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Sensitivity of dynamic aging on the climate effects of black carbon aerosols over East Asia in summer

 by Gao et al.

Black carbon aerosols (BC) substantially affect regional and global climate and environmental systems through interacting with radiation and cloud. Since BC aging strongly modulates aerosol lifetime and aerosol–climate feedbacks, this study further explores how aging processes alter the climatic impacts of BC over East Asia during summer by implementing a dynamic aging scheme within the regional climate model RegCM-Chem. A set of numerical experiments is conducted to quantify the direct, indirect, and combined (total) climatic effects of BC over the East Asian domain. This work effectively addresses a major uncertainty in current climate simulations. The manuscript is well structured with a solid experimental framework and systematic analyses. The results underscore the vital importance of BC aging representations in regional climate modeling and offer valuable implications for constraining uncertainties in BC climatic effect assessments. Overall, this is a rigorous and scientifically valuable study. I therefore recommend the publication of this manuscript following minor revisions, with detailed comments provided below.

1. The manuscript implemented the dynamic BC aging scheme based on the parameterization developed by Fierce et al. (2017) and Ghosh et al. (2021). As noted in the Introduction, several aging schemes have been developed and applied in previous modeling studies. I suggest that the authors should clarify why the schemes is more suitable for the study and strengthen the scientific justification of the methodology.

2. The reliability of the adopted dynamic aging scheme requires further statement. In Eq. (2), the two parameters (kcond and kcoag) related to condensation and coagulation processes appear to be prescribed constants. The authors should provide more details regarding how these parameters were derived and whether their applicability under East Asian atmospheric conditions has been evaluated.

3. The study evaluates the simulated BC concentrations under the default aging scheme and the dynamic aging scheme (Exp. 4 and Exp. 8) and shows the comparison with observations in Fig. S1. It seems that introducing the dynamic aging scheme leads to only limited improvement in BC concentration simulations and the authors also acknowledge this. However, considering that improving BC aging timescales is one of the motivations in this study, the authors should provide a more comprehensive evaluation of simulated BC using statistical metrics both spatially and temporally.

4. The impacts of BC aging on aerosol concentration, column burden, optical properties and cloud activation capability strongly depend on how hydrophobic and hydrophilic BC are represented in the model. Detailed information about the prescribed parameters of these two BC categories should be provided.

5. More detail on the statistical testing methodology would strengthen confidence in the reported significance of the results (e.g., treatment of temporal autocorrelation and effective sample size).

6. Eq. (4), how the sulfate condensation flux is addressed in the model?

7. Section 3.4, please explicitly state what baseline state the radiative forcing in this study is compared against (e.g., relative to preindustrial era?).

8. Some writing errors. Line 296, “hotpots” should be “hotspots”. Line 305, “polluter” should be “polluted”. Line 644, “aera” should be “area”.