Particle Detectors
CIMS
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]