This paper presents a simulation-based particle age evolution during a major haze event in China. Although the method is not completely new, the application itself provides some interesting dimensions of the evolution of haze events and demonstrates how particle age information could be used to tackle atmospheric chemistry and physics problems associated with aerosol dynamics. However, there are some concerns regarding the current version of the manuscript. One major issue is the lack of model validation of the predicted particle age, which makes it difficult to determine whether the age numbers should be interpreted as quantitative or qualitative. I would recommend reconsidering this manuscript after the following comments are addressed:

Line 95， this description is confusing. Do you mean you implemented the CMAQ version method that expanded from UCD/CIT back to UCD/CIT?

Line 113: Since primary and secondary particles are tracked separately, does it mean there are no interactions simulated between the primary and secondary particles? Such as coagulation.

Line 120: CMAQ does not use a sectional method, what is the major challenge or novelty associated with the implementation of adopting the CMAQ method to UCD/CIT?

Line 122: How many age bins are used? Is there a limited age range? Does the model allow particles of different sizes and ages to interact with each other? If that is true, how that works?

Line 132: This is a short period, does it implies that this model is not designed for long-term simulation?

Line 135: Figure 1 should be referenced here.

Line 154: I am not sure if I understand the age bin updating frequency design strategy here. What is the point to make simulations with different updating intervals? Isn't it obvious that the simulation with updating intervals of 1 h provides the highest age resolution? And how the combination of simulation results works? "By replacing the low time resolution simulations with corresponding high time resolution results", won't that mean everything is replaced by the 1-hour result? In that case, why bother to run simulations of larger intervals? And how do those simulations address uncertainty problems as mentioned in the discussion section?

Line 160, It doesn't sound right for a "burden" to be "~3 times slower", I would say "the burden is ~ times higher".

Line 180: Does it implies that IMR can be used as a marker of particle age? Is IMR from observation compared with modeled results? Is there a way to check or validated the simulated particle age hours with observation?

Figure 2, the wind direction (the north I assume) is not marked in the upper panel (a). Also, NCP and YRD are not explained in the map. I am assuming that the red boundary is NCP and the blue boundary is YRD.

Line 218: Is the statistic based on hourly results or daily?

Line 228: What is "SNA"?

Line 250: What is the reason for the whole NO3- discussion here? Is it justified why only NO3- is investigated in Figure 4? Some clarification is needed.

Line 260-270, I think the time scale used in Figures 5 and 6 is not very helpful for diurnal observation, I would recommend a 24-hour scale averaged diurnal concentration curve as additional or supporting information. It is difficult to identify the "12:00 to 16:00 LT 28 December" on those figures. Also, when referring to "Beijing" and "Shanghai", do you mean one location in the city or the average information of the whole city domain?

Line 282. YRD is not mentioned in Figures 5 and 6, do you mean "Shanghai"? The author should clarify the regional name and its exact reference throughout the paper.

Line 281: I think this is both true for Shanghai and Beijing and it is normal that SOA is older, since "SOA" is secondary. What needs to be explained is the reason for the opposite, why sometimes the age of "SOA" is older than "POA" and "EC", and that could be the real deal of this study.

Line 297: I don't think that is a good explanation of the NO3- and NH4+ age pattern.  Ammonium sulfate would have the same behaviors in this case. Maybe the difference comes from the different emission patterns between NOX and SOX. Also in Line 375 of the discussion, if SO4 is formed upwind, where is the location of this "upwind"? The author should provide the emission map of SOX as supporting information. Also, if this is the right explanation than it does not apply to the whole simulation period and domain. The claims in Line 294 should be limited to only "Shanghai" and periods like Dec. 31 and Jan. 1st.

Line 344: I would like to see the spatial distribution of averaged particle ages in a map for different periods on ground level and also vertically averaged.

Summary:

The concept of atmospheric age, which refers to the time that has passed since the emission or formation of an air pollutant, has not received much attention in previous studies. This manuscript attempted to addresses this gap by updating an age-resolved UCD/CIT model and using it to simulate the evolution of age distribution for different PM_2.5 components during a severe regional haze episode in eastern China. The particle age information presented in this manuscript is interesting and provides a unique perspective on the formation and evolution of regional haze event, which is meaningful for future research in atmospheric chemistry and physics. Moreover, the manuscript is well written and easy to follow. I recommend to accept this manuscript after some minor revisions.

Comments:

1. Lines 67-68: How the define the atmospheric age of secondary particles? Is that based on the time since the emission of precursors or the formation of secondary aerosols?
2. Lines 79-80: The key factor contributes to the changes in the hydrocarbon ratios is the difference in reaction rates of different hydrocarbons with oxidants such as OH radicals. Please modify this sentence to make it more clear.
3. Lines 90-96: The author mentioned the dynamic age-bin updating algorithm used in the UCD/CIT model is based on the method used previously in the CMAQ model. Is there any difference between them? It’s better to provide more details.
4. Lines 123-126: How many age bins are used? What’s the time interval between different age bins? Also, What’s the highest explicit age?
5. Line 130: τ_i in equation (2) should be .
6. Lines 145-146: How to convert total emissions into different size bins? Please provide more details.
7. Lines 155-156: So there are a total of 5 cases? Why not directly use the high time resolution simulation, such as with a time interval of 1 hour?
8. Line 191: Figure S1 should be Figure S2. So as Figure S2 in Line 196 and Figure S3 in Line 201. The authors should check this throughout the manuscript.
9. Line 229: How to convert the measured OC to OM? Is that based on an assumption of OM/OC ratio? Please clarify.
10. Figure 3: What time period does the result shown in Figure 3e correspond to? Additionally, the colors used for SO₄^2- and NH₄⁺ are too similar, making it difficult to differentiate between them. Please change.
11. Lines 316-317: I can not find a “more uniform” vertical distribution for the period of accumulation stage from Figure 7. On the contrary, during the regional transport stage, the vertical distribution in the NCP region appears to be more uniform. This is because the air pollutants ahead of the cold front was lifted from the ground to the upper level.
12. Lines 335-336: What is “fresh” particle? In Line 272, the author mentioned fresh particles refer to particles with atmospheric age < 24 h, while they are particles with an age of less than 12 h here. Please clarify.
13. Line 375: It should be “NO₃^- formed locally in YRD”.
14. Mistake in the reference in Line 558.