the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Aug 2025
Larger than expected organic acid yields from the multi-generation oxidation of petrochemical alkenes
Abstract. Alkenes are primary pollutants in petrochemical source atmospheres, and their atmospheric chemistry is of great importance for tropospheric ozone and secondary organic aerosol formation. Hence, combining quantum chemical calculations and kinetic modelling, we investigated the oxidation mechanism and kinetics of 2-butene (BU), as one of the most important alkenes, and its impact on the environment. The mechanism results show that OH addition is the dominant pathway for cis- and trans-isomers of BU, and then the corresponding OH-adducts are attacked by O2 to produce peroxy radicals, which further react with NO to form acetaldehyde and hydroxyalkyl radicals. Different from the one adopted in current atmospheric models, addition of hydroxyalkyl radicals by O2 and NO to form acetic acid proceeds with a smaller barrier than that for H-abstraction by O2 to form acetaldehyde. A lifetime of less than a few hours (< 4 hours) for BU is estimated in the petrochemical regions. Kinetic modelling demonstrates that oxidation of BU is predicted to yield significant amounts of organic acids (> 56 %) in the petrochemical areas, larger than those are currently recognized, even in environments with low NO concentrations. Our results reveal that the OH-initiated oxidation of BU contributes importantly to organic acid budgets, particularly in the petrochemical regions, bridging the gap in organic acid budgets.
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RC1: 'Comment on egusphere-2025-3182', Anonymous Referee #1, 29 Aug 2025
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This is an interesting work that investigates the multi-generation OH-initiated oxidation of 2-butene and its impact on the formation of organic acids in the petrochemical regions. Mainly it finds that larger than expected organic acid yields from the multi-generation oxidation of 2-butene in the petrochemical regions, originated from a novel pathway for acetic acid formation. It combines quantum calculation and photochemical box model simulation to deliver the critical thermodynamic and kinetic parameters for atmospheric models and advances our understanding of petrochemical-alkene chemistry. The manuscript is well organized and clearly written; its topic is timely and appropriate for publication.
My major comments related to the following aspect of the work:
Line 62: It would be helpful if the authors provide the atmospheric concentration of 2-butene to illustrate that 2-butene is abundance in petrochemical regions.
Lines 85–87: The manuscript states that multi-conformer transition-state theory (MC-TST) was used for “pathways involving multiple conformers,” but it is not clear how the authors identified which reaction channels possess multiple conformers. A brief description of the conformational search protocol and the energetic or structural criteria used to retain conformers (e.g., energy cut-off, rotational barriers, symmetry considerations) should be added to the Methods section. Besides, Line 136: DLPNO//M06-2X is not fully introduced anywhere.
It would be valuable to elaborate the reasons why the temperature range of 240-340 K is selected to calculate the kinetics. Does the negative correlation for the rate constant refer to the total k value? Since other values actually show a positive correlation, it would be helpful to elaborate on this further.
How sensitive are the calculated rate constants to variations in environmental parameters such as temperature, pressure, or the concentration of OH radical? An exploration of these dependencies under atmospherically relevant conditions would strengthen the applicability of the findings.
It is proposed that alkoxyl radicals (cis-RO and trans-RO) decompose into one CH3CHO and one CH3CHOH radical, whereas the Master Chemical Mechanism (MCM v3.3.1) assumes the direct cleavage yielding two CH3CHO. Please supply the transition-state geometry and corresponding barrier for the “two CH3CHO” pathway, and compare its rate constant with the CH3CHO + CH3CHOH pathway to justify why the latter is favored under the studied conditions.
Minor comments:
Line 71: “reactants” should be “Rs”;
Line 79: “Frisch 2009” should be “Frisch et al. 2009”;
Line 83: “tunneling” should be “tunnelling”;
Line 100: “the optimization” should be “the optimized structure”;
Line 115: Table S1 appears after Table S12 (Line 92), and Tables S2, S3 and S4 do not appear in the text.
Line 120: “trans-BU” should be “that of trans-BU”;
Line 122: the subscript of “cis-TS” should be “cis-TSadd1”;
Line 134: The reference “Sims et al., 1994” does not match the citation “Atkinson, 2000” in Figure 2;
Line 214: “Atchem-2” should be “Atchem 2”. Please check and revise accordingly.
Citation: https://doi.org/10.5194/egusphere-2025-3182-RC1 -
RC2: 'Comment on egusphere-2025-3182', Anonymous Referee #2, 04 Sep 2025
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The work presented in the manuscript consists of a theoretical reaction kinetics investigation of the atmospheric photo-oxidation of 2-butene, an important petrochemical volatile organic compound in the atmosphere. The authors report a detailed mechanistic exploration, which is well depicted in the form of potential energy diagrams. The manuscript also discusses the atmospheric implications of their findings, making use of box model simulations. The computational methods employed in the evaluated work are adequate, and the text is clear and concise, which is commendable. However, there is one major issue to be addressed that potentially changes the main message of the manuscript, that is, the high yield of organic acids from petrochemical alkenes. I therefore recommend the reviewed manuscript to be reconsidered for publication after this major issue is addressed.
Page 7-8:
"According to the previous study (Parandaman et al., 2018), CH3CHOH·radical undergoes the H-abstraction pathway (R16) to form CH3CHO and HO2 radical. However, the corresponding ΔE# a and ΔEr values are calculated to be 11.27 kcal mol-1 and 11.69 kcal mol-1, respectively, and the pathway of O2-addition to CH3CHOH radical (R17) is barrierless and largely exothermic (-34.11 kcal mol-1) to yield peroxy radical (ER-O2) (Figure 4). More importantly, the subsequent NO-association of ER-O2 (R18) is also barrierless and largely exothermic (-24.16 kcal mol-1). Therefore, peroxy nitrite (ER-O2NO) is the dominant product of the subsequent reaction of CH3CHOH via R17 and R18 rather than CH3CHO and HO2 radical via R16."The authors talk about a H-abstraction pathway (R16), which I assume to be the one involving direct abstraction by O2 (according to Figure 4), and reference an energy barrier value for calculated for it by Parandaman et al. (2018). However, the reaction investigated in the cited work is a concerted elimination reaction (where the H atom transfer occurs alongside cleavage of the C-O(O) bond), whose transition state is connected to the peroxyl radical, and not to the alkyl radical + O2 as assumed by the authors. This means that formation of acetaldehyde + HO2 can proceed via a step-wise addition/elimination mechanism (CH3CHOH + O2→ E2-RO2→ CH3CHO + HO2), in which case an energy barrier of 11 kcal/mol associated with the second step is actually quite low (meaning that the reaction is very fast). Furthermore, Parandaman et al. (2018) report calculations done only for geminal diols, and not for the specific system investigated in the reviewed work. That being said, the reaction of O2 with alpha-hydroxyethyl radical has already been studied previously by theory and experiments: See e.g. da Silva et al., 2009 (https://doi.org/10.1021/jp903210a) and Zádor et al., 2009 (https://doi.org/10.1016/j.proci.2008.05.020). According to their results, the energy barrier to the HO2-elimination reaction is also quite low for E2-RO2 (11.4 kcal/mol), which I must say is a typical observation for this type of reaction pathway. On top of that, formation of E2-RO2 is 34 kcal/mol downhill in energy, so that the barrier to HO2-elimination is submerged under the entrance energy level (see Fig 1a in https://doi.org/10.1016/j.proci.2008.05.020) and the overall reaction rate may be enhanced due to chemical activation.
My point here is that, while I agree with the authors that addition will almost certainly outcompete direct H-abstraction during the initial attack of O2 on CH3CHOH, I argue that the subsequently formed peroxyl radical E2-RO2 will (very likely) react via the concerted elimination mechanism to yield acetaldehyde + HO2 so rapidly that it outcompetes reaction with NO even at extremely high NOx levels, so that formation of acetic acid via the route shown in Figure 4 becomes a negligible channel. The authors should then calculate the rate coefficient for this reaction, or at least take already calculated values from references mentioned above, and include it in their box model simulations. If, with the inclusion of this elimination reaction, the acetic acid forming channel turns out to be indeed minor, then the following sections may need to be reworked around the new results.Minor comments:
Page 2, paragraph 2:
The authors discuss the importance of the atmospheric oxidation of alkenes to air quality and climate in comparison to that of isoprene. However, important biogenic volatile organic compounds in the atmosphere, such as isoprene and most monoterpenes, are themselves also alkenes. I suggest that the authors include a mention to this fact in their introduction to further emphasize that their work is focused on an alkene from petrochemical origin, rather than biogenic.Page 2, line 40:
"For example, the reaction of isoprene 40 with nitrate radicals (NO3) produces the high nitrogen-containing monomers and dimers, ..."
What do the authors mean by "high nitrogen-containing monomers and dimers"? Please rewrite for clarity.Page 3, line 69:
"Harmonic vibrational frequencies were performed at the same level to verify the nature of transition state (NIMAG=1) and minimum (NIMAG=0), ..."
I understand that "NIMAG" stands for "number of imaginary frequencies", however, a reader who is less familiar with the methods may be confused with the acronym. Please write out what is meant with it.Page 4, Line 100:
"the optimization of geometries for all single points (SPs) involved in these two reactions"
Do the authors mean "stationary points"?Figure 2:
The experimental data shown with light blue and light pink overlaps with theoretical data and is not that easy to see. I suggest using a different color scheme for those data points to aid visualization.Page 5, lines 131-132:
"There is a negative correlation between rate constants and temperatures in the temperature range of 240 - 340 K, attributable to the presence of the pre-reactive complexes."
I would say that the negative correlation is due to the presence of a submerged reaction energy barrier rather than to the presence of a pre-reactive complex alone. Note that the transition state of OH-addition pathways, which dominate the overall rate coefficients, are lower in energy than the respective reactants. The rate coefficients of H-abstraction pathways, however, do not have submerged energy barriers and thus display positive temperature dependences (according to Figure 2).Page 6, line 167:
"undergoes dissociation (cis-R13), isomerization (cis-R14), and H-abstraction (cis-R15) yield acetaldehyde"
missing word: "... to yield .."Citation: https://doi.org/10.5194/egusphere-2025-3182-RC2
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