The manuscript presents results of a comprehensive field campaign at two different altitudes, the foot (150 m a.s.l.) and the summit (1534 m a.s.l.) of Mt. Tai (Shandong province, China). Mt. Tai locates in the middle of the NCP with a relatively high pollution level. The measured HONO diurnal profile shows a daytime peak at 12:30 local time, which is interesting since HONO diurnal profiles would typically peak during the night and early morning in more polluted regions. The topic is of interest to the scientific community and is suitable for publication in ACP after addressing the comments below.

The authors claim that OH+NO gas-phase reaction accounts for only 8% of measured HONO, and that 70-98% of the unknown HONO sources can be attributed to vertical transport from ground surfaces. However, the authors didn’t show/present the OH values used to calculate the OH+NO reaction rate, and they didn’t consider this reaction when calculating the net production of OH from HONO. The authors used an unjustified circular assumption that OH loss in the OH+NO reaction at the ground will be recycled back to OH at a higher altitude without any valid calculation of HONO lifetime vs transport time from the ground to the summit. The authors claim that they calculated HOx budget, although they only calculated gross HONO photolysis and O3 photolysis. (primary sources of OH only). I suggest the authors limit their discussions to HONO sources and sinks, and that they should account for NO+OH reaction in calculating HONOpss or assume several OH values around those published earlier to calculate their uncertainties. Otherwise, the manuscript is publishable after addressing these comments.

Specific comments:

Page 15, Line 320: The authors didn’t justify the use of OH-j(O¹D) correlation from previous publications to calculate OH in this study. Although some studies showed a good correlation, it still may not be a good proxy for OH given the large variation in the obtained slops. The authors use a circular argument that OH is not important since NO+OH is not important, to justify the uncertainty associated with their approach. At which OH levels does the NO+OH reaction accounts for 8%?

Maybe, it is safer to either simulate OH using a box model or use a range of OH levels around those reported previously by Kanaya et al. (2009) to show that it is not important, as they claim This is a major issue that the authors need to address before continuing with their calculations of unknown HONO sources.

The authors used several assumptions to calculate the contribution of different HONO sources to measured HONO levels. Most importantly is the photolysis of pNO3, for which the authors used a range of enhancement factors (EF) that ranges from 1 to ~15.6, accounting for 0.6 to 9.6%, depending on EF, leaving ~93% of HONO unknown sources unknown. I think. A major uncertainty here is related to HONOpss, which the authors didn’t sufficiently address, which affects the unknown fraction HONO.

Page 18, lines 404-412: The authors’ argument of OH recycling via HONO photolysis as the source of OH at higher altitude is not justified and is flawed. The authors didn’t provide information about the HONO lifetime vs the transport time to this altitude. I think this whole paragraph should be just deleted.

Page 18, line 414: provide a reference…

Page 19, lines 418-420: This long sentence is not clear at all….either provide all relevant information or leave it for the accompanying paper. Otherwise, HONO net photolysis should be used to calculate HONO relative contribution to OH primary sources.

Page 19, lines 428-438: Again, this is all irrelevant if HONO net contribution is not calculated.

Page 19, lines 425-445: replace HOx with OH since you HONO and O3 photolysis are sources of OH only, not HO2.

This paper presents comprehensive field campaign which was performed in summer at the foot (150 m a.s.l) and the summit (1534 m a.s.l) of Mt. Tai (Shandong province, China). The author performed the analysis of HONO budget and found strong unknown HONO sources. Constraints on the kinetics of aerosol-derived HONO sources were discussed and their contribution to HONO formation were negligible. The vertical transport form the ground to the summit levels and heterogeneous conversion of NO₂ was proposed to support the remaining majority of unknown HONO sources. The subject is suitable for publication in ACP and I would recommend the paper is accepted after the author have addressed the following concerns.

Specific comments:

1. Instrumentation: Low levels of HONO were measured by LOPAP technique with detection limit of 1.5 pptv at the summit of Mt. Tai in summer 2018. The QA and/or QC for LOPAP instrument should be stated to guarantee data quality.
2. Anthropogenic emissions: The author stated that low NO_x/NO_y of 0.43 ± 0.28 indicated aged air masses and small impact of anthropogenic emissions. However, NO and NO_x were measured simultaneously at the summit station. Why did not the author utilize NO/NO_x to evaluate the influence of nearby anthropogenic emissions. Moreover, the rapid increase in pollutants (HONO, NO, NO₂, NOy, CO, PM_2.5) was observed on 29 July. Low NO concentrations (1-2 ppbv) were observed at high O₃ levels (~50 ppbv), which should originate from local emissions. However, the author stated the high HONO levels could come from the heterogeneous conversion. The author should reexamine the data and explore the sources of increased pollutants.
3. Figure 5: The data of HONO and J(NO₂) for summit and foot station were measured at different periods. Whether it is appropriate to exhibit the data at different periods together in the figure? The measured data at different periods were different. Is such comparison meaningful?
4. Page 13, line 285-290: The author stated that south wind could enhance the upslope valley breeze wind because higher wind speeds (>5 m s^-1) were observed at the summit station than at the foot of the mountain (> 2 m s^-1). However, the wind speeds are generally higher at the summit station, which requires detailed explanation by the author.
5. Page 16, line 347-348: “Note that the uncertainty of …”. I don’t quite understand this sentence. Section 3.6 stated the contribution of photolysis of HONO and O₃ to OH. Please give the explanation.
6. The author calculated the enhanced uptake coefficient of NO₂ on the aerosol surfaces. The dark uptake of NO₂ on the aerosol surface could be considered to evaluate the influence of heterogeneous reaction on the aerosol surfaces since the dark uptake coefficient of NO₂ were mostly investigated.
7. Page 19, Section 3.6: The author only calculated the contribution of the photolysis of HONO and O₃ to OH and not HO_x. The HO_x should be replaced by OH.
8. Page 21, line 470: What dose λ_a stand for? It is γ_a?