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
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
"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
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
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
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