We propose to develop a "black carbon" (soot) monitor for measuring non-volatile particulate emissions from gas turbine engines employing a proprietary optical extinction measurement technique based on cavity attenuated phase shift spectroscopy (CAPS) operting in conjunction with a differential mobility analyzer. The singular aspect of the CAPS approach is that extinction is measured by determining shifts in the phase angle of a modulated light beam instead of changes in the intensity of the ransmitted light caused by the presence of particulates. This aspect makes the sensor immune to either abrupt or gradual changes in the intensity of the light caused by temperature or pressure fluctuations or light source deterioration. Furthermore, the sensor rarely needs to be calibrated -- i.e., its span value, a function of the optical properties of the particles themselves, remains virtually constant. This sensor does not rely on the deposition of particles on a filter and requires little maintenance. The monitor will be able to measure the size dependent particle mass concentration in the sub-micrograms per cubic meter with a sampling period of only 1 second.