This paper summarizes aqueous-phase reaction studies of butenedial as it reacts with OH^- (at high pH) and NH₃ / NH₄⁺ (“NH_x,” at neutral and acidic pH).  In both cases, brown carbon is produced.  Rates are compared with wet deposition as sinks for butenedial, and it is concluded that reaction with NH_x can compete with wet deposition at mildly acidic pH or high aerosol-phase NH_x concentrations.   The atmospheric significance of butenedial should be better established at the beginning of the article, but in general, the work is extremely thorough and the conclusions are convincing.

Specific comments:

Line 41:  The introduction contains many chemical justifications for studying aqueous-phase chemistry of butenedial, but this sentence is the only atmospheric justification given:  “Larger, complex dicarbonyls are also thought to be important products of biomass burning and fossil fuel combustion (Arey et al., 2009; Aschmann et al., 2011, 2014; Gómez Alvarez et al., 2007, 2009; Volkamer et al., 2001; Yuan et al., 2017), but they have rarely been studied or quantified in the atmosphere. ”  It would be helpful if the authors could here address the question of whether butenedial has ever been reported in an aerosol field study or a chamber oxidation study.  If not, why was it specifically selected?  It would seem odd to study the chemistry of a reactant that has not yet been identified in the atmosphere or in a chamber study.

Line 51:  The ease of using bulk-phase methods to study aerosol-phase chemistry is described here, but the drawbacks of using only these methods should also be described.  These include lack of sites for surface chemistry to take place.

Line 21, 61-64, 317 and 320:  According to Noziere et al., 2009, and Sedehi et al., 2013, the quadratic dependence of the rate on glyoxal concentrations is only observed at high concentrations, and switches to 1^st order at concentrations below the molar range.  The argument (made three times in the manuscript and referenced as a key difference from glyoxal in the abstract) that the quadratic rate law contributes to the reaction shutting off at low concentrations is therefore not sound.  The paragraph in the discussion should be rewritten to avoid a reliance on this idea.  But I agree with the rest of the statement at line 62.

Line 108:  The manuscript states that no overlapping products were observed between OH^- and NH_x experiments.  However, the OH^- reaction products were detected using negative ion mode, and the NH_x reaction products were detected using positive ion mode on a different mass spectrometer.  Were the two reactions tested using the other mass spectrometric methods to screen for these overlapping products?  I did not find a clear answer to this question in the manuscript or supplement.

Technical Corrections:

Line 148:  The manuscript states that 0.5 M sodium carbonate (Na₂CO₃) – sodium bicarbonate (NaHCO₃) buffer was added to every reaction solution with ammonium.  Table 1 states that a pH range of 3 to 9 was studied, but the carbonate buffer could only be used at the upper end of this range.  How was pH experimentally set to values less than 8?

Line 182:  This sentence refers to a fitting procedure described in section 2.3.1, but the sentence is part of section 2.3.1.  A typo?

References Cited

Noziere, B., P. Dziedzic and A. Cordova (2009). "Products and kinetics of the liquid-phase reaction of glyoxal catalyzed by ammonium ions (NH₄⁺)." Journal of Physical Chemistry A 113 (1): 231-237.

Sedehi, N., H. Takano, V. A. Blasic, K. A. Sullivan and D. O. De Haan (2013). "Temperature- and pH-dependent aqueous-phase kinetics of the reactions of glyoxal and methylglyoxal with atmospheric amines and ammonium sulfate." Atmospheric Environment 77: 656-663.

General comments:

This manuscript describes results from an analysis of the reactions that can occur for butenedial in aqueous solution as a function of pH and as a function of NHx concentration. The chemical changes were tracked with NMR combined with some MS to help identify products and a chemical scheme along with reaction kinetics are provided. Overall, this work demonstrates the need for additional studies on different types of dicarbonyls that are atmospherically relevant as the behavior of butenedial does not follow what would be predicted based on prior studies of a-dicarbonyls like glyoxal and methylglyoxal. This is a well written and clear study that builds on prior work. I would recommend acceptance in ACP after the following minor comments are addressed:

Minor comments:

1. The accretion products from betenedial/OH- were observed to be brown immediately, and a portion of the MS is provided for the samples (figure S4). Were there any nitrogen containing peaks observed to form in this sample in the other mass ranges? I am concerned about trace ammonia from the room since very small concentrations would be needed if the chromophores have a large absorption cross section.
2. The concentration ranges here were reasonably high, what do these concentration ranges correspond to in the atmosphere? There is some discussion about rainwater in India, given the much lower concentrations for organics that can be found in rainwater, would the authors expect to see the same types of chemistry?
3. On page 7, it is stated that the 1H-NMR spectra shows a buildup of signal in the baseline which increases and spreads out with respect to the chemical shift over time. With the data overlaid and colored the way it is, this is very difficult to see in the figure. Also in Figure S5, there is a note that there are two expanded regions, but these are not shown in the figure.
4. The numerical values for the last few supplemental figures appear to be off in the manuscript (there is no Figure S16).

Hensley and coworkers studied the reaction of butenedial, which has been observed in lab studies and ambient air, with OH- and water in the absence of NHx and in ammonium sulfate (AS) solutions. Products and rates were monitored with 1H HMR. The butenedial was synthesized. LC-TOF-MS data were also taken to analyze products. This is a solid study. Generally I found the discussion of the chemistry, or analytical assignments, could have been more specific and clear. I have the following comments that should be addressed prior to publication.

Can “kinetic mechanism” be changed into something more clear like “kinetic modeling mechanism”?

Can the authors provide more information into how products such as pyrrolinone were assigned? More discussion is need to support major reaction route and products in Figure 1. How were products confirmed or ruled out based on NMR shifts? I see that the SI has some assignments, but the text should refer to the SI when these assignments are called on, and it’s not clear how these assignments were made and no citations to analogous compounds in the literature. It’s a good idea to also discuss any limitations when it comes to unambiguously assigning structure from NMR shifts in a complex mixture and or from m/z.

Some of the figures/tables in the SI are not referred to in the main text. For example, when the authors mention the solutions turn brown they should probably refer to the figures they provided in the SI, otherwise the reader does not know to look.

92 What is the concentration of butenedial in ambient air? Is it observed in the gas or condensed phase? How do these concentrations compare with the initial concentrations the authors chose for this work, and if they are very different, please discuss how this work can extrapolate to the ambient environment.

99 “react with OH-“ is stated multiple times, without discussion about how. Please summarize what is known about how OH- reacts with the moieties of interest, and please discuss the specific mechanism. Same thing with NHx.

149 The mass spectra show a number of odd peaks. Did the authors rule out 2N products and how did they confirm 0N products? It would be good to specifically address whether this system is anticipated to form 2N cyclic compounds like in other dialdehyde systems. What is the mass precision after calibration in order to differentiate between 0-2N? (the assigned mass and peak shown in S4 are roughly 5 ppm off)

193-200 Can the authors provide citations for any of these reactions that they are suggesting, such as OH- reacting with BD to form a hydroxyacid (and by which pathway, i.e., addition/abstraction and where) and then ultimately leads to oligomeric light absorbing products (again, by which pathways). And what do the oligomeric light absorbing products look like?

236 Reaction R3 is an example of an accretion product given in this work. Its assignment was shown in Figure S6/S8 and associated discussion. The text should reference the carbon/proton assignments in S6, and discuss how those assignments were made (including any NMR reference tables used and for which proxy molecules). Also instead of “produces” it should say “proposed to produce” because these are only tentative assignments after all. Please insert a few sentences to discuss the specific mechanism by which this product can form, via reaction R3 with OH-.

Even though the solutions look visibly brown, the authors did use a lot of carbon material initially. What is the mass absorption coefficient (MAC) of these reactions and how do they compare to other brown carbon (e.g., from Updyke 2012)?

320 clarify “favorable separation”

345 I see that the authors considered the wet deposition lifetime of aerosols (~ 1 week) from Seinfeld and Pandis. I’d  also suggest to calculate the wet deposition lifetime of the molecule based on equation 12 in this work (https://acp.copernicus.org/preprints/acp-2021-137/) to better motivate the importance of aqueous partitioning for this compound compared to its gas phase photolysis.

330 the lifetimes decrease with increasing pH. Can the authors discuss relevance of this reaction to aerosol water, which tends to be acidic (such that BD would have lifetimes of > 4 h, and would photolyze before that) but would be the locations where one would find higher NH4+. Comparatively pH 6 might be cloud droplet range, but then ionic strengths are low.