The manuscript by Li et al. simultaneously reported concentrations and stable nitrogen isotope and oxygen isotopes compositions of atmospheric NO₃^- and concentrations and nitrogen isotopes compositions of atmospheric NH₄⁺ in PM_2.5 samples collected in Guangzhou from May 2017 to June 2018. Then, authors restrained nitrogen isotope fractionation values of the process of NH₃ to formed NH₄⁺ and NO_x to formed NO₃^-. Finally, using the IsoSource model, authors quantified the relative contributions of major sources of NH₃ and NO_x to atmospheric NH₄⁺ and NO₃^-, respectively. Authors found the focus of NH₃ reduction should be on anthropogenic combustion sources especially on biomass burning, which might be responsible for the lag of the decline in deposition of air pollutions behind the reduction in emission. Additionally, despite a series of measures to reduce emissions of NO_x, fossil fuels, as the main energy for production and living, will still inevitably emit a large amount of NO_x. Authors emphasized that the emission of atmospheric inorganic nitrogen is largely related to anthropogenic combustion sources. The development and promotion of clean energy and efficient use of biomass are conducive to the deep reduction of atmospheric nitrogen. I believe that this result is meaningful and would make a substantial contribution to the field. The manuscript is generally well-organized in structure. If the following comments are adequately addressed, I believe the manuscript could be accepted to Atmospheric Chemistry and Physics.

(1) Lines 112-113: The author needs to provide the analytical accuracy of isotopes nitrogen and oxygen isotopes.

(2) Nitrogen isotope fractionation values of the process of NH₃ to formed NH₄⁺ and NO_x to formed NO₃^-­­­ are key parameters for quantifying the relative contributions of major sources of NH₃ and NO_x to atmospheric NH₄⁺ and NO₃^-. The calculation methods for the two parameters should be include in the text of manuscript. In addition, it is necessary to give readers detailed data of each parameter, especially the fractionation value.

(3) Authors should explain why these four sources are selected as main sources of atmospheric NO₃^- and these six sources are selected as main sources of atmospheric NH₄⁺?

(4) Lines 176-178: Does the combustion of sugarcane leaf emit NH₄⁺ directly or emit NH₃ and then formed NH₄⁺?

(5) Lines 236-237: The sources apportionment results of atmospheric NO₃^- in Figure c does not correspond to that in Figure b.

(6) Lines 272-273: Why does the author only consider the OH radical oxidation and N₂O₅ hydrolysis pathway to NO₃^-, and not consider other pathways? The author needs to explain.

This paper estimated the relative contributions of main sources to ammonium and nitrate aerosols in a subtropical megacity of South China using stable N isotope analysis. They found that anthropogenic activities (e.g., coal combustion, biomass burning and vehicle exhaust) are important sources and should be considered seriously in future for the improvement of air quality. In my opinion, few studies simultaneously reported 15N signatures for both NH4+ and NO3- and I think this one-year dataset is valuable and probably will improve our knowledge on the sources of air pollution. I support its publication after some minor revisions.

Line 66-68: The dominant source of atmospheric NH3 highly depends on the scale of study area. For example, the dominant emitter of NH3 in the whole China should be the agricultural source; while the dominant emitter may be the vehicular emission for a city site. Therefore, cautions need to be taken when you describe this sentence.

Line 122-126: Many d15N-NH3 endmembers of sources were collected by passive samplers. Did you correct these values when you conducted the source apportionment? Also, the endmembers and source numbers are important parameters for 15N-derived source apportionment model and I suggest you add these in the main manuscript.

Line 149: Fig 1. Can you please highlight/mark the seasonal periods in this figure? I think this will improve the readability because you mentioned the seasonal values.

Line 157: “average+”?

Line 163: It would be better to provide the way you got the NH3 concentration in the main manuscript.

Line 238/274 (Fig2, Fig3): again, please highlight/mark the seasonal periods (spring, summer, autumn, and winter)

Line 291-292: Why you defined BeiChengHuang Island and Heshan as the sites receiving strong anthropogenic impact? These two sites are not located in cities and should be impacted less anthropogenic activities than megacities such as Beijing and Guangzhou.

Line 311-313. This explanation sounds reasonable. I suggest you add the references to support the facts you mentioned here (stricter vehicle emission standard, promotion of new electric vehicles etc.).

Line 324-325: “The contribution of biomass burning and vehicle was stable through a year.” The vehicular emission, in my opinion, is likely constant because people drive cars in all seasons. However, the biomass burning activity generally is highly related with seasons. Can you make some explanations on this?