10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
A Single-pass RGB Differential Photoacoustic Spectrometer for Aerosol Absorption Measurement
ZHENHONG YU, Gregory Magoon, William Brown, James Assif, Richard Miake-Lye, Aerodyne Research, Inc.
Abstract Number: 21 Working Group: Instrumentation
Abstract We developed a single-pass RGB photoacoustic spectrometer for aerosol absorption measurement at 671nm (red), 532nm (green) and 473nm (blue). This instrument uses the differential photoacoustic absorption spectroscopic (DPAS) technique, in which signals due to light absorption of total PM + gaseous samples and those of gaseous samples alone are measured simultaneously in two identical acoustic cells. The difference between the photoacoustic signals yields the aerosol absorption coefficients at the three wavelengths. This measurement approach eliminates the interferences from the light-absorbing gaseous species as well as the surrounding low-frequency background acoustic noises.
In this design, laser beams of three diode-pumped solid state lasers at the RGB wavelengths with superb beam quality were merged into a single beam via dichroic mirrors before passing through the photoacoustic cells. The laser beam at each color was amplitude modulated separately at 50% duty cycle around the resonance frequency of the photoacoustic cells via a square-waveform control on their electrical DC power controls. Frequency-division multiplexing was used to combine the three-color photoacoustic signal, allowing simultaneous absorption monitoring at the three wavelengths. To accomplish that, the modulation frequencies were slightly different by approximately 5 – 15 Hz.
Custom-designed miniature microphone array detectors were used to measure photoacoustic signals at each cell. Each detector contained four silicon-based microelectromechanical system (MEMS) microphones, which were integrated onto a printed circuit board with voltage regulators, band-pass filters and low-noise voltage preamplifiers.
Data acquisition for the RGB DPAS system was carried out via a fast-response and high-resolution DAQ device, in which an analog-to-digital converter (ADC) was used to convert the analog voltage outputs from the PCB into 24 bit-resolution digital outputs for digital signal processing. Instead of using the conventional phase-sensitive detection scheme, we utilized least-squares (LS) analysis to extract signal parameters.
Signal responses of the RGB DPAS instrument were calibrated with mono-disperse carbon black samples. Based on the Allan variance analysis, detection sensitivities (2δ) of 0.3 Mm-1 at 671nm, 0.3 Mm-1 at 532nm and 1.2 Mm-1 at 473nm have been achieved in 100s data acquisition for the RGB DPAS instrument. The single-pass configuration allows us to determine aerosol absorption angstrom exponent (AAE) directly from photoacoustic signals and laser power measurement, without calibration of signal response.