NACS Instrument
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Neutral Atmospheric Composition Spectrometer (NACS)
The NACS instrument is a quadrupole mass spectrometer combined with a cryosorption pump, which reduces the pressure in the quadrupole volume sufficiently to permit the mass spectrum to be obtained. The first measurements of the gas composition in the Earth’s mesosphere and lower thermosphere were obtained by this instrument in 1970. The measurements from 65 to 125 km altitude included profiles of N2, O2, O, O3, CO2, H2O and Ar. The ratio of Ar/N2 provided our first direct determination of the location of the turbopause, where turbulence mixing of the atmosphere ceases to dominate and molecular diffusion describes the separation between lighter and heavier molecules/atoms at higher altitudes. The turbopause has been observed at altitudes between 84 and 110 km depending on the location and the geophysical conditions. Severa
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