Tutorial: A photoacoustic spectrometer to measure aerosol light absorption

Accurate measurement of aerosol light absorption is essential to reduce uncertainties associated with aerosol radiative forcing in climate models. Contemporary techniques to measure aerosol light absorption continue to suffer from systematic errors and biases that are difficult to quantify. Photoacoustic spectrometry offers a first-principles approach for in-situ measurement of aerosol light absorption. The photoacoustic effect takes place when particles are illuminated by an amplitude-modulated laser, whereupon the particles absorb some amount of incident laser beam energy. Most of the absorbed energy, typically, is released to the surrounding gas as heat, creating pressure waves (sound) of an intensity proportional to that of the modulated laser power. These pressure waves are detected using a sensitive acoustic sensor in real time. This tutorial outlines the working principle of a basic photoacoustic spectrometer (PAS) from a fundamental standpoint, and elucidates its construction by taking into consideration the different design constraints and optical configurations. Methods for data acquisition and signal processing are explained in detail. The tutorial concludes with a brief discussion on PAS calibration techniques, determination of the instrument's limit of detection, and the intrinsic limitations of the technology.