| 15 Jul 2026
Black Carbon Aging Sustained by Chemical Evolution in Wintertime Haze over the East Asian Outflow: Evidence from Airborne Measurements
Abstract. Evaluating aging processes remains crucial for air quality management. We conducted airborne observations over the Yellow Sea during a haze event to elucidate how dynamic chemical transitions govern the physical aging of refractory black carbon (rBC).
Over December 15–17, 2025, the flight-mean NR-PM1 and rBC mass concentration varied from 3.8 ± 1.3 μg m⁻³ to 18.9 ± 5.8 μg m⁻³ and 201.2 ± 41.6 ng m⁻³ to 581.8 ± 136.4 ng m⁻³, respectively. During a NO3- explosion (mean 8.7 μg m-3), the maximum NO3- concentration coincided with peak rBC internal mixing parameter, Fthick under high RH (74 %) sustained since the previous evening, exceeding NH4NO3 deliquescence RH. These conditions likely promoted nighttime aqueous NO3- formation on rBC surfaces. With northerly winds shift, the SO42- mass fraction rose sharply, nonetheless, Fthick continued to elevate alongside the benzene/toluene ratio. Throughout this evolution, the organic aerosol fraction increased progressively, suggesting a constant supply of condensable mass via multi-channel pathways. Coating sensitivity analysis revealed that smaller rBC (<155 nm) responded linearly during the early stages, while larger rBC (>156 nm) exhibited a delayed sensitivity. Both were most sensitive to NO3-, NH4+, and organic aerosol, with aromatic and oxygenated VOCs playing key roles in organic coating. These findings suggest that physical rBC aging is fundamentally governed by major aerosol composition and size-dependent mass requirements.
Our findings highlight the need for an integrated mitigation framework targeting NOx, aromatic VOCs, and BC.
Lim et al. present observations of non-refractory aerosol from an aerosol mass spectrometer, black carbon, and limited volatile organic compounds (VOCs) from a proton transfer reaction mass spectrometer from 6 research flights that was conducted across three days over the Yellow Sea. They use the observations to investigate trends and driving factors of black carbon coating thickness. Currently as written, the paper is lacking detailed analysis, appropriate comparisons and discussions within broader literature, and appropriate understanding and description of the chemical and physical drivers impacting the observed trends in the various aerosol component reported here, as discussed below. Due to this, the paper does not appear ready or appropriate for ACP.
1) Though OpenAI is acknowledged for grammar and language editing, either another AI tool or a person should review the article for grammar, flow, and understanding prior to next submission.
2) Introduction could use more motivation and clarification about the problem and the uncertainty. As written, it was not even clear that much of the introduction was based on observations in East Asia or Yellow Sea region. Further, how does this problem occur outside just East Asia?
3) The methods need substantially more information. What is the diameter and residence time for the aerosol measurements? Was ram heating used for a drier? How was cloud sampling treated? Was inlet forward or backward facing? Was same inlet used for PTR as the aerosol instruments? How were the different instruments time-aligned? What "bounce" was used for AMS data? What lens was used for AMS (PM1 or PM2.5)? What was the particle transmission through all the lines? Were cals conducted before or after each flight, each day, only at beginning and end of campaign? Why are charges included in the description of nitrate, sulfate, and ammonium? As both inorganic and organic species can lead to the ions observed for these three aerosol species, charged symbols should not be included as there is inherent uncertainty how much organic nitrate, organic sulfate, and organic reduced nitrogen species maybe contributing to the whole.
4) The authors state many values and that they are different. However, looking at the values, it is hard to discern if they are statistically different or not. The error bars in Fig. 2, 4, and 5 generally makes it appear that there is minimal statistical difference in coating thickness, make it difficult to determine if the changes observed during the flights are real.
5) Much of the meteorological and physicochemical discussions about the aerosols are not cited and incomplete for what may be occurring. Examples include:
a) Consider winds at 4 m/s (9 mph) to be stagnant.
b) Discussing the relative humidity observed at the moment the aerosol is observed. Aerosol have a strong hysteresis effect; therefore, it is important to know the airmass history in order to say if the aerosol is "deliquesce" or "effloresce" at that moment.
c) There is discussion about residual and boundary layer over water at over 1 km. Combination of winter and being over water, it is not intuitive or clear what height the boundary layer, residual layer, and free troposphere would be. Further, would the marine boundary layer be the same boundary layer as what the air had experienced over land, or would there be a decoupling?
6) Statements are made without references, either from further analysis or from prior studies. One example is in line 243, saying the air is coming from coal-combustion regions. What leads to this conclusion? Another example is line 275 - 278, where the authors discuss the partitioning of NH3 and HNO3 to aerosol. Where is the NH3 coming from, especially if this is the residual layer? NH3 has a relatively short lifetime, and being in the wintertime, biological sources of NH3, both from agriculture and marine, should be minimal.
7) Fig. 4 and 5 are uninterpretable. It is now clear how size resolved coating for each component of aerosol measured by AMS is determined, and why some panels are missing data while other panels are not missing data. The combination of unclear how organics fthick is different from sulfate and the lack of statistical analysis leads to a disconnect between conclusions and results/discussions.
8) The interpretation of the PTR species for sources of organic aerosol is limited.
A) It's unclear how PTR measured "light" VOCs or what "light" VOCs are, as PTR is only sensitive generally to alkenes >C3, aromatics, BVOCs, and OVOCs.
B) The interpretation of OVOCs being a source of organic aerosol does not reflect the OVOCs being measured, as these OVOCs have too high vapor pressure to ever partition to the aerosol phase.
9) Lack of any back trajectories or other airmass history also generally limits the understanding and interpretation of the overall study.
10) Both the abstract and conclusion do not fully follow the author guidelines (https://www.atmospheric-chemistry-and-physics.net/policies/guidelines_for_authors.html). This includes the use of abbreviations without defining, comparison and context of the study, caveats, and implications.