This is a chamber study on SOA formation promoted by atmospheric ammonia. This manuscript investigates the influence of NH₃ on secondary organic aerosol (SOA) formation during the photo-oxidation of mixed anthropogenic (n-heptylcyclohexane) and biogenic (α-pinene) volatile organic compounds using a large outdoor photochemical smog chamber. The study demonstrates that NH₃ significantly accelerates VOC degradation and enhances SOA production by facilitating nucleation and participating in particle-phase reactions, including the formation of nitrogen-containing light-absorbing substances such as imidazoles. The work addresses an important gap in understanding NH₃-involved interactions in complex mixed precursor systems and provides valuable insights into the formation of light-absorbing aerosols in AVOC-BVOC mixtures. The experimental design using a large-scale outdoor chamber is a notable strength, offering realistic atmospheric conditions. Overall, this research contributes meaningfully to the scientific basis for assessing NH₃ effects on the co-oxidation of anthropogenic and biogenic gases in urban environments.

1. It is not clear if these figures are from specific experiments (which ones) or an average of all experiments (which wouldn't make sense). Are these averages from all experiments, or data from specific experiments in Fig.1? Are there corresponding blank measurements from the chamber that are used for a baseline subtraction?
2. What were the starting concentrations of all trace gases in a clean chamber? How were the trace gases measured? When were filter collected from the chamber? This is not clear in the text, the whole first 3.5 hours?
3. Figure 8 is scaled by Nitrogen number which may create a massive bias in that plot toward nitrogen containing species. Why is this done?
4. How do the authors make an estimation of the mass concentration of the different functional groups in Figure 4?
5. The figure caption should explain the content of the figure, and the meanings of different symbols or patterns should be clarified. For example, what do the solid and hollow patterns represent in Figure 1(b) and 1(c)?

In this study, the key role of NH₃ on SOA formation during the photo-oxidation of mixed VOCs of anthropogenic (n-heptylcyclohexane) and biological (α-pinene) sources was revealed by a large-scale outdoor photochemical smog chamber experiment. The results showed that NH₃ significantly accelerated the degradation of VOCs and increased the quantity and mass concentration of SOA by promoting nucleation and particle-phase reactions (e.g., acid-base interaction, Maillard reaction). It especially enhanced the production of nitrogenous light-absorbing substances (e.g., imidazoles), which may contribute to climatic impacts through the radiative effect of brown carbon and cloud interaction. Meanwhile, NH₃ inhibits the formation of OOMs by mediating the gas-particle partitioning of medium molecular weight compounds, which hinders the cross-reactions necessary for OOMs production. Additionally, elevated temperatures inhibit SOA production. The study elucidates how NH₃ affects the co-oxidation of ambient anthropogenic and biogenic gases, and deepen the understanding of its role in SOA generation, particularly light-absorbing aerosols, in the AVOC-BVOC mixed system. It is recommended that the paper be accepted after major revision. In addition, there are some issues that need to be clarified, which are list below:

1. The NH₃ concentrations tested (74.85–748.5 ppb) span a wide range. Does the particle size distribution reveal any systematic trends with NH₃ concentration?
2. Why was n-heptylcyclohexane chosen to represent AVOC? Is its chemical structure (cyclically branched alkanes) prevalent in the actual urban atmosphere?
3. Is the mixing effect of other BVOCs (e.g. isoprene) considered for α-pinene as a typical representative of BVOCs? Are the experimental results generalizable?
4. Were multiple replications of the experiment performed to verify the stability of the results?
5. Some of the terms (e.g., "VOC/NOx ratio") are not clearly explained in their abbreviated meanings, which may affect the understanding of lay readers.