A fast way to measure PAN
Scientists hope to learn
more about this important atmospheric chemical and its connection
It’s been more than four decades since
scientists became aware of PAN, a chemical in the atmosphere
associated with ground-level ozone. But they could never
devise a way to quickly measure it—until now.
In a breakthrough with important
ramifications for understanding pollution, a team of researchers
at NCAR and the Georgia Institute of Technology has designed
a new type of mass spectrometer that can measure small
amounts of PAN in two seconds or less. The previous method,
which used gas chromatography, took about two minutes—an excruciatingly long period of
time if you’re flying through a plume of pollution
and want to measure more than one sample.
Frank Flocke and Aaron Swanson
“It’s really a revolutionary step,” says
ACD’s Frank Flocke, a leader of the research team with
The new instrument, called PAN-CIMS (for PAN chemical ionization
mass spectrometer) will be put through its paces this summer
during a series of international air quality projects in
New England and Europe.
PAN, or peroxyacetyl nitrate, is a naturally occurring but
uncommon chemical in the atmosphere that exists in quantities
of a few parts per billion in polluted areas down to about
one part per trillion or less over remote oceanic areas.
It remained unknown to chemists until the 1950s, when it
was identified during ground-level ozone episodes. Scientists
now know it can irritate the eyes and throat, as well as
What makes PAN and other peroxyacyl
nitrates particularly intriguing is they have no known
direct sources. Instead, they are formed in the atmosphere
from reactions that involve hydrocarbons and nitrogen oxides—which
also are the ingredients of ground-level ozone.
Differences in the chemical structures of PAN-type molecules
can be used to glean insights into the sources of ozone,
an important pollutant. For example, both terrestrial plants
and human sources like cars and factories emit hydrocarbons.
But the PAN-type molecules associated with the biogenic emissions
differ from those associated with anthropogenic emissions.
PAN is important for another reason. A stable molecule at
cold temperatures, it can travel for thousands of miles high
in the atmosphere before descending, breaking apart, and
releasing nitrogen oxide. As a result, it acts as a transport
agent of pollution and is partly responsible for high levels
of ground-level ozone in otherwise pristine regions.
Frank has focused on PAN for much of his 12 years
at NCAR. In the 2000 Texas Air Quality Study, he
analyzed different types of PAN compounds to conclude
that industrial emissions, not isoprene from plants,
were contributing significantly to high ozone levels
in Houston (see the March 2002 issue of Staff Notes
But most of his work has relied on gas chromatographs
to measure PAN concentrations. Gas chromatography
involves sending air through a column that separates
individual components, primarily by molecular weight.
The components that come out of the column are then
measured and quantified as a function of time.
Although NCAR’s PAN gas chromatograph
is one of the fastest and most sensitive instruments
of its kind,
it takes about two minutes to analyze the important
PAN species. In contrast, common methods for measuring nitrogen
oxides and ozone take only about a second. “PAN
has always been the weak link,” Frank says.
A few years ago, Frank and Greg Huey began discussing
a different approach: a mass spectrometer. Their
idea was to heat the PAN molecules to split them
apart, ionize the resulting radicals (fragments of
the original molecule), and record their relative
abundance by measuring the respective ion.
The challenge, however, was
to keep the unstable radicals in existence long
enough to measure them. “The
tricky part is to get them to react with iodide before
they all disappear,” Frank says. This required
numerous adjustments to pressure, temperature, and
To create the PAN-CIMS instrument,
Frank worked closely with postdoctoral researcher
Aaron Swanson in ACD, and with Georgia Tech’s David Tanner and Darlene
Slusher, as well as Greg. Other ACD staffers helped
design key instrument components, including graduate
student Sandra Lopes, Roger Hendershot, Rudy Montoya,
William Bradley;, and John Vanderpol. NOAA’s
Jim Roberts, an expert on PAN and organic nitrates,
also provided guidance.
The 300-pound spectrometer cost about $150,000 to
build. NSF, NOAA, and NASA provided the funding.
The PAN CIMS instrument.
Air comes in from the left and is drawn into the
thermal dissociation unit. The large cylinder to
the right is the vacuum housing for the mass spectrometer.
(Courtesy Frank Flocke.)
The real test
a PAN-CIMS unit operated by Georgia Tech performed
up to expectations at a ground-based trial in the
Duke Forest in North Carolina during a study led
by ACD’s biogenics
group. But this summer comes the real test. The
instrument will be flown aboard a NOAA P-3 aircraft
in the Northeast as part of a series of international
experiments designed to better understand the formation
of air pollution in the northeastern United States
and how it gets transported around vast stretches
The experiments include the
$9 million New England Air Quality Study (NEAQS),
which will probe the sources and types of pollution
affecting the New England area. Led by NOAA’s Aeronomy Laboratory and
the University of New Hampshire, it will be combined
with another field project: the Aeronomy Lab’s
Intercontinental Transport and Chemical Transformation
The resulting international project, coordinated
by several U.S. and European agencies, will use 12
aircraft, a research ship, balloons, satellites,
and a network of ground-based instruments to look
at sources of air pollution in the Northeast and
the transport and chemical evolution of air masses
across the Atlantic Ocean. A related European project,
the Intercontinental Transport of Pollution, will
look at the downwind impacts of U.S. emissions on
The speed of PAN-CIMS means
Frank and his colleagues will have their work cut
out for them when it comes time to analyze the
resulting data. In past field trips, a research
aircraft would sometimes fly through a plume of
smoke before it could even analyze a sample of
air for PAN. This time, Frank calculates he will
be looking at 1,800 to 3,600 samples per hour of
six to seven types of PAN compounds—which could
be an enormous amount of data since researchers are
planning on 180 flight hours.
But he thinks the results
will be worth it. “We’ve
never been able to look at this before,” he
says. “We couldn’t measure the plumes
because we were
What’s next for PAN-CIMS?
Because it is so fast, the instrument is ideal
for aircraft studies. It will likely be deployed
during the Megacity Impacts on Regional and Global
Environments experiment (MIRAGE), an ACD-led study
that will be conducted in and around Mexico City
The instrument may also be used to determine whether
plants can take in organic oxidized nitrogen compounds,
such as PAN. This will help scientists better understand
the cycle of these important pollutants.
“With one-second resolution or better,” Frank
says, “the instrument is perfectly suited for
flux measurements.” •David
on this summer's experiments
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