This study investigates the key controlling factors nitrate formation in YRD region during wintertime based on field observation and box model. They found large ALWC significantly promoted the uptake of N2O5 and gas-to-partition of gaseous HNO3, the partitioning coefficient of which varied with pH values of particles. The model calculation showed that N2O5 uptake contribute to the major fraction of particulate nitrate formation in this region during the pollution periods. Further analysis on the correlation of nitrate with its precursors indicated the controlling effect on nitrate formation resulted from atmospheric oxidation, which could be the availability of ozone and OH radical. A comparison over various parameters associated with nitrate formation made between the data before and during the epidemics also provided confidence for the results derived above.

Overall, this work provides valuable data on analyzing nitrate formation. It shows the dominant contribution from N2O5 uptake to nitrate formation in YRD region which might be different from other regions in China, and reinforces the importance of atmospheric oxidation on mitigating secondary pollution. I would recommend publication of this paper in Atmospheric Chemistry and Physics after the following comments are well addressed.

Specific comments:

Line 240~243: A constant dilution rate for model is inappropriate. For example, the dilution should be significantly enhanced during the breakup of nocturnal boundary layer in the morning at sunrise. It therefore could influence the calculated abundance of long lifetime species, like particulate nitrate, and change the relative contribution from different pathways. Suggest the parameterization of dilution rate constant varying with PBL for a more accurate quantification.

Line 338: The sentence of “The nitrate formation mechanism is different during the different time of a day” is a wrong statement, as the chemical mechanism should be basically the same throughout the day while the dominant formation pathway could change. It should be rephrased or deleted since it is closed to following sentence.

Line 340~350: There are two major problems on the evaluation of nighttime nitrate formation pathway. First, the concentration of particulate nitrate observed during nighttime is composed of both daytime remainder and nighttime formation, as it is a long lifetime species. Thus the positive correlation of particulate nitrate concentration with [NO2]²×O₃ might fail to represent the contribution from N2O5 uptake pathway. Second, what is the time resolution of data points showed in Figure 6? If it is one hour, the level of [NO2]²×O₃ at the point just after sunset, when nighttime formation of nitrate starts, should be the highest over the night under a stable condition without transports. The positive correlation tends to unreasonable accordingly. Suggest replacing the point-to-point correlation with nighttime averages correlation. Similar problems also apply to the daytime cases.

Line 473~474: References as to the statement that reduction of NO2 could result in the increase of O3 and OH radical are suggested to be provided here.

Line 522~523: Please explain why regional transport with more aged air plume leads to higher NOR and SOR values before the epidemic periods than that during the epidemic periods? It seems confusing to readers.

Zang et al., present a comprehensive study to identify the major nitrate formation pathways and their key controlling factors during the winter haze pollution period in the eastern YRD, China using two-year (2018-2019) field observations and detailed observation-constrained model simulations. They find that high atmospheric oxidation capacity is the reason for the winter nitrate aerosol pollution in YRD region in China. And N₂O₅ uptake contributes 60-70% in urban and suburban sites in polluted days. The analysis of the observation data is sound, I only have some comments to the model simulations.

1. Line 24-27, The quantification of nitrate formation importance is derived from pollution episodes only. The campaign average result should be much more different. Please clarify it.
2. The model includes the dry deposition of HNO3, it seems that the authors want to simulate the variation the particle nitrate. I am very interesting whether the modelled nitrate comparable with the observation. Is it possible to provide more details about the intercomparison? In addition, when calculating the contribution of nitrate formation, are you just accumulate the nitrate production rate during a certain period from different channel? Are the only represent the formation potential without considering the dry deposition, what is the role of the dry deposition in the model simulation since it cannot influence any result in the paper?
3. The heterogeneous chemistry is well considered in the model simulation, such as the N2O5 and NO2 uptake mechanism, but limited by the observation, the importance of these reactions cannot be confirmed, If the field measurement of N2O5 or ClNO2 are available, the result would be more insightful with smaller uncertainties. Here, I suggest the author provide more information about the parameterized N2O5 uptake and ClNO2 yield in the main text or SI, which could help people to connect the further observation studies that quantifying N2O5 uptake coefficient and/or ClNO2 yield.
4. Monoterpene is very reactive to NO3 radical, and we notice that monoterpene was not included in the model simulation, although the monoterpenes concentration may be low during the winter due to low temperature, but it maybe still have large contribution to the NO3 loss and affect the budget, I encourage the authors do some sensitivity tests to assess the impacts to N2O5 uptake and following nitrate formation.
5. Line 249-250, why only constrain the sum NO and NO2, if the NO and NO2 not constrained separately but only the sum, I guess the modeled nocturnal NO always be zero when O3 over ppb. While in fact NO spikes by local emission always observed in urban regions during the nighttime, which would lead to a bias of nitrate formation from N2O5 uptake (possibly an overestimation).
6. Figure 8 case 1, the observed NO2 during daytime and nighttime had a lower and higher biases, are they mean the modelled nitrate during the daytime is lower and nighttime is higher. This phenomenon also happened in case 3.
7. Line 81 or change to “and”
8. Line 361 weaker change to “weak”.
9. Line 346 the value 15000 misses the unit, may be pppbv³.