This manuscript studied HONO concentrations and its sources in urban Beijing during autumn and winter of 2022. The results showed that NO2 heterogeneous reaction on ground was the dominant HONO source. Vehicle emissions and nitrate photolysis also contributed to HONO concentrations. In general, the research is interesting, the results and discussions are sounds. Here are some technique comments need to be addressed before it can be accepted.

1. L78, studied.
2. L114-115, the NO concentrations measured by chemiluminescence NOx analyzer is ok. But the analyzer could overestimate NO2 concentrations due to include other oxidized nitrogen. You can calibrate the data using the method in JGR: Atmospheres, 127, e2021JD036379. https://doi.org/10.1029/2021JD036379.
3. L123-124, delete the Wolfe et al before the bracket. It is the same for other similar references, such as Yan et al. (Yan et al., 2015), Zhang et al. (Zhang et al., 2019b), etc.
4. Figure 1 caption, it is better to define the meaning of DHP, PEP, and CLP. The meaning of the color bar in the 2^nd sub-figure should also be clarified.
5. L163-169, can you shortly explain the reasons why the pollutants concentrations are so low?
6. The value used in Table 2 should be listed.
7. L382-385, did you find any relationships between HONO concentrations and solar radiation? If you have, please show the data. Please refer to the publication: Explainable Machine Learning Reveals the Unknown Sources of Atmospheric HONO during COVID-19, ACS EST Air 2024, 1, 1252−1261.
8. The implications of the research should be clarified. Such as in L419-433, the results may indicate that control vehicle emissions could be an effective measures to reduce air pollution, while more measures should be integrated during the haze periods.

The manuscript uses measurements of gas-phase species and aerosols together with meteorological parameters to separate the measurements into three distinct periods. For each period they use a model to evaluate the sources of HONO and how they vary between the three phases due to changes in anthropogenic emissions. My major concern with the study is the use of a NOx analyzer (Thermo Fisher 42i), which according to the manual uses a molybdenum converter for the NO2 measurements. This is problematic since molybdenum converters are known to overestimate NO2 due to conversion of PANs and other nitrogen containing compounds and NO2 is a key component of the analysis. Additionally, some clarification is required throughout the manuscript, which is commented as minor comments or technical comments to improve the readability of the manuscript.

I recommend the manuscript be published when these concerns are addressed.

General comments:

I would suggest changing “anthropogenic activities variations” to “variations in anthropogenic activities” throughout the paper as it is easier to read.

Major comments:

Line 114-115: Since the NOx analyzer uses a molybdenum converter, it also converts organic nitrates into NO2/NO and potentially also particulate nitrates. Do you somehow take that into account when using the NO2 measurements? How often is the sensitivity of the different channels calibrated? If it isn’t taken into account, can you estimate an uncertainty on the measurements?

Minor comments:

Line 40: Something seems to be missing in the following sentence: “The more severe pollution, and the higher contribution of HONO to primary OH radicals (70–92 %).”

Line 56-63: The part of the paragraph between “Over the last decade” and “simultaneous control of both PM2.5 and O3” require some grammatical rephrasing. If the climate policies were implemented prior to another past event, then the use of past perfect tense is good, however, when writing “Over the last decade” then it should just be written in past tense. If you add when the air pollution control focus switched in line 62, then that becomes the other event in the past.

Line 61: In the sentence “the nitrate (NO3 - ) concentration and its proportion in PM2.5 had increased”, what do you mean by nitrate? Is it particulate nitrate, organic nitrates, inorganic nitrates, nitrate radicals? I would suggest defining it the first time you use it, since it is used throughout the manuscript.

Line 136-137: You write “As shown in Table 1, the highest HONO concentration in this study was generally higher than in other studies.”, however, 30% of the previous studies have higher maximum HONO concentrations than your study according to Table 1, so maybe rephrase it to represent that.

Figure 1: Please define DHP, PEP, and CLP here since the figure is described before the definitions. And describe the colourbar.

Line 195: When you mention NO3- formation, do you mean particulate nitrate?  Because if it is particulate nitrate, do you then mean that particulate nitrate formation leads to PM2.5 pollution or does this part of the sentence only refer to the low WS and BLH?

Line 201-203: You write “During the DHP and PEP, NO, NO2, CO and NH3 showed significant peaks during the morning rush hour (7:00–8:00 LT) due to vehicle emissions, then remained at lower levels throughout the daytime until concentrations began to rise again during the evening rush hour and built up during the night.”. This seems like a overgeneralisation since both NO2 and NH3 only show small if any enhancement during the morning rush hour in the PEP phase, CO doesn’t reach lower levels after the increase during DHP and as you write in the following sentence NO doesn’t increase at nighttime during DHP.

Line 205: NO does not look lower during CLP.

Line 209: What do you mean by vertical contrast and horizontal comparison?

Line 269-270: You write “exhibited significant increases in the evening (~19:00 LT) and early morning (~6:00 LT) during the DHP and PEP”, but in Figure 3a it looks like DHP is continuously increasing over the night and PEP is fairly flat.

Line 305-307: Maybe add that the enhanced oxidation of organic and inorganics during DHP is consistent with the high O3 concentrations observed.

Table 2: The references used for the parameterization should be mentioned.

Line 340-342: You write in the SI that you use EF=30 for the photolysis of particulate nitrate, however, studies have reported values between 1 and 700 for aerosols (Ye et al., 2016, Romer et al., 2018, Ye et al., 2017) and up to 1700 for urban grime (Baergen and Donaldson, 2013). Recent studies have found that the enhancement factor (EF) for photolysis of particulate nitrate depends on different aerosol parameters and for example decrease with increasing particulate nitrate (Andersen et al., 2023, Sommariva et al., 2023, Rowlinson et al., 2025). These dependencies are not incorporated in your model and would maybe give a different effect than what you observed (increasing importance of photolysis of particulate nitrate to the HONO formation with increasing particulate nitrate). While it is probably outside the scope of this paper to investigate the impact of different parameterizations of the EF, it would be good with a couple of sentences to discuss these effects and how it might impact your results.

C. Ye et al., Rapid cycling of reactive nitrogen in the marine boundary layer. Nature532, 489–491 (2016).

C. Ye et al., Photolysis of particulate nitrate as a source of HONO and NO_x. Environ. Sci. Technol.51, 6849–6856 (2017)

P. S. Romer et al., Constraints on aerosol nitrate photolysis as a potential source of HONO and NO_x. Environ. Sci. Technol.52, 13738–13746 (2018).

A. M. Baergen, D. J. Donaldson, Photochemical renoxification of nitric acid on real urban grime. Environ. Sci. Technol.47, 815–820 (2013).

S. T. Andersen et al., Extensive field evidence for the release of HONO from the photolysis of nitrate aerosols.Sci. Adv.9, eadd6266(2023)

R. Sommariva et al., Factors Influencing the Formation of Nitrous Acid from Photolysis of Particulate Nitrate. JPCA 127, 9302-9310 (2023)

M. J. Rowlinson et al., Observations of tropospheric HONO are incompatible with understanding of atmospheric chemistry, EGUsphere [preprint] (2025)

Line 424-425: You write “NO3- photolysis accounted for 12.6 %, 11.8 %, and 4.8 %, consistent with PM2.5 concentrations in three periods, and indicating increasing NO3- fractions in PM2.5 under more polluted conditions.”, but is that really what you mean? Since the NO3 is approximated based on the mass fraction of PM1 in PM2.5 (line 100 in the manuscript) and you use the same EF to determine the HONO production for NO3- photolysis is it not just an indication that you have significantly more aerosols available with increasing pollution?

Technical comments:

Line 19-20: Change “a comprehensive observation” to “comprehensive observations”

Line 25: I would suggest writing particulate nitrate as pNO3 instead of NO3- to avoid people misunderstanding it for NO3 radicals.

Line 65: change “development of second pollutions in Beijing” to “development of secondary pollution in Beijing”

Line 71: delete “had” in “Hereby, we had conducted”

Line 75: change “During this observations” to either “During these observations” or “During this campaign” or “During this observation period”

Line 78: replace “to” with “on” in “impact to secondary”

Line 78: replace “studies” with “studied”

Line 82: replace “provided” with “provide”

Line 123: remove “(Wolfe et al., 2016)” in “refer to Wolfe et al (Wolfe et al., 2016)”

Line 129: replace “illustrated” with “illustrates”

Line 132: replace “observation” with “campaign” or “observation period”

Line 132: Add “the” to write “due to the span across”

Line 135: delete “the” in front of Beijing

Line 147-148: replace “when” with “where” to write “there were 6 days O3 pollution where the daily maximum”

Line 151: replace “accompanying” with “accompanied”

Line 181: replace “parameter” with “parameters”

Line 200: replace “gases” with “gas” and add “the” to write “between the three periods”

Figure 2 and 3 text: replace “line graphs” with “lines”

Line 213 and 414: I would suggest adding “mixing ratios” after higher

Line 217, 224, 225, 226 and 227: I would suggest adding “the” in front of HONO concentration

Line 233: add “the” before Beijing

Line 261, 285, 287 and 292: I would add “period” after observation

Line 275: replace “cleaner period” with “cleaner periods”

Line 294: replace “was” with “were”

Line 296-297: remove the double references “Yan et al. (Yan et al., 2015) and Zhang et al. (Zhang et al., 2019b)”

Line 320: replace “further” with “which”

Line 379, 380, 412, 414, 418, 424, 428: I would add “the” before “three periods”

Line 397: Replace “declined” with “declines”

Line 419: Add “relative” before contribution since the absolute contribute is higher during the other two periods

Line 421: Add “the” before dominant

Line 431: “significantly reproduced” should be replaced by “significantly improved the agreement with”