the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Aug 2025
Key Role of Nitrogen-containing Oxygenated Organic Molecules (OOMs) in SOA Formation Evidenced by OH/NO3-induced Terpinolene Oxidation
Abstract. Oxygenated organic molecules (OOMs), generated from the oxidation of various biogenic volatile organics with diverse yields, are a great contributor to SOA formation. Terpinolene is an isomeride of limonene, with an even higher SOA yield. Herein we investigated the elaborate oxidation mechanism of terpinolene by OH and NO3, elucidating the new formation mechanism of OOMs and their yield profiles based on the newly-built zero-dimensional chemical model under three typical atmospheric conditions. For terpinolene oxidation by OH, H shift imposes restrictions on continuous autoxidation, instead by the reactions with HO2/NO/NO2 resulting in chain termination. For the reaction of terpinolene with NO3, the transfer of the radical center via bond breaking, triggering a new round of autoxidation, is newly found to be pivotal in the formation of nitrogen-containing OOMs with high yields. The effective saturation concentration (C*) of nitrogen-containing OOMs is mostly lower than the OOMs formed by OH oxidation, more easily distributed into particle phase. The estimated C* of the generated OOMs are distinctly varied among OOM isomers, which emphasizes the necessity of determining their molecular structures, peculiarly the number of rings. The comparative analysis of OH-initiated (daytime) and NO3-driven (nocturnal) terpinolene oxidation mechanism, highlighted the formation of nitrogen-containing OOMs, would be conducive to molecular structures identification of OOMs in atmospheric monitoring and atmospheric chemical model refinement.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3219', Anonymous Referee #1, 15 Sep 2025
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RC2: 'Comment on egusphere-2025-3219', Anonymous Referee #2, 03 Oct 2025
This study explores terpinolene’s oxidation mechanisms initiated by OH (daytime) and NO₃ (nocturnal) radicals， finding H shift limits continuous autoxidation in OH-induced reactions while radical center transfer via bond breaking drives new autoxidation for high-yield nitrogen-containing OOMs in NO₃-induced ones. it also finds nitrogen-containing OOMs have lower C* (more prone to enter particle phase), significant C* differences among OOM isomers emphasize determining molecular structures (especially ring numbers), and comparing the two mechanisms aids OOM identification in atmospheric monitoring and model refinement. It can be published after addressing the following concerns:
Abstract: Line 3. Higher SOA yield than what? Higher than other limonene-based systems? What is the specific magnitude of this higher yield? SOA yield varies significantly depending on multiple factors and is not a fixed value.
Introduction: Authors should review existing literature to clarify whether nitrogen-containing species have lower or higher saturation concentrations compared to non-nitrogen-containing species. Additionally, is organic nitrate or peroxy organic nitrate considered in this context?
Method: The approach for evaluating uncertainties associated with the calculations is missing.
Line 153: Previous studies suggest that NO₂ release was derived from experimental data, whereas HONO release in this study is based on energy barrier calculations. If NO₂ was indeed measured experimentally, should this be more reliable than theoretical calculations? If so, how can the calculation results in this study be validated? Conversely, if NO₂ was not measured directly (e.g., its signal could be interfered with by HONO), then HONO should be the correct pathway. Are you implying that the NO₂ measurements reported in previous studies originated from HONO signals?
HONO can release HONO upon photolysis—do you have any data to demonstrate the yield of HONO from this process?
Line 236: NO₂ is not specified; I assume its concentration is set to 0 in your model? In forested areas and remote regions with low NO levels, most NO is converted to NO₂. NO₂ can undergo photolysis to form NO, so total NOₓ (NO + NO₂) should be a more representative metric.
I am also curious why NO concentrations in forests are set to be lower than in "remote areas". Soil NOₓ emissions are a substantial source of NOₓ in forest environments.
Line 237: Why is HO₂ concentration fixed at 50 ppt across all scenarios? HO₂ is an intermediate radical, and its formation and levels depend on factors such as VOCs and NOₓ. It is not typically constrained to a fixed value in box models.
Line 349: Can you provide a more detailed description of the results presented in Figure 8? For example, are you stating that C₁₀H₁₆O₄ (classified as an OOM) is identified as an SVOC with a C* of 1 in your analysis, but would be categorized as an SVOC with a C* of 2.5 if calculated using the functional group method?
Line 350: Which method for volatility estimation—molecular formula-based or functional group-based—is more reliable? Can you discuss the reasons for the discrepancies between these two methods? Additionally, does organic nitrate influence volatility calculations?
Results section: Organic nitrate is a key component of nitrogen-containing OOMs. Given this, I recommend focusing discussions on organic nitrate (RONO₂) OOMs specifically, rather than attributing all relevant observations to "nitrogen-containing OOMs" in general.
Citation: https://doi.org/10.5194/egusphere-2025-3219-RC2
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The clarification of the formation mechanism of HOMs/OOMs is of great importance for atmospheric chemistry. In this study, the authors investigated the elaborate oxidation mechanisms of terpinolene initiated by OH and NO3, elucidating the new formation mechanism of OOMs, the molecular structures of the products, and their time-dependent yields and volatility. These findings contribute to the molecular structure identification of OOMs in atmospheric monitoring and the refinement of atmospheric chemical models. Overall, the work is well-presented and innovative. I recommend publication after the following issues are addressed: