A look at the late, great campaign for measuring mid-continent greenhouse gases
One day in March 2000, a Cessna 172 took off from a little airstrip in Ponca City, Oklahoma. The intent: Measure air samples of dried aerosol particles at nine different altitudes to determine their light-scattering and absorption properties.
The data were collected on behalf of the In Situ Aerosol Profiles (IAP) campaign, an investigation of atmospheric composition. It was led by John Ogren of the National Atmospheric and Oceanic Administration on behalf of the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a U.S. Department of Energy scientific user facility.
This humble beginning—one pilot, two instruments, and a plane the size of a family sedan—turned into 16 years of data-gathering flights that ended in the fall of 2016. Recorded in that long interval were the vertical profiles of aerosols (and later carbon cycle gases) in the atmosphere over the Southern Great Plains (SGP) atmospheric observatory, and how they change over time.
“I’m not aware of a greater set of profiles for aerosols and carbon cycle gases,” said Beat Schmid, ARM Aerial Facility (AAF) manager at Pacific Northwest National Laboratory.
In 2002, a 2-flask sampler was added to the Cessna 172 payload. At two different altitudes, the sampler collected the campaign’s first airborne carbon cycle gases measurements for the Lawrence Berkeley National Laboratory Carbon Project.
“At the beginning it was very primitive,” said Schmid of the patterned three-hour Cessna flights involving minimal instrumentation. “But the big thing was to fly those patterns consistently.”
By 2005, with help from ARM, the aging plane was replaced with a bigger, faster, turbo-powered Cessna 206 contracted by AAF. That made room for additional and more sophisticated instruments for collecting data at higher altitudes. In 2006, the 2-flask sampler was upgraded to a 12-flask version.
Then in 2007, a continuous carbon dioxide analyzer was placed aboard the Cessna. That made the plane, at the time, the only airborne U.S. platform for systematic continuous measurements of greenhouse gases.
Every Week, Flying Horizontal Legs
By the time that first Cessna lifted off the ground, the broad outline of the way carbon cycle gases were measured above SGP was established: Twice a week, fly in daisy-like vertical flight patterns at different altitudes, from 500 feet to (later) 17,500 feet. Fly these 5-to-10-minute horizontal legs only in good weather, and never into clouds. (DOE safety rules forbade a single-pilot aircraft from flying into clouds.)
Data from lower elevations allow scientists a look at the fine details of carbon cycle gases and the role of the land surface as a source or sink of carbon. Measurements from higher altitudes are helping modelers understand how greenhouse gases are transported at the scale of continents.
A detailed, representative picture of how greenhouse gases are distributed vertically in the atmosphere is important. It heads off potential errors in the way carbon dioxide radiative forcing and surface fluxes are represented in earth system models.
IAP Segues to ARM-ACME
At the end of 2007, after 597 research flights, the IAP campaign concluded. It was replaced by the first ARM Airborne Carbon Measurements (ARM-ACME) field campaign, funded by the ARM Facility and DOE’s Terrestrial Ecosystem Science program.
Over time, there were six iterations of ARM-ACME. One of these campaigns, ACME-V, focused on Alaska; the other five were designed to inventory carbon dioxide, methane, and other climate-affecting trace gases in weather-charged terrain spanning Oklahoma and Kansas. Ground-based instruments at the SGP Central Facility complemented ARM-ACME’s aerial record of changes in greenhouse gases in the atmospheric column.
A few basic imperatives informed ARM-ACME: Establish a long-term baseline of measurements from vertical profile flights. Add in data from air-land interactions. Feed data to national carbon observing networks, such as CarbonTracker, in order to capture long-term trends for carbon dioxide mixing ratios.
In 2010, for cross-validation purposes, ARM-ACME added a second carbon dioxide analyzer. That brought the campaign closer to the 0.1 parts per million precision (ppm) standard achieved in laboratories and at ground-based observation sites.
It is notoriously difficult to measure minute changes in carbon dioxide levels, which globally are today around 400 ppm.
“We’re looking for a very small change in that number,” smaller than about a tenth of a percent, said Sébastien Biraud, an atmospheric scientist at the Lawrence Berkeley Laboratory who was principal investigator for the ARM-ACME campaigns. But in the end, he added, the ARM-ACME’s aerial probes “achieved unprecedented accuracy.”
The Little Plane That Could
The measurements the little Cessna scooped from the sky became the only data regularly validated by continuous in situ ground measurements.
“These were the most intensive observations being made nationally,” said Biraud.
He was lead author of a 2013 study outlining the 2002-2012 record of airborne carbon dioxide observations in the air over the SGP observatory. In the last five years of that period, ARM-ACME gathered up more than 400 vertical profiles of carbon dioxide.
Biraud’s review noted the strong influence of land surface fluxes on levels of atmospheric carbon dioxide. How big a sink, and how big a source, he asked, are these terrestrial factors? That is still one of the greatest uncertainties in predicting 21st century climate change. With its coordinated and validating look at land, air, and satellite measurements of greenhouse gases, ARM-ACME was helping provide an answer.
Next Step, Last Step
ARM-ACME VI, with Biraud as principal investigator, started in October 2015 and closed out a year later. It was the latest and the last iteration of a de facto 16-year campaign with a multifaceted mission: Record greenhouse gases in the atmosphere. Document changes. Contribute to improving climate models. Underpin research on regional carbon budgets. And be a focal point for validating new remote sensing instruments for ground, airborne, and satellite programs.
ARM-ACME VI also focused on gathering data on methane, a greenhouse gas that in Oklahoma is linked to regional oil and gas drilling and to fugitive emissions. The measurements helped close a methane data gap in the south-central United States, where some emissions estimates may be just a third of real totals.
So far, the list of publications related to ARM-ACME totals 28. (Biraud had a part in at least 26 of them.) There is more published work on the way, thanks to a data set so large, novel, and robust, said Schmid. “It will be used heavily.”
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The ARM Climate Research Facility is a national scientific user facility funded through the U.S. Department of Energy’s Office of Science. The ARM Facility is operated by nine Department of Energy national laboratories.