In this study, the authors analyzed the chemical composition changes during the pandemic in ten urban and rural sites, and compared the HONO concentration level before and during the emission control period. The authors found that the HONO decline was relatively insignificant compared to its precursors and a detailed calculation shows that the enhanced production rate of aqueous phase reaction partially offset the effect of lower precursors. By comparing the atmospheric acids and bases concentrations, the authors suggested that the enhanced level of NH3 and elevated aerosol pH due to less acidic components in the atmosphere was the reason for the higher HONO production rate. It can be one of the possible reasons, while there are several important issues that the authors did not have enough discussion or provide clear explanation. Some analysis and explanations are too simplified to give the assessment of the quality of this study.

Major issues:

The direct emission HONO was estimated based on the vehicle emission factors and NOx concentration level, which should reflect a general situation of normal human activities. However, during the pandemic, the emission factors could change very significantly if only necessary activities were allowed to be carried out. The authors did not mention emission profile change before and during the pandemic, which could lead to the overestimation of the effect of other pathways.

Supplement Line 107: it is very challenging to pick a representative OH concentration to represent the general situation. The authors also suggested in the introduction that OH radical concentration could change during emission control as part of atmospheric oxidizing capacity changes. While the authors did not mention such an approach in their HONO production calculation. In addition to other reaction pathways, another possibility is the change of reaction rates, like OH concentration levels and higher temperature (the authors only mentioned H and K temperature dependence but did not mention k1 temperature dependence, which could be important). The authors should fully discuss the possibilities of the changes in reaction rate and possible sinks.

It is also questionable about the contribution of NH3 concentration changes to the total pH changes. Temperature, relative humidity, and other salts could also contribute to pH changes. It was not mentioned how the sensitivity tests of Line 264-275 were done and the interpretation of the results was also unclear. The authors did not give a complete pH comparison like NHx levels, only provided two sites in Figure 4. The authors mentioned the increase of pH 0.4 and 0.1 for U-ZK and R-PY sites respectively. However, based on the NH3 levels shown in Table 1 and the relationship mentioned in Song et al. (2019): ∂pHi/∂[NH3(g)]≈0.4/[NH3(g)], the NH3 concentration changes was only responsible for 0.13 unit pH change in U-ZK (less than half). The pH changes of most sites, if only considering NH3 levels changes in Table 1, can be calculated to be around 0.1 with the exception of R-SQ where NH3 concentration nearly doubled.

Figure 4, the maximum and minimum values provided little information of the whole pH variations. A box and whisker plot is more useful to identify the general trends and variations. And there were frequent situations of maximum pH higher than 7, which could not be explained by higher NH3 concentrations. Instead, it could be from the strong influence of dust components. If that situation happened frequently enough (hard to judge now based on the information given), it could be the dust components that are actually responsible for the high pH.

It should also be mentioned that the approach of the authors used to estimate AWC(org) is sensitive to the parameters chosen, such as OM/OC ratio, density, and kappa parameter. Normally, the term AWC(org) is small enough so that its influence is limited, while it is possible the uncertainty associated with the parameters chosen became big enough when inorganic salts become depleted and the relative contribution of OM got enhanced.

Minor issues:

The definition of TNHx is different in Line113 and Line 228.

Line 42, the study cited is the result based on a field campaign.

Figure 2, the max and min as error bars provide little information about the general trends, and there are negative values.

Line 215, it is hard to judge if agricultural activity got weakened or not. The NH3 concentration change could be due to less farm activity like less frequent animal feces cleaning, relatively higher temperature or a different regional transportation pattern.

Song, S., Nenes, A., Gao, M., Zhang, Y., Liu, P., Shao, J., Ye, D., Xu, W., Lei, L., Sun, Y., Liu, B., Wang, S., and McElroy, M. B.: Thermodynamic modeling suggests declines in water uptake and acidity of inorganic aerosols in Beijing winter haze events during 2014/2015–2018/2019, Environmental Science & Technology Letters, 6, 752-760, 10.1021/acs.estlett.9b00621, 2019.

This study reported that there was a noticeable increase in NH₃ concentrations during the COVID-19 pandemic. In addition to the meteorological conditions, the significant decrease in sulfate and nitrate concentrations enhanced the portioning of NH4+ to NH3, which enables enhanced particle pH values and in turn accelerate the redox reactions between NO2 and SO2 to form HONO. The article have several major issues and should be considered carefully.

Major comments:

1. In the introduction, the author comments that the exact relationship between NOx, NH3 and AOC remains unclear. However, it's a lengthy description of the changes in NH3 and pH before and during the epidemic and there is no detailed discussion on the specific impact on AOC. In short, the research problems pointed out in the introduction have not been fully explored in the study, and many conclusions are very far-fetched.
2. In lines 296-297, the paper argues that HONO has other sources and that the process of NO2 reacting with SO2 to generate HONO is currently insufficient evidence. In addition, this reaction is affected by pH, so how much does this contribution to HONO affect atmospheric oxidation? This discussion is also sorely lacking.
3. About HONO sources calculation, there are also many issues. The emission of motor vehicles at different stations varies greatly, so it is unreasonable to use 0.65% as the emission factor of HONO at all stations.
4. The uptake coefficient of NO2 on surfaces is not mentioned.
5. The same OH concentration are used at all station is also controversial.
6. In the supplement, is the equation (4) utilized in the calculation?
7. The J_HONO and J_nitrate used are suggested to be described in detail.