Direct Measurements of Covalently Bonded Sulfuric Anhydrides from Gas-Phase Reactions of SO3 with Acids under Ambient Conditions

Sulfur trioxide (SO3) is an important oxide of sulfur and a key intermediate in the formation of sulfuric acid (H2SO4, SA) in the Earth's atmosphere. This conversion to SA occurs rapidly due to the reaction of SO3 with a water dimer. However, gas-phase SO3 has been measured directly at concentrations that are comparable to that of SA under polluted mega-city conditions, indicating gaps in our current understanding of the sources and fates of SO3. Its reaction with atmospheric acids could be one such fate that can have significant implications for atmospheric chemistry. In the present investigation, laboratory experiments were conducted in a flow reactor to generate a range of previously uncharacterized condensable sulfur-containing reaction products by reacting SO3 with a set of atmospherically relevant inorganic and organic acids at room temperature and atmospheric pressure. Specifically, key inorganic acids known to be responsible for most ambient new particle formation events, iodic acid (HIO3, IA) and SA, are observed to react promptly with SO3 to form iodic sulfuric anhydride (IO3SO3H, ISA) and disulfuric acid (H2S2O7, DSA). Carboxylic sulfuric anhydrides (CSAs) were observed to form by the reaction of SO3 with C2 and C3 monocarboxylic (acetic and propanoic acid) and dicarboxylic (oxalic and malonic acid)-carboxylic acids. The formed products were detected by a nitrate-ion-based chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (NO3--CI-APi-TOF; NO3--CIMS). Quantum chemical methods were used to compute the relevant SO3 reaction rate coefficients, probe the reaction mechanisms, and model the ionization chemistry inherent in the detection of the products by NO3--CIMS. Additionally, we use NO3--CIMS ambient data to report that significant concentrations of SO3 and its acid anhydride reaction products are present under polluted, marine and polar, and volcanic plume conditions. Considering that these regions are rich in the acid precursors studied here, the reported reactions need to be accounted for in the modeling of atmospheric new particle formation.

Significant influence of water molecules on the SO3 + HCl reaction in the gas phase and at the air-water interface

The products resulting from the reactions between atmospheric acids and SO3 have a catalytic effect on the formation of new particles in aerosols. However, the SO3 + HCl reaction in the gas-phase and at the air-water interface has not been considered. Herein, this reaction was explored exhaustively by using high-level quantum chemical calculations and Born Oppenheimer molecular dynamics (BOMD) simulations. The quantum calculations show that the gas-phase reaction of SO3 + HCl is highly unlikely to occur under atmospheric conditions with a high energy barrier of 22.6 kcal mol-1. H2O and (H2O)2 play obvious catalytic roles in reducing the energy barrier of the SO3 + HCl reaction by over 18.2 kcal mol-1. The atmospheric lifetimes of SO3 show that the (H2O)2-assisted reaction dominates over the H2O-assisted reaction within the altitude range of 0-5 km, whereas the H2O-assisted reaction is more favorable within an altitude range of 10-50 km. BOMD simulations show that H2O-induced formation of the ClSO3-⋯H3O+ ion pair and HCl-assisted formation of the HSO4-⋯H3O+ ion pair were identified at the air-water interface. These routes followed a stepwise reaction mechanism and proceeded at a picosecond time scale. Interestingly, the formed ClSO3H in the gas phase has a tendency to aggregate with sulfuric acids, ammonias, and water molecules to form stable clusters within 40 ns simulation time, while the interfacial ClSO3- and H3O+ can attract H2SO4, NH3, and HNO3 for particle formation from the gas phase to the water surface. Thus, this work will not only help in understanding the SO3 + HCl reaction driven by water molecules in the gas-phase and at the air-water interface, but it will also provide some potential routes of aerosol formation from the reaction between SO3 and inorganic acids.

Measurement of atmospheric nanoparticles: Bridging the gap between gas-phase molecules and larger particles

Atmospheric nanoparticles are crucial components contributing to fine particulate matter (PM_2.5), and therefore have significant effects on visibility, climate, and human health. Due to the unique role of atmospheric nanoparticles during the evolution process from gas-phase molecules to larger particles, a number of sophisticated experimental techniques have been developed and employed for online monitoring and characterization of the physical and chemical properties of atmospheric nanoparticles, helping us to better understand the formation and growth of new particles. In this paper, we firstly review these state-of-the-art techniques for investigating the formation and growth of atmospheric nanoparticles (e.g., the gas-phase precursor species, molecular clusters, physicochemical properties, and chemical composition). Secondly, we present findings from recent field studies on the formation and growth of atmospheric nanoparticles, utilizing several advanced techniques. Furthermore, perspectives are proposed for technique development and improvements in measuring atmospheric nanoparticles.

Computational chemistry of cluster: Understanding the mechanism of atmospheric new particle formation at the molecular level

New particle formation (NPF), which exerts significant influence over human health and global climate, has been a hot topic and rapidly expands field of research in the environmental and atmospheric chemistry recent years. Generally, NPF contains two processes: formation of critical nucleus and further growth of the nucleus. However, due to the complexity of the atmospheric nucleation, which is a multicomponent process, formation of critical clusters as well as their growth is still connected to large uncertainties. Detection limits of instruments in measuring specific gaseous aerosol precursors and chemical compositions at the molecular level call for computational studies. Computational chemistry could effectively compensate the deficiency of laboratory experiments as well as observations and predict the nucleation mechanisms. We review the present theoretical literatures that discuss nucleation mechanism of atmospheric clusters. Focus of this review is on different nucleation systems involving sulfur-containing species, nitrogen-containing species and iodine-containing species. We hope this review will provide a deep insight for the molecular interaction of nucleation precursors and reveal nucleation mechanism at the molecular level.

The driving effects of common atmospheric molecules for formation of prenucleation clusters: the case of sulfuric acid, formic acid, nitric acid, ammonia, and dimethyl amine

How secondary aerosols form is critical as aerosols' impact on Earth's climate is one of the main sources of uncertainty for understanding global warming. The beginning stages for formation of prenucleation complexes, that lead to larger aerosols, are difficult to decipher experimentally. We present a computational chemistry study of the interactions between three different acid molecules and two different bases. By combining a comprehensive search routine covering many thousands of configurations at the semiempirical level with high level quantum chemical calculations of approximately 1000 clusters for every possible combination of clusters containing a sulfuric acid molecule, a formic acid molecule, a nitric acid molecule, an ammonia molecule, a dimethylamine molecule, and 0-5 water molecules, we have completed an exhaustive search of the DLPNO-CCSD(T)/CBS//ωB97X-D/6-31++G** Gibbs free energy surface for this system. We find that the detailed geometries of each minimum free energy cluster are often more important than traditional acid or base strength. Addition of a water molecule to a dry cluster can enhance stabilization, and we find that the (SA)(NA)(A)(DMA)(W) cluster has special stability. Equilibrium calculations of SA, FA, NA, A, DMA, and water using our quantum chemical ΔG° values for cluster formation and realistic estimates of the concentrations of these monomers in the atmosphere reveals that nitric acid can drive early stages of particle formation just as efficiently as sulfuric acid. Our results lead us to believe that particle formation in the atmosphere results from the combination of many different molecules that are able to form highly stable complexes with acid molecules such as SA, NA, and FA.

Atmospheric Sulfuric Acid Dimer Formation in a Polluted Environment

New particle formation (NPF) contributes significantly to atmospheric particle number concentrations and cloud condensation nuclei (CCN). In sulfur-rich environments, field measurements have shown that sulfuric acid dimer formation is likely the critical step in NPF. We investigated the dimer formation process based upon the measured sulfuric acid monomer and dimer concentrations, along with previously reported amine concentrations in a sulfur-rich atmosphere (Atlanta, USA). The average sulfuric acid concentration was in the range of 1.7 × 10⁷-1.4 × 10⁸ cm^-3 and the corresponding neutral dimer concentrations were 4.1 × 10⁵-5.0 × 10⁶ cm^-3 and 2.6 × 10⁵-2.7 × 10⁶ cm^-3 after sub-collision and collision ion-induced clustering (IIC) corrections, respectively. Two previously proposed acid-base mechanisms (namely AA and AB) were employed to respectively estimate the evaporation rates of the dimers and the acid-amine complexes. The results show evaporation rates of 0.1-1.3 s^-1 for the dimers based on the simultaneously measured average concentrations of the total amines, much higher than those (1.2-13.1 s^-1) for the acid-amine complexes. This indicates that the mechanism for dimer formation is likely AA through the formation of more volatile dimers in the initial step of the cluster formation.

DFT Study of the Formation of Atmospheric Aerosol Precursors from the Interaction between Sulfuric Acid and Benzenedicarboxylic Acid Molecules

Dicarboxylic acids are ubiquitous products of the photooxidation of volatile organic compounds which are believed to play a significant role in the formation of secondary organic aerosols in the atmosphere. In this paper, we report high-level quantum investigations of the clustering properties of sulfuric acid and benzenedicarboxylic acid molecules. Up to four molecules have been considered in the calculations, and the behavior of the three isomers of the organic diacid species have been compared. The most stable geometries have been characterized together with the corresponding thermodynamic data. From an atmospheric point of view, the results of the DFT calculations show that the organic diacid molecules may significantly enhance the nucleation of small atmospheric clusters, at least from an energetic point of view. In this respect, the phthalic acid isomer seems more efficient than the two other isomers of the benzenedicarboxylic acid, in particular because the internal distance between the two carboxyl groups in the organic diacids appears to play an important role in the stabilization of the H-bond network inside the corresponding heterocluster formed with sulfuric acid molecules.

Organic acid-ammonia ion-induced nucleation pathways unveiled by quantum chemical calculation and kinetics modeling: A case study of 3-methyl-1,2,3-butanetricarboxylic acid

Nucleation of organic acids (OAs) and H₂SO₄ is an important source for new particle formation in the atmosphere. However, it is still unclear whether organic acids can produce nanoparticles independent of H₂SO₄. In this study, 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) was adopted as a model of OAs. Pathways of clustering from MBTCA, ammonia and ions (NH₄⁺ and NO₃^-) to form a 1.9 nm nucleus were investigated by quantum chemical calculation and kinetic modeling. Results show recombination of charged clusters/ions plays an essential role in the nucleation processes. Cluster formation rates increase by a factor of 10³ when NH₃ increases from 2.6 × 10⁸ molecules·cm^-3 (under clean conditions) to 2.6 × 10¹¹ molecules·cm^-3 (under polluted conditions), as NH₃ can stabilize MBTCA clusters and change ion compositions from H₃O⁺ to NH₄⁺. Although the proposed new mechanism cannot compete with H₂SO₄-NH₃-H₂O or H₂SO₄-OA nucleation currently, it may be important in the future with the decline of SO₂ concentration.

The potential mechanism of atmospheric new particle formation involving amino acids with multiple functional groups

Amino acids are recognized as significant components of atmospheric aerosols. However, their potential role in atmospheric new particle formation (NPF) is poorly understood, especially aspartic acid (ASP), one of the most abundant amino acids in the atmosphere. It has not only two advantageous carboxylic acid groups but also one amino group, both of which are both effective groups enhancing NPF. Herein, the participation mechanism of ASP in the formation of new particle involving sulfuric acid (SA)-ammonia (A)-based system has been studied using the Density Functional Theory (DFT) combined with the Atmospheric Clusters Dynamic Code (ACDC). The results show that the addition of ASP molecules in the SA-A-based clusters provides a promotion on the interaction between SA and A molecules. Moreover, ACDC simulations indicate that ASP could present an obvious enhancement effect on SA-A-based cluster formation rates. Meanwhile, the enhancement strength R presents a positive dependence on [ASP] and a negative dependence on [SA] and [A]. Besides, the enhancement effect of ASP is compared with that of malonic acid (MOA) with two carboxylic acid groups (Chemosphere, 2018, 203, 26-33), and ASP presents a more obvious enhancement effect than MOA. The mechanism of NPF indicates that ASP could contribute to cluster formation as a "participator" which is different from the "catalytic" role of MOA at 238 K. These new insights are helpful to understand the mechanism of NPF involving organic compounds with multiple functional groups, especially the abundant amino acids, such as the ASP, in the urban/suburban areas with intensive human activities and industrial productions and therefore the abundant sources of amino acids. Furthermore, the NPF of the SA-A-based system involving amino acid should be considered when assessing the environmental risk of amino acid.

Interactions of sulfuric acid with common atmospheric bases and organic acids: Thermodynamics and implications to new particle formation

Interactions of the three common atmospheric bases, dimethylamine ((CH₃)₂NH), methylamine (CH₃NH₂), ammonia (NH₃), all considered to be efficient stabilizers of binary clusters in the Earth's atmosphere, with H₂SO₄, the key atmospheric precursor, and 14 common atmospheric organic acids (COAs) (formic, acetic, oxalic, malonic, succinic, glutaric acid, adipic, benzoic, phenylacetic, pyruvic, maleic acid, malic, tartaric and pinonic acids) have been studied using the density functional theory (DFT) and composite high-accuracy G3MP2 method. The thermodynamic stability of mixed (COA)(H₂SO₄), (COA)(B1), (COA)(B2) and (COA)(B3) dimers and (COA)(H₂SO₄)(B1), (COA)(H₂SO₄)(B2) and (COA)(H₂SO₄)(B3) trimers, where B1, B2 and B3 refer to (CH₃)₂NH, CH₃NH₂ and NH₃, respectively, have been investigated and their impacts on the thermodynamic stability of clusters containing H₂SO₄ have been studied. Our investigation shows that interactions of H₂SO₄ with COA, (CH₃)₂NH, CH₃NH₂ and NH₃ lead to the formation of more stable mixed dimers and trimers than (H₂SO₄)₂ and (H₂SO₄)₂(base), respectively, and emphasize the importance of common organic species for early stages of atmospheric nucleation. We also show that although amines are generally confirmed to be more active than NH₃ as stabilizers of binary clusters, in some cases mixed trimers containing NH₃ are more stable thermodynamically than those containing CH₃NH₂. This study indicates an important role of COA, which coexist and interact with that H₂SO₄ and common atmospheric bases in the Earth atmosphere, in formation of stable pre-nucleation clusters and suggests that the impacts of COA on new particle formation (NPF) should be studied in further details.

Nucleation mechanisms of iodic acid in clean and polluted coastal regions

In coastal regions, intense bursts of particles are frequently observed with high concentrations of iodine species, especially iodic acid (IA). However, the nucleation mechanisms of IA, especially in polluted environments with high concentrations of sulfuric acid (SA) and ammonia (A), remain to be fully established. By quantum chemical calculations and atmospheric cluster dynamics code (ACDC) simulations, the self-nucleation of IA in clean coastal regions and that influenced by SA and A in polluted coastal regions are investigated. The results indicate that IA can form stable clusters stabilized by halogen bonds and hydrogen bonds through sequential addition of IA, and the self-nucleation of IA can instantly produce large amounts of stable clusters when the concentration of IA is high during low tide, which is consistent with the observation that intense particle bursts were linked to high concentrations of IA in clean coastal regions. Besides, SA and A can stabilize IA clusters by the formation of more halogen bonds and hydrogen bonds as well as proton transfers, and the binary nucleation of IA-SA/A rather than the self-nucleation of IA appears to be the dominant pathways in polluted coastal regions, especially in winter. These new insights are helpful to understand the mechanisms of new particle formation induced by IA in clean and polluted coastal regions.

2-Methyltetrol sulfate ester-initiated nucleation mechanism enhanced by common nucleation precursors: A theory study

Aerosol samples from all over the word contained 2-methyltetrol sulfate ester (MTS). We investigated the role of MTS in new particle formation (NPF) with aerosol nucleation precursors, including sulfuric acid (SA), water (W), ammonia (N), methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA). The analysis was performed using quantum chemical approach, kinetic calculation and molecular dynamics (MD) simulations. The results proved that the molecular interactions in the clusters were mainly H-bonds and electrostatic interaction. The negative Gibbs free energy changes for all the studied MTS-containing clusters indicated that the formation of these clusters was thermodynamically favorable. The stability of the clusters was evaluated according to the total evaporation rate. Here, (MTS)(SA) and (MTS)(W) were the most and least stable cluster, respectively. MD simulations were used for time and spatial analysis of the role of the MTS-SA system. The results indicated that MTS can self-aggregate or absorb SA molecules into clusters, larger than the size of the critical cluster (approximately 1 nm), suggesting that MTS can initiate NPF by itself or together with SA.

Assessment of the DLPNO Binding Energies of Strongly Noncovalent Bonded Atmospheric Molecular Clusters

This work assesses the performance of DLPNO-CCSD(T0), DLPNO-MP2, and density functional theory methods in calculating the binding energies of a representative test set of 45 atmospheric acid-acid, acid-base, and acid-water dimer clusters. The performance of the approximate methods is compared to high level explicitly correlated CCSD(F12*)(T)/complete basis set (CBS) reference calculations. Out of the tested density functionals, ωB97X-D3(BJ) shows the best performance with a mean deviation of 0.09 kcal/mol and a maximum deviation of 0.83 kcal/mol. The RI-CC2/aug-cc-pV(T+d)Z level of theory severely overpredicts the cluster binding energies with a mean deviation of -1.31 kcal/mol and a maximum deviation up to -3.00 kcal/mol. Hence, RI-CC2/aug-cc-pV(T+d)Z should not be utilized for studying atmospheric molecular clusters. The DLPNO variants are tested both with and without the inclusion of explicit correlation (F12) in the wavefunction, with different pair natural orbital (PNO) settings (loosePNO, normalPNO, and tightPNO) and using both double and triple zeta basis sets. The performance of the DLPNO-MP2 methods is found to be independent of PNO settings and yield low mean deviations of -0.84 kcal/mol or below. However, DLPNO-MP2 requires explicitly correlated wavefunctions to yield maximum deviations below 1.40 kcal/mol. For obtaining high accuracy, with maximum deviation below ∼1.0 kcal/mol, either DLPNO-CCSD(T0)/aug-cc-pVTZ (normalPNO) calculations or DLPNO-CCSD(T0)-F12/cc-pVTZ-F12 (normalPNO) calculations are required. The most accurate level of theory is found to be DLPNO-CCSD(T0)-F12/cc-pVTZ-F12 using a tightPNO criterion which yields a mean deviation of 0.10 kcal/mol, with a maximum deviation of 0.20 kcal/mol, compared to the CCSD(F12*)(T)/CBS reference.

Oxidation Enhances Aerosol Nucleation: Measurement of Kinetic Pickup Probability of Organic Molecules on Hydrated Acid Clusters

We investigate the uptake of the most prominent biogenic volatile organic compounds (VOCs)-isoprene, α-pinene, and their selected oxidation products-by hydrated acid clusters in a molecular beam experiment and by DFT calculations. Our experiments provide a unique and direct way of determination of the surface accommodation coefficient (α_S) on the proxies of ultrafine aerosol particles. Since we are able to determine unambiguously the fraction of the clusters to which the molecules stick upon collisions, our α_S is a purely kinetic parameter disentangling the molecule pickup from its evaporation. Oxidation increases the α_S of VOCs by more than an order of magnitude, because oxidized compounds form hydrogen bonds with the clusters, whereas the interactions of the parent VOCs are weaker and nonspecific. This work provides molecular-level insight into the condensation of single molecules into atmospheric particles, which has important implications for aerosol nucleation and growth.

Theoretical study of the cis-pinonic acid and its atmospheric hydrolysate participation in the atmospheric nucleation

cis-Pinonic acid (CPA), one of the major photooxidation products of α-pinene, is believed to contribute to the formation of aerosols formed over forested areas. In the current study, we implement quantum chemical calculation to investigate the interaction between sulfuric acid (SA) and CPA as well as the hydrolysate of CPA (HCPA) in the presence of water or ammonia in the atmosphere. The lowest free energy configurations, reactants, transition states, intermediates, and products were optimized at 298/278K and 1atm at the M06-2X/6-311+G(3df,3pd) level. Our results show that one CPA molecule might initially nucleate with SA molecules and subsequently participate in the formation and growth of the new particle in the form of HCPA. More than one HCPA molecule may be involved in the critical nuclei. Furthermore, the hydrolysis reaction of CPA can be effectively catalyzed by SA and nitric acid (NA) in presence of water, which significantly increases the HCPA content in the atmosphere and subsequently promotes the particle nucleation. Overall, the current study elucidates a new mechanism of atmospheric nucleation driven by CPA and its hydrolysate.

Elucidating the mechanism and kinetics of the water-assisted reaction of nitrous acid with hydroxyl radical

The rate constant of the HONO + OH reaction is slightly increased by hydration.

Benchmarking sampling methodology for calculations of Rayleigh light scattering properties of atmospheric molecular clusters

We present four different computational methods for benchmarking the sampling and Rayleigh light scattering of hydrogen bonded atmospheric molecular clusters.

Gas phase transformation from organic acid to organic sulfuric anhydride: Possibility and atmospheric fate in the initial new particle formation

New particle formation (NPF) process has been observed frequently in various environments and produces a large fraction of atmospheric aerosols. However, the chemical species participating in the nucleation as well as the corresponding nucleation mechanism in the atmosphere still remain ambiguous. Recent research by Leopold et al. shows that cycloaddition reaction of SO₃ to carboxylic acids could contribute to the formation of organic sulfuric anhydride which would have lower vapor pressure compared with the corresponding carboxylic acid and hence kick-start new particle formation in the gas phase. In the present study, energy profile for the formation of 3-methyl-1,2,3-butanetricarboxylic sulfuric anhydride (MBTCSA) through the cycloaddition of SO₃ to 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) has been investigated using computational methods. As a result, such a process would be effectively barrierless for one of the terminal carboxy group and has very low energy barriers for the other two carboxy groups (0.6 and 2.8 kcal/mol, respectively), indicating the whole process is a plausible gas phase pathway to MBTCSA formation. Furthermore, by evaluating the stability of the generated atmospheric clusters through topological and kinetic analysis, interaction between atmospheric nucleation precursor with MBTCSA is found to be more thermodynamically favourable and stronger than those with sulfuric acid and MBTCA which is identified from further-generation oxidation of a-pinene. Hence MBTCSA is speculated to be a potential participator in the initial new particle formation and the further particles growth.

Robust metric for quantifying the importance of stochastic effects on nanoparticle growth

Comprehensive representation of nanoparticle dynamics is necessary for understanding nucleation and growth phenomena. This is critical in atmospheric physics, as airborne particles formed from vapors have significant but highly uncertain effects on climate. While the vapor–particle mass exchange driving particle growth can be described by a macroscopic, continuous substance for large enough particles, the growth dynamics of the smallest nanoparticles involve stochastic fluctuations in particle size due to discrete molecular collision and decay processes. To date, there have been no generalizable methods for quantifying the particle size regime where the discrete effects become negligible and condensation models can be applied. By discrete simulations of sub-10 nm particle populations, we demonstrate the importance of stochastic effects in the nanometer size range. We derive a novel, theory-based, simple and robust metric for identifying the exact sizes where these effects cannot be omitted for arbitrary molecular systems. The presented metric, based on examining the second- and first-order derivatives of the particle size distribution function, is directly applicable to experimental size distribution data. This tool enables quantifying the onset of condensational growth without prior information on the properties of the vapors and particles, thus allowing robust experimental resolving of nanoparticle formation physics.

The potential role of malonic acid in the atmospheric sulfuric acid - Ammonia clusters formation

Malonic acid (MOA), one of the major dicarboxylic acids (DCAs) in aerosols, has been identified experimentally and computationally to be a strong acid. However, its potential role in the atmospheric clusters formation is still ambiguous. Hence, the participant mechanism of MOA on the formation of atmospheric sulfuric acid (SA)- ammonia (A) clusters was investigated by combining computational methods with atmospheric cluster dynamics code (ACDC). The most stable molecular structures obtained at the M06-2X/6-311++G(3df,3pd) level of theory shows that the added MOA molecule in the SA-A-based clusters presents a promotion on the interactions between SA and A molecules. ACDC simulations indicate directly an obvious enhancement strength R_MOA on the clusters formation rates at 218 K and the concentration of MOA ([MOA]) larger than 10⁸ molecules cm^-3, up to five orders of magnitude. Meanwhile, enhancement strength of MOA is compared with that of glycolic acid, and as expected, MOA presents a superior enhancement strength. Both R_MOA and the compared enhancement strength (r_com) present a positive dependency on [MOA] and a negative dependency on [SA]. With the increase of [A], both R_MOA and r_com (except at [SA] = 10⁴ molecules cm^-3) first increase, reaching the maximum value and then decrease. Finally, a catalytic participant mechanism of MOA where MOA acts as a mediate bridge for the formation of pure SA-A-based clusters has been identified by tracing the main growth pathways of the system.

The role of nitric acid in atmospheric new particle formation

Nitric acid, an air pollutant with strong acidity and oxidizability, can be found in considerable quantities in the gas and aerosol phase. Understanding the role of nitric acid in atmospheric new particle formation is essential to study the complicated nucleation mechanism. Using density functional theory combined with the Atmospheric Clusters Dynamic Code (ACDC), the role of nitric acid in the formation of new particles has been investigated under different atmospheric conditions (different precursor concentrations and temperatures). The results show that nitric acid can form clusters with sulfuric acid and ammonia by hydrogen bond or even proton-transfer interactions. The concentrations of clusters involving nitric acid can be comparable with those of sulfuric acid-ammonia-based clusters, considering the thermodynamic stability combined with the realistic atmospheric concentrations of precursors. Within the atmospheric concentration range, nitric acid can enhance the formation rates of sulfuric acid-ammonia clusters, especially at low temperature, low sulfuric acid concentration and high ammonia concentration. In addition, the new particle formation mechanism indicates that nitric acid can contribute to the cluster formation and the role of nitric acid in the cluster formation pathway is as a "bridge" connecting the smaller and larger clusters.

Guanidine: A Highly Efficient Stabilizer in Atmospheric New-Particle Formation

The role of a strong organobase, guanidine, in sulfuric acid-driven new-particle formation is studied using state-of-the-art quantum chemical methods and molecular cluster formation simulations. Cluster formation mechanisms at the molecular level are resolved, and theoretical results on cluster stability are confirmed with mass spectrometer measurements. New-particle formation from guanidine and sulfuric acid molecules occurs without thermodynamic barriers under studied conditions, and clusters are growing close to a 1:1 composition of acid and base. Evaporation rates of the most stable clusters are extremely low, which can be explained by the proton transfers and symmetrical cluster structures. We compare the ability of guanidine and dimethylamine to enhance sulfuric acid-driven particle formation and show that more than 2000-fold concentration of dimethylamine is needed to yield as efficient particle formation as in the case of guanidine. At similar conditions, guanidine yields 8 orders of magnitude higher particle formation rates compared to dimethylamine. Highly basic compounds such as guanidine may explain experimentally observed particle formation events at low precursor vapor concentrations, whereas less basic and more abundant bases such as ammonia and amines are likely to explain measurements at high concentrations.

Uptake of methanol on mixed HNO3/H2O clusters: An absolute pickup cross section

The uptake of atmospheric oxidized organics on acid clusters is relevant for atmospheric new particle formation. We investigate the pickup of methanol (CH₃OH) on mixed nitric acid-water clusters (HNO₃)_M(H₂O)_N by a combination of mass spectrometry and cluster velocity measurements in a molecular beam. The mass spectra of the mixed clusters exhibit (HNO₃)_m(H₂O)_nH⁺ series with m = 0-3 and n = 0-12. In addition, CH₃OH·(HNO₃)_m(H₂O)_nH⁺ series with very similar patterns appear in the spectra after the methanol pickup. The velocity measurements prove that the undoped (HNO₃)_m(H₂O)_nH⁺ mass peaks in the pickup spectra originate from the neutral (HNO₃)_M(H₂O)_N clusters which have not picked up any CH₃OH molecule, i.e., methanol has not evaporated upon the ionization. Thus the fraction of the doped clusters can be determined and the mean pickup cross section can be estimated, yielding σ_s¯≈ 20 Ų. This is compared to the lower estimate of the mean geometrical cross section σ_g¯≈ 60 Ų obtained from the theoretical cluster geometries. Thus the "size" of the cluster corresponding to the methanol pickup is at least 3-times smaller than its geometrical size. We have introduced a method which can yield the absolute pickup cross sections relevant to the generation and growth of atmospheric aerosols, as illustrated in the example of methanol and nitric acid clusters.

Closed-Shell Organic Compounds Might Form Dimers at the Surface of Molecular Clusters

The role of covalently bound dimer formation is studied using high-level quantum chemical methods. Reaction free energy profiles for dimer formation between common oxygen-containing functional groups are calculated, and based on the Gibbs free energy differences between transition states and reactants, we show that none of the studied two-component gas-phase reactions are kinetically feasible at 298.15 K and 1 atm. Therefore, the catalyzing effect of water, base, or acid molecules is calculated, and sulfuric acid is identified to lower the activation free energies significantly. We find that the reactions yielding hemiacetal, peroxyhemiacetal, α-hydroxyester, and geminal diol products occur with activation free energies of less than 10 kcal/mol with sulfuric acid as a catalyst, indicating that these reactions could potentially take place on the surface of sulfuric acid clusters. Additionally, the formed dimer products bind stronger onto the pre-existing cluster than the corresponding reagent monomers do. This implies that covalent dimerization reactions stabilize the existing cluster thermodynamically and make it less likely to evaporate. However, the studied small organic compounds, which contain only one functional group, are not able to form dimer products that are stable against evaporation at atmospheric conditions. Calculations of dimer formation onto a cluster surface and the clustering ability of dimer products should be extended to large terpene oxidation products in order to estimate the real atmospheric significance.