The manuscript is a case study on the behaviour of nitrate and ammonium aerosols during transport. Based on the comparison between the chemical composition of MPs sampled simultaneously (with 4h resolution) in a mountain site and a nearby site located at low altitude, the authors describe the processes that soluble inorganic compounds undergo. During transport, the NH4NO3 volatilizes, the formed HNO3 reacts with the dust, forming coarse nitrate, and the available NH3 reacts with the NH4HSO4 forming (NH4)2SO4. The isotopic study points to unidirectional reactions and additional ammonia partition into the particulate phases reducing aerosol acidity during transport.

A weakness of the article is that some references of interest are missing. A very recent paper on variation of 15N in total particulate N by Wiedenhaus et al., 2021, presents results related to the enrichment of 15N different from those presented in this article, probably because Wiedenhaus et al have determined 15N for N total. Given the close relationship with the present manuscript, this document should be referenced and similarities/differences should be discussed.

The volatilization of NH4NO3 and the formation of coarse nitrate is a well-known process by interaction on HNO3 with coarse sodium and calcium aerosols. Authors did not mention any previous reference to these processes. Authors shall include classical references such as Harrison and Pio 1983, Pakkanen 1996, and others (line 351 and others),

The increase in NO3 at the mountain site during September 12 and 13 is partially attributed to an external input. This may affect the interpretations of the origin of the nitrate based on the comparison between the averaged concentrations obtained in the studied sites. Therefore, this episode must be excluded for the calculation of the averages used for the interpretation of the modifications of the ions suffered during the transport from the lower levels to the mountain site.

Ammonia concentrations are not shown. However, some conclusions are based on the differences between the concentrations measured in MF and MS (Lines 407-408). These values, of key interest for the interpretation of data, must be presented or a reference must be included.

Minor corrections

Throughout the text, the authors used the term "surface" (surface pollutants…) to refer to the low-elevation site, located in the valley. This is not appropriate. The measurements made at both sites are made at surface level, although at different heights with respect to sea level. I would distinguish between mountain and valley sites, or between high and low elevation sites.

Line 92: WHO: add year: WHO, 2021, and add the refence in the reference list:”

Lines 155 and 159: add altitude to the coordinates: e.g. (34º 32’N. 110º5’E, 400 m a.s.l.)

Line 309: delete parenthesis

Line 326: with latitude? Or with altitude?

Figure S2: please, better describe the reason of including Xi’an and Dazhou in Figure S2

Line 407: add reference for observational NH3 data

Line 453-455: any explanation for these differences?

Line 460-462 and figure 9: were the samples collected 12-13 September discarded? Origin of pollutant at the two sites may differs during this period.

References:

Harrison R. M. and Pio C. A. (1983) Size-differentiated composition of inorganic atmospheric aerosols of both marine and polluted continental origin. Atmospheric Environment 22, 1733-1783.

Pakkanen, T. A. 1996. Study of formation of coarse particle nitrate aerosol. Atmos. Environ. 30 (14):2475–82.

Hanna Wiedenhaus, Laura Ehrnsperger, Otto Klemm & Harald Strauss (2021) Stable 15N isotopes in fine and coarse urban particulate matter, Aerosol Science and Technology, 55:7, 859-870 DOI: 10.1080/02786826.2021.1905150

WHO 2021. WHO global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. Geneva: World Health Organization; 2021.

The manuscript of "Different physicochemical behaviors of nitrate and ammonium during transport: a case study on Mt. Hua, China" by Wu. et al. reported the concentration of PM_2.5 and composition of secondary inorganic aerosols at different altitudes. The authors investigated the physicochemical behaviors of nitrate and ammonium during the transport process. While the methods are reasonable and the conclusion convinced, there are several questions need to be clarified before publication, especially, more detail descriptions were needed.

• Lines 39 and 65: Please change “mountain food” to “mountain foot”.
• Line 138: The “x” in “NOx” should be italic and subscript. Please unify the expression in the text.
• Line 199: “As revealed in previous studies, …”. Please add references here.
• Line 314: “As summarized in Table 1, the water-soluble ion level …”. Please indicate which site is describing.
• Lin 318-320: “Notably, this elevated contribution of WSIs was mostly attributed to secondary inorganic ions (sulfate, nitrate and ammonium, (SNA))”. Note that the nitrate contribution is reduced (Figure 5).
• Line 332-334: “Furthermore, distinct nitrate size distributions were also observed between the different sites in the summertime of 2020.” Please add references here. And if the data is unpublished, please indicate here.
• Line 337: In addition to R², please give p-values.
• Line 351: As Figure S3 shows, the diurnal total SNA pattern at the MS site exhibited a daily maximum value instead of MF.
• Line 355: “…, though these peaks lagged behind those observed at the MS site by 4 hours, further substantiating the vertical transport of these pollutants.” The description here is wrong.
• Line 393: “As can be inferred from earlier studies……” Please add references here.
• “Based on trace gas observations, the f_NO2 values of the air aloft were very high due to…”. Please describe the data quantitatively and indicate the source of the data.
• Figure 5: Remaining contributes 31-41.1% of PM2.5, but specific species are not indicated.
• Figure 5(b) : Remaining is not clear in the picture.
• Figure S1 (b): The right axis title should be “O3 at MF site”.

This work investigated PM_2.5 and nitrogen isotope composition at a high-elevation site of Mt. Hua and a nearby surface site. By comparing the analysis results of the two sites, the authors proposed a conceptual model to illustrate the different behaviors of nitrate and ammonium during vertical transport. NH₄NO₃ decomposes into gaseous NH₃ and HNO₃ during the transport, followed by heterogeneous reactions of HNO₃ and dust, leading to a shift of nitrate from fine to coarse particles. This chemical process has already been well documented in many previous studies.

Additionally, the partitioning of ammonia toward the particle phase during vertical transport neutralized HSO₄^- from the surface and reduced the aerosol acidity.

Due to the lack of direct evidence, synchronous PM characterizations at two sampling sites at different elevations can hardly explain chemical-dynamic processes. Here are my major comments.

This study assumes that the vertical transport of surface aerosols is the primary source of aerosols at the high-elevation site (MS site).

Besides the vertical transport, aerosols at the mountain top might also come from the subsidence of air parcel and horizontal transport, which is a possible cause for the difference in aerosol compositions and size distributions between mountaintop and ground surfaces.

As no atmospheric modeling was conducted to simulate the transport of air parcels, at least the meteorological field of the sampling location, we might not assure that the NH₄NO₃ observed at the MS area is coming from the air parcels from the MF area.

Maybe the difference in PM composition is also caused by source variations, not only by the difference in physicochemical behaviors. Could the authors rule out the possibility of source variation? 

Have the authors considered the time scales of vertical transport and atmospheric reactions mentioned in this work? Maybe the transport time between the two sites is shorter than the chemical-dynamic processes proposed in this work.

Lines 124-125. Are there any studies investigating chemical-dynamic processes that drive haze episodes in the lower troposphere using observations at high-elevation mountain sites? Please provide examples.

Such observations might only reflect the difference in aerosol chemical composition and properties between ground and high-elevation sites, but not the chemical-dynamic process.

Lines 330-334. Why does the similarity in mass concentrations of sulfate and ammonium at the two sites indicate further formation during the transport? Couldn’t these two ions be formed at the MS site?

Lines 335-336, many components show increased concentrations with elevation, like Na⁺ and K⁺.

Except for NO₃^-, only Ca⁺, OC, and PM showed decreasing trends.

Lines 351-357, the shift of fine mode nitrate toward the coarse mode was well documented in previous work.

Did the temperature increase with elevation? Since the dissociation of NH₄NO₃ tends to happen in warmer periods or areas, why high-elevation nitrate exhibited a bimodal pattern, but not the surface nitrate? Are the size distribution data of PM at the two sites available? The size distribution of nitrate might be closely related to the PM size distribution.

Lines 367-373, could the decrease in nitrate concentration at the MS site be partly caused by dilution during the transport? As shown in Figure S1, NO₂ concentrations at the MS site are way lower than the MF site, and no information on NH₃ and SO₂ concentrations was available. 

Lines 405-408. Why was the MS site an ammonia-poor environment? NH₃ was not measured in this study, and sulfate and nitrate were almost fully neutralized by NH₃.

Lines 409-423. The authors provided a possible explanation for the change in the chemical forms of sulfate and bulk PM_2.5 pH from MF to the MS site.

However, have the authors considered the change in aerosol water content from the MF to the MS site? Would the changes in chemical forms of sulfate and pH be caused by variations in aerosol water content?

Lines 425-436. The authors mentioned that the temperature decreased from MF to the MS site (line 433), why is volatile NH₄NO₃ easily converted to gaseous NH₃ and HNO₃ during the transport?

Theoretically, more NH₄NO₃ should be formed through gaseous reactions at the MS site with lower temperatures.

Lines 453 – 455, would the authors explain the inconsistent calculation compared to Lindaas et al. (2021)? Could it be attributed to the large uncertainties in the empirical calculations of P_HNO3×P_NH3 and Kp?

In this work, the authors always assumed that NH₄NO₃ was in an aqueous state. But Kp is the dissociation constant of dry salts. Will aerosol water content impact the dissociation of NH₄NO₃?

Lines 460-494.

The sources of NH₄⁺ and NO₃^- at the high-elevation site are probably not the same as those collected at the MF site, and this might lead to the difference in δ¹⁵N-NH₄⁺ and δ¹⁵N-NH₄⁺ values at the two sampling sites.

Minor corrections.

Lines 60-61, abstract. Define MS and MF where they first appear.

Lines 165-167, what type of aerosols was sampled? PM_2.5, PM₁₀, or TSP.

Line 342, typo, “so much”, not “so such”.

Line 453, typo, Change “that” to “than”.