We propose to develop a highly sensitive and compact monitor to measure light absorption from particulate matters. The fundamental of the proposed device is based on differential photoacoustic absorption spectroscopy (DPAS). It will be able to be remotely controlled and operated at high altitude. Background interference from NO2 absorption and acoustic noise of aircraft engines will be significantly reduced due to a specific acoustic cell design. The Phase I project will provide a proof-of-concept demonstration. The ultimate goal of this study is to design and construct a portable PM absorption monitor for NASA's airborne measurement programs. The tasks of the proposal will include: 1) Construction of a laboratory prototype DPAS PM absorption monitor; 2) Development of an absorption calibration scheme; 3) Comparative study on optical absorption of laboratory soot particles with multi-angle absorption photometer (MAAP); and 4) Performance evaluation at a variety of sampling pressure. This SBIR Phase study will be performed by Aerodyne Research, Inc., in collaboration with United Technologies Research Center (UTRC). Both companies have been major participants in past NASA/FAA-sponsored aircraft emissions programs with extensive experiences in the study of PM optical absorption as well as soot formation and evolution in the atmosphere.