Nitrate fills atmosphere
Emily Elliott, an assistant professor in the department of geology and planetary science at the University of Pittsburgh, led a team of researchers in investigating the link between the presence of nitrogen in the atmosphere and vehicle emissions.
The three-year study was published in American Chemical Society on Oct. 20.
Nitrogen is not only an essential constituent of key industrial compounds such as nitric acid, but is also an integral component of all living tissues and proteins.
A nitrate, which is one of the many salts found in nitric acid that forms as a result of the oxidation of nitrogen, is made up of one nitrogen and three oxygen atoms. Stable isotopes of nitrate form when an additional neutron is added to nitrate, changing its mass number.
Any form of precipitation can lead to nitrate deposition in rivers and valleys by transporting nitrate from its source to other locations. This increase in nitrate deposition causes nitrogen levels in the atmosphere to rise.
In the past, tracking fuel emission from motor vehicles has been an effective method of assessing levels of nitrate deposition in urban areas.
However, Elliott’s study suggests that a more profound indicator of nitrate levels is the presence of stable nitrate isotopes across the midwestern and northeastern United States.
Stable nitrate isotopes are generally emitted from the combustion of fossil fuels, coal burning, lightening, and biomass burning. These isotopes are also present in any concentration of nitrate that reaches the earth by means of precipitation.
Elliott said, “These stable isotopes cause different reaction rates in the biological composition of nitrate. We took weekly rainwater samples from different regions and observed distributions of isotopes in rainwater to test the nitrate concentration in spatial and temporal variations.”
These samples of rainwater showed that most nitrate deposition was located across the northeastern United States.
Furthermore, researchers found that this nitrate deposition was mostly generated by stationary sources, including electricity generation and the burning of coal. However, nitrate deposition from stationary sources was generally higher in western states than in eastern states.
According to the published study, “global atmospheric emissions of nitrogen oxide have increased dramatically during the past 150 years.” This study suggests that the effects of these increasing levels of nitrogen are drastic and almost irreversible.
High levels of nitrogen in populated areas are toxic to human health and almost deadly to marine life. Historically, they have been responsible for wiping out populations of algae and small fish, as well as giving rise to hypoxia.
Hypoxia is the enrichment of surface water with high amounts of nitrogen, thereby depleting the water of oxygen. An extreme case of hypoxia is anoxic sea water, which has dangerously low levels of oxygen and is thus a significant threat to oceanic ecosystems.
Elliott and her team developed a new approach to detecting traces of nitrogen oxide in the atmosphere through extensive lab work using a mass spectrometer.
A mass spectrometer is a device that provides researchers with a specific measurement of isotope composition. It performs this measurement in the order of ionization, acceleration, deflection, and finally, detection.
Elliott said that this research also depended on geographical information and statistical data.
“We used bacteria to reduce the nitrate in rainwater to a gas form to test the samples for nitrate concentration,” said Elliott.
Developed by Princeton University a few years ago, this method of converting nitrate into a gas has two major functional benefits.
“Firstly, it requires very small volumes of rainwater for testing. Secondly, it acts upon even minimal concentrations of nitrates. These characteristics are perfect for rainwater,” Elliott said.
The publication states that “large areas of the landscape potentially receive atmospheric nitrate deposition inputs in excess of what one would infer from existing monitoring data alone.”
Researchers fear that levels of nitrogen in the atmosphere will keep increasing as they have in the past, eventually leading to massive destruction of biological life cycles as the nitrogen displaces oxygen in the air.
This study offers a way to trace the fate of nitrogen oxides from the stationary sources. It provides a wider scope for locating sites with heavy nitrogen concentration, as well as potential treatment.
According to a University of Pittsburgh press release, researchers investigated precipitation samples from 33 sites. All of these sites are a part of the National Atmospheric Deposition Program, an organization that studies the contamination of precipitation.
Elliott said that most sites from this study were located in rural areas and were not subject to heavy emissions from vehicle exhaust.
This condition rules out the possibility that such high levels of nitrogen in the atmosphere arose solely from active sources, such as vehicle emissions.