Zhang et al. conducted a comprehensive case study in a coastal region to investigate HONO formation mechanism. The diurnal cycle of HONO is well simulated and successfully explained by the model. Results are convincing and sound. The manuscript reads very clear. I think it in general deserves an ACP publication, while I still have a few comments and some minor text-revising suggestions as below:

Comments:

1. It is interesting to observe the peak HONO at noon. However, if I understand correctly, it is from a monthly average in no-raining days. I am wondering if this diurnal pattern is consistent across individual days, or if multiple distinct patterns exist but are masked by the averaging. From a rough inspection of Fig. 2, the diurnal patterns seem to vary from day to day. I think authors should at least demonstrate that the noon-peak HONO occurred on most days. Even better, they are encouraged to do more analysis to examine 1) if model can capture different diurnal patterns and 2) if the underlying mechanisms can also be explained.
2. It might be insufficient to only discuss the HONO chemical production when investigating the impacts of ship emissions on HONO diurnal cycles. I am wondering whether the diurnal cycle of land-sea wind could also play a role in transporting NOx/HONO?

Minor suggestions:

1. Line 20: please specify the model name in the abstract
2. Lines 39-40 and Lines 48-49: The new reactions of the photo- and dark-oxidation of NOx should also be listed. In the reactions of HONO removal, the HONO oxidized by OH should also be included, as it is used in the following analyse (Fig. 5a).
3. Lines 56-58: what are the associated conclusions of this previous study?
4. Lines 102-103: It is not clear what is the ‘seven-day loop cycle’? What ‘systemic biases’ authors want to avoid?
5. Table 2. Please clarify that the 120 is from the ratio between measured NO3- photolysis rates and the default HNO3 photolysis rate.
6. Line 215. Figure 3b? Meanwhile, it looks like the BASE experiment also shows noon-peak HONO. Could you explain why it is? Could it be possible to imply that the diurnal emissions/transport also contribute to the noon-peak HONO?
7. Lines 314-316. Which one is daytime and which one should be nighttime?
8. Lines 422-423. Please specify the ratio of HONO to NOx of mobile sources are supposed to be higher or lower?
9. The uncertainty of soil HONO emissions should also be discussed, especially there are large cropland areas in the study domain.

This paper conducted a one-month HONO observation at a suburban site in coastal Fujian, combined with an improved WRF-Chem simulation, to systematically investigate the mechanism of high noontime HONO and the impact of shipping emissions on regional oxidizing capacity. The overall idea is complete, with close integration of observation and simulation, and the results are of great significance for a deep understanding of the HONO source mechanism in coastal areas and for quantifying the contribution of shipping emissions to regional atmospheric oxidizing capacity. However, this study still has many shortcomings in model settings, discussion depth and expression, and the authors need to carefully revise the paper to ensure the reliability and rationality of the results. The specific comments are as follows:

1. Section 2.1 states that the observation site is about 25 km from the Taiwan Strait. However, during the daytime, HONO quickly dissipates due to photolysis, with a typical lifetime of only a few tens of minutes. The researchers also reported low wind speeds during the observationperiod (average WS = 2.1 ms^-1). A simple transport calculation gives: the transport time from the ocean to the observation site is 3.3 hours (t = 25000/2.1/3600), which is one order of magnitude longer than the daytime HONO lifetime. Under this condition, any HONO directly emitted over the Strait is expected to decay significantly before reaching the receptor site. Therefore, the paper may overestimate the contribution of daytime shipping emissions to observed HONO concentrations and atmospheric oxidizing capacity.
2. L89-L90:NO₂was measured using 17i, also using chemiluminescence, which will overestimate NO₂ concentration and should be corrected. In addition, the concentration units of the same species in the manuscript should be unified. Was the concentration of NO, an important precursor of HONO, measured? Why is it not shown?
3. L134-136: In most studies the NO₂uptake coefficient on the ground is smaller than that on the aerosol surface. The authors should provide sufficient reasons for this choice. In addition, the light-enhanced NO₂uptake coefficient is generally on the order of 10^-5, and in some studies 1×10^-3 has only been used as the upper limit of NO₂ heterogeneous reactions. This value will seriously overestimate the contribution of NO₂ heterogeneous reactions, and it is recommended that the authors reconsider the value. In addition, selecting 1.45% as the emission factor is also significantly higher than the commonly used 0.8%. The authors should calculate the corresponding emission factor based on field observations to increase the rationality of the value.
4. The IOA index increased from 0.62 (BASE) to 0.69 (REV), which is not very high. At the same time, in Fig. 3a and 3b, the fit of the REV simulation results with the observations is poor, and the simulated values are significantly higher than the observed values on many days. These simulation results are difficult to convince readers. Did the authors consider the effect of rainy days when calculating the model evaluation index? The authors did not clearly state this. In the diurnal variation diagram of Fig. 3b, why are the three curves shifted?
5. Why do the authors not consider the removal pathway of HONO deposition, especially since nighttime HONO removal is mainly the deposition process.
6. The authors should provide PM_2.5concentrations to support the conclusion that NO₂heterogeneous reactions on the aerosol surface contribute little.
7. In the updated HONO sources, the parameter values should be explicitly provided or the calculation process shown. For example, how were the S/V of ground and aerosol surfaces calculated?
8. In Section 3.3.2 the authors explain “…meaning that shipping emissions contributed less to coastal NOx during the daytime.” However, the daytime HONO production rate is relatively high. In theory, as an important precursor of HONO, if the impact of NOx from shipping emissions is low, even if there are light-enhanced reactions, the HONO production rate should be limited. Therefore, the high daytime HONO production rate cannot be explained by “light-dependent reaction pathways.” At the same time, the explanation in Section 3.3.3 is also not valid.
9. Sensitivity analysis was not sufficiently carried out. The authors should scale the various parameters used by a certain proportion and then analyze how this parameter change affects the contribution of HONO sources or the impact on OH/O₃concentrations. The uncertainty analysis in Section 3.5 is not an explanation of the reasons for the parameter values, but should involve sensitivity experiments for the parameter values and discussion of their impact on HONO production rate, OH and O₃.
10. L373-L374:After adding HONO sources in the model, the daytime maximum OH concentration increased to 12.1×10⁶molecules cm^-3, significantly higher than OH concentrations observed in southern China in May, which further challenges the rationality of the parameter values in the updated HONO parameterization scheme. It also shows that the enhancement effect of HONO on O₃ in this study is significantly higher than previously reported ranges, which should also be considered in terms of the rationality of the parameters used.
11. The authors quantified the increments of HONO, NOx, and NO₃^-from shipping emissions, but there is a lack of spatial comparison analysis with actual shipping routes/port areas. It is recommended to add route or port distribution maps in the SI, and group the analysis by wind direction, to explore the modulation effect of nearshore O₃return/reaction on HONO and NOx.
12. Some minor errors: L45 “organic volatile organic compounds (VOCs)” is incorrect; L107 misstates, not Fig. 2b; where is Fig. 3c; L349-L350 and L363-L365 both mention the average daily OH radical production rate, but the values are completely different. The authors should carefully check and distinguish them.

Zhang et al. present a comprehensive analysis of the production and loss mechanisms of HONO, incorporating the updated mechanism into the WRF-Chem model over the coastal region of Fujian, southeastern China. Their measurements, supported by modeling, quantify unusually elevated daytime HONO levels. Through a series of sensitivity simulations, the study further examines the impact of shipping emissions on HONO, as well as the contributions of HONO to OH radical production and O₃ formation. Overall, the manuscript is well written and recommended for publication in ACP. Some specific comments are:

1. Lines 16-18: Briefly mention why HONO concentration is lowest around noon.
2. In Introduction discuss HONO patterns outside China and cite studies reporting any unusual trends elsewhere as well.
3. Lines 37-39: It will be informative to include reaction rates/photolysis coefficients with references for all reactions discussed in the manuscript.
4. Line 39: In reaction R3, replace v by Greek letter /nu, if possible.
5. Line44: Probably authors meant Additionally and not Adaptationally?
6. Lines 59-60: Mention the major reactions contributing to lower HONO concentrations in the noon time in polluted urban environments, as discussed in the previous studies.
7. Lines 60-61: Please add the reference for the study reporting inverse HONO diurnal variation; if it is Zhong et al. (2023), cite it here as well.
8. Lines 61-65: Discuss Zhong et al.'s suggested reasons for the unexpected HONO diurnal peak.
9. Lines 64-65: It would be worth discussing in conclusions - the key differences in HONO formation and loss mechanisms between coastal and urban polluted regions.
10. Lines 68-69: Please mention the typical number / quantitative of HONO emissions from shipping activities.
11. Lines 85-86: Do the authors consider this unusual daytime high HONO pattern in coastal regions to be seasonal, or the results valid across all seasons? Additionally, please discuss the applicability of these findings to other coastal regions worldwide in the conclusions.
12. Lines 102-103: It would be helpful if the authors could further elaborate the statement that the simulation was conducted in a seven-day loop to avoid systemic biases.
13. Lines 135-137: Are the daytime gamma values assumed or based on reported studies? Please provide relevant citations.
14. Lines 174-176: It would be nice to mention the modelled HONO concentrations as well for Beijing to assess how well they match observations.
15. Line 185-186: Are the definitions of non-rainy and rainy days based on a specific metric?
16. Lines 190-191: It would be informative to include standard deviations in Fig. S2, particularly for the HONO, NO₂, and O₃ plots.
17. Lines 193-194: Discuss the possible reasons for high O₃ at 4 pm despite low meteorological parameters?
18. Lines 215-216: Please mention correct figure number.
19. Figure 3: To avoid confusion, it would be better to label the legends as REV and BASE, respectively, instead of 'Mean model' for both.
20. Line 224: In Figure 4, NO₂ values for BASE and REV appear very similar and closer to each other than to the observations. Does this mean the updates have little effect on NO₂ concentrations? Notably, the 24-hour mean NO₂ for BASE seems slightly closer to observations than REV. Please clarify.
21. Lines 240-242: It would be helpful to include a supplementary plot of the net production rate (Production − Loss) to show when production dominates.
22. Line 347: Along with OH production rates, it would be informative to show OH loss rates too, since NO_y, HO_y, CO, CH₄, and VOCs significantly consume OH.