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Sensitive detector identifies missing nitrogen oxide pollutant in atmosphere, UC Berkeley chemists report
22 March 2002

By Robert Sanders, Media Relations

Berkeley - Along the route from tailpipe through atmosphere, nitrogen oxides - collectively known as NOx compounds - react with hydrocarbons to form a variety of nitrogen-containing pollutants, including nitric acid, the cause of acid rain. Until now, however, as much as half the resulting nitrogen oxide has been unaccounted for in the atmosphere, leaving air pollution models incomplete.

Chemists at the University of California, Berkeley, think they have found the missing nitrogen oxides with the aid of the most sensitive detector of nitrogen dioxide (NO2) in the world.

Deploying the detector in downtown Houston and in a remote Sierra Nevada forest, they detected large amounts of organic nitrogen oxide (NO) compounds, alkyl nitrates, that were thought to be only a minor constituent of smog. In the forest, these alkyl nitrates included chemicals such as isoprene nitrate, which could only come from combining hydrocarbons emitted by trees with tailpipe emissions of NOx, presumably from the city of Sacramento, which is upwind of the forest. In Houston, other alkyl nitrates are formed by combining NOx with industrial hydrocarbon chemicals.

The UC Berkeley team's description of the instrument and their analysis of test data from a forested research plot in the Sierra will appear in the March 2002 issue of the Journal of Geophysical Research-Atmospheres. The article was posted on the Web this week.

"Nitrogen oxide radicals are the major species controlling production of photochemical smog and subsequent chemical reactions in the atmosphere," said Ronald C. Cohen, professor of chemistry and leader of the UC Berkeley research team. "Until now, though, no one had really looked at what are, in some places, the most abundant NO-containing chemicals in the atmosphere.

"Our technique allows us to identify the molecules in the atmosphere and then build models of air pollution that are more accurate, that have the right chemistry. With the right chemistry, we can get better predictions. We hope our device for measuring NO compounds is a better tool for following pollution."

Cohen also is a professor in UC Berkeley's Department of Earth and Planetary Science, a faculty scientist in the Energy and Environment Technologies Division at Lawrence Berkeley National Laboratory and a member of the Berkeley Atmospheric Sciences Center.

This is another instance, he said, of how man-made NOx compounds react with natural hydrocarbons from vegetation to produce ozone smog, affecting not only human health but causing global climate change.

"Ozone is toxic to humans, in particular those with asthma," Cohen said. "In addition, ozone in the troposphere has doubled in the past century, contributing 10 to 15 percent of the human additions to the greenhouse effect. All of this is driven by NOx."

Cohen is trying to make a more compact and cheaper nitrogen oxide detector for routine air pollution monitoring in urban areas. Today's smog monitors measure essentially the sum total of all nitrogen oxides in the air, and are unable to break this down into the specific amount of each NO-containing chemical.

"Cutting NO emissions actually works to reduce smog and greenhouse gases, and today's NO detectors are OK for monitoring that," Cohen said. "But if we want to understand quantitatively the effect of local pollution on the global scale, we need to know how and in what form NO is transferred to the rest of the globe. We need instruments like this to identify the classes of nitrogen oxides."

The nitrogen dioxide detector has other possible uses, too, including as a sensitive detector of the nitrogen compounds in explosives.

Nitrogen oxides from auto and smokestack emissions, among other sources, react readily with hydrocarbons to form compounds that eventually lead to ozone. As former President Ronald Reagan asserted, trees are a big part of the problem - hydrocarbons from trees are just as effective as exhaust hydrocarbons in reacting with NOx to create ozone.

"Reagan was telling only half the story," Cohen said. "Trees alone do not cause pollution, trees in combination with NOx emissions, which are caused by people, cause pollution."

On the way from NOx and hydrocarbons to ozone, the nitrogen oxide is converted to various forms, including nitric acid, which falls out in rain, and peroxy(acyl)nitrates, which travel widely around the globe because they do not dissolve in rainwater. Together with unreacted NOx, these account for between 50 and 90 percent of all the NO in the atmosphere. The identity of the remaining chemical reservoir of NO was a mystery.

To solve the mystery, Cohen and his laboratory colleagues spent about four years developing a detector that could simultaneously measure all the different NO compounds in air. What he came up with is a device that flash heats an air sample at the tip of an inlet to convert NO-compounds to nitrogen dioxide (NO2), and then measures the amount of NO2 by hitting the sample with a tunable dye laser and measuring the amount of fluorescence.

By flash heating the air at different temperatures, he takes advantage of the fact that different NO compounds decompose to NO2 at different temperatures. He thus is able to measure separately the levels of nitric acid, peroxynitrates, and the sum of all alkyl and hydroxyalkyl nitrates.

The technique, which Cohen refers to as thermal dissociation-laser induced fluorescence (TD-LIF), can monitor NO compounds continuously with sensitivity down to 30 parts per trillion. This is 1000 times more sensitive than needed for today's pollution monitoring.

He and his group have used the device on an airplane flying over Canada and Greenland, as well as on the ground in urban Houston and in a mountain forest, the University of California's Blodgett Forest Research Station in El Dorado County. Interestingly, the relative amounts of NO compounds are about the same in Houston as in the Sierra Nevada, though the levels are four times greater in the city.

"It was shockingly similar what we found in the forest and the city," Cohen said.

Cohen's colleagues on the paper are UC Berkeley graduate students Douglas A. Day, Michael B. Dillon and Joel A. Thornton, and staff scientist Paul J. Wooldridge.

The research is supported by the National Aeronautics and Space Administration.