HONO is a very important nitrogen-containing reactive species in the atmospheric environment, which has a very great influence on atmospheric oxidability. At present, there are still many unknowns about the formation mechanisms of HONO, and it is very necessary to carry out HONO-related studies.

Major comments:

1. The title of this manuscript emphasizes "marine source", but the discussion of the effects of marine source on the observed HONO only takes up a very small part of this study, which in fact only shows that the large difference between the simulated and observed HONO under the "sea case" conditions is unexplained, while the contribution of the marine source and the mechanisms are not studied.
2. The accuracy of the model simulations is the key to the discussion in this paper. How can the accuracy of the model simulation be determined in the scenario where there are clearly 2 peaks in HONO during the observation period, but the model can only explain one of them
3. The authors only set the HONO concentration to zero in the "without HONO" scenario, which in fact still has the effect of HONO in the model. I think that the HONO-related reactions should be turned off instead of just setting HONO concentration to zero.

minor comments:

1. 1. Additional studies on marinesource need to be added in the introduction section.
2. What are the definition criteria for daytime time ranges (7:00-17:00) in this study? The light intensity is already so low at 18:00 that the photolysis reaction has stopped?
3. Line167: How are the contribution of vehicle emissions to HONO adjusted based on the environment and traffic density?
4. Line212：The dry deposition of the other components is not considered?
5. Can the authors explain the reason for the second HONO peak in the late afternoon?
6. How the VOC data is used in the model, since the VOC measurements have such a low time resolution?

General comments:

HONO is an important source of OH radicals in the atmosphere. Elucidating the characteristics and formation mechanisms of HONO is vital to understand the OH budget of OH. By combining modeling and field studies, Zhong et al provide evidence of a significant unidentified daytime marine source of HONO. Further, this missing HONO source is likely photochemical induced. This work has important implications for atmospheric chemistry in coastal and marine areas and will motivate further work on this topic in due course. This manuscript is well written and ACP is an appropriate venue. I would recommend the paper for publication after the following issues are addressed.

Specific comments:

Line 1: Should it be “the presence of a daytime marine source”?

Line 53: It would be helpful to introduce the missing source of daytime HONO.

Lines 137-138: Would the data averaging procedure introduce uncertainties to the subsequent analysis considering that different period of time was chosen for different days?

Lines 152: What are the unconstrained species?

Lines 161-164: What is the HONO/NOx value used in this study?

Lines 250-252: There is another peak of HONO at around 18:00. What are the potential reasons?

Lines 268-270: I am curious about the performance of the updated OBM on non-polluted periods. Do the model results of HONO agree well with the observations?

Lines 340-342: Though the authors focus on the sources of HONO during the daytime, the nighttime observations may provide constraints on the non-photochemical sources of HONO. Could the model capture the high nighttime HONO/NO2 ratio in the “sea case”?

Lines 374-376: The interfacial chemistry may lead to a high uptake coefficient of NO₂ on aerosol. Previous laboratory studies have found that the uptake coefficient of NO₂ on the aerosol surface can reach 2×10^-4 (see Liu and Abbatt (2021); Yabushita et al. (2009) and references therein).

Figure 4: Do the blue lines in Figure 4c,d represent the observed HONO production rate?

Technical comments:

Line 116: Give the full words of VOC in the first appearance.

References

Liu, T., & J. P. D. Abbatt (2021), Oxidation of sulfur dioxide by nitrogen dioxide accelerated at the interface of deliquesced aerosol particles, Nature Chemistry, 13(12), 1173-1177.

Yabushita, A., S. Enami, Y. Sakamoto, M. Kawasaki, M. R. Hoffmann, & A. J. Colussi (2009), Anion-Catalyzed Dissolution of NO₂ on Aqueous Microdroplets, The Journal of Physical Chemistry A, 113(17), 4844-4848.