General Comments:

The authors provided an analysis for the sources of abrupt PAN enhancement at Shangdianzi (SDZ) station on Oct 20 and Oct 25, 2020. The influences from regional transport were evaluated using CO as an inert tracer; the contributions from various precursors are then calculated based on observations. The authors found CH3CHO oxidation by OH is the major pathway of PAN formation at the SDZ site. I recommend the paper for publication after consideration of the points below.

Specific Comments:

1. Improvements in the language are suggested.
2. As shown in the title, the scientific importance of this paper relies on the validity of the conclusion to represent rural North China Plain during the cold season haze events. However, it is unclear whether the analysis based on a single station during pollution events (3 days) has enough representation.
3. Lines 236-251: More details should be provided for the calculation in this section. The Phys is calculated with dCO/dt * PAN/CO in Eq. 2. However, I assume Eq. 2 should consider PAN and CO along the transport pathway, particularly, the source domain, instead of SDZ site itself.
4. Section 3.4: The authors indicated that the contribution from CH3CHO oxidation by OH is larger than that from other precursors. I am wondering whether the total contributions from precursors can explain the observed PAN enhancement, quantitatively?

Technical Comments:

1. Line 130-136: improvement in the language is suggested as continuous “In addition” and “Here” are used.
2. Line 146: It seems Zhu et al. 2018 is not southern coast region.
3. Line 163: which were 10.6 and 7.7 times larger than those during clean days.
4. Line 213: the increases of PAN on pollution days are impossibly caused by direct PAN transport.
5. Lines 214-215: rephrase this sentence: “Here, direct transport impact is restricted to PAN that has been formed in other regions, but exclusive of PAN precursors”.
6. Line 228: what is “urban city”?
7. Lines 240-241: rephrase this sentence: “It should be noted that the calculation of physical transport impact aims at PAN that has been formed outside of the SDZ site”.

This manuscript investigated the occurrence of PAN, typical VOCs, PM_2.5, HONO and various trace gases during cold season haze events in the North China Plain, to elucidate the cause of rapid increase of PAN.  The article was well written with pre-established methodology. The authors provide some useful information about the formation of wintertime PAN. Overall this manuscript should be accepted after the authors address the issues below.

• Title: I don’t think it is a representative results for cold-season, exactly in autumn, based on the limited observation data.

• Highlight the new findings of this study. The authors should demonstrate the creative results, especially to differentiate those in previous studies. I think, studies on the occurrence of PAN have been widely obtained. The authors should introduce more studies about them, and discuss the formation mechanisms of PAN, especially in autumn.

• Analytical method appeared adequate; however some key procedural and QA/QC details on the observation of PAN, typical VOCs, HONO and various trace gases are missing. Please provide more detailsin the manuscript.

• The authors mentioned that, “Formaldehyde (HCHO) photolysis dominates the daytime HOx production thus contributing to fast photochemistry of PAN”. Limited VOCs species were measured in this study. How about the contributions of other VOCs species? Other studies had found that acetaldehyde was regarded as the most important precursor of PAN during winter in Beijing, could you explain it?

• For production rates of HOx and PA under different pollution level, do you compare them with OBM-MCM analysis?

• The authors are suggested to discuss the impact of environmental factors (including temperature, wind speed and SLP, etc) on the pollution characteristics of PAN.

General comments:

This paper explored enhancements in PAN over the North China Plain at the Shangdianzi (SDZ) site for two October days. After meteorological analysis, the authors determined that direct PAN transport was impossible given the timing of prevailing southerlies and used CO as a tracer to confirm. Local enhancements in photochemistry were determined to be the source of enhanced PAN. Authors used observations of various precursor species to calculate respective contributions and found that oxidation of acetaldehyde by the hydroxyl radical was the main pathway for PAN formation at the SDZ site. 

I recommend this paper for publication given the following minor edits:  

Specific comments: 

• The title suggests generalizability to all cold-season haze events, though 3 October days were examined. Additional observation/analysis is necessary in order to determine generalizability to all cold-season pollution events with PAN enhancements. Revision to the wording of the title is suggested. 
• The authors concluded that conditions were anomalously warm and wet for days of study, though title suggests "cold-season". As a reader, I question whether results are generalizable to "cold-season" days when anomalously warm and wet conditions are not present. 
• Figure 3b -- it is unclear as to what "clean days" refers to. 
• Improvements in grammar and language are suggested. 

Technical comments: 

• Line 116 - listed chemical species naming is not consistent with respect to chemical formula and English name use.
• Line 185 - lack of consistency in naming wind directions. "Southwesterly" should be used rather than "Northwestern". "Southwesterly" should be followed by "winds" to be complete (i.e. Southwesterly winds)
• Line 186 - sentence reads “..the SDZ site was in the south of a strong” should read “...the SDZ was to the south of a strong..”
• Line 187 - sentence reads “The southwesterly on 10/25 was caused by a weak high-pressure system with anticyclone in the southeast” should read “the southwesterly winds on 10/25 were caused by a weak high-pressure system with an anticyclone in the southeast.”
• Line 195 - “meridian” should be “meridional” 
• Line 217 - Wind direction naming consistency --  “southern wind” should be changed to “northerly wind” if this is what is meant.
• Lines 240-245 are not interpretable by the reader because the meaning of (Chem + Phys), (Chem), (Phys), were not clearly defined.
• Line  335 - “while our results was based on” should be “while our results were based on” 
• Line 358, 395 - wind direction naming consistency needed.