10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
A Differential Photoacoustic Spectroscopic (DPAS) Technique for Aerosol Light Absorption Measurement in the Presence of Light-absorbing Gaseous Species
ZHENHONG YU, Gregory Magoon, William Brown, James Assif, Richard Miake-Lye, David Liscinsky, Aerodyne Research, Inc.
Abstract Number: 22 Working Group: Carbonaceous Aerosol
Abstract We developed a differential photoacoustic absorption spectroscopic (DPAS) technique to measure light absorption from particulate matter (PM) at the spectral region from 360nm to 1064nm, in the presence of light-absorbing gaseous species. It is capable of operating in a single-wavelength or multiple-wavelength configuration. The dual-cell DPAS system contains two identical single-pass photoacoustic cells to measure the photoacoustic signals due to light absorption of total PM + gaseous samples and that of gaseous samples alone, separately. The precise dimension and single-pass configuration of the photoacoustic cells guarantee that optical path length, resonance frequency, cavity quality of the two cells are virtually identical. The resulting differential photoacoustic signal can be used to determine the light absorption purely from the PM species. This measurement method eliminates the interferences from the light-absorbing gaseous species as well as the surrounding low-frequency background acoustic noises.
A high-power continuous-wave diode-pumped solid state (DPSS) laser with good beam quality at 532nm was used as the light source of the DPAS system. The laser output was amplitude modulated at the resonance frequency of the photoacoustic cells via a waveform control on its electrical DC power supply. Given the small dimension of the silicon-based microelectromechanical system (MEMS) microphone, we found that a linear-array detector with multiple microphones can be applied as the photoacoustic detector, which doubles the S/N relative to a single microphone detector.
Photoacoustic signals of the DPAS monitor at 532nm were calibrated with NO2 gas standards. Based on an Allan analysis, a detection sensitivity (2δ) of 0.30 Mm-1 can be achieved in 100s data acquisition at 532nm.
Using the Jet Burner Test Stand (JBTS) facility at the United Technologies Research Center (UTRC), we measured light absorption by the aviation soot emissions from a representative high-temperature and high-pressure test combustor for aircraft auxiliary power units (APU). The DPAS measurement results at 532nm, under the high gaseous NO2 conditions, were then compared to the determination of soot mass concentrations from an AVL Micro Soot Sensor (MSS). An excellent linear correlation between the measurements from two instruments was observed. The mass absorption coefficient (MAC) of the aviation soot was determined to be 7.4 ± 0.1 m2 g-1, in good agreement with the expected value of 7.6 ± 0.6 m2 g-1.