the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Mar 2024
Development of a detailed gaseous oxidation scheme of naphthalene for SOA formation and speciation
Abstract. Naphthalene is the most abundant polycyclic aromatic hydrocarbon (PAH) in vehicle emissions and polluted urban areas. Its atmospheric oxidation products oxygenated compounds potentially harmful for health and/or contributing to secondary organic aerosol (SOA) formation. Despite its impact on air quality, its complex structure and lack of data mean that no detailed scheme of naphthalene gaseous oxidation for SOA formation and speciation has yet been established. This study presents the construction of the first near-explicit chemical scheme for naphthalene oxidation by OH including kinetic and mechanistic data. The scheme redundantly represents all the classical steps of atmospheric organic chemistry (i.e. oxidation of stable species, peroxy radical formation and reaction and alkoxy radical evolution) integrating therefore fragmentation or functionalization pathways and the influence of NOx on secondary compound formation. Missing kinetic and mechanistic data were estimated using structure-activity relationships (SARs) or by analogy with existing experimental or theoretical data. The proposed chemical scheme involves 392 species (237 stable species, including 93 % of the major molar masses observed in previous experimental studies) and 503 reactions with products. A first simulation reproducing experimental oxidation in oxidation flow reactor under high NOX conditions shows a simulated SOA mass in the same order of magnitude as experimentally observed, with an error of -12 %.
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Status: open (until 02 May 2024)
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RC1: 'Comment on egusphere-2024-711', Anonymous Referee #1, 25 Apr 2024
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The authors present a well-conceived and thorough chemical mechanism for gas-phase oxidation of naphthalene and for the resulting SOA formation. Comparisons are made between the mechanism and results from an OFR reactor. This is a very thorough and careful study, with much attention to detail and to what is available in the literature. The work is well presented, with (among other things) a very logical structure and naming system. I recommend publication, subject to consideration of the comments below.
My first comment is really the only one of significance: It is true that GECKO-A does not handle naphthalene or PAH’s in general. That said, once the aromaticity of one of the fused rings is broken (e.g., from OH addition), the GECKO-A website appears to me capable of at least dealing with the remainder of the species encountered here, albeit without full recognition of all the aromatic-like chemistry that ensues in some cases. At a quick glance, it did look to me though that species like 2NaOort and 2NaOpar were treated in GECKO-A as is done in this work. Also, MechGen (http://mechgen.cert.ucr.edu), conceptually similar to GECKO-A, appears to treat naphthalene chemistry explicitly (although I know much less about the details). So, while I am sure that there are still many unique aspects to the current work, I think that at the least some mention of the other systems and comparisons with them should be done. (I realize that a brief mention of this is made near the end of the paper, dealing with continuation of the chemistry beyond what is outlined in this manuscript).
Remaining comments and suggestions:
Line 12: Perhaps change text to read: “Its atmospheric oxidation products are oxygenated compounds potentially harmful for health and/or for contributing to secondary organic aerosol …”
Line 103: “A rate coefficient of …” would sound better.
Line 106: ‘includes’ is spelled incorrectly.
Line 128: I think the term ‘carbonyl radical’ is used on some occasions when what is really meant is ‘carbon-centered’ radical.
Figure 3 (for example): The ring-closure chemistry is being applied to radicals of structure R-COCO(.). Could CO elimination occur with these species, as happens with CH3COCO(.) formed from methylglyoxal?
Since NaOPEN is a major product, it must be formed somehow, but the H-shift shown in Figure 4 (middle left) doesn’t seem quite right to me. (The first step, moving H from the alcohol to the hydroperoxide is very likely endothermic).
Line 255: should be 18%, not 12% I think.
Line 257: I think you mean by HO2 elimination, not hydrogen abstraction?
Overall, the figures are very clear and well labeled. There are a few places however where additional labeling can be done – e.g., 2NaOOOBp in Figure 7. Also, the different cases could be labeled in Figure 8 to further guide the reader.
It appears to me that the ring closures proposed are in most cases competing with very fast processes, such as O2 addition to an RCO radical or CO2 elimination from RCO2. Can the authors provide any further justification for these processes? (I would guess that the competing processes are happening on sub-microsecond time scales).
Line 325 – I think you mean acyl peroxy here, rather than acyloxy.
Citation: https://doi.org/10.5194/egusphere-2024-711-RC1
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