Fundamentals of Aerosol Photoacoustic Spectroscopy
Final Report Abstract
One of the most important and widespread techniques that enables optical absorption measurements of particles is photoacoustic (PA) spectroscopy. The photoacoustic effect derives from the interaction between an electromagnetic wave (usually light) and matter: when the optical energy is absorbed, it causes heating and thermal expansion. The technique is characterized by a large dynamic range, linear signal response, and higher sensitivity for aerosol light absorption measurements than techniques based on extinction-minus-scattering methods. In this work, a new, high-precision PA system was developed. The new experimental setup allows us to accurately monitor the light extinction through two photodetectors placed before and after the PA cell. The entire system was explicitly designed to keep the atmospheric parameters of temperature and humidity under precise control. In the first step, the system was tested using a gaseous sample of ethanol vapor at different levels of relative humidity (RH). The scope of such a preliminary investigation was to assess and quantify the effect of humidity on PA signal generation. In order to investigate the same phenomenon on PA signals from aerosols, graphite aerosol produced by a spark discharge generator was used as a sample instead. Any possible variation of the photoacoustic signal due to the uptake of water or other substances by aerosols was evaluated by coating graphite particles with a layer of stearyl alcohol. The experimental work was accompanied by simulations performed with the Finite Element Method (FEM) simulation software COMSOL Multiphysics (Version 5.5 and 6.2, comsol.com). The research activities conducted within the project fostered the theoretical knowledge of the aerosol PAS technique and contributed to improving the instrumentation currently used for measuring aerosol light absorption.
