Please see the attached pdf file for the review in detail. 

This manuscript presents an evaluation of BrC (brown carbon) absorption in the GISS ModelE Earth System Model using aircraft campaign measurements of black carbon (BC), organic aerosol (OA), and BrC. By comparing average vertical profiles, the authors find that the model systematically underestimates BrC absorption, even after correcting for biases in carbon monoxide and BC. To investigate this issue, they explore a range of sensitivity tests, including variations in the OA:OC ratio, BrC aging, the water-soluble fraction of emitted BrC, and assumptions about the relative fraction of emitted/browner/bleached BrC. Despite these efforts, model biases persist in comparison to the in situ observations. While the topic is important and the dataset valuable, several critical issues must be addressed before I can make a recommendation.

Major issues:

1. Model Description of BrC Optical Properties. The paper lacks any discussion of the BrC optical properties implemented in the model, such as the mass absorption coefficient (MAC) values. Since these assumptions directly influence BrC absorption estimates, they should be explicitly described and justified. There is very little discussion on browning and bleaching of BrC. How exactly is it implemented? Is it in agreement with current understanding of the timescale of browning and bleaching?

Additionally, there is no discussion of how dry and wet deposition processes are handled for BrC species, which is essential for interpreting vertical profile comparisons.

2. Emission Corrections. The corrections applied for CO and BC based on wildfire attribution are not well justified. For some of the aircraft campaigns, wildfires are not the only significant sources of BC and CO. Without proper source attribution, applying such corrections could introduce further uncertainties.

3. Possible Causes of Model Bias. While the authors test several model parameters, their exploration of potential causes for the underestimation of BrC absorption remains somewhat narrow. Other important factors could include the assumed MAC values and their spectral dependence, photobleaching rates, emission injection heights, numerical diffusion in the model, and general representation errors in model transport and mixing. A broader discussion of these potential factors would strengthen the analysis.

Minor issues:

4. The use of dashed lines in the vertical profile figures is confusing. The captions state that dashed lines “indicate altitudes with no data,” yet lines are drawn—implying interpolated or assumed values. This needs to be clarified.

Overall, the model-observation comparison comes across as rather superficial. The paper presents a potentially useful dataset-model comparison, but it currently lacks sufficient detail in model description and scientific interpretation. I recommend a major revision to address the above issues before the manuscript can be considered for publication.