| 26 Sep 2025
Development of the SiMPLE-PAS: A low-cost, 3-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, and, when combined with a scattering or extinction method, provides a measure of the aerosol single scattering albedo, 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 open-source options wherever possible. In this manuscript, 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 $500 USD. The instrument is, to the best of our knowledge, the first 3D-printed PAS for aerosols and 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-minute, 2-σ), respectively, that 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.
General
The manuscript describes the development of a low-cost, 3-wavelength photoacoustic spectrometer (SiMPLE-PAS). The emphasis is on engineering design choices (mechanical, electrical, and software), with some laboratory validation and a limited field deployment. While the realization of the device is technically competent, I do not find significant novelty from a scientific instrumentation perspective: the underlying working principle is that of a standard photoacoustic instrument, and the use of 3D-printed parts and consumer electronics is incremental rather than conceptually new. The authors also do not clearly articulate the specific need or scientific problem that this instrument addresses beyond low cost.
That said, the work could still be of interest to Aerosol Research if framed as a reproducible, open-source, educational, or accessibility-focused contribution. To reach that point, the manuscript requires major revision, both in structure and in content.
Manuscript length and structure
At ~12,000 words (not accounting figures and tables), the manuscript is 4,000–6,000 words too long. Sections should be significantly shortened or moved to the supplement. Moreover, the current organization is confusing: for example, calibration methods are embedded in the Results section rather than Methods. The paper should follow standard structure before detailed discussion is considered.
Reproducibility and open-source availability
If the intention is to provide a community-sensor-type instrument, all essential resources (software, CAD files, PCB designs) must be openly available in a long-term, independent repository. “Available by request” is not sufficient. An assembly guide with photographs would further enhance reproducibility and impact.
Calibration and evaluation
The methods used to calibrate and evaluate the device are not sufficiently thorough or clearly explained. In particular, the lack of a conventional field evaluation with side-by-side reference instruments is unfortunate, as this is typically the best way to obtain a general understanding of the instrument response characteristics (e.g. susceptibility to varying relative humidity and temperature, long-term drift, influence of aerosol composition etc). I encourage the authors to seriously consider whether such an evaluation could be arranged. The comparison of Ångström exponents with denuded and non-denuded samples is not without interest, but the measurement arrangement introduces multiple sources of uncertainty that make explicit conclusions about the device performance difficult to draw. Below are some specific concerns related to the O₃ evaluation:
Recommendation
I recommend major revision. The manuscript is not suitable for publication in its current form, but with substantial shortening, restructuring, open-source dissemination of design files, clearer justification of calibration, and a more thorough evaluation against reference instruments, it could become publishable.