The manuscript by Jiang et al. investigates the oxidation of indole by selected oxidants (OH radicals and O₃) with/without NO₂. The authors report the chemical composition and optical properties of indole SOA (ind-SOA) under the investigated conditions. In the presence of NO₂, the ind-SOA yields decreased by more than a factor of two, but the mass absorption coefficient at 365 nm of ind-SOA was 5 times higher than that of the SOA form without NO₂. The global emission factors of indole could be around half of the emissions of the most abundant amines, trimethylamine. However, there are only limited studies investigating the formation of SOA and BrC from the oxidation of indole. Overall, this study would be a valuable addition to a better understanding of the ind-SOA formation mechanisms and the influence of NO₂ on the chemical composition and light-absorbing characteristics of ind-SOA. The results may be particularly important for areas with abundant indole emissions, such as large animal husbandries and maize fields. The manuscript is well-presented, and it could be accepted for publication after considering the comments below.

Line 90: To clarify how the OH concentrations were calculated, the authors could consider adding a few sentences explaining the methodology used.

Lines 95-99 and Figure S2: The O₃ was injected into the chamber at around 600-800 ppb in the REF and seed experiments, while in the Seed-NO₂ experiment, it was initially added at around 100 ppb and then increased to 600-800 ppb after 30 minutes. The authors may want to provide an explanation for this difference.

Line 100: It would be helpful if the authors could provide more information about the background samples and whether they would react with the reactants.

Line 117: Were the estimated trace gas and particle wall losses corrected?

Lines 132 and 152: Why the methanol and acetonitrile were used to extract the filter samples for different analyses? It would be beneficial if the authors could explain their rationale for selecting these solvents and discuss any potential solvent effects.

FIGAERO-CIMS part: The manuscript does not mention the mass resolution of the instrument used. Additionally, while the authors assumed a uniform sensitivity for different compounds, it is possible that sensitivities vary by order of magnitude. It would be helpful if the authors could provide references from the literature supporting their assumption or consider rephrasing statements regarding “XXX% of CIMS detected compounds.” Furthermore, it would be interesting to know if thermal desorption caused any fragmentation of the compounds and if multimodal thermograms were observed.

Line 172: What would be the reasons for the slightly lower SOA yield in the AS seed experiment than that in the REF experiment? Line 183: What is the seed concentration used in Montoya et al.? Would different seed concentrations play a role in the different yields?

Figure 1b: When calculating the effective density of indole SOA by comparing the AMS and SMPS data, would the seed density affect the results? Was it excluded?

Figure 3: It was mentioned in the figure caption that the Y-axis scale shows the fraction of CxHyOzN_1-2 of the total ion intensity, but there are compounds without N atom shown in the Figure.

Line 223: The author attributed the common ions C₆H₄+ and C₅H₃+ to be fragmented from 3-nitroindole or C₁₆H₁₂O₄N₄ (Figure S8), but these ions were also observed in REF and AS experiments.

Figure 4: Please check the caption about the description of the color used in the Figure. For example, “The unassigned chromophores (red)”.

Line 249: 3-nitroindole contributed 76% of compound signals detected by a CIMS, and ~50% of the BrC absorption. Would this indicate there are compounds with low signal intensities that contribute even more than 3-nitroindole to the BrC absorption?

Comments to Jiang et al egusphere 2023

This manuscript by Jiang et al. explores chemical composition, formation mechanisms and optical properties of 

ind-SOA BrC produced from  oxidation of  indole in a environmental chamber at atmospherically relevent conditions

with/without NO2. They observed that in the presence of NO2, the SOA yields decreased by more than a factor of 

two but the mass absorption coefficient of ind-SOA BrC at 365 nm was 5 times higher as compared to the ind-SOA BrC

formed without NO2. The global emissions of Indole is half of  one of the most abundant amines, i.e., trimethylamine. 

Despite its significant presence in the atmosphere, the chemical composition, formation mechanism, and 

optical properties of ind-SOA including its BrC remain poorly understood. The study is valuable for atmospheric 

chemistry and climate modelling community, particularly for areas with high Indole emissions, such as animal 

husbandry, maize and rice fields and tea manufacturing areas.  This manuscript is well written, well-presented, 

and could be accepted for publication after considering the following comments:

Line 85-90: How did you make sure that there was no interaction between methanol/indole mixture? How will the

volatilization of methanol will affect wall losses? Did you do blanks? Please elaborate.

Line 90: Did you use any tracer for OH concentration calculation? If yes, what tracer? Add a brief discussion about OH concentration calculation. 

Section 3.2 (Line 195-210): You have used acetonitrile extracted ind-SOA BrC in UPLC-PDA analysis (section 3.3). However, BrC

extraction efficiency in methanol and acetonitrile could be significantly different from each other. Why did you not compare ind-SOA BrC optical properties in methanol and acetonitrile?  

Line 205-210: The MAC values in REF and AS were similar between online-PAS and offline-Aqualog measurements but not for AS-NO2. Why, elaborate? 

Figure 4: How did you calculate the fraction of individual chromophores (known, unassigned, unresolved) to total indi-SOA absorption? Add a brief discussion.

Figure 4c: Typo-error "Unassiged"

Line 283: "However, in presence of NO2, a significant shift occurs, and 3-nitroindole becomes the dominant compound, comprising up to 76% of the chemical composition." I think it's 76% of the total CIMS species, not the total composition.