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Nitrogen dioxide and the hydroxyl radical combine in the atmosphere to make either nitric acid or peroxynitrous acid; the so-called branching ratio of these two chemicals is important in models of ozone production. Credit: Caltech/Mitchio Okumura
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A team of scientists led by researchers from the
California Institute of Technology (Caltech) and NASA's Jet Propulsion
Laboratory (JPL) have fully characterized a key chemical reaction that affects
the formation of pollutants in smoggy air. The findings suggest that in the
most polluted parts of Los Angeles—and on the most polluted days in those
areas—current models are underestimating ozone levels, by between 5% to 10%.
The results—published in the journal Science—are likely to have "a
small but significant impact on the predictions of computer models used to
assess air quality, regulate emissions, and estimate the health impact of air
pollution, " says Mitchio Okumura, professor of chemical physics at
Caltech and one of the principal investigators on the research.
“This work demonstrates how important accurate laboratory
measurements are to our understanding of the atmosphere,” added JPL Senior
Research Scientist Stanley P. Sander, who led that team's effort.
The key reaction in question in this research is the
reaction between nitrogen dioxide, NO2, and the hydroxyl radical,
OH. In the presence of sunlight, these two, along with volatile organic
compounds (VOCs), play important roles in the chemical reactions that form
ozone.
Until the last decade or so, it was thought that NO2
and OH combine only to make nitric acid, HONO2, a fairly stable
molecule with a long lifespan in the atmosphere. "HONO2, or
nitric acid, dissolves in rainwater, so that the molecules get washed
away," Okumura explains. "It's basically a sink for these radicals,
taking them out of the ozone equation and thus slowing down the rate of ozone
formation."
Chemists had suspected, however, that a second reaction
might occur as well: one that creates a compound called HOONO, otherwise known
as peroxynitrous acid. HOONO is much less stable in the atmosphere, falling
apart quickly after being created, and thus releasing the OH and NO2
back for use in the ozone-creation cycle.
But what was not known with any reasonable
certainty—until now—is how fast these reactions occur, and how much HONO2
is created relative to the amount of HOONO created. Those relative amounts are
known as the branching ratio, so called because OH and NO2 can
chemically transform, or branch, into either HONO2 or HOONO.
Enter the Caltech and JPL teams. The JPL team took the
lead on measuring the rate at which the OH + NO2 reaction produces
both HONO2 and HOONO. They did this using "an advanced chemical
reactor built at JPL that was designed to measure reaction rates with very high
accuracy," says Sander.
Once the scientists had determined the combined reaction
rate for the two possible products—coming up with rates that are on the higher
rather than the lower end of the scale of previous estimates—the Caltech group
took the lead to try to uncover the branching ratio, or the ratio of the rates
of the two separate processes.
Using a powerful laser measurement technique called
cavity ringdown spectroscopy, the team was able to detect both products being
created in the lab in real time, says Okumura. "We could start the
reaction and watch, within microseconds, the products being formed," he
says. "That allowed us to measure the species immediately after they were
formed, and before they got lost in other side reactions. That is what allowed
us to figure out the branching ratio."
Because HOONO was not a well-studied molecule, another
key was using state-of-the-art theoretical calculations; for this, the authors
enlisted Anne McCoy, professor of chemistry at The Ohio State University.
“Solving this atmospheric chemistry problem required us to use many tools from
modern chemical physics,” says Okumura.
"This work was the synthesis of two very different
and difficult experiments," adds Andrew Mollner, the Science paper's first author and a
former Caltech graduate student who is now at the Aerospace Corporation.
"While neither experiment in isolation provided definitive results, by
combining the two data sets, the parameters needed for air-quality models could
be precisely determined."
In the end, what they found was that the loss of OH and
NO2 is slower than what was previously thought—although the
reactions are fast, fewer of the radicals are going into the nitric acid sink
than had been supposed, and more of it is ending up as HOONO. "This means
less of the OH and NO2 go away, leading to proportionately more
ozone, mostly in polluted areas," Okumura says.
Just how much more? To try to get a handle on how their
results might affect predictions of ozone levels, they turned to Robert Harley,
professor of environmental engineering at the University of California,
Berkeley, and William Carter, a research chemist at the University of
California, Riverside—both experts in atmospheric modeling—to look at the
ratio's impact on predictions of ozone concentrations in various parts of Los
Angeles during the summer of 2010.
The result: "In the most polluted areas of L.A.," says Okumura,
"they calculated up to 10% more ozone production when they used the new
rate for nitric acid formation."
Okumura adds that this strong effect would only occur
during the times of the year when it's most polluted, not all year long. Still,
he says, considering the significant health hazards ozone can have—recent research
has reported that a 10 ppb increase in ozone concentration may lead to a four
percent increase in deaths from respiratory causes—any increase in expected
ozone levels will be important to "people who regulate emissions and
evaluate health risks." The precision of these results reduces the
uncertainty in the models—an important step in the ongoing effort to improve
the accuracy of the models used by those policymakers.
Okumura believes that this work will cause other
scientists to reevaluate recommendations made to modelers as to the best
parameters to use. For the team, however, the next step is to start looking at
"a wider range of atmospheric conditions where this reaction may also be
very important."
Sander agrees. "The present work focused on
atmospheric conditions related to urban smog—i.e., relatively warm temperatures
and high atmospheric pressure," he says. "But the OH + NO2
reaction is important at many other altitudes. Future work by the two groups
will focus on the parts of the atmosphere affected by long-range transport of
pollution by high-altitude winds (the middle and upper troposphere) and where
ozone depletion from man-made substances is important (the stratosphere)."
In addition to Okumura, Sander, Mollner, McCoy, Harley,
and Carter, the other authors on the Science
paper, "Rate of Gas Phase Association of Hydroxyl Radical and Nitrogen
Dioxide," are postdoctoral fellow Lin Feng and graduate student Matthew
Sprague, both from Caltech; former JPL postdoctoral researchers Sivakumaran
Valluvadasan, William Bloss, and Daniel Milligan; and postdoctoral fellow
Philip Martien from the University of California, Berkeley.
Their work was supported by
grants from NASA, the California Air Resources Board, and the National Science
Foundation, and by a NASA Earth Systems Science Fellowship and a Department of
Defense National Defense Science and Engineering Graduate Fellowship.
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