ESRL Quarterly Newsletter - Spring 2010

Setting the Standard

Global Monitoring Division standards laboratories key to global atmospheric science collaboration

Brad Hall's laboratory sits on special underground pillars, to better isolate the room from any activities taking place outdoors. Hall and his colleagues create meticulously calibrated standards of halocarbons and other trace gases found in the atmosphere. Some of their balances are precise enough to pick up the grease on an errant fingerprint.

"In our old lab, when a truck drove up to the loading dock, all my balances went off scale," Hall said. "I had to make standards at night."

Brad Hall in lab

Tertiary standard cylinders containing various amounts of nitrous oxide.

Hall prepares standards of chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and other gases. He and his colleagues create cylinders containing specific amounts of the materials, then send them around the world for scientists to use in calibrating their own measurement systems. That ensures that measurements made in China, on an airplane science mission, and on the ground in Antarctica are all comparable.

Many of the chemicals his team targets are ozone-depleting substances, released by human activity and now regulated by the Montreal Protocol because they damage Earth's protective ozone layer. Some are also greenhouse gases, of growing international interest.

A canister of a pure HCFC.
Brad Hall attaches the canister to a vacuum apparatus, to pull the gas into a micropipette about 1 mm in diameter. Immersing the pipette in liquid nitrogen cools and liquifies the gas.
Hall seals the micropipette with a flame.
A sealed micropipette is ready to be weighed (this is Hall's
An extremely precise balance.

1. A canister of a pure HCFC.
2. Brad Hall attaches the canister to a vacuum apparatus, to pull the gas into a micropipette about 1 mm in diameter. Immersing the pipette in liquid nitrogen cools and liquifies the gas.
3. Hall seals the micropipette with a flame.
4. A sealed micropipette is ready to be weighed (this is Hall's "practice" version, filled with red water).
5. An extremely precise balance.

"In the mid-1980s, there were huge differences among measurements and instruments," Hall said. "The idea of standards is to tie atmospheric measurements to a known, stable value. You have to be able to trust your measurements, especially if you're looking at a trend over, say, 20 years."

His colleague Duane Kitzis (Cooperative Institute for Research in Environmental Sciences) works with a standards team that focuses on carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO). The NOAA carbon dioxide scale is based on the measurement of the pressure of CO2 extracted from air ("manometric method") while other gases such as halocarbons and methane are based on standards prepared by weighing gases into cylinders ("gravimetric method").

Using these methods, Hall, Kitzis and their colleagues create in-house "scales" covering a range of concentrations needed to measure the range of atmospheric trace gas species.

These so-called "primary" standards are used only to calibrate secondary and tertiary mixtures of gases. Secondaries also remain at ESRL; tertiary standards are sent and used worldwide.

Kitzis is currently coordinating a WMO "Round Robin intercomparison," an international contest of sorts, to see how accurately various laboratories around the world measure unknown samples of CO2, CH4, CO, and nitrous oxide, N2O. Kitzis created three sets of cylinders containing amounts of carbon dioxide that only he knows. Every participating laboratory gets the cylinders for a few weeks, and reports results to "referee" Linxi Zhou at the Chinese Meteorological Administration, who does not know the actual concentrations. Results are tallied when all laboratories are finished. Laboratories use results to identify problems in instrument calibration or measurement methods, Kitzis said.

The images on this page show several steps in the process Hall uses to make standards gravimetrically, by measuring precise weights of tiny quantities of condensed gases. The posed pictures give a sense of how the work is done, but Hall can't let visitors in the lab when he's making actual samples. "It takes too much concentration," Hall said.

"Making the standards is not rocket science," Hall said. "Making them reproducible is tricky, and learning how to make them stable over long periods of time is the hardest."

ESRL's standards and standard-making processes are linked to NIST, the National Institute of Standards and Technology, its neighbor in Boulder's Department of Commerce campus.