Where tailpipe emissions end up

By Robert Sanders,Public Affairs

17 April 2002 | After they leave the tailpipe, 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. But for a long time as much as half the resulting nitrogen oxide has been unaccounted for in the atmosphere, leaving air pollution models incomplete.

Now campus chemists, aided by the most world’s most sensitive detector of nitrogen dioxide, think they have found the missing nitrogen oxides.

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 — including chemicals such as isoprene nitrate formed when hydrocarbons emitted by trees combine with tailpipe emissions of nitrogen oxide. The latter presumably come from the city of Sacramento, which is upwind of the forest. In Houston, other alkyl nitrates are formed by combining nitrogen oxide with industrial hydrocarbon chemicals.

“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,” said Ronald Cohen, professor of chemistry and leader of the research team. “… We hope our device for measuring NO compounds is a better tool for following pollution.”

This is an example 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, who is trying to make a more compact and cheaper nitrogen oxide detector for routine air pollution monitoring in urban areas. The nitrogen dioxide detector could also potentially be used to detect 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 – as 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 nitrogen oxide emissions, which are caused by people, cause pollution.”