Development of the SiMPLE-PAS: a low-cost, three-wavelength photoacoustic spectrometer for aerosol absorption

Abstract

Photoacoustic spectroscopy (PAS) has become a common method for measuring aerosol absorption and is one of the few techniques capable of directly measuring absorption by suspended aerosol particles at ambient concentrations. When multiple wavelengths are used, PAS provides a way of measuring the absorption Ångström exponent (AAE) and, when combined with a scattering or extinction method, provides a measure of the aerosol single-scattering albedo (SSA), and both AAE and SSA are important parameters in climate models. Despite this utility, few commercial PAS instruments are available, and no multi-wavelength commercial instruments are currently available. Thus, most extant PAS instruments are custom-built and therefore come with considerable cost and development time and require access to machine shops capable of fabricating the needed components. The goal of this work was to provide a blueprint for a low-cost, multi-wavelength PAS for measurement of the aerosol AAE both in the laboratory and in the field. In an effort to create an instrument with a low barrier to entry, we aim to use low-cost, readily available components and use open-source options wherever possible. In this paper, we present the SiMPLE-PAS, a single-pass, multi-wavelength, portable, and low-expense photoacoustic spectrometer that uses low-cost electronics and a 3D-printed cell to meet these design goals. The instrument has a total bill-of-materials cost on the order of USD 500. The instrument is, to the best of our knowledge, the first 3D-printed PAS for aerosols and likely the lowest-cost PAS to date. The instrument performed well in laboratory validation experiments and showed good agreement with measurements of aerosol absorption by the previously developed MultiPAS-IV instrument when co-located at the second Georgia Wildland Fire Simulation Experiment (G-WISE 2) during April 2025. The instrument shows competitive detection limits of 0.63, 1.99, and 0.55 Mm-1 for the blue, green, and red channels (10 min, 2σ), respectively, which will allow it to measure both ambient and laboratory-generated aerosols. The SiMPLE-PAS therefore provides a low-cost, accessible photoacoustic spectrometer that offers to lower the barrier to entry for groups wishing to measure aerosol absorption, whether in the laboratory or in the field.