Extension of Low-Cost Optical Particle Counter Measurement Range by Refractive Index Correction

Low-cost optical particle counters (OPCs) are calibrated by manufacturers using a fixed refractive index (R.I.) to measure particle size based on light scattering onto a photodetector. While this R.I. is suitable for urban environments, nominal settings can introduce bias when measuring high concentrations of non-urban aerosol particles, such as Saharan dust. Two uncrewed aircraft systems (UAS) were deployed simultaneously during dust event in Orounda, Cyprus on 6 April 2022 by the University of Tübingen and Cyprus Institute. In-situ profiling of aerosol load was performed using an OPC-N3 and a Universal Cloud and Aerosol Sonde System (UCASS) sensor, respectively. Whereas the UCASS utilized an R.I. tailored to mineral dust (1.52+0.0020i), the OPC-N3 used a R.I. for urban air (1.50+0.00i).

An analysis of the sensitivity of the particle size distribution to the R.I. was conducted using real components between 1.48-1.56 and imaginary components between 0.0000i and 0.0028i. Application of a standard R.I. for saharan-derived mineral dust (1.53+0.0015i) improved agreement between the two systems, suggesting that observational discrepancies could be attributed in part to the differences in nominal R.I.. Furthermore, the backscatter coefficients derived from the corrected OPC systems are in good agreement with those obtained from the CIMEL CE376 lidar; however, optimal agreement was achieved between UAS and lidar when a greater imaginary component of 0.0024i was utilized.

To better understand the generalizability of this R.I. and sampling ability of the OPC-N3, a subsequent ground-based campaign was conducted at the Cyprus Institute, in which OPC-N3 observations were compared with a TEOM and a Palas Fidas 200. Correction with a R.I. of 1.53+0.0024i achieved the best cross-instrument agreement.

Under this correction, the bin denotation of the OPC-N3 was extended from 40 µm to 124 µm, with significant detection of particles with diameters reaching 86 µm during the 6 April 2022 dust event. These findings illustrate that a refractive index correction substantially enhances the ability of the OPC-N3 to measure and provide evidence of coarse, super-coarse, and giant mode particles.