Differential Photoacoustic Particle Absorption Monitor

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.

The primary NASA need for this technology is to measure PM light absorption at different altitudes for its Airborne Measurement program. At present, PM absorption is measured by collecting sample on a filter subtract and measuring light extinction and scattering of the collected samples during the airborne measurements. This method suffers from a number of intrinsic errors such as low sensitivity, ambient-pressure sampling and scattering light interference. The proposed DPAS technique will be far more sensitive than the filter-based techniques, and is capable of sampling at different pressures and providing 1s data acquisition measurement on absorption. In addition, emissions of small particulates (PM10 and PM2.5) are regulated by the EPA. The impact of particulate emissions from aircraft engines, which have a direct effect on radiative forcing, is magnified by the fact they are typically emitted in the upper troposphere and lower stratosphere where their influence is greatest. Past NASA programs such as EXCAVATE, APEX, UNA-UNA, and AAFEX have had as a major focus of their work, the measurement of black carbon (BC) emissions from civilian aircraft engines. The proposed DPAS PM absorption monitor can be used as a BC emission monitor to provide a cost-efficient, compact, fast and real-time BC measurement on aircraft engines.

We expect that the DPAS PM absorption monitor developed under this program will significantly benefit the scientific community interested in characterizing the radiative properties of ambient aerosols. The ability of one instrument to simultaneously measure particle absorption with good time resolution and high precision will enable continuous measurements of the particle optical absorption that can be directly used by regional and global climate forcing models. In combination with the Cavity Attenuated Phase-Shift (CAPS) extinction monitor, single particle albedo of ambient aerosols could be directly determined. The development of a DPAS PM absorption monitor would have a large impact on the ambient air quality monitoring community. PM2.5 and PM10 levels must be routinely monitored as part of ambient air pollution monitoring programs. Current techniques rely on use of collection filters and subsequent weighing, a procedure which is labor intensive (and thus expensive) and prone to human error. There are 5000 sites in the U.S. alone which must monitor particulates under the mandate of the Clean Air Act of 1970. Particulate emissions from stationary combustors like power plants must also be monitored and an automated DPAS instrument would provide considerable savings over current technology, which costs on the order of $80,000-100,000 per unit!

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