UCAR Staff Notes: A fast way to measure PAN

June 2004

A fast way to measure PAN

Scientists hope to learn more about this important atmospheric chemical and its connection to pollution

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 Georgia Tech’s Greg Huey.

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 damage plants.

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.

“The weak link”

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 Monthly).

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 flow.

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.

PAN

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

Last year, 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 of the globe.

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 study.

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 Europe.

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 too slow.”

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 in 2006.

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 Hosansky

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