An aldehyde as a rapid source of secondary aerosol precursors: Theoretical and experimental study of hexanal autoxidation
Abstract. Aldehydes are common constituents of natural and polluted atmospheres, and their gas-phase oxidation has recently been reported to yield highly oxygenated organic molecules (HOM) that are key players in the formation of atmospheric aerosol. However, insights into the molecular level mechanism of this oxidation reaction have been scarce. While OH initiated oxidation of small aldehydes, with two to five carbon atoms, under high NOx conditions generally leads to fragmentation products, longer chain aldehydes involving an initial non-aldehydic hydrogen abstraction can be a path to molecular functionalization and growth. In this work, we conduct a joint theoretical-experimental analysis of the autoxidation chain reaction of a common aldehyde, hexanal. We computationally study the initial steps of OH oxidation at the RHF-RCCSD(T)-F12a/VDZ-F12//ωB97X-D/aug-cc-pVTZ level, and show that both aldehydic (on C1) and non-aldehydic (on C4) H-abstraction channels contribute to HOM via autoxidation. The oxidation products predominantly form through the H-abstraction from C1 and C4, followed by fast unimolecular 1,6 H-shifts with rate coefficients 1.7 × 10−1 s−1 and 8.6 × 10−1 s−1, respectively. Experimental flow reactor measurements at variable reaction times show that hexanal oxidation products including HOM monomers up to C6H11O7 and accretion products C12H22O9−10 form within 3 seconds reaction time. Kinetic modeling simulation including atmospherically relevant precursor concentrations agrees with the experimental results and the expected timescales. Finally, we estimate the hexanal HOM yields up to seven O atoms with mechanistic details through both C1 and C4 channels.
Shawon Barua et al.
Status: open (until 10 Apr 2023)
- RC1: 'Comment on egusphere-2023-128', Robin Shannon, 10 Mar 2023 reply
Shawon Barua et al.
Shawon Barua et al.
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This work details an interesting pathway for HOM formation and involves robust theory and experiment working in tandem which is nice to see. The work is well presented and of good scientific quality. I am not sufficiently qualified to comment any further on the experimental techniques used however broadly speaking the theoretical methods seem approariate given the moderately large number of heavy atoms.
My only queries and comments regard the way the multiconformer approach is presented. I know that in some formulations of multiconformer approaches, corrections are applied to ensure the correct hindered rotor limit but I do not see such corrections in expression 1. Are these included or are the conformer partition functions calculated entirely within the harmonic oscilator aproximation? If the conformer hindered rotors are not accounted for then expression 1 could be achieved simply by putting the seperate conformers into MESMER. Additionally if every conformer is treated as harmonic then you also have the potential issue of overcounting of states when you reach the hindered rotor regime.
It would also be nice to see some consideration of the hindered rotation potentials? do you have these? I do appreaciate that this is quite a large system and since this paper has a large experimental part I do not consider hindered rotors crucial to publication, however if you have not done a hindered rotor treatement then the comparison between a single well model and a multiconformer model is a little missleading since strictly speaking the conformers are equilibrated and the multiconformer model is simply a way of approximating the fully coupled configuration space of each species. Related to this a brief look at your MESMER input shows each species has a symmetry number of 1? By not including hindered rotors im slightly concerned you are not accounting for the three fold periodicity in any methyl rotations for example although correct me if these all cancel out between reactants and TS? In summary the lack of consideration of hindered rotors while potentially pragmatic in this case leads to potential pitfalls. At the least I would ask the authors to clarify some of these points and make minor adjustments to the text acknowledging some of these issues. Otherwise I am very happy to reccomend publication of this manuscript.