Optimizing the Performance of Aerosol Photoacoustic Cells using a Finite Element Model. Part 1: Method Validation and Application to Single-Resonator Multipass Cells

Abstract

Photoacoustic spectroscopy is the technique-of-choice for non-contact and in situ measurements of light absorption coefficients for aerosols. For most aerosol photoacoustic (PA) detectors, a key process is the amplification of the acoustic pressure wave generated from light absorption through excitation of a pressure eigenmode of a PA cell. To our knowledge, no modelling of the acoustics, sensitivity or signal-to-background ratio (SBR) has been performed for the PA cells applied commonly to aerosol absorption measurements. In this Part 1 manuscript, we develop a finite element method (FEM) framework to simulate the acoustic response and SBR of photoacoustic cells. Furthermore, we validate this modelling framework by comparing FEM predictions of single-resonator PA cells with measurements using a prototype single-resonator cell, the geometry of which can be readily adjusted. Indeed, single-resonator cells are applied commonly to aerosol absorption measurements. We show that our model predicts accurately the trends in acoustic properties with changes to cell geometry. We investigate how common geometric features, used to supress detection of background and noise processes, impact on the SBR of single-resonator PA cells. Such features include using multiple acoustic buffer volumes and tuneable air columns. The FEM model and measurements described in this paper provide the foundation of a companion paper that reports the acoustic properties and optimization of a two-resonator PA cell used commonly in aerosol research.