The Atmospheric Composition Monitor Assembly (ACMA) is an instrument for monitoring the composition of the atmosphere in Space Station Freedom. It is part of the Environmental Control and Life Support System. The ACMA is composed of the following major instruments: Major Constituent Analyzer (MCA); Carbon Monoxide Monitor (COM); Trace Contaminant Monitor (TCM); and Particle Counter Monitor (PCM). The MCA is a mass spectrometer based system that provides continuous monitoring of major atmospheric constituents (O2, CO2, N2, H2O) and of hydrogen and methane. The COM is a non-dispersive infrared (NDIR) instrument for the continuous monitoring of trace levels of carbon monoxide, The TCM is a gas chromatograph-mass spectrometer (GCMS) for monitoring trace levels of contaminants, The PCM monitors the concentration of particulates in the 0.5 to 100 micron range using a light scattering technique.