Dimethylamine Clusters Research Articles

Rapid iodine oxoacid nucleation enhanced by dimethylamine in broad marine regions

Abstract. Recent experiments have revealed a vital nucleation process of iodic acid (HIO3) and iodous acid (HIO2) under marine boundary layer conditions. However, HIO3–HIO2 nucleation may not effectively drive the observed rapid new particle formation (NPF) in certain coastal regions influenced by urban air masses. Dimethylamine (DMA) is a promising basic precursor to enhance nucleation considering its strong ability to stabilize acidic clusters and the wide distribution in marine atmosphere, while its role in HIO3–HIO2 nucleation remains unrevealed. Hence, a method combining quantum chemical calculations and Atmospheric Cluster Dynamics Code (ACDC) simulations was utilized to study the HIO3–HIO2–DMA nucleation process. We found that DMA can preferentially accept the proton from HIO3 as a basic precursor in the most stable configurations of HIO3–HIO2–DMA clusters. Kinetically, the participation of DMA in the cluster formation pathways of the iodine oxoacid system could be significant at the 10−1 to 1 pptv level of [DMA]. Furthermore, DMA can enhance the cluster formation rates of the HIO3–HIO2 system in marine and polar regions near DMA sources more than 103-fold. Compared to the classical nucleation mechanism, the HIO3–HIO2–DMA mechanism exhibits strong nucleation ability, worthy of consideration as a promising mechanism in marine and polar regions rich in amine sources. The newly proposed HIO3–HIO2–DMA ternary mechanism might provide an explanation for some missing fluxes of atmospheric iodine particles.

Hydrogen bond network structures of protonated dimethylamine clusters H+(DMA)n (n = 3-7).

Infrared spectroscopy of protonated dimethylamine clusters, H+(DMA)n, (n = 3-7), and their Ar-tagged clusters was performed in the NH and CH stretching vibrational region to explore their hydrogen bond network structures. A stable isomer search and vibrational spectral simulations of the clusters were also carried out to support the interpretations of the observed spectra. Weakly hydrogen-bonded NH stretching vibrational bands, which are characteristic of cyclic structures of small-sized protonated clusters, are observed in the spectra of the Ar-tagged clusters of n ≥ 5, while only linear chain type structures are suggested for the Ar-tagged clusters of n = 3-4 and the bare clusters of all the sizes. These results demonstrate that the size and temperature dependence of the hydrogen bond network structures of the protonated dimethylamine clusters is analogous to that of protonated monohydric alcohol clusters.

A study on the fragmentation of sulfuric acid and dimethylamine clusters inside an atmospheric pressure interface time-of-flight mass spectrometer

Abstract. Sulfuric acid and dimethylamine vapours in the atmosphere can form molecular clusters, which participate in new particle formation events. In this work, we have produced, measured, and identified clusters of sulfuric acid and dimethylamine using an electrospray ionizer coupled with a planar-differential mobility analyser, connected to an atmospheric pressure interface time-of-flight mass spectrometer (ESI–DMA–APi-TOF MS). This set-up is suitable for evaluating the extent of fragmentation of the charged clusters inside the instrument. We evaluated the fragmentation of 11 negatively charged clusters both experimentally and using a statistical model based on quantum chemical data. The results allowed us to quantify the fragmentation of the studied clusters and to reconstruct the mass spectrum by removing the artifacts due to the fragmentation.

Interaction between succinic acid and sulfuric acid–base clusters

Abstract. Dicarboxylic acids likely participate in the formation of pre-nucleation clusters to facilitate new particle formation in the atmosphere, but the detailed mechanism leading to the formation of multicomponent critical nuclei involving organic acids, sulfuric acid (SA), base species, and water remains unclear. In this study, theoretical calculations are performed to elucidate the interactions between succinic acid (SUA) and clusters consisting of SA-ammonia (AM)∕dimethylamine (DMA) in the presence of hydration of up to six water molecules. Formation of the hydrated SUA⚫SA⚫ base clusters is energetically favorable, triggering proton transfer from SA to the base molecule to form new covalent bonds or strengthening the preexisting covalent bonds. The presence of SUA promotes hydration of the SA⚫AM and SA⚫AM⚫DMA clusters but dehydration of the SA⚫DMA clusters. At equilibrium, SUA competes with the second SA molecule for addition to the SA⚫ base clusters at atmospherically relevant concentrations. The clusters containing both the base and organic acid are capable of further binding with acid molecules to promote subsequent growth. Our results indicate that the multicomponent nucleation involving organic acids, sulfuric acid, and base species promotes new particle formation in the atmosphere, particularly under polluted conditions with a high concentration of diverse organic acids.

Molecular-Level Understanding of Synergistic Effects in Sulfuric Acid-Amine-Ammonia Mixed Clusters.

The abundance and basicity of a stabilizing base have shown to be key factors in sulfuric acid driven atmospheric new-particle formation. However, since experiments indicate that a low concentration of ammonia enhances particle formation from sulfuric acid and dimethylamine, which is a stronger base, there must be additional factors affecting the particle formation efficiency. Using quantum chemistry, we provide a molecular-level explanation for the synergistic effects in sulfuric acid-dimethylamine-ammonia cluster formation. Because of the capability of ammonia to form more intermolecular interactions than dimethylamine, it can act as a bridge-former in sulfuric acid-dimethylamine clusters. In many cluster compositions, ammonia is more likely to be protonated than dimethylamine, although it is a weaker base. By nanoparticle formation rate simulations, we show that due to the synergistic effects, ammonia can increase the particle formation rate by up to 5 orders of magnitude compared to the two-component sulfuric acid-amine system.

Infrared spectra of neutral dimethylamine clusters: An infrared-vacuum ultraviolet spectroscopic and anharmonic vibrational calculation study.

Infrared-vacuum ultraviolet (IR-VUV) spectra of neutral dimethylamine clusters, (DMA)n (n = 2-5), were measured in the spectral range of 2600-3700 cm-1. The experimental IR-VUV spectra show NH stretch modes gradually redshift to 3200-3250 cm-1 with the increase in the cluster size and complex Fermi Resonance (FR) pattern of the CH3 group in the 2800-3000 cm-1 region. Ab initio anharmonic vibrational calculations were performed on low-energy conformers of (DMA)2 and (DMA)3 to examine vibrational coupling among CH/NH and to understand the Fermi resonance pattern in the observed spectra features. We found that the redshift of NH stretching mode with the size of DMA cluster is moderate, and the overtone of NH bending modes is expected to overlap in frequency with the CH stretching fundamental modes. The FR in CH3 groups is originated from the strong coupling between CH stretching fundamental and bending overtone within a CH3 group. Well-resolved experimental spectra also enable us to compare the performance of ab initio anharmonic algorithms at different levels.

Infrared spectroscopic probing of dimethylamine clusters in an Ar matrix

Amines have many atmospheric sources and their clusters play an important role in aerosol nucleation processes. Clusters of a typical amine, dimethylamine (DMA), of different sizes were measured with matrix isolation IR (infrared) and NIR (near infrared) spectroscopy. The NIR vibrations are more separated and therefore it is easier to distinguish different sizes of clusters in this region. The DMA clusters, up to DMA tetramer, have been optimized using density functional methods, and the geometries, binding energies and thermodynamic properties of DMA clusters were obtained. The computed frequencies and intensities of NH-stretching vibrations in the DMA clusters were used to interpret the experimental spectra. We have identified the fundamental transitions of the bonded NH-stretching vibration and the first overtone transitions of the bonded and free NH-stretching vibration in the DMA clusters. Based on the changes in vibrational intensities during the annealing processes, the growth of clusters was clearly observed. The results of annealing processes indicate that DMA molecules tend to form larger clusters with lower energies under matrix temperatures, which is also supported by the calculated reaction energies of cluster formation.

Molecular understanding of sulphuric acid–amine particle nucleation in the atmosphere

Nucleation of aerosol particles from trace atmospheric vapours is thought to provide up to half of global cloud condensation nuclei. Aerosols can cause a net cooling of climate by scattering sunlight and by leading to smaller but more numerous cloud droplets, which makes clouds brighter and extends their lifetimes. Atmospheric aerosols derived from human activities are thought to have compensated for a large fraction of the warming caused by greenhouse gases. However, despite its importance for climate, atmospheric nucleation is poorly understood. Recently, it has been shown that sulphuric acid and ammonia cannot explain particle formation rates observed in the lower atmosphere. It is thought that amines may enhance nucleation, but until now there has been no direct evidence for amine ternary nucleation under atmospheric conditions. Here we use the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN and find that dimethylamine above three parts per trillion by volume can enhance particle formation rates more than 1,000-fold compared with ammonia, sufficient to account for the particle formation rates observed in the atmosphere. Molecular analysis of the clusters reveals that the faster nucleation is explained by a base-stabilization mechanism involving acid-amine pairs, which strongly decrease evaporation. The ion-induced contribution is generally small, reflecting the high stability of sulphuric acid-dimethylamine clusters and indicating that galactic cosmic rays exert only a small influence on their formation, except at low overall formation rates. Our experimental measurements are well reproduced by a dynamical model based on quantum chemical calculations of binding energies of molecular clusters, without any fitted parameters. These results show that, in regions of the atmosphere near amine sources, both amines and sulphur dioxide should be considered when assessing the impact of anthropogenic activities on particle formation.

Free energy barrier in the growth of sulfuric acid–ammonia and sulfuric acid–dimethylamine clusters

The first step in atmospheric new particle formation involves the aggregation of gas phase molecules into small molecular clusters that can grow by colliding with gas molecules and each other. In this work we used first principles quantum chemistry combined with a dynamic model to study the steady-state kinetics of sets of small clusters consisting of sulfuric acid and ammonia or sulfuric acid and dimethylamine molecules. Both sets were studied with and without electrically charged clusters. We show the main clustering pathways in the simulated systems together with the quantum chemical Gibbs free energies of formation of the growing clusters. In the sulfuric acid-ammonia system, the major growth pathways exhibit free energy barriers, whereas in the acid-dimethylamine system the growth occurs mainly via barrierless condensation. When ions are present, charged clusters contribute significantly to the growth in the acid-ammonia system. For dimethylamine the role of ions is minor, except at very low acid concentration, and the growing clusters are electrically neutral.

Amine substitution into sulfuric acid – ammonia clusters

Abstract. The substitution of ammonia by dimethylamine in sulfuric acid – ammonia – dimethylamine clusters was studied using a collision and evaporation dynamics model. Quantum chemical formation free energies were computed using B3LYP/CBSB7 for geometries and frequencies and RI-CC2/aug-cc-pV(T+d)Z for electronic energies. We first demonstrate the good performance of our method by a comparison with an experimental study investigating base substitution in positively charged clusters, and then continue by simulating base exchange in neutral clusters, which cannot be measured directly. Collisions of a dimethylamine molecule with an ammonia containing positively charged cluster result in the instantaneous evaporation of an ammonia molecule, while the dimethylamine molecule remains in the cluster. According to our simulations, a similar base exchange can take place in neutral clusters, although the overall process is more complicated. Neutral sulfuric acid – ammonia clusters are significantly less stable than their positively charged counterparts, resulting in a competition between cluster evaporation and base exchange.

Enhancing effect of dimethylamine in sulfuric acid nucleation in the presence of water – a computational study

Abstract. We have studied the hydration of sulfuric acid – ammonia and sulfuric acid – dimethylamine clusters using quantum chemistry. We calculated the formation energies and thermodynamics for clusters of one ammonia or one dimethylamine molecule together with 1–2 sulfuric acid and 0–5 water molecules. The results indicate that dimethylamine enhances the addition of sulfuric acid to the clusters much more efficiently than ammonia when the number of water molecules in the cluster is either zero, or greater than two. Further hydrate distribution calculations reveal that practically all dimethylamine-containing two-acid clusters will remain unhydrated in tropospherically relevant circumstances, thus strongly suggesting that dimethylamine assists atmospheric sulfuric acid nucleation much more effectively than ammonia.