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As part of the refurbishment, the aircraft received a new, much- needed, low-turbulence inlet for collecting air samples. Why the excitement about the new inlet? Because the air that flows into an inlet must be drastically slowed down—from about 100 to 4–5 meters per second—before it reaches the collection instruments. In most inlets, the slowdown takes place in a funnel- shaped diffuser that inevitably creates turbulence. The swirling motion doesn't affect particles that are small enough to behave more or less like air molecules, but it slams larger particles into the diffuser walls, where they stick. As much as 90% of large particles can be lost that way. The scientific consequences are significant: In the first ACE field program, for example, scientists were unable to collect the big sea-salt particles that play a role in sulfate reactions, and thus unable to understand the sulfur budget in its entirety.
The new inlet has porous walls made of bonded stainless steel pellets. When the airflow hits the walls, as much as 80% of it simply passes through and is returned to the open air by a plenum (a chamber whose pressure is greater than the outside atmosphere). The remaining 20%, however, is a laminar flow that retains the natural distribution of particle sizes.
Russell Seabaugh and his colleagues at Denver University developed the inlet with the help of Barry Huebert (University of Hawaii) and Jack Fox (ATD). The low-turbulence inlet was flight-tested last July over the Caribbean, and it outperformed three other inlets in collecting dust and sea-salt particles. From the test, it appears the new inlet will be able to measure particles up to at least 7 microns in size. As Huebert notes: "ACE-Asia is the first experiment in which we really have some assurance that we're getting super-micron-sized particles."
• Carol Rasmussen
Unless otherwise noted all images are copyrighted by the University Corporation for Atmospheric Research / National Center for Atmospheric Research / National Science Foundation.
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