Art of the Inlet
Precision engineering and innovation for airborne science
It’s tricky to study the atmosphere from a research airplane speeding faster than 325 feet per second through thin air. Researchers rely on “inlets” to draw samples into custom instruments onboard—and designing those inlets can take exquisite attention to detail. “Some of them are dead simple, but some are full-on instruments in their own right,” said ESRL’s Tom Ryerson (Chemical Sciences Division, CSD).
It’s not just a matter of avoiding jet exhaust: If an inlet isn’t designed correctly, particles could shoot by the opening, evading observation by instruments onboard. Gases might chemically react with the lining of an inlet tube, or temporarily absorb enough to mess up measurements.
As NOAA’s WP-3D research airplane—studded with inlets—headed out for the CalNex mission in California this spring, it stopped for a few days at the Rocky Mountain Region Airport in Broomfield, CO. ESRL researchers installed and tested equipment on a practice flight before the mission, a major study of the nexus of air quality and climate change in California. Ryerson, Carsten Warneke (CSD and CIRES), and Chuck Brock (CSD) also showed curious colleagues around the airplane, explaining the science and art of airplane-based sampling.
“This is an experiment,” Ryerson said, pointing to an oddly angled inlet on the right side of the WP-3D. Researchers have struggled for years to obtain accurate measurements of nitrogen oxide compounds as a group, Ryerson said.
“You want to measure them all in the gas phase, but you also want to exclude the aerosols, some of which have nitrogen components. That’s very hard to do.”
His inlet uses a ring-shaped point-of-entry to smooth the airflow, and also angles the main tube, which is roughly parallel to the aircraft body, to make that smoothed airflow race even faster. The idea is to trick lightweight aerosols into speeding past the gas intake, in the center of that tube, so that only true gases are sucked in.
The angle of Ryerson’s inlet can be adjusted mid-flight, and he based his design, in part, on the pattern of sticky oil residue on a research airplane (similar to the WP-3D) that flew through the Kuwaiti oil fires in the early 1990s. When that plane landed, the oil smudges quickly dripped off—but the photograph someone took as the airplane landed traced the pattern of airflow over the airplanes’ body—key information for Ryerson’s instrument. His experimental inlet is flying for the first time during CalNex.
Stainless steel tubes three-eights of an inch in diameter collect air samples for many instruments on board—those measuring carbon monoxide, for example (that gas can be used to calculate the age of an airmass or to help trace its source) and carbon dioxide (a key greenhouse gas). Ryerson puts both CO and CO2 in the “non-sticky” gas category. “You could sample those through old socks and still get a good measurement,” he joked.
For some instruments, steel inlets must be lined with Teflon—necessary if you’re sampling ozone, for example. “Steel eats ozone,” Ryerson said. Other Teflon inlets are heated so that water can’t condense inside and “blur” the measurements of “stickier” gases such as sulfur dioxide, nitric acid, and ammonia.
The LTI, low-turbulence inlet
“There are only a handful of these in the world,” Chuck Brock said, pointing to an arrow-shaped inlet emerging from the left side of the WP-3D. Brock is in charge of a suite of instruments on the WP-3D designed to study many aspects of particles (aerosol) in the atmosphere: their mass, size distribution, light-absorption and extinction behavior, and whether they can take up water and help form clouds. These measurements are critical for understanding how particulate pollutants are transported and transformed in the atmosphere, and how they affect climate directly (by absorbing or scattering light) and indirectly (via clouds).
Brock’s inlet needed to slow the airflow from about 100 m/s down to 10 m/s, without creating turbulence. Turbulence allows larger aerosol particles, with greater inertia, to spin off into walls, changing the composition of samples that pass through to the instruments. So the low-turbulence inlet has porous walls that remove air from the turbulent edges of the tube, allowing a central, laminar core of air to flow into instruments. Only about 35 percent of the air mass pulled into the tip of the inlet makes it to the instruments inside the plane.