Zhai et al. present a novel perspective on the impact of coarse particulate matter (PM) on the formation of fine PM nitrate in the North China Plain and the Seoul Metropolitan Area. The authors report that anthropogenic coarse PM, particularly fugitive dust from construction, which has not been considered in atmospheric chemical transport models, is capable of more efficiently absorbing gas-phase nitrate acid during the summer, resulting in a suppression of fine PM nitrate formation. Consequently, reducing emissions of coarse PM could lead to an unanticipated increase in fine PM nitrate levels, despite the decrease in NO_x and NH₃ emissions. The findings suggest the need for more extraordinary efforts to control NO_x and NH₃ emissions as fugitive dust pollution is controlled. The manuscript is well-organized, and the conclusions drawn are solid and well-supported by the data and simulation results. Therefore, I recommend the manuscript for acceptance with minimal revisions.

1. While the manuscript clearly outlines the relationships between coarse PM, fine PM, and gas-phase NO_x and NH₃, it would be helpful for readers if the authors could provide a diagram illustrating the tradeoffs associated with reducing emissions of coarse PM and NO_x/NH₃.

2. Line 160 suggests that the absorption of HNO₃ by coarse PM is three times more efficient than dry deposition, with alkalinity not being a limiting factor. However, it is not clear what the limiting factor is. Given that the available surface area typically restricts reactive uptake, the authors could provide clarification on how the surface area of coarse PM compares to that of fine PM, and which factor (e.g., surface area, chemical composition, relative humidity/temperature, etc.) limits the simulation.

This paper presents an interesting analysis of the influence of coarse mode aerosol on the distribution of nitrate in the East Asian atmosphere. They use detailed aircraft and ground measurements during KORUS-AQ to show that a significant amount of nitrate appears to be associated with coarse mode particles and that including a more realistic representation of coarse mode particles reduces the GEOS-Chem model’s high bias in fine mode nitrate. The analysis is also extended to simulations over the North China Plain. Overall the data and interpretation are clearly presented and I think the paper should be published following some clarifications, as requested below.

I found it difficult to follow the description of the approach for including coarse mode particles in the model on lines 142-147. Were the network observations applied as emissions or concentrations? How did the inclusion of the coarse mode in the lowest model level influence higher altitudes in the model domain? Was the linear interpolation in time, space?

The authors should clarify the definition of PM_1-4 nitrate from the DC-8 observations. The AMS only measures submicron semi-volatile nitrate whereas the SAGA measures all PM₄ nitrate. It appears, though is not stated, that the difference between the SAGA and AMS nitrate measurements is used to define PM_1-4 nitrate. What if there is refractory PM₁ nitrate? Figure S2 does not rule out this possibility because the AMS would likely not be sensitive to nitrate associated with refractory minerals in dust. What if there is semi-volatile supermicron nitrate? Would this impact the measurement model comparisons (it’s unclear from line 151 if the model nitrate is PM1 or any ammonium nitrate)? These details are unlikely to impact the qualitative outcomes of the analysis, but it would be useful to have more precision in the definitions.

Related to the points above, the authors note that the impact of including coarse mode aerosol that can uptake HNO₃ is less impactful in the NCP, but in this example they compare PM_2.5 nitrate rather than PM₁ nitrate. Very little is said about the observations from China, but are these measurements limited to semi-volatile nitrate, or would they also include dust nitrate in the fine mode?

In the analysis of the overall sensitivity of fine mode nitrate to recent emissions trends, the authors explain the more modest sensitivity of nitrate to dust by the abundance of NH₃ and kinetic arguments about mass transfer (lines 207-212). I find this surprising as I would assume that in the model the ammonium nitrate remains semi-volatile, whereas the coarse nitrate formation is irreversible. In that case, it doesn’t matter how quickly the ammonium nitrate forms, given sufficient time the presence of a reactive coarse mode would still siphon off fine mode nitrate, unless there are other competing sinks?