Impact of refractive index corrections of the OPC-N3 on Saharan dust aerosol particle size distributions

This study investigates observations of coarse particles during a Saharan dust event using low-cost optical particle counters (OPC) mounted on uncrewed aircraft systems (UAS). Measurements from an AlphaSense OPC-N3 and a Universal Cloud and Aerosol Sounding System (UCASS) are compared, and the impact of refractive index (R.I.) correction on OPC observations is examined. On 6 April 2022, a dust layer existing from 1250 to 2500 m a.g.l. advected over Orounda, Cyprus. Two UAS performed vertical profiles to examine meteorological and particle dynamics. Refractive index (R.I) corrections were applied to AlphaSense OPC-N3 size distributions to assess effects on aerosol particle load estimates. The correction substantially altered apparent number and volume concentrations, reducing discrepancies to an additional UAS (UCASS). Whereas varying the real part of the R.I. had a minor influence on the apparent distribution, sensitivity to the imaginary component was more pronounced. Assuming a R.I. of 1.53 + 0.0015i improved the fit of derived quantities with lidar and a TEOM observations; however, 1.53 + 0.0024i provided closer cross-instrument alignment, suggesting a higher imaginary component for the observed dust. The correction highlights long-range transport of super-coarse aerosol particles, and suggests that the OPC-N3’s ability may be extended to detect particles with geometric mean diameter up to three times larger than nominal bin classifications during Saharan dust events. Observations from a 2025 dust event further support these results. R.I. corrections improve interpretation of OPC-N3 data and enhance its ability to characterize coarse, super-coarse, and giant particles, ultimately supporting aerosol particle transport dynamic studies and reducing persistent knowledge gaps.