Response to Reviewer Comments

We thank the reviewer for the constructive comments and suggestions, which helped us improve the clarity and quality of the manuscript. In this response letter, we provide point-by-point responses. Our replies to the reviewers’ comments are presented in blue font, while the corresponding revisions in the manuscript are indicated in blue italics with yellow highlights, together with the updated line numbers.

General comments

This study presents a unique dataset of aircraft measurements of BC and its microphysical properties over the Yellow Sea, consistently conducted throughout several years. The results will be useful for constraining the assumptions and parameterizations of size-distribution and mixing state of BC in aerosol-climate models and in aerosol remote sensing. I can recommend publication after incorporating the following comments.

Response: We sincerely thank the reviewer for the positive assessment and constructive feedback. The manuscript has been carefully and thoroughly revised in accordance with these comments.

Major comments

Comment:

In Section 1 or Section 3, the authors should add references of observational studies on the relationship between the microphysical properties and wet removal efficiency of BC in the same region, for example, https://doi.org/10.1029/2012GL052034. It should help the interpretation of the decrease of rBC MMD with APT or altitude observed in this study.

Response:

We appreciate this valuable suggestion. There have been several insightful studies which deal with BC’s properties in relation to APT and transport efficiency (TE).

Moteki et al. (2012) found, from aircraft observations over the Yellow and East China Seas, that as BC TE decreases, larger-mass BC particles are preferentially removed, resulting in a smaller count median diameter (CMD) during uplift from the planetary boundary layer to the free troposphere. An analysis of the relationship between BC TE and APT revealed a significant negative correlation (R² = 0.88) at altitudes above 2 km, and TE decreased with increasing APT as altitude increased (Oshima et al., 2012). In line with these studies, seasonal differences in precipitation and uplift strongly influence BC concentrations, and TE decreases as APT increases, indicating that the seasonal variability of BC is largely controlled by the seasonality of wet deposition (Kondo et al., 2016).

In the revised manuscript, we have incorporated the aforementioned references and expanded the discussion on the relationship between BC microphysical properties and wet removal efficiency based on these studies.

L 65–76:

“Recent aircraft-based observations have revealed diverse vertical and regional characteristics of BC particles. For example, BC particle diameters were found to be smaller near the surface over urban areas (Lamb et al., 2018), whereas in rural regions, BC tended to decrease in size with increasing altitude (Lu et al., 2019). Regional differences in BC mass concentration have also reported between the Atlantic and Pacific Oceans (Katich et al., 2018), and substantial removal of up to 98 % of BC was observed in Asian summer monsoon outflow (Berberich et al., 2025). These findings provide valuable insight into combined effects of emission characteristics, meteorology, and removal processes under real-world atmospheric conditions. More specifically, aircraft observations over the Yellow and East China Seas revealed that BC transport efficiency (TE) declines during uplift from the planetary boundary layer (PBL) to the free troposphere (FT), preferentially removing large-mass BC particles and thus reducing the count median diameter (CMD) (Moteki et al. (2012). At altitudes above 2 km, TE showed a strong negative correlation with accumulated precipitation along air mass trajectory (APT), with R² = 0.88 (Oshima et al., 2012). Building on this, Kondo et al. (2016) demonstrated that seasonal variations in precipitation and uplift patterns strongly modulate BC concentrations.”

L 369–370:

“Seasonal differences in precipitation and uplift significantly influence BC concentrations (Kondo et al., 2016) and physical properties (Moteki et al., 2012).”

Individual comments

L17: refractive BC → refractory BC

It has been corrected.

L18: height-dependency → height-dependence

It has been corrected.

L24: reduction → decrease

It has been corrected.

L25: These findings emphasize → These observations reflect

It has been corrected.

L26: You can remove “We believe that”

It has been removed and revised as follows.

L25–26:

“These findings provide valuable observational constraints for improving model representations of the size distribution and mixing state of ambient BC particles in the outflow regions.”

L27: “the physical realism of models”: Please be more specific.

It has been revised to the following sentence.

L 25–26:

“These findings provide valuable observational constraints for improving model representations of the size distribution and mixing state of ambient BC particles in the outflow regions.”

L34: “IPCC WG1 AR6”: Is this an accepted style for referencing the IPCC report? Please check. Should it be “First Author name YYYY”?

Thank you for pointing this out. The IPCC report citation was previously given as “IPCC WG1 AR6,” and the correct citation was inadvertently omitted during sentence revision. We have corrected this to (Masson-Delmotte et al., 2021) in accordance with ACP reference style.

L 29–30:

“The pace of recent climate change has intensified, prompting extensive efforts across scientific, political, and societal domains to curb its impacts (Masson-Delmotte et al., 2021).”

L 30–32:

“However, the nonlinear and interconnected nature of the climate system presents substantial challenges to accurately predicting outcomes and implementing effective mitigation strategies (Masson-Delmotte et al., 2021; Steffen et al., 2018).”

L 35–37:

“BC is a carbonaceous aerosol that is directly released into the atmosphere from incomplete combustion of biomass, fossil fuels, and biofuels. It absorbs light strongly in the visible and near-infrared spectrum (Masson-Delmotte et al., 2021).”

L39: “0.14 W m^-2”: Please use the minus-sign “−” instead of the hyphen “-“.

We have corrected this case as well as all other instances in the manuscript where the minus sign was typed as a hyphen, ensuring consistency throughout the text.

L39: You need one or more references for the BC radiative forcing “+0.14 W m^-2”.

We have added the appropriate reference.

L 34–35:

“Black Carbon (BC) is a significant SLCF that contributes roughly 0.14 W m⁻² of radiative forcing, as assessed in Masson-Delmotte et al. (2021).”

L45: estimated at → estimated to be

It has been corrected.

L47–L50: “Historical trends prior to 2000 show … ECLIPSE inventory.”: I couldn’t understand this sentence. Please re-word it to be more concise.

It has been revised into two shorter sentences for clarity:

L 44–47:

“BC emissions declined in North America and Europe prior to 2000, based on the Coupled Model Intercomparison Project (CMIP) inventory (Eckhardt et al., 2023). In contrast, the ECLIPSE (Evaluating the Climate and Air Quality Impact of Short-Lived Pollutants) inventory indicates a sustained increase in Asian emissions over the past two decades (Klimont et al., 2017).”

L68: mass distribution → mass concentration

It has been corrected.

L82: “CO/CO₂ ratio below 0.1%”: Using % for ratio (not fraction) is uncommon.

We directly calculated CO/CO₂ ratios as ‘ppmv/ppmv × 100’ and consistently report them in percent. It was initially intended to compare previous studies who reported CO/CO₂ ratios in % over the region including the Yellow Sea (e.g., Halliday et al., 2016). To clarify, we added the following sentence when CO/CO₂ is discussed in the manuscript. We believe this addition resolves potential ambiguity while allowing consistent use of % throughout the text.

L 377–378:

“In this study, CO/CO₂ ratios are expressed in percent (ppmv/ppmv × 100).”

References:

Halliday, H. S., Thompson, A. M., Wisthaler, A., Blake, D. R., Hornbrook, R. S., Mikoviny, T., Müller, M., Eichler, P., Apel, E. C., and Hills, A. J.: Atmospheric benzene observations from oil and gas production in the Denver-Julesburg Basin in July and August 2014, J. Geophys. Res. Atmos., 121, 11,055-011,074, https://doi.org/10.1002/2016JD025327, 2016.

Kondo, Y., Moteki, N., Oshima, N., Ohata, S., Koike, M., Shibano, Y., Takegawa, N., and Kita, K.: Effects of wet deposition on the abundance and size distribution of black carbon in East Asia, J. Geophys. Res. Atmos., 121, 4691-4712, https://doi.org/10.1002/2015JD024479, 2016.

Masson-Delmotte, V., Zhai, P., Pirani, A., Connors, S. L., Péan, C., Berger, S., Caud, N., Chen, Y., Goldfarb, L., Gomis, M. I., Huang, M., Leitzell, K., Lonnoy, E., Matthews, J. B. R., Maycock, T. K., Waterfield, T., Yelekçi, O., Yu, R., and Zhou, B. (eds.): Summary for Policymakers, in: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 3–32, https://doi.org/10.1017/9781009157896.001, 2021.

Moteki, N., Kondo, Y., Oshima, N., Takegawa, N., Koike, M., Kita, K., Matsui, H., and Kajino, M.: Size dependence of wet removal of black carbon aerosols during transport from the boundary layer to the free troposphere, Geophys. Res. Lett., 39, https://doi.org/10.1029/2012GL052034, 2012.

Oshima, N., Kondo, Y., Moteki, N., Takegawa, N., Koike, M., Kita, K., Matsui, H., Kajino, M., Nakamura, H., Jung, J. S., and Kim, Y. J.: Wet removal of black carbon in Asian outflow: Aerosol Radiative Forcing in East Asia (A-FORCE) aircraft campaign, J. Geophys. Res. Atmos., 117, https://doi.org/10.1029/2011JD016552, 2012.

L 344–359:

“In line with these studies, our findings suggest that aged rBC particles transported to high altitudes (above ~3 km) undergo significant physical transformation largely due to wet scavenging processes such as precipitation and cloud interaction, highlighting the strong sensitivity of both their size and mixing state to these removal mechanisms.

As a case, during F211125 in the autumn, a sharp decrease in M_rBC and MMD was observed above ~2 km, coinciding with an increase in APT to ~2–6 mm (Figure S7). This alignment suggests that rBC particles, particularly larger ones, were scavenged by precipitation, leading to the observed reductions in M_rBC and MMD. This episodic case (F211125) influenced the average shape of the autumn vertical profile (Figure 4). However, two mixing-state parameters (F_thick and R_shell/core) did not follow the same vertical patterns as M_rBC and MMD. Compared to the other seasons, both F_thick and R_shell/core remained consistently high with altitude on all three autumn flight days (Figure S7), likely due to the combined effects of (i) enhanced photochemical production of coating materials and shorter transport distance than winter and (ii) the relatively low solubility of those coatings, etc. Meanwhile, the vertical gradient of R_shell/core was steeper than that of F_thick. This can be attributed to their size-selection difference in methods: F_thick is determined from all detected rBC particles (~70–510 nm), whereas R_shell/core is derived from rBC cores in the 140–220 nm range. Preferential wet scavenging of larger particles at upper altitude likely contributed to the steeper decline of R_shell/core with altitude at some extent. ”

L 427–434

“This airmass origin-varying physical properties of rBC particles influenced their vertical profiles (Figure 4). In winter, Chinese air masses dominated below 2.5 km (74 %), whereas above this altitude, the LRT contribution rises to 88 % (Figure S7), accompanied by sharp decreases in both F_thick and R_shell/core (Figure 4). In contrast, during spring, together with potentially enhanced photochemical reaction at upper altitude, the contrast in source regions was less pronounced, leading to more moderate vertical gradients in mixing-state parameters. These findings provide clear evidence that the vertical structure of rBC mixing state is fundamentally shaped by combustion characteristics at the source region and thus vary seasonally with changes in air mass origin. This structure is further modified by atmospheric processes such as wet scavenging and photochemical aging.

L504 in Conclusion:

“Our observational results from YS provide the clear evidence that the vertical structure of rBC’s mixing state was fundamentally shaped by combustion characteristics at the source region and further modified substantially by atmospheric processes such as wet scavenging and photochemical aging.”

1. Figure 5a: The transport processes of gases and aerosols differ below and above the boundary layer. Explain how the authors justify using the same fitting result to represent the results at different heights. Additionally, discuss the reason for Russia's significantly lower r2 compared to other regions.

We acknowledge that transport processes differ between the planetary boundary layer (PBL) and the free troposphere (FT).

In an earlier version of the analysis, we applied separate fits for data within the PBL and in the FT, but except for a few cases such as Russia, the slopes and correlations were nearly identical to those obtained with a single fit. As shown in Figure 5a, the data points from the PBL and FT are distinguished by different markers, yet the relationship between M_rBC and CO is well described by essentially the same slope, which justified using a single fitting line to represent both altitude ranges.

Regarding the relatively low R² for Russia (0.34), several factors may contribute:

• Source heterogeneity: High R² values primarily imply that M_rBC and CO originate from similar combustion sources. A lower R² suggests that the two species may have different emission sources or that the sampled air masses were influenced by multiple plumes during transport. Russia covers a very large geographical area, and emissions differ substantially between its western and eastern regions. Such heterogeneity may reduce the correlation.
• Trajectory classification uncertainty: When we separate Russian air masses by altitude, the R² in the FT improves to 0.68, while it remains low (0.18) in the PBL. Air masses entering the Yellow Sea from Russia generally travel at higher altitudes; those sampled in the PBL are more likely to have interacted with plumes from Mongolia or China on route, which can weaken the M_rBC–CO correlation.

These combined factors plausibly explain the lower R² for Russia compared with other source regions. We have added this discussion to the manuscript.

L 443–449:

“In addition, Russia-sourced air exhibited notable vertical contrasts in both tracer ratios and rBC-CO correlation. While the overall rBC-CO correlation was relatively weak (R² = 0.34; Figure 5), this likely reflects the heterogeneous emission sources across the vast Russian territory and the influence of mixed plumes during transport. However, when separated by altitude, the correlation improved significantly in the FT (R² = 0.68) but remained low in the PBL (R² = 0.18), suggesting greater mixing with other continental plumes at lower altitudes. This vertical pattern was further supported by lower rBC/CO and higher CO/CO₂ slopes in the PBL, likely influenced by additional CO emissions along the transport pathway, while the opposite was observed in the FT.”

References:

Choi, Y., Kanaya, Y., Park, S. M., Matsuki, A., Sadanaga, Y., Kim, S. W., Uno, I., Pan, X., Lee, M., Kim, H., and Jung, D. H.: Regional variability in black carbon and carbon monoxide ratio from long-term observations over East Asia: assessment of representativeness for black carbon (BC) and carbon monoxide (CO) emission inventories, Atmos. Chem. Phys., 20, 83–98, https://doi.org/10.5194/acp-20-83-2020, 2020b.

Gu, J., Zhang, Y., Yang, N., and Wang, R.: Diurnal variability of the planetary boundary layer height estimated from radiosonde data, Earth Planet. Phys., 4, 479–492, https://doi.org/10.26464/epp2020042, 2020.

Hu, K., Zhao, D., Liu, D., Ding, S., Tian, P., Yu, C., Zhou, W., Huang, M., and Ding, D.: Estimating radiative impacts of black carbon associated with mixing state in the lower atmosphere over the northern North China Plain, Chemosphere, 252, 126455, https://doi.org/10.1016/j.chemosphere.2020.126455, 2020.

Hu, K., Liu, D., Tian, P., Wu, Y., Li, S., Zhao, D., Li, R., Sheng, J., Huang, M., Ding, D., Liu, Q., Jiang, X., Li, Q., and Tao, J.: Identifying the fraction of core–shell black carbon particles in a complex mixture to constrain the absorption enhancement by coatings, Environ. Sci. Technol. Lett., 9, 272–279, https://doi.org/10.1021/acs.estlett.2c00060, 2022.

Liu, D., Zhao, D., Xie, Z., Yu, C., Chen, Y., Tian, P., Ding, S., Hu, K., Lowe, D., Liu, Q., Zhou, W., Wang, F., Sheng, J., Kong, S., Hu, D., Wang, Z., Huang, M., and Ding, D.: Enhanced heating rate of black carbon above the planetary boundary layer over megacities in summertime, Environ. Res. Lett., 14, 124003, https://doi.org/10.1088/1748-9326/ab4872, 2019.

Qu, Y., Han, Y., Wu, Y., Gao, P., and Wang, T.: Study of PBLH and its correlation with particulate matter from one-year observation over Nanjing, Southeast China, Remote Sens., 9, 668, https://doi.org/10.3390/rs9070668, 2017.

Tian, P., Liu, D., Zhao, D., Yu, C., Liu, Q., Huang, M., Deng, Z., Ran, L., Wu, Y., Ding, S., Hu, K., Zhao, G., Zhao, C., and Ding, D.: In situ vertical characteristics of optical properties and heating rates of aerosol over Beijing, Atmos. Chem. Phys., 20, 2603–2622, https://doi.org/10.5194/acp-20-2603-2020, 2020.