General comments:

The paper titled “Multiple pathways for the formation of secondary organic aerosol in North China Plain in summer” by Gu et al. reported an summer field observation in Handan City, in which four types of SOA were resolved using the PMF method dealing with the OA mass spectra data from a a soot particle long time-of-flight aerosol mass spectrometer. In addition, the variations and evolution processes of so-called SOA factors were discussed and their formation pathway were then deduced. Although a series of similar studies using AMS data had been reported in NCP previously, this study provided a more detail source appointment results in distinguish SOA as freshly (less-oxidized) and aged (more-oxidized) factors, and further directly associated with the formation pathway (photochemistry and aqueous-phase). With this, a better understanding of ambient SOA formation and aging in complex pollution with high oxidation capacity were gained. The manuscript was well written and presented clearly. Therefore I recommend the publication of Gu et al. work after some issues were clarified and revised.

Specific and technical comments:

1. Line 167-170. The new named four SOA factors are interesting, that different from previous studies using same AMS data set resolved by PMF. But the identification of those SOA fators were missed here, or a brief description but not enough appeared in the end of section 3.1. I think the identification of SOA factor is the most important and the foundation of this study, thus a detail description should be provided here. For example, why do the authors directly name the OOA factor as photochemistry and aqueous-phase formed? What the difference between the primary-related SOA and CCOA, both of which showed pronounced peaks of PAH ions?
2. Line 260-262. The transformation processes of POA and fresh SOA factor to phochem-SOA is interesting, it deserve more discussions here. Exploring the diurnal pattern of these factors during a special episode would be a choice, such as that conducted in Li et al. 2020. https://doi.org/10.1016/j.atmosenv.2019.117070.
3. Line 277-282. The aqueous-phase chemistry may also contributed to the fresh-SOA, but if so it is unclear what the difference of aqueous-phase in fresh-SOA and aq-SOA?
4. Line 289-290. It is subjective to conclude that the photochemistry is more efficient in elevating the oxidation degree of OA, as the correlations were analyzed during different periods (P2 and P3), during which other factors like primary emissions and transportation would be different and also effect the O:C ratio of OA.
5. Line 308-310. It is unclear where the photochemistry formation of SOA occurred. Please clarify it and revise this sentence.
6. Line 351-353. I do not think the transformation of POA to SOA could be deduced based solely on the negative correlation of each other. In fact, as showed in Fig 7, the negative correlation between phochem-SOA and POA would be expected, as the formation period of phochem-SOA usually occurred during the noontime when the boundary layer was much developed, while the POA usually decreased via horizontal and vertical diffusion. It is also supported by the better correlations in P2, which defined as high-Ox period and also has better diffusion conditions. Please clarify it.

This manuscript investigates the formation of secondary organic aerosol in the summertime North China Plain (NCP). The authors observed that both photo-oxidation and aqueous-phase chemistry contribute to the formation of SOA in NCP. Results highlight that the SOA formation is related to residential coal combustion, and the SOA formed could be further oxidized rapidly under high-RH conditions. Overall, the manuscript is well-written. The results are interesting. I recommend the manuscript be considered for publication only after my following comments are fully addressed.

1. What were the concentrations of inorganic cations such as Fe, Mg, and Ca? How would it affect the calculation of ALWC?

2. What was the pH of the aerosol particles? Did it contribute to the photochemical and aqueous formation of SOA? Please discuss.

3. Lines 167-170: A sentence is needed here to briefly illustrate how these SOA factors were defined. For example, what was fresh SOA? Was it LOOOA?

4. It seems that fresh-SOA and aq-SOA were quite similar (Figures 2, 4, 5, S4). Why did the authors separate them into two factors? Please clarify.

5. Lines 212-224: The authors mentioned that the primary-related SOA might be transformed from locally emitted POA, as suggested by the PAH ions in the primary-related SOA. What was the correlation of PAH ions in the primary-related SOA and POA? It seems that the patterns are quite different.

In addition, in my view, the correlations between the “primary-related SOA” and CO, NO2, and HOA were not high. The highest R-value was only 0.6, with R2 < 0.5. Could it be just primarily emitted OA?

Also, why did the “primary-related SOA” mostly peak at night? Please explain.

6. Lines 274-277: If SP-LTof-AMS collected PM2.5, how can aq-SOA in droplets be sampled and analyzed? Please provide the size distribution of the particle sampled. The authors can compare the size distribution of aq-SOA to sulfate or other inorganic ions to support their conclusions.

7. Figure S3: Please explain why aq-SOA concentration decreased with ozone concentration, and why phochem-SOA concentration decreased with ALWC?

8. Figure 5: How could aq-SOA be formed when ALWC was 0?

9. Figure 6: There was a substantial amount of phochem-SOA at night. Can the authors discuss if it was possible that “phochem-SOA” can also be formed at night? Could it be just SOA formed via ozone oxidation? Maybe using a different name for “phochem-SOA” would avoid confusion.

10. Most of the results provided in this study are based on correlation analysis. Although I believe that correlations provide valuable insights, the results from correlations solely may not be conclusive and convincing enough. Can the authors provide additional evidence (e.g., from the perspective of chemical composition and tracers) to support the conclusions of this study?