General comments:

This paper reports observations of HONO in a suburban location in the YRD region over a period of several weeks in summer. The authors find that photo-induced NO₂ conversion on the ground dominated the HONO production during the daytime, and NO₂ hydrolysis on the ground surface was the major source of nighttime HONO. Meanwhile, the authors employ a box model to investigate what contribution RO_x and O₃ derived from HONO makes to the radical chemistry at their measurement site. These results are meaningful for the development of HONO investigation. However, there also existed some problems the authors need to improve the manuscript before its publication in ACP.

Specific comments:

1. I am curious about the observation time. In the Method section, the observation period is introduced to be from May 14 to June 20, 2018, but Figure 2 only presents the observed parameters from May 23 to June 18, 2018, whereas the box model simulates the period of May 28-June 12, 2018. Why?

2. Importantly, in the calculation of HONO unknown source strength, the HONO deposition was not considered, why?

3. In the section of vehicle emission, the calculated average contribution of vehicle emission to observed HONO could reach 15%, but it did not appear in the HONO budget, why? According to the HONO budget result, direct emission might be the second most important source for HONO.

4. a few technical comments and typos:

Line 2: The secondary HONO should be removed in the title.

Line 43: SOA should be presented as its full name when it appeared at the first time.

Line 47: …HONO was a vital OH precursor not only in the early morning but also throughout the day.

Line 52/87: varied – various

Line 53: remove “to explain HONO”

Line 76: remove “typically”

Line 80: heterogeneous nitrate/HNO₃ photolysis on varied surfaces – adsorbed nitrate/HNO₃ photolysis

Line 82: heterogeneous – adsorbed

Line 98/258/321/345/348/356/418/439/474: write the right format (e.g., Fu et al., (2019) found…) for the references.

Line 150: throughout the paper – in this study

Line 155: an instrument model is needed for the portable weather station.

Line 162: try – trying

Line 180: the reaction of NO₂ and OH is missing in D(O_x)

Line 186/348: relative humidity has been abbreviated in line 154.

Line 191: it is difficult to derive the deduction of “VOCs are abundant” from “the MAXIMUM diurnal averaged HCHO concentration”. Please rephrase the sentence.

Line 197: the Class-II limit values are corresponding to the maximum 8-hour averaged O₃, rather than O₃ concentration.

Line 207: What does mean the average peak concentration of OH? For example, in the study of Zhang et al., (2022a), the OH concentration of 2.7*10^6 cm^-3 represented the average OH radical concentration at noontime (11:00-13:00). Comparison should be performed at the same level.

Line 220: was possibly the reason – was the possible reason

Line 227: need the reference for the HONO lifetime. Generally, nocturnal HONO lifetime is relatively long (several hours).

Line 235: higher concentration of O₃ production – higher O₃ production

Line 255: Jinan – Ji’nan

Line 307: photolytic – photo-related?

Figure 2: the order of magnitude for OH is not 10^(-6) but 10^6.

Figure 13: the meaning of legend should be stated one by one.

General comments

The manuscript examined the potential formation pathways for HONO and their impacts on ozone production in a suburban site in the China YRD region during the summertime, using sophisticated field measurements and constrained box model tests. They found that the traditional OH+NO pathway only largely underestimate the observed HONO concentrations. Within several potential pathways, photo-induced NO2 conversion on the ground is mostly likely the missing HONO source during the day, and NO2 hydrolysis on the group surface is the major missing source at night. The study also indicated a significant HONO contribution from direct vehicle emissions. They also assessed the contributions of the missing sources to the ozone production, suggesting an important role of HONO in aggravating ozone pollution. The topic is important and relevant. The dataset and analysis are comprehensive and valuable in improving the understanding the secondary pollutions and control policies. This paper is within the scope of ACP and might be of great interest to the broad atmospheric science community. I have a few questions and comments that should be answered before it can be considered for publication.

Specific comments:

Line 36: Should it be “increased by 88%”? (12.6-6.7)/6.7 = 88%

Line 64: “which is typically less than 2% NOx emissions” is confusing. Did you mean the HONO/NOx ratio is typically less than 2%?

Section 3.7 HONO Budget: why did you not include direct emissions, such as vehicle emission, into the HONO production rate? It seems vehicle emission contributed significantly to this site (~15%).

Technical corrections:

Line 28: change it to be “more likely due to”.

Line 37: Should it be “indicating HONO evidently enhanced O₃ production”?

Line 42: please define “SOA”, also “VOCs”, “PAN” …in the following text.

Line 52-53: may change the sentence to be “several HONO sources, including …, have been proposed”.

Line 52, 80, 87: replace “varied” with “various”.

Line 98: please correct the format of the citation.

Line 105: from 28 to 76 is more than doubling.

Line 314: Should “Sa” here be the “aerosol surface area density”, rather than “aerosol surface-to-volume ratio”?

Line 345: please correct the format of the citation.

Line 353: change “whether” to be “regardless of whether”.

Figure 2: Some values on y-axis of CO, PM2.5, and NO2 overlaps with each other. Please fix that.

Table 1: Why the Reaction of NO2 hydrolysis only gives 0.5 HONO for 1 NO2 reacted?

Ye et al. examined HONO chemistry and its impact on ozone formation using a field campaign measurement and box modeling. They found a high HONO/NOx ratio of 0.17 around noon coinciding with high J(O¹D), which suggests the importance of photo-induced sources for HONO formation. This is furthered verified by statistical analysis and box modeling with updated parameterization. They also demonstrated HONO chemistry can greatly enhance net ozone production by 45%.

Overall, this is a well-executed study and the key conclusions are reasonably defended. In particularly, the observational constraint for HONO chemistry from EXPLORE-YRD campaign adds important evidence to the understanding of HONO formation. However, I suggest the authors to discuss more broadly the HONO formation chemistry under different chemical conditions. I would recommend its publication after revision.

The authors highlighted HCHO production from NO2 heterogeneous conversion at ground. But how is daytime PM2.5 concentration during EXPLORE-YRD? I am wondering if the importance of aerosol update will increase over a severe PM2.5 pollution episode. Some discussion on the application of key conclusion from this study is required.

L43: please spell out “SOA”

L166-167: any reference for “a lifetime of 8 hours”?

L251: this argument should be further justified.

In Fig.1, The website of TROPOMI NO2 data product should be provided.