This work studied the dicarboxylic acids (diacids) and related compounds from fresh and aged biomass burning (BB) source samples. The emission factors of each diacid and their carbon isotopic ratios were also calculated, which suggested the secondary formation nature of oxalic acid and related species. This work presents some interesting results given the uncertainty of direct emission of diacids from BB source samples. However, several issues need to be clarified before the consideration of acceptance.

1. The major concern is the setup of aging experiments of source samples in the chamber. Two and seven days are quite long for the oxidation of organics to diacids under high loadings of oxidant. Although authors had cited two references to support this setup, the detailed conversions from VOCs to oxalic acid were almost completed in two days from the ratios of 50.8 and 64.5 given in this work, so authors need to clarify this issue.
2. Authors need to explain more clearly why the direct measurement of diacids from fresh BB samples is so important in the introduction in lines 83-86.
3. Authors said “investigate reactions of volatile organic carbon compounds (VOCs) with oxalic acid and intermediates that form in the aging process” is one of the aims of this work. However, the detailed compositions and abundances of VOCs were not presented in the paper, and the supporting evidences were mainly from linear regressions between the reduction of VOCs and increase of oxalic acid. This was insufficient to give such important conclusion.
4. How do you set the RH in the chamber study?

Minor comments:

1. Line 230-231: rewrite.
2. Line 232: “This also is a likely reason”, grammatical error.
3. Line 251-252: the” rapid” is contradictory to the description in lines 250-251.
4. Line 292: “photochemically oxidization rune pronounced”. I can’t understand.
5. Line 327: “reported the a heaviest”, rewrite.

This manuscript reported emission factors (EF) of dicarboxylic acids and related compounds from fresh and aged aerosols from burning rice, maize and wheat straw, and stable carbon isotope (δ¹³C) composition of these emissions. The authors provided a very interesting data, the EF of aged samples was significantly higher than that of fresh samples, and the molecular distribution was also significantly different. The carbon isotope of dicarboxylic acid was measured and the results showed that the δ¹³C value became more and more positive with the increase of aging degree. These results indicated that dicarboxylic acids and related compounds is largely produced by secondary photochemical processes rather than direct emissions in biomass burning. In my opinion, the MS can be eventually accepted for publication if below questions are adequately addressed.

Here are my detail comments:

1. In 2.1. “Preparation and collection of fresh and aged BB aerosols” section, more detail should be provided on the biomass burning. eg. Line 119~121, “A portion of the diluted smoke was drawn through a quartz fiber filter (47 mm diameter, Whatman QM/A, Maidstone, UK) at 5 L min⁻¹ using a mini-Vol PM_2.5 sampler (Airmetrics, OR, USA) to capture fresh emission…”PM_2.5 sampler for collecting fresh samples is given in the manuscript. What is the sampler for PM_2.5 aging aerosols? In which combustion phase was collected PM_2.5samples? How was the smoke diluted?

Lines 121-124 “…and another portion (~9 L min⁻¹) was drawn into a 19-L cylinder PAM-OFR (with a diameter of 20 cm and length of 60 cm) to simulate atmospheric aging.” What is the residence time of aged biomass burning aerosol in OFR? Is the residence time of 2-day aging the same as that of 7-day aging?

2. In 3.1. “Emission factors for dicarboxylic acids, ketocarboxylic acids, α-dicarbonyls” section, Lines 201-205, C₄ and C₉ were most abundant in fresh biomass burning samples, why not C₂? Please explain in detail. 
3. In 3.2. “Effects of atmospheric aging processes” section, EF_aged of maize at 7 days was lower than that of at 2 days, which was different from that of wheat and rice straw. This phenomenon should be explained in this paper.
4. In 3.3. “Comparisons of diagnostic ratios of dicarboxylic acids in fresh and aged aerosols” section, Lines 300-301, Why is the degradation rate of Gly faster than that of mGly? How to reflect the consistency of Gly/mGly?
5. In 3.4. “Stable carbon isotopes” section, Lines 330-333, There was no significant difference in the isotopic data for simulating 2- and 7-day aging reactions. Do you think two days of aging simulation is enough?
6. In 3.5. “Relationships between volatile organic carbon compounds and dicarboxylic acids” section, the authors only discussed the correlation coefficient (R) between VOCs and dicarboxylic acids. The correlations are meaningful, but there is more information that can be interpreted from the regressions. Take a consideration at the regression analyses to see if they show anything interesting?
7. In Conclusions, I think the conclusions ends a bit too abruptly, you should be to discuss what impact these results have on climate change or pollution control.

In addition, in Line 386-388, The author mentioned “These results suggest that the dicarboxylic acids in the atmosphere largely originated from secondary photochemical processes as opposed to primary emissions from biomass burning.” You should emphasize why this is a significant finding.

8. During the 2-day and 7-day aging experiments, fresh samples were collected, and whether there were differences in the collection of fresh samples. As shown in Table 1, EF results for fresh samples are different. Is the experiment reproducible?
9. Some items of the manuscript are to be improved to avoid any errors in expression and stylistic phrases:

Line 28 “Biomass burning (BB) is a significant source for dicarboxylic acids (diacids) and…” The dicarboxylic acids are abbreviated as diacids, but the abbreviation is not used elsewhere in the abstract. Is it necessary to mention this abbreviation?

Line 41 “…and the C₂ δ¹³C became isotopically heavier during aging.”

Line 230~231 “These results are further evidence that PM_2.5 oxalic acid…”