When using optical particle counters (OPCs) to measure airborne particles, the unknown complex refractive index is one of the main sources of uncertainty in particle sizing. This manuscript addresses this issue by providing correction factors for specific instruments. Although all but one of the instruments are from the same manufacturer, the procedure used to obtain the correction factors is clearly described and could be applied to other spectrometers with different scattering geometries.

Specific comments:

1. The nominal measuring range of the UHSAS (Table 1) is 0.06–1 μm. Where do the marine and urban aerosol data shown in Figure 4 come from?
2. Please use a larger font for the x- and y-axis labels in all figures to improve readability.
3. I suggest examining the papers cited below and including them in the Introduction to better highlight the development of the issues related to the topic of this contribution.

Numerical and experimental study of the performance of the dual wavelength optical particle spectrometer (DWOPS), Nagy et al. https://doi.org/10.1016/j.jaerosci.2007.02.005

Optical particle spectrometry—Problems and prospects, Szymanski et al. https://doi.org/10.1016/j.jqsrt.2009.02.024.

Real-Time Determination of Absorptivity of Ambient Particles in Urban Aerosol in Budapest, Hungary, Nagy et al. https://doi.org/10.4209/aaqr.2015.05.0356

The subject of the present manuscript is relevant with Aerosol Research topic’s, specifically addressing the issue of the impact of complex refractive indices on the size classification of optical particle counters by providing a dataset of correction factor. The approach described in this paper is thoroughly delineated and may be generalized to other optical particle counters (e.g. the OPS3330) and certain low-cost sensors.

Comments:

1-I recommend to check Grimm description. According to the GRIMM publication, a single elliptical mirror is utilized, with light scattering being collected between 60 and 120°. Such opening angle may change the results describe in this paper. Indeed, Mie oscillations are expected to be more pronounced.

2-Please note that Equation 1 refers to the geometric bin diameter, not the midpoint diameter.

3-Use coherent writing for geometric diameter along the paper: Dgeom and after Dgeo

4-Line 65: OPC is based on single-particle counting. Please suppress “ensembles”

5-Please provide justification for the sentence appearing on lines 137 to 139

6-In Table 1, I believe that 'direct beam' and 'reflected beam' should be inverted. The range of 30-150° (GRIMM) corresponds to the angular aperture where light is reflected by the mirror. The range of 81-98° corresponds to the angular aperture where light is directly collected by the detector.

7-Line 210 to 222: please add units

8-Figure 2 does not contain any dotted lines. Please correct the text on lines 298-299.

9-Figure 3 does not contain any black lines. Please correct the figure legend.

10-The majority of low-cost sensors have a poor size resolution (five bins) and significant sizing errors caused by the position of particles in the laser beam (see references below). As far as I am aware, only a few LCSs have a good degree of classification accuracy and size resolution (OPC-N3, OPC-R2, SDS029). This aspect should be considered during the discussion.

Ouimette, J. Arnott, W.P., Laven, P., Whitwell, R., Radhakrishnan, R., Dhaniyala, S., Sandink, M., Tryner, J., and Volckens, J. 2024. Fundamentals of low-cost aerosol sensor design and operation. Aerosol Sci. Tech. 58 (1): 1-15. doi: 10.1080/02786826.2023.2285935

Pribošek, J. and G. Röhrer. 2018. Estimation of the particle sizing error due to particle position in an integrated PM2.5 optical particle counter. Proceedings 2 (13): 850-. doi: https://doi.org/10.3390/proceedings2130850

11-Sphericity is a rare particle shape characteristic. A brief discussion on the potential impact of shape on the correction factor would be beneficial. I would encourage the authors to investigate this aspect, as this would represent a significant step forward and useful.

12-It’s not possible to access the MIE routine via the provided link; furthermore, the DOI does not respond.

13-It can be hard to read the labels and axes of the graph. Please check all the figures, including the one in the supplementary material.