This study uses statistical methods and model simulations in an attempt to explain the decreasing mode diameter (DMD) events that were preceded by new particle formation (NPF) in western Saudi Arabia. The study area has frequent NPF and DMD events, and thus the collected data are of special interest in the study of aerosol formation and growth, as well as apparent particle shrinkage. The manuscript is well organized and written with clear hypotheses, but could be shortened and improved by focusing on the major findings. The presented methodology is reasonable and includes new analytical approaches to elucidating the causes of DMD events. The presented results are explained in detail and adequately discussed with existing work. The summary and conclusions, however, could be more specific and quantitative. My specific comments are given below:

1. P4, L12, P11, L4-6, P14, L24: To put the observations in perspective, it would be helpful to provide the typical ground-level SO2 and PM concentrations at the study area and in Jeddah (or the region).
2. P7, Table 1: Why is the T0 set at 278K? Please consider giving the exact numbers or steps of Psat, instead of “…” Also, please replace “…” with “to” or “–“ in the text.
3. If applicable, please consider providing relevant references to the presented equations.
4. P13, L5-6: Please be specific. Pearson correlation coeff.? Chi-square?
5. Fig. 2: This is a nice and interesting illustration of the stated hypotheses. It would be more informative if the typical transport characteristics could also be considered in the graph, e.g., the average wind speed during the DMD events, and how far into the sea at the onset of NFP. I suggest adding this info in the discussion sections (e.g., P17, L13, and Fig. 6).
6. P23, L9: Fig. 6 is informative. But what about including a plot of the probability of observing DMD particles? Would that be more straightforward for discussion?
7. Section 4.3.1: The simulated outcomes of the model somewhat do not seem to capture the expected results or relationships, except wind speed. Please consider discussing the important variables that may be missing in the model. This is crucial for subsequent discussion about the model predictions in the following sections.
8. Discussion: Based on the observed and model results, is it possible that the DMD phenomena are a result of measurements at the edge (i.e., boundary) of the NPF event? In other words, the NPF event is analogous to a large-scale “plume” event, in which condensable vapors and hence particle growth is strongest in the middle of the plume (i.e., source region), and decreases to the lowest at the edge (consider a plume traveling over a fixed measurement site).
9. Summary and conclusions: It would be helpful to provide the specific boundary conditions ([SO2], CS, WS, WD, T, BLH, Cvol, Cnon, etc.) that favor the occurrence of DMD in the study area. These could be used as a reference for other studies.

The MS deals with decreasing mode diameter NPF (DMD) events in a special atmospheric environment. These events are important to formulate and check ideas on the volatility properties of the chemical species involved in the particle growth. The special environment is characterised by a very high relative occurrence frequency of NPF events of 73% without evident seasonal tendency. Of them 76% were identified as DMD events. These characteristics are related to the local properties at the site. The MS covers a complex study with a nice adaptation of physicochemical approaches. The findings are valuable and are discussed in detail. It is relevant and of interest for the scientific community. I can definitely propose its acceptance. The following comments can be advised to the authors for consideration.

1. The MS seems to be long and dense. The authors may want to reorganise it in a way that it focuses in the main text on the main relationships, on the conditions for their validity, on discussions of the bounding conditions and on their physical meaning, and on the findings, results and conclusions. The detailed derivation of the relationships could be in the Appendix. In this way, the authors would better communicate the messages. The Summary and conclusions section should be shortened to primary messages.

2. The MS could be reorganised also from structural point of view. The part P13, L26 – P14, L9 belongs more to the site description than to Hypotheses based on our previous results. Further parts (for instance P4, L6) are redundant in an article.

3. The actual results were obtained under very specific atmospheric and geographic conditions. Could the spatially localised (local) nucleating plume affect its dynamic development? In that sense, the MS could advantageous be complemented with a brief discussion on the representativity of its finding. Could they be valid for other, more ordinary locations?

4. Paragraph P37, L15-19 are not completely clear. Reformulation is requested.

5. Formal comments. Altitude is missing in P4, L10. Insert space after semicolons in many places (e.g. in references and P4, L16). Internal brackets in P4, L17 are redundant, while semicolon is needed.