Reviewer Comments to Ha et al.

Heterogeneous chemistry of HONO formation and sink in the atmosphere is one of the least quantified issues in tropospheric chemistry, which induce serious uncertainty in the global and regional CTM to predict O₃ formation, CH₄ lifetime, and so on. The present article implements the heterogeneous HONO chemistry into the chemistry-climate model CHASER to show the inclusion of HONO chemistry reduces the model bias against the measurements for PM_2.5, NO₃^-/HNO₃, NO₂, OH, O₃ and CO in the lower troposphere.

Since the importance of heterogeneous production and loss of HONO has rarely been treated by a global CTM, it is interesting and worthwhile to evaluate the role of the chemistry of HONO in a global scale, and the present study is a challenging effort toward the direction. The most serious problem of the present article, however, is that the effects of HONO chemistry in the global atmosphere are discussed in 3.1 and 3.2 without enough validation of the processes and assumed uptake coefficients for heterogeneous formation and loss of HONO. At the present stage of understanding of heterogeneous HONO chemistry, selection of appropriate processes and uptake coefficients to reproduce the HONO concentration in the urban and remote field measurements should be the starting point for the model discussion. I am afraid that the present article does not fulfill such requirement.

Therefore, I rather reject the present version of the paper for publication, but do encourage the authors to revise the paper considering the following comments and resubmit the paper after conducting appropriate recalculation.

Specific suggestions for improving the paper:

1. Many field observations and regional CTMs have revealed that the concentrations of daytime HONO in urban area is much higher than expected by models considering only dark heterogeneous formation process (e.g., Lee et al., 2016; Lu yet al., 2018; quoted in the present paper). Since photochemical heterogeneous formation is now widely accepted to be important as a source of HONO, it should be taken into account as one of the important processes in the global model as well. The present paper discusses such photochemical process only later in 3.1.5.
2. Further, it has been generally accepted that every surface including soil dust and PM₅, and the earth’s ground would be effective for the formation of HONO. These processes should be taken into account in the global model as well.
3. As for the quantitative choice of uptake coefficients of heterogeneous processes, it is suggestive to parameterise them to reproduce the observational HONO concentration in the flied. The sensitivity of uptake coefficient of the heterogeneous formation processes would be high in the boundary layer in polluted areas, validation in urban areas should be performed at first by the global model comparing with the results of regional models. Then, validation for vertical profiles by aircraft measurements of EMeRGe and Atom1 should be made before the discussion of the effect of HONO chemistry in the lower troposphere in general.
4. Although the present paper refers limited laboratory studies on the heterogeneous dark reactions of HONO formation and loss on specific aerosol such as soot, the uncertainties are more than an order of magnitude, and it is not appropriate to select particular values for the “standard” run as in Table 2. Considering the large uncertainty of the uptake coefficients at this stage, the empirical approach mentioned above is suggested to be followed in the future paper.

over

A review report for manuscript entitled “Implementation of HONO into the chemistry-climate model CHASER (V4.0): roles in tropospheric chemistry” by Phuc et al., 2021.

 The study investigates the impacts of including HONO three formation paths (gas phase, heterogeneous, and emissions sources) on the levels of PM2.5, SO4^2-, HNO3, CO, O3, in the model compared to measurements from different platforms. However, the study does not account for major HONO sources via night and daytime heterogeneous NO2 conversion on-ground sources, and nitrate photolysis. In addition, the study presents the results of comparison but without a sufficient or reasonable explanation of these differences. This is a major issue, and the authors should carefully review the article and provide adequate clarifications accounting for HONO impact on each species beyond increase or decrease in species concentrations.

Specifics

Abstract: remove “for the first time” since HONO has been parameterized in several previous studies (e.g., Elshorbany et al., 2012; Zhang et al., 2021).

Page 1, line 24: the 51% contribution of heterogeneous NO2 to HONO formation on aerosol, and emissions sources are very high compared to current literature (e.g., Zhang et al., 2021).

Page 1, line 30: Why does reducing NOx (NO2+NO) reduce the atmospheric oxidation capacity? For instance, reducing NO2 would increase OH in high NOx conditions. Please clarify.

Page 6, line 169: I don’t see the value of the OLD simulations which assumed no HONO chemistry since all models have at least gas-phase HONO chemistry (OH+NO=HONO; HONO=OH+NO; and HONO+OH).

Page 6, table 2: How is the heterogeneous loss of HONO (R6) leads to NO?

Page 7, line 175: I suggest you differentiate NO from NO2 in model calculations throughout the paper.

Page 9, line 230: Please report the calculated surface aerosol density for each aerosol type.

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Page 10, lines 238-247: Please elaborate on “O3-reducing effects of HONO chemistry”. The authors should explain how HONO would reduce O3 when its photolysis is a source of OH, supposedly increasing the oxidation capacity?

Page 11, line 262-264: The authors are advised to explain the causal factors leading to increase or decrease in the impacted species (PM2.5, SO4^2-, HNO3, O3..etc) rather than stating the numbers.

Page 11, line 285: Again, an explanation of causal factors is missing.

Page 14, line 344: Again, the authors should explain how the inclusion of gas-phase HONO sources led to increased CO in some regions but decreased CO in other regions…

Page 17, line 382 and Figure 7: Figure 7 does not show the vertical profiles of simulated HONO. I also don’t think that HONO will have any impacts at 200 hPa??

Page 21, lines 445: HONO values at these heights are extremely high. HONO values of 70 ppt at 2000m are almost impossible given its ground-based sources and its fast photolysis. Authors should show some evidence that these numbers are reasonable.

References:

Zhang, S., Sarwar, G., Xing, J., Chu, B., Xue, C., Sarav, A., Ding, D., Zheng, H., Mu, Y., Duan, F., Ma, T., and He, H.: Improving the representation of HONO chemistry in CMAQ and examining its impact on haze over China, Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2021-47, 2021.

Elshorbany, Y. F., Steil, B., Brühl, C., and Lelieveld, J.: Impact of HONO on global atmospheric chemistry calculated with an empirical parameterization in the EMAC model, Atmos. Chem. Phys., 12, 9977-10000, doi:10.5194/acp-12-9977-2012, 2012.