We propose to design, build and test an optical extinction monitor for the detection of spacecraft cabin particulates. This monitor will be sensitive to particle sizes ranging from a few nanometer to tens of micrometers in diameter. Designed to utilize commercial off-the-shelf components, the monitor, once calibrated, will require no recalibration and only periodic baseline determinations, a process which can be automated as part of the operation of the instrument. It employs no consumables. This monitor employs cavity attenuation phase shift technology and involves the use a light emitting diode coupled to a low-loss optical cavity. The Phase I project will involve a proof-of-principle demonstration followed by the monitoring of ambient particulates in an urban area.