professor of geology and planetary science in the University of Pittsburgh’s School of Arts and Sciences, recommends that urban areas and roadways be specifically monitored for nitrogen deposition. Nitrogen oxides can contribute to a wide variety of environmental and health ills. Nitrate — which forms when exhaust from vehicles and smokestacks oxidizes in the atmosphere — is an important contributor to acid rain and can result in stream and soil acidification, forest decline, and coastal water degradation. Other researchers recently reported that ecosystems recover much slower from this acid rain damage than previously thought (earlier post).

Elliott and her colleagues conducted the first large-scale application of a method for determining the source of atmospheric nitrate on rain and snow samples from 33 precipitation collection sites across the Midwestern and Northeastern United States, including Pennsylvania. The sites belong to the National Atmospheric Deposition Program (NADP), a cooperative of private organizations and U.S. government agencies that analyzes precipitation for chemicals such as nitrogen, sulfur, and mercury from more than 250 sites in the United States, Puerto Rico, and the Virgin Islands.

Although vehicles are the single largest source of nitrogen oxides in this region, the researchers found by analyzing the stable isotope composition of nitrate that the primary source of nitrate in their samples were stationary sources, such as power plants and factories, located hundreds of miles away. Stationary sources pump pollutants high into the atmosphere where they can be transported for long distances before falling to the ground. Vehicle exhaust is released close to the ground and more likely deposited over shorter distances near roadways. Most monitoring sites in the NADP network are deliberately located in relatively rural settings away from urban, industrial, or agricultural centers.

The amount of nitrate pouring over the cities and busy roadways thick with vehicles could be higher than monitoring data at most NADP sites reflect, and it is possible that a significant amount of this atmospheric nitrate finds its way into sensitive water supplies:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: fossil fuels :: nitrogen oxides :: nitrates ::

The researchers give the example of vulnerable water bodies such as the Ohio River or Chesapeake Bay. In aquatic ecosystems, excess nitrate can promote an overgrowth of oxygen-consuming algae and lead to an oxygen deficiency in the water known as hypoxia. Hypoxia kills marine creatures and creates 'dead zones' akin to the lifeless area of the Gulf of Mexico at the mouth of the Mississippi River. Determining the fate of major sources of nitrogen emissions is necessary to develop sound regulatory and mitigation strategies for both air and water quality, Elliott said.

The results highlight the need to improve our understanding of the fate of vehicle emissions—one way we can do this is by expanding monitoring networks to include more urban sites, Elliott said, adding that both vehicle and stationary sources are major contributors to air pollution in the region studied.

Elliott said that future research will further characterize the isotopic ratios of nitrogen oxides from various emission sources and quantify how these values change during transport and with different emission controls. She is looking for industrial partners who can provide samples from smokestacks for analysis.

Additionally, Elliott is interested in establishing an urban precipitation monitoring site in Pittsburgh to assess pollution sources that contribute to nitrate deposition in the Pittsburgh region.

Other researchers involved in this project are from the U.S. Geological Survey, the University of California at Berkeley, the NADP, and Cornell University, and the Institute of Ecosystem Studies in New York.

References:
E. M. Elliott, et al., "Nitrogen Isotopes as Indicators of NOx Source Contributions to Atmospheric Nitrate Deposition Across the Midwestern and Northeastern United States", Environ. Sci. Technol., ASAP Article 10.1021/es070898t S0013-936X(07)00898-X, October 20, 2007

Biopact: Research finds recovery from acid rain much slower than expected - September 29, 2007