This manuscript addresses how aerosol composition, specifically organic fraction, can influence CCN closure results. The authors obtain aerosol composition and CCNC measurements from aircraft observations in different field campaigns, ACE-ENA and HI-SCALE. The sites differ in their environments, resulting in differing composition and changes in CCNC observations and closure. The results highlight the importance of understanding regional aerosol sources in order to reduce the gap between observed CCNC results and model results. The paper can be a great asset for future studies bridging the gap between modeled and observational CCN. I do have a few questions/a comment, which are attached in a pdf.

General Comments:

In this study, the authors investigate how variations in aerosol composition affect CCN prediction accuracy, demonstrating that the dependence of CCN closure with organic mass fraction differs between midlatitude marine and continental environments. This paper advances understanding of aerosol activation and emphasizes the need to incorporate size-resolved chemical composition and mixing state information into global climate models to improve CCN prediction. The authors thoroughly discuss all data and the conclusions are well supported. The paper is well written with a few minor corrections. I have a few questions and comments:

Specific comments:

Line 120, “CCN data was measured by a Continuous-Flow Streamwise Thermal Gradient CCN Counter (CCN-200, Droplet Measurement Technologies) at prescribed supersaturation levels during flight operations;” How was the instrument supersaturation calibrated and how often was this calibration performed? 

Can the authors comment on why they chose to assume complete internal mixing? The authors have a thorough discussion of why this assumption tends to overestimate CCN concentrations, but no discussion of why they did not investigate any other mixing state assumptions. 

Line 129, “Each instrument contributes its most reliable size range and overlapping regions harmonized to yield a continuous distribution spanning from a few nanometers to several micrometers (Mei et al., 2024).” Can the authors specify what the “most reliable size range” for each instrument is? 

Line 140, “We perform leg-mean values and percentiles (5, 10, 25, 50, 75, 90, 95) for measurements obtained in each flight leg, and use leg-mean values in most part of this study. The results are robust if median values (50th percentile) are used (not shown).” Can the authors clarify what they mean by “in most part”? Are the median values preferred for being deemed “robust”?

For the MAM3 model parameters, how do these values, specifically density of organics and kappa of organics, contribute to overestimation of predicted CCN concentrations? What are the typical range of values assigned to ρ_org and κ_org in continental and mid-latitude marine environments? 

Line 331 “It is surprising that the relationships with MF_org are dramatically different at the two sites.” Can the authors comment on the potential influence of the two campaigns occurring during different seasons on the dependence of CCN closure ratio on MF_org? Does this possibly contribute to the large difference between the two sites? 

Line 363, “Thus, a higher MF_org likely indicates conditions dominated by primary organic aerosol loading, often accompanied by suppressed NPF activity, leading to more larger particles and fewer small particles (Fig. 8b). While these compensating changes may leave the total aerosol number concentration nearly unchanged, they reduce the fraction of particles that can be activated as CCN, resulting in a lower CCN activation fraction at higher MF_org during the HI-SCALE campaign.” It makes sense that a greater organic fraction would lead to a lower activation fraction due to reduced bulk hygroscopicity, however the discussion of these particles being larger preceding this statement is slightly confusing. An increase in the number of larger particles would seemingly lead to an increase in the CCN activation fraction as larger particles are more likely to exceed the critical diameter and activate. 

Minor Comments and Technical Corrections: 

Line 27, “Atmospheric aerosols, as tiny solid or liquid particles suspended in the atmosphere, exert substantial influence on Earth’s climate system (Intergovernmental Panel on Climate Change (IPCC), 2023).” This sentence is missing the word “defined” after aerosols. 

Line 228, “This is also consistent with Fig. 3b, that showing the accumulation-mode aerosols were much more numerous in summer than in winter while the Aitken-mode aerosols were similar or even slightly less for particles of ~ 20 nm in diameter.”  This sentence should read “This is also consistent with Fig. 3b, showing that the …” 

For Figures 3c and 4c is it possible to show the median lines in black? The red is hard to see. 

Line 384, “It shows that the median κ_ccn is rarely dependent on MF_org, which is inconsistency with the assumption of internal mixing and fixed hygroscopicity for each AMS-measured composition.” Inconsistency should be replaced with “inconsistent”. 

Line 424, “ At lower supersaturations, the critical diameter is larger with only larger particles (accumulation mode) can be activated” With should be replaced with “and” so that the sentence reads: “At lower supersaturations, the critical diameter is larger and only larger particles (accumulation mode) can be activated”