(539g) Assessing Low-Cost Optical Particle Counter Performance Metrics

Particulate matter (PM) pollution remains a global health concern, particularly due to the risks associated with fine particles like PM2.5, which can penetrate deep into the respiratory system. Exposure to PM2.5 has been linked to cardiovascular diseases, asthma, increased morbidity, and premature mortality. According to the Global Burden of Diseases (GBD) report from 2019, approximately one million premature deaths annually were attributed to long-term PM2.5 exposure between 2000 and 2019 [1–2]. Accurate monitoring of PM concentration is therefore essential for assessing both indoor and outdoor air quality.

Optical particle counters (OPCs) are widely used instruments for PM monitoring. Unlike aerosol photometers, which measure bulk scattering from multiple particles, OPCs operate as single-particle counters, detecting and sizing individual particles as they pass through a laser beam. The light scattered by each particle is collected and converted into an electrical pulse, which correlates with particle size and is classified into size bins. Using Mie theory, these measurements are used to estimate mass concentrations [3-4]. Low-cost OPCs, typically priced under $100, are often designed with simplified optics, fewer size bins, and limited detection ranges. While this makes them economically viable and accessible, it can lead to compromised measurement accuracy and reliability [5-6]. The performance of OPCs is influenced by various factors, including environmental conditions, aerosol properties, design geometry, and calibration methods. High humidity can lead to particle hygroscopic growth, causing overestimation of PM levels [7]. Additionally, the optical configuration and internal flow geometry of the OPC can significantly affect detection efficiency [8-9].

This study investigates how internal flow behavior and geometrical design influence particle sampling, detection, and measurement accuracy in two commercial low-cost OPC models. The main objective is to evaluate whether particles entering the OPC inlet are successfully guided through the detection region and accurately recorded. To achieve this, computational fluid dynamics (CFD) simulations were carried out in ANSYS Fluent to replicate the airflow and particle trajectories inside each OPC. Three key performance metrics were defined to assess device performance: sampling ratio (the ratio of particle concentration entering the OPC to the ambient concentration), detector region ratio (the ratio of particle concentration within the detection region to that at the inlet), and measurement efficiency (an indicator of how well light scattering from a detection region can represent the detection region concentration). A simple light scattering model was applied to particles in a detection region and the scattered light incident on the detector was approximated. A representative calibration is done by employing a nonlinear regression calibration for a sample of simulation data and the corresponding scattered light.

Detailed CAD models of the two OPCs were created based on physical measurements and imported into ANSYS Workbench. A polyhedral mesh with three inflation layers was generated, and a mesh independence study confirmed the adequacy of the mesh resolution. In the simulations, air was modeled as the continuous phase and solid carbon particles as the discrete phase. The flow was laminar and incompressible. A particle volume fraction of 3.4×10⁻⁶ supported the assumption of one-way coupling, and a Stokes number of 5.5×10⁻⁶ indicated that particles would follow the air streamlines closely.

A surface injector distributed particles randomly across the inlet surface, following a Rosin-Rammler size distribution with a mean diameter of 1.25 µm, a range from 0.01 to 2.5 µm, and a spread parameter of 3.5. Simulations were run for at least 500 seconds simulations time and time-averaging was applied to smooth fluctuations in particle concentrations and improve data stability. A simplified light-scattering model was then used to relate the concentration in the detection region to the amount of scattered light reaching the photodetector.

Simulation results will be presented for both OPC A and OPC B, highlighting the critical influence of internal flow dynamics and geometric design on the performance of low-cost OPCs. The insights gained from this research will guide future design improvements, including better alignment of light sources and photodetectors, and optimized flow channel geometry.

References
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