Photoacoustic measurement of the optical absorption of aerosols
Open Access
- Author:
- Case, John
- Graduate Program:
- Acoustics
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- October 24, 2022
- Committee Members:
- Andrew Barnard, Program Head/Chair
Robert William Smith, Thesis Advisor/Co-Advisor
David Carl Swanson, Committee Member
Daniel C. Brown, Committee Member - Keywords:
- Photoacoustics
Helmholtz Resonator
Acoustics
Optical Absorption
Aerosols - Abstract:
- Atmospheric optical absorption, including the contribution from aerosols, is important in modeling propagation of laser beams and for climate research. Photoacoustic methods have been used in the past for this purpose, as they directly measure absorption, rather than the sum of absorption and scattering. In the case of a photoacoustic measurement approach, acoustic operating frequency determines the sensed aerosol size range. The design of a windowless double-Helmholtz photoacoustic aerosol sensing instrument, operating at an acoustic frequency near 150 Hz, with the capability to measure an optical absorption with a 1/e length of 1 Mm, will be presented. In this design, environmental noise limits the instrument sensitivity. To improve immunity to environmental noise with an open resonator, an aluminum acoustic enclosure was built around the instrument, with welded vacuum flanges for inputs and outputs. Inlet and outlet acoustic mufflers were designed to limit the in-band noise entering the sensing resonator. A modulated 10 W 1064 nm fiber laser was used as the excitation source, and a high-sensitivity microphone placed within the open-cell cavity was used to measure the internal photoacoustic pressure fluctuations. The instrument was modeled using various computational tools and then was constructed and tested. The current instrument was able to measure absorption coefficients from black carbon aerosols down to 10 Mm^-1. An unwanted internal photoacoustic signal was also detected, and methods to describe and mitigate the effects of this signal are described. Methods are described to move the lower detection range closer to the design level.