This study used different types of instruments with mass spectra, combining with a DMA and AAC for size selection and investigated the effective density of aerosols, during the Beijing 2022 Olympic Winter Games for the impacts of emission controls on particle mixing state and density. The results provide information on changes in aerosol compositions and mixing state due to emission control. A few points need to be addressed before it can be accepted.

Major:

1) The method in deriving the effective density for different compositions should be given more details. The used equations have not been clearly explained. It is not clear how you have linked the effective density with certain composition.

2) The instrument setup should be given in front in the main texts, with more explanation why running AAC and DMA in parallel. Why the density has been derived using two methods.

3) The effective density from 1.26 to 1.20 is not significantly different, as emphasized in the abstract.

4) Why high molecular weight OA has a lower effective density.

5) What can the findings tell from environmental policy point of view? 

Others:

1. Please indicate the full name where the abbreviation first appears in the abstract, such as EC and OC on line 17, EC-NC and KEC-N in line 19, and ECOC-NC in line 20.
2. Please give the full name of NR-PM1 when it appears for the first time.
3. Line105-114, Why to use DMA connecting with SPAMS and AAC to connect with SPAMS separately to obtain effective particle size? Why are two different instruments required?
4. Could you provide a detailed explanation how Equation 2 is derived?
5. Line 212-214, Please indicate the corresponding figure number for the conclusive numerical results provided by the authors.
6. Please specify what the color bar in Figure S5 stands for.
7. Section 3.3 is quite confused. The title indicates that the study focused on the effective density of aerosols during the Olympic Winter Games. However, the effective density is only briefly mentioned but not related to the event.
8. As this study mainly focused on the BC-containing particles, a few quite related studies also measured the shape of BC-containing particles in Beijing, which showed more spherical particles when polluted (Hu et al., EST Letters, 2022, 10.1021/ acs.estlett.2c00060), and also the aerodynamic size-selected compositions and density by AAC (Yu et al., ACP, 2022, 10.5194/acp-22-4375-2022). These studies could be referenced to support some of your conclusions.
9. 2 is not used in the texts.
10. Please explain the many significantly high values of PA_sulfate/PA_nitrate in Figure 6.

The manuscript " Mixing state and effective density of aerosol particles during the Beijing 2022 Olympic Winter Games " mainly investigates the impacts of emission controls on particle mixing state and density by deploying a single particle aerosol mass spectrometer in tandem with a differential mobility analyzer and an aerodynamic aerosol classifier during the Beijing 2022 Olympic Winter Games (OWG). In general, the paper is well written and presented in a logical way. It is of general interest for Atmospheric Chemistry and Physics related communities. I therefore recommend publication of this paper in Atmospheric Chemistry and Physics after some revisions. My comments are listed as follows: 

Specific Comments:

1. Line 16: “showed” should be changes to “show”. Besides, I suggest the authors to use present tense in writing a scientific article. I recommend to use external proof reading before submission of the revised version.
2. Lines 16-17: the meaning of “Total- EC”, “Total-OC” and “Total-ECOC” should be given here, also including other abbreviations (e.g., EC-NS, KEC-N, and ECOC-NS).
3. Line: 75-76: a detailed explanation for “DMA (model 3085A, TSI Inc.) and SPAMS (Hexin Analytical Instrument Co., Ltd.), AAC (Cambustion Ltd.)” should be given.
4. Lines 88-89: “The detailed operations of AE33 and HR-ToF-AMS, and the data analysis are given in Xu et al. (in preparation).”, some descriptions are needed because the paper of Xu et al. is in preparation.
5. Line 106: what are the sources of the two approaches of calculating the ρ_eff in this study? Besides, why do the authors use two approaches to calculate the ρ_eff?
6. Lines 498-502: the meaning of wind direction (WD) values should be described; the colors in curves of eBC and NR-PM1 cannot be well distinguished; “hit rare” is “hit rate”? Why “both size and hit counts after 2.10 are divided by 4”? Additionally, all the meaning of symbols (e.g., green cross in Fig. 1a) and lines (e.g., blue and green lines with arrows in Fig. 1a) occurred in figures should be described.

1. Line 158: “diurnal cycle” could be more reasonable than “diurnal trend”.
2. Line 254-255: Why did the emission controls during OWG lead to the increases in aged and regional particles? A reason analysis is expected in discussions. Thus, corresponding to the data analysis results, an analysis of mechanism behind the phenomenon is needed.
3. A discussion on the uncertainties of data quality and analysis should be included in the “conclusions” section.