Evans et al. show the effects of flaming and smoldering biomass combustion on the emission chemical composition. Moreover, they also show differences after aging. Overall, the study is well-designed, and the paper is well-organized. The findings will benefit the community by helping them understand the effects of biomass-burning aerosols on climate. I have a few minor comments that I hope the author can consider.

1. It seems like the experiments have CO and CO2 measurements. If this is true, it will be better to quantify the combustion condition based on modified combustion efficiency (MCE), and I am interested to see the correlation between chemical composition and MCE.

2. For section 2.1.3, is there any reason why you don't use water:MeOh solution to extract the filter? If your samples were initially extracted by methanol, how would that affect water-soluble but methanol-insoluble species? And could you provide an estimation of how much organic will be lost during the process? 
3. For Figure 1, I suggest adding a legend of markers as you did for other figures.
4. I think eq. 5-7 are duplicates of equ. 2-4. 

Evans et al. investigated the effects of photochemical aging on biomass-burning organic aerosol (BBOA) under smouldering and flaming conditions. A new semi-quantitative UPLC-ESIMS workflow was used for the chemical characterisation of the fresh and aged samples. Overall, the study fits into the scope of Atmospheric Chemistry and Physics. However, some critical information is missing in the experimental designs and discussion, potentially lowering the scientific quality of the work. The manuscript requires major revision before it can be considered for publication.

Major comments

1. Definition of atmospheric ageing: Please define “atmospheric ageing” in terms of oxidant exposure in the study. Please specify how OH radical was generated and how OH concentration was estimated in the chamber. What is the range of OH exposures or oxidant concentrations in the experiments?
2. Line 139 to 141: How were the “flaming dominated” and “smouldering dominated” phases distinguished? Please specify what parameters you use. For example, modified combustion efficiency is a commonly used variable to differentiate flaming and smouldering. If possible, please include modified combustion efficiency in Table 1.
3. Lines 143 to 144: In the fresh experiments, the POA filter samples were directly taken from the wood burner flue without dilution. However, in the aged experiments, the POA sample was diluted after entering the chamber, potentially altering its chemical composition. Compounds with high volatilities will likely partition into the gas phase after dilution. Therefore, in terms of composition, the POA samples collected in fresh experiments were very likely to differ from those in aged experiments. In my opinion, a proper way to obtain a POA sample would be to sample the chamber before the lights are on. Also, the fresh (flue) and aged samples were obtained from experiments conducted over two different days. The manuscript does not provide evidence or data showing that the compositions of fresh emissions and/or combustion efficiencies were comparable over the two days (i.e., fresh vs. aged conditions). Therefore, it is unreasonable to compare the aging products observed by the ESI-Orbitrap-MS with the undiluted fresh emission samples. For example, in sections 3.2 and 3.3, the observed decreases in chemical compound concentrations in aged samples can be attributed to both the evaporation of initial POA materials after entering the chamber and the oxidation of POA after turning on the UV lights.

Minor comments

1. Line 35 to 37: Is there any more up-to-date information on the solid fuel percentage of POA in London? The information was more than ten years ago, and wood-burning activities in 2020 were compared with information from 2010.
2. Lines 105 – 107: The sentence “This semi-quantitative methodology had… in this study” should be moved into either the method or discussion part.
3. Table 1. What were the temperatures and relative humidities inside the chamber for each experiment? How did the author control the temperature and relative humidities and ensure they are similar among different experiments?
4. Line 157 to 158: What is the pore size of the filter? Will a flow rate of 3 m³ min^-1 cause filter breakthroughs or reduce the collection efficiency of the OA sample?
5. Line 157 to 158: How long was the transportation process? How did the author ensure that filters were not contaminated during transportation? Please specify whether blanks were prepared to correct for the potential contamination of the samples.
6. Line 165: How was the OC measured here? Does that refer to the total organic content measured by the AMS? If so, please correct the terminology (non-refractory organic) here and elsewhere to avoid misunderstanding.
7. Line 210: Please provide proper citations when using third-party software.
8. Line 214: Apart from elemental ratios, does the author have constraints on double bond equivalents? If so, please specify it here as well.
9. Lines 244- 252: The effort of comparing findings between studies is appreciated. To strengthen the discussion, I recommend including literature data in Figure 1 for comparison.
10. Lines 258-259: It is unclear why the conversion of NO to NO₂ can indicate the oxidation of VOCs here. More background information needs to be provided for clarification.
11. Line 266 to 267: What observations in Figure 1 showed that the OA contains POA, oPOA and SOA? How can the author differentiate the above three species using AMS data? Was positive matrix factorisation used for separation? Please clarify.
12. Lines 283 to 284: Eq. 2 and 3 differ from the equations stated in the cited reference and its erratum (Koch and Dittmar, 2006). Please check whether the current study's calculations were based on the correct equations. Please specify if these equations were modified based on the original ones.
13. Figures 2 and 3: The colour code for different DBEs is confusing and reduces the readability of the figure. Please consider grouping the DBEs into several categories, as the significance of reporting DBEs individually is unclear.  
14. Line 338 to 339: Please confirm whether these two equations are correct. If they differ from Eq. 2 and 3, please use other terms to describe DBE_AI and C_AI expressed in Eq. 5 and 6.
15. Most of the results in Sections 3.2 and 3.3 agree with the literature. What are the novelties of the current study? If the results are similar, what is the advantage of using the semi-quantitative workflow suggested by the current research to investigate photooxidation reactions?
16. Since AMS data is available, I would appreciate it if an analysis of organic nitrates could be carried out. This information will potentially help explain the changes in CHON compounds during aging.

Technical Comments

1. Legends and texts in figures in the main text and appendix are currently too small to read. Please increase them for readability.
2. Label the subfigure with letters “a), b), c), etc.” so that it is clear which subfigure is referred to in the main text.
3. Line 128: It should be “6 kW”.
4. Line 133: Harsher to what? What was compared with?
5. Line 151: Please specify what had been injected in the sentence “Following injection…”.
6. Lines 153 – 155: The sentence “The smoke was aged for approximately 6 hours… for offline chemical composition analysis” is difficult to follow. Please rewrite it.

1. Line 167 to 168: The experiment dates shown in Table 1 are April and August. Please check if the “September campaign” was a typo or if it refers to something else.
2. Line 174: It is supposed to be “SO₄^2-".

1. Line 183: Higher aerosol mass loading to what? What was compared with?
2. Lines 223 – 225: Please use a table or figure to summarise the scaling factor for each window.
3. Figure A1 a: NO₃^-
4. Figure A1 b: Please describe what the inset plot refers to in the caption.
5. Figure 1: Please provide legends of the figure. For example, it’s unclear what the difference is between the circle and square points. Please use different marker styles or separate them into three sub-figures. It’s hard to distinguish the three marker types in Figure 1.