In this manuscript the chemical and physical characteristics of O3 (O3SPD), PM2.5 (PM2.5SPD) polluted days and O3 and PM2.5 (O3&PM2.5PD) co-polluted days over BTH are investigated by using the 3-D global chemical transport model (GEOS-Chem). This manuscript is clearly written and well organized, and its conclusions are interesting.

• The simulated PM2.5 components including NO3 - , NH4 +, SO4 2- , BC, and OC are compared against observed PM2.5 concentrations, and the comparison shows that the simulated PM2.5 had a NMB of -26.9%. Even with the adjusted thresholds, percentages of observed polluted days for PM2.5SPD shown in Figure c are lower than for O3SPD and O3&PM2.5PD. Is the underestimation attributable to some missing primary aerosols?
• In the analysis two oxidation indicators (sulfur oxidation ratio and nitrogen oxidation ratio) are used, but not assessed. As observed SO2 and NO2 concentrations are available at CNEMC, model performance for SO2 and NO2 is suggested to be evaluated.
• Figure S5a shows the hourly variations of PBLH (m) averaged in all model-captured O3SPD (blue), PM2.5SPD (yellow), and O3&PM2.5PD (purple). Average PBLH at noon time for O3SPD and O3&PM2.5PD is over 2000m, why are they so high? Figure S5b shows the daily anomaly of PBLH for O3SPD and O3&PM2.5PD at night time exceeds -500m, while at noon time over 1000m. How does PBLH usually change over BTH?
• Figure S6 shows the vertical profile of SO4 2- chemical production. Why is SO4 2- chemical production larger at high levels than at low levels? Is it associated with cloud or high relative humidity? How is SO2 concentration distributed vertically? How to understand the difference between O3SPD and PM2.5SPD?
• It is interesting to see Figure 8a that O3 levels for O3SPD are lower than for O3&PM2.5PD. Does it mean high PM2.5 leads to in crease in O3? Figure 8b also shows BC is well mixed vertically up to ~819h Pa. Is it an average for all selected days?

In recent years, decreases in PM_2.5 but increases in O₃ over eastern China make the co-occurrences of PM_2.5 and O₃ polluted days (O₃&PM_2.5PD) an important issue related to human health. In this work, Dai et al. explored the chemical and synoptic characteristics of O₃&PM_2.5PD in Beijing-Tianjin-Hebei (BTH) region within a GEOS-Chem framework. They provided comprehensive analysis and concrete details in the differences among PM_2.5 alone polluted days (PM_2.5SPD), O₃ alone polluted days (O₃SPD) and O₃&PM_2.5PD. Results are novel and of scientific significance. I would like to suggest publication after addressing my comments below:

Major Concerns:

1. I suggest authors to separate Section 3.3 into two or three parts, where the chemical characteristics, vertical profile and process analysis are described respectively. The current demonstration looks not very logistic and thus makes it hard to follow.
2. Section 3.2, GEOS-Chem still significantly underestimates peak PM₅ concentrations as shown in Fig. 3d. Which PM_2.5 components dominate such underestimates? I’m worried that GEOS-Chem incapacity in simulating peak PM_2.5 could significantly influence the following analysis related to the differences in SO4^2- and NO3^- among O₃SPD, PM_2.5SPD and O₃&PM_2.5PD. At least more evaluation and discussions are necessary.
3. Lines 351-354 and Fig. 6, compared to O₃&PM₅PD, less S was oxidized into SO4^2-during PM_2.5SPD and less N was oxidized into NO₃^- during O₃SPD. Such differences also reflected in the PM2.5 components in Fig. 6. Are there any explanations about that? In addition, I’m curious what are the dominant oxidation pathways (e.g. SO₂ oxidation through H₂O₂, O₃, OH or NO₂) of SO₂ and NOx in GEOS-Chem? Can pathways be different among O₃SPD, PM_2.5SPD and O₃&PM_2.5PD?
4. In Fig.9, I’m confused about the totally different diffusion profile in SO4^2- relative to NO3^-and NH4⁺. In the PBL, air pollutants are supposed to diffuse following concentration gradients. For NO3^- and NH4⁺, strong chemical production happened in upper layers (913-771 hPa), where diffusion contributions at this altitude were negative, meaning the diffusion of new-generated NO3^- and NH4⁺ diffused through PBL. It is reasonable. However, SO4^2- diffusion were still positive at altitude where chemical production was strong, which seems against the concentration gradients. It might also be related to the constant SO4^2- profile in Fig. 8, which is interesting but I could not find clear explanations in this manuscript.
5. I suggest authors to summarize some highlights logistically in conclusions, e.g. what are the major differences in chemical mechanisms among O₃SPD, PM₅SPD and O₃&PM_2.5PD? What meteorological factors or synoptic patterns drives the differences? Also, although authors made very comprehensive analysis, one important question remained not very clear to me: Why O₃&PM_2.5PD only occurred at part of the O₃SPD or PM_2.5SPD? Which one among chemical mechanisms, vertical profile and meteorology drives the differences?

Specific Comments:

1. Lines 48-49: Natural sources also have significant contributions to PM_2.5.
2. Line 61: 'observations' should not be capitalized.
3. Lines 334-342: I suggest authors to add a table or figure in the main text or supplementary to show the OH evaluation.
4. Line 548: From the traditional synoptic definition, WPSH in eastern China should be regions with 500hPa geopotential height larger than 5880 m (or larger than 1520 m at 850hPa). I don’t think the high pressure here is WPSH.
5. Line 553: Northeast Cold Vortex is not necessary to abbreviate since it no longer appeared in the manuscript.
6. Figure 12 and S10: I wonder could the synoptic patterns be clearer if using anomalies rather than absolute values?