Rate Coefficients for the Reactions of OH Radicals with a Series of Alkyl-Substituted Amines

Formation of Nitrosamines from the Heterogeneous Reaction of Nitrous Acid and Organic Amines in Indoor Environments

Carcinogenic nitrosamines have been widely studied due to their risk to human health. However, the universality and evolutionary processes of their generation, particularly concerning their secondary sources, remain unclear at present. We demonstrated through laboratory flow tube experiments that corresponding nitrosamines were generated from heterogeneous reactions of nitrous acid (HONO) with five structurally diverse amines commonly found indoors, including diphenylamine (DPhA), dibenzylamine (DBzA), dioctylamine (DOtA), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), and N-phenyl-1-naphthylamine (PANA). The heterogeneous reaction rate constants of DBzA and DOtA with HONO (∼70 ppb) were 1.21 × 10-3 and 2.13 × 10-3 min-1 at 30% relative humidity (RH), resulting in a lifetime of 13.8 and 7.8 h. As compared to higher RH (∼80%), more nitrosamines were produced from the reaction of HONO with surface-sorbed DBzA, DOtA, 6PPD, and PANA at lower RH (30%), with product yields ranging from <0.1% to 0.5%. Furthermore, we observed the formation of nitroso-6PPDs and nitro-6PPDs during room air exposure of 6PPD in a genuine indoor environment, in addition to various other transformation products indicative of reactions of 6PPD with HONO, NOx, and ozone indoors. This study confirmed the universality of the heterogeneous reaction of surface-sorbed amine with HONO as a source of nitrosamines indoors.

Insights into the role of the H-abstraction reaction kinetics of amines in understanding their degeneration fates under atmospheric and combustion conditions

Amines, a class of prototypical volatile organic compounds, have garnered considerable interest within the context of atmospheric and combustion chemistry due to their substantial contributions to the formation of hazardous pollutants in the atmosphere. In the current energy landscape, the implementation of carbon-neutral energy and strategic initiatives leads to generation of new amine sources that cannot be overlooked in terms of the emission scale. To reduce the emission level of amines from their sources and mitigate their impact on the formation of harmful substances, a comprehensive understanding of the fundamental reaction kinetics during the degeneration process of amines is imperative. This perspective article first presents an overview of both traditional amine sources and emerging amine sources within the context of carbon peaking and carbon neutrality and then highlights the importance of H-abstraction reactions in understanding the atmospheric and combustion chemistry of amines from the perspective of reaction kinetics. Subsequently, the current experimental and theoretical techniques for investigating the H-abstraction reactions of amines are introduced, and a concise summary of research endeavors made in this field over the past few decades is provided. In order to provide accurate kinetic parameters of the H-abstraction reactions of amines, advanced kinetic calculations are performed using the multi-path canonical variational theory combined with the small-curvature tunneling and specific-reaction parameter methods. By comparing with the literature data, current kinetic calculations are comprehensively evaluated, and these validated data are valuable for the development of the reaction mechanism of amines.

Autoxidation mechanism for atmospheric oxidation of tertiary amines: Implications for secondary organic aerosol formation

Tertiary amines are one kind of identified amines in the atmosphere. Here, the atmospheric oxidation mechanism and kinetics of tertiary amines were investigated by using computational methods. As proxies of these amines, trimethylamine (TMA) and triethylamine (TEA) have been selected. Results indicate that N-containing peroxy radicals (NRO₂⋅), which are key intermediates in ⋅OH initiated oxidation of TMA and TEA, can follow a so-called autoxidation mechanism (a chain reaction of H-shift followed by O₂ addition) even on the condition of high NO/HO₂⋅ concentration. Such unique mechanism can be ascribed to the ability of N-atom in facilitating the unimolecular H-shift of NRO₂⋅ and the absence of H-atoms on N-atom. However, different from TMA reaction system, the pathway dissociating into fragmental products can compete with the autoxidation pathway for TEA system. More importantly, TEA reaction system cannot lead to the formation of products with high O/C ratio due to the autoxidation pathway terminated by the release of fragmental molecules. Such difference can be corroborated by previously observing lower secondary organic aerosol yield of TEA oxidation than that of TMA oxidation. The unveiled mechanism enhances current understanding on atmospheric fate of amines and autoxidation mechanism.

Mechanism and predictive model development of reaction rate constants for N-center radicals with O2

Atmospheric oxidation of NH_x-containing (x = 1, 2) compounds can produce N-center radicals, a precursor of toxic nitrosamines. The reaction rate constant (k_O2) with O₂ has been considered as an important parameter to determine the nitrosamines yield in the subsequent reactions of N-center radicals. However, available k_O2 values of N-center radicals are limited. Here, a three-step scheme including mechanistic analysis and kinetics calculation of the reactions of 28 various N-center radicals with O₂, and model development was taken to solve the k_O2 data shortage. Mainly employed tools include highly cost-expensive coupled-cluster theory (CCSD(T)), kinetic model and statistics. The results indicate that the direct H-abstraction pathway is the most favorable for the reactions of all considered N-center radicals with O₂. The specific molecular conformation and the C-H bond energy of the N-center radicals are two important factors to determine k_O2 values. Based on the mechanistic understanding of k_O2 values, a quantitative structure-activity relationship (QSAR) model of k_O2 values was developed. The model has satisfactory goodness-of-fit, robustness and predictive ability. The determined k_O2 values and the in silico methods provide a scientific base for assessing formation risk of toxic nitrosamines in the atmosphere.

Piperazine Enhancing Sulfuric Acid-Based New Particle Formation: Implications for the Atmospheric Fate of Piperazine

Piperazine (PZ), a cyclic diamine, is one of 160 detected atmospheric amines and an alternative solvent to the widely used monoethanolamine in post-combustion CO₂ capture. Participating in H₂SO₄ (sulfuric acid, SA)-based new particle formation (NPF) could be an important removal pathway for PZ. Here, we employed quantum chemical calculations and kinetics modeling to evaluate the enhancing potential of PZ on SA-based NPF by examining the formation of PZ-SA clusters. The results indicate that PZ behaves more like a monoamine in stabilizing SA and can enhance SA-based NPF at the parts per trillion (ppt) level. The enhancing potential of PZ is less than that of the chainlike diamine putrescine and greater than that of dimethylamine, which is one of the strongest enhancing agents confirmed by ambient observations and experiments. After the initial formation of the (PZ)₁(SA)₁ cluster, the cluster mainly grows by gradual addition of SA or PZ monomer, followed by addition of (PZ)₁(SA)₁ cluster. We find that the ratio of PZ removal by NPF to that by the combination of NPF and oxidations is 0.5-0.97 at 278.15 K. As a result, we conclude that participation in the NPF pathway could significantly alter the environmental impact of PZ compared to only considering oxidation pathways.

Ozone and ozone/vacuum-UV degradation of diethyl dithiocarbamate collector: kinetics, mineralization, byproducts and pathways

The diethyl dithiocarbamate (DDC) collector, a precursor of toxic N-nitrosamines, is detected in flotation wastewaters usually at the ppm level. In this study, the O₃ and O₃/Vacuum-UV (O₃/VUV) processes were compared to investigate the efficient removal of DDC with a low risk of N-nitrosamine formation. The results showed that 99.55% of DDC was removed at 20 min by O₃/VUV, and the degradation rate constant was 3.99 times higher than that using O₃-alone. The C, S and N mineralization extents of DDC using O₃/VUV reached 36.36%, 62.69% and 79.76% at 90 min, respectively. O₃/VUV achieved a much higher mineralization extent of DDC than O₃-alone. After 90 min of degradation, O₃/VUV achieved lower residual concentrations of CS₂ and H₂S, and released lower amounts of gaseous sulfur byproducts compared to O₃-alone. The solid phase extraction and gas chromatography-mass spectrometry (SPE/GC–MS) analysis indicated that the main byproducts in O₃/VUV degradation of DDC were amide compounds without the detection of N-nitrosamines. The avoidance of N-nitrosamine formation might be attributed to exposure of UV irradiation and enhanced formation of ˙OH radicals in the O₃/VUV system. The degradation pathways of DDC were proposed. This work indicated that O₃/VUV was an efficient alternative treatment technique for the removal of DDC flotation collector with low risk of N-nitrosamine formation.

The diethyl dithiocarbamate collector, a precursor of toxic N-nitrosamines, can be effectively degraded by the O₃/VUV with satisfactory mineralization extents.