The work is done and presented well. There are no major questions about the quality of the work. The main question this reviewer has is "why"? The manuscript is about fitting functions to a multimodal particle size distribution. This is basic data analysis and all major scientific software suites include this capability. An argument was made that the presented work simplifies the process so that the user does not have to learn how to find the peaks (which in itself could be problematic since now there's a black box doing the analysis). However, the code is written in R (using existing R packages for analysis - so again, where's the novelty?) which limits its usefulness to those using R (as opposed to e.g., Python or Matlab). As such, there is little scientific merit to be reviewed here. In conclusion, this manuscript does not offer anything novel to meet the standards of AMT and qualify as a full research paper. The manuscript could be re-submitted as a technical note or the focus could be shifted to analyzing the data from SPL using the R functions described.

The work done by Rapp et al. is presented clearly and coherently and I have no major issues with the writing itself or the structure of the manuscript. However, I cannot recommend its publication as a research paper and would recommend its resubmission as a technical note. While the method presented in the manuscript could be of use to some, it is not novel enough for a full research article and the study itself presents little new additional research. In addition, it is only available for R, which undoubtedly limits the number of potential users.

Other comments:

1. Fig. 1 and 2: the fit seems to be poorer for smaller diameters. The limitations imposed by size or low concentrations and their implications on the interpretation of the fitted modes could be discussed.

2. No comparison with other mode fitting methods is presented. Some comparative results would increase the value of the study.

3. The study is focused on introducing a R-based curve fitting algorithm for modal aerosol size distributions. The paper would be more useful were some actual examples of using the algorithm given. In addition, detailed description of the outputs of the algorithm should be provided.