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Instrument: CIMS : Chemical Ionization Mass Spectrometer
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The Chemical Ionization Mass Spectrometers (CIMS) instrument has
two independent detection channels. For CRYSTAL/FACE both
channels are configured for measurements of ambient nitric acid
(HNO3). For HNO3 detection, reagent ions SiF5 - are generated
and mixed into the ambient air sample. Strongly bound cluster
ions HNO3 ?SiF5 - which are formed in the reaction HNO3 + SiF5 -
↔HNO3 ?SiF5 ?can be detected with high sensitivity and
selectivity. The product ion abundance is proportional to the
ambient HNO3 mixing ratio. The response of each detection
channels is calibrated several times in flight by standard
addition of HNO3 via a HNO3 permeation cell. The baseline of
each channel is determined by replacing the ambient air sample
with dry nitrogen.

Sample inlets are located in an airfoil-shaped inlet pylon that
extends 36 cm below the bottom of the

WB-57 fuselage pallet at positions which are far beyond the
aircraft boundary layer. One inlet is facing backward and is
only sensitive to gas-phase HNO3. The other inlet is facing
forward and is sensitive to particulate HNO3 as well.

[Summary provided by NASA]