Optimizing the performance of aerosol photoacoustic cells using a finite element model. Part 2: Application to a two-resonator cell
Photoacoustic spectroscopy (PAS) measures aerosol absorption in a noncontact manner, providing accurate absorption measurements that are needed to improve aerosol optical property representations in climate models. Central to PAS is resonant amplification of the acoustic pressure wave generated from laser-heated aerosol transferring heat to surrounding gas by a photoacoustic cell. Although this cell amplifies pressure sources from aerosol absorption (signal), it also amplifies noise and background sources. It is important to maximize the cell signal-to-background ratio (SBR) for sensitive absorption measurements. Many researchers have adopted the two-resonator cell design described by Lack et al. (2006). We show that the uncertainty in PAS measurements of aerosol absorption using this two-resonator cell is significantly degraded by its large sensitivity to background contributions from laser scattering and absorption at the cell windows. In Part 1, we described the use of a finite element method (FEM) to predict cell acoustic properties, validated this framework by comparing model predictions to measurements, and used FEM to test various strategies applied commonly to single-resonator cell optimization. In this second part, we apply FEM to understand the excitation of resonant modes of the two-resonator cell, with comparison measurements demonstrating accurate predictions of acoustic response. We perform geometry optimization studies to maximize the SBR and demonstrate that the laser–window interaction background is reduced to undetectable levels for an optimal cell. This optimized two-resonator cell will improve the sensitivity and accuracy of future aerosol absorption measurements.