the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 May 2026
Heterogeneous Nitrosation Reactions of Amines Driven by Dinitrogen Tetroxide: A Missing Source of Particulate Nitrosamines
Abstract. Nitrosamines are highly carcinogenic and reactive nitrogen-containing pollutants that are widely detected in atmospheric particulate matter; however, their formation mechanisms remain poorly understood. Herein, we elucidate previously unrecognized yet kinetically viable heterogeneous mechanisms for nitrosamine formation via amine-mediated reactions with dinitrogen tetroxide (N2O4) at the air–water interface, using Born–Oppenheimer molecular dynamics simulations. The reactions proceed via two distinct pathways: (i) barrierless N-nitrosation of methylamine (MA) or dimethylamine (DMA) by N2O4, directly yielding nitrosamine cations and nitrate ions (NO3−); and (ii) MA/DMA-mediated hydrolysis of N2O4 produces interfacial HONO rapidly within ~2–16 ps, which can further react with amines to form nitrosamines, albeit with a relatively high reaction barrier of 7.65 kcal mol−1. Overall, the amine-mediated interfacial N-nitrosation reactions proceed rapidly and may represent an important source of particulate nitrosamines. Meanwhile, amine-mediated interfacial hydrolysis of N2O4 is a potential source of HONO, proceeding through the combined effects of interfacial water bridging and strong basicity of amines. Our findings reveal a previously overlooked role of heterogeneous interfacial chemistry in elevated particulate nitrosamine formation and coupled HONO production, with important implications for urban reactive nitrogen cycling and its representation in chemical transport models.
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Status: open (until 08 Jul 2026)
- RC1: 'Comment on egusphere-2026-2401', Anonymous Referee #1, 10 Jun 2026 reply
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Chi et al. present a well-designed theoretical study on the heterogeneous formation mechanisms of carcinogenic nitrosamines at the air–water interface. Using ab initio molecular dynamics simulations, the authors demonstrate that amines (MA/DMA) can rapidly react with dinitrogen tetroxide (N2O4) at aqueous aerosol surfaces, resulting in the direct formation of particulate nitrosamines and nitrate. This study provides in-depth mechanistic insights into the particulate nitrosamine production in urban atmospheres. Additionally, combination of BOMD and metadynamics simulations enables a precise monitoring of these ultrafast interfacial processes, and the accompanying wave function analyses enhance mechanistic interpretation. Overall, this manuscript is well-designed and contains substantial theoretical results. The obtained theoretical results are of significant importance to atmospheric multiphase chemistry and aerosol interfacial processes. The reviewer recommends publication of this manuscript in Atmos. Chem. Phys. after the following minor concerns are addressed:
1. Schemes 1 and 2 are very informative and greatly aid the understanding of the proposed reaction mechanisms. However, Scheme 2 (iii) should be explicitly referenced and discussed in Section 3.2 to better direct the readers to the mechanistic pathways presented. In addition, the authors should further clarify why the reaction pathway shown in Scheme 2 (iii) is not feasible. Notably, the preformed complex involved in Scheme 2 (iii) appears to be the same as that shown in Scheme 1 (ii). In Scheme 1 (ii), collision of this complex with the air–water interface directly leads to nitrosation at the interface. However, this does not adequately explain why hydrolysis cannot occur when the same complex interacts with the air–water interface. A more detailed mechanistic explanation distinguishing the feasibility of nitrosation versus hydrolysis pathways would strengthen the interpretation of the interfacial reaction mechanisms.
2. It is recommended to cite recent key studies in the “introduction” to offer a clear picture regarding the “reaction-accelerating effect of the air-water interface”.
3. The conclusion effectively summarizes the key findings. Please explicitly reiterate the “missing source” aspect in the final paragraph to tie it back to the Abstract and resonate with the readers.
4. The environmental implications of simultaneous HONO and nitrate formation deserve further emphasis. The manuscript primarily focuses on nitrosamine production, while the concurrent formation of HONO and nitrate is not strictly discussed. From an atmospheric chemistry perspective, this coupled production pathway could be highly important because it simultaneously influences oxidation capacity, aerosol nitrate burden, and reactive nitrogen cycling. The authors may consider expanding this discussion in the Conclusion section to better highlight the broader implications of the revealed chemistry beyond nitrosamine formation alone.
5. The discussion of HONO-mediated nitrosation could be strengthened to better contextualize its atmospheric significance. Although the HONO + DMA pathway is identified as secondary due to its higher free-energy barrier, the calculated interfacial rate constant is still reported to be 7-8 orders of magnitude faster than the corresponding aqueous-phase value. This represents a remarkably strong interfacial enhancement. The authors may therefore consider discussing under what atmospheric conditions this secondary pathway could nevertheless become important, such as in aged aerosols or HONO-rich nighttime environments.
6. Typos and non-scientific corrections:
a). Line 73: Add the missing article “the” before “plane-wave basis set”: “...while that for the plane-wave basis set was set to 280 Ry.”
b). Section 3.3: In the sentence “Although the calculated energy barrier... is 7.65 kcal mol−1... which is significantly lower...”, the relative clause is awkward. Please split this into two sentences or rephrase to “Although the calculated energy barrier is 7.65 kcal mol−1—a value significantly lower than...”
c). Table S1: Please specify “Vibrational frequencies” in the header instead of “Calculated frequencies”.