Sulfuric Acid Clusters Research Articles

Path-integral molecular dynamics simulations of small hydrated sulfuric acid clusters H 2SO 4·(H 2O) n ( n = 1–6) on semiempirical PM6 potential surfaces

A recently developed semiempirical PM6 method was applied to study small hydrated sulfuric acid clusters. Various low-energy structures of the H 2SO 4·(H 2O) n ( n = 1–9) clusters were optimized at this level and then compared to previous ab initio and density-functional theory studies in order to understand the applicability of the PM6 method in describing proton-transfer processes as well as hydrogen-bonded structures in the clusters. Although the PM6 method seems to somewhat overemphasize bifurcated hydrogen-bonded structures, moderately good agreement was obtained. Quantum path-integral molecular dynamics simulations for the H 2SO 4·(H 2O) n ( n = 1–6) clusters were subsequently performed directly using PM6 potential energies and their gradients. It was found that the acid dissociation probability increases with an increase in the cluster size, as expected, and that so-called contact-ion-pair structures are dominant in the proton-dissociated clusters. The importance of nuclear quantum effects in the cluster structures and proton-transfer processes is demonstrated.

Importance of the Number of Acid Molecules and the Strength of the Base for Double-Ion Formation in (H2SO4)m·Base·(H2O)6 Clusters

Sulfuric acid and water clusters are important for new particle formation in the atmosphere. Recent experimental studies demonstrate that critical clusters in diverse atmospheric environments contain two acid molecules and may also include additional N-containing molecules (i.e., a base). We use first-principles molecular dynamics simulations to show that the presence of two sulfuric acid molecules in (H2SO4)m x base x (H2O)6 clusters is always sufficient to form a double ion, whereas a single acid molecule, even in the presence of a base, is not.

The role of ammonia in sulfuric acid ion induced nucleation

Abstract. We have developed a new multi-step strategy for quantum chemical calculations on atmospherically relevant cluster structures that makes calculation for large clusters affordable with a good accuracy-to-computational effort ratio. We have applied this strategy to evaluate the relevance of ternary ion induced nucleation; we have also performed calculations for neutral ternary nucleation for comparison. The results for neutral ternary nucleation agree with previous results, and confirm the important role of ammonia in enhancing the growth of sulfuric acid clusters. On the other hand, we have found that ammonia does not enhance the growth of ionic sulfuric acid clusters. The results also confirm that ion-induced nucleation is a barrierless process at high altitudes, but at ground level there exists a barrier due to the presence of a local minimum on the free energy surface.

Measurement of the Thermodynamics of the Hydrated Dimer and Trimer of Sulfuric Acid

We report measurements of the equilibrium constants, K(p), for the water mediated clustering of two and three sulfuric acid molecules for atmospheric temperatures and relative humidities. Limits for K(p) for the formation of the sulfuric acid tetramer and higher clusters as well as the kinetics of small sulfuric acid cluster growth are also presented. These results put strong constraints on the rate of nucleation in the atmosphere via the H(2)SO(4)/H(2)O neutral mechanism. We show that the neutral nucleation of H(2)SO(4) and H(2)O is slower than ion-induced nucleation of H(2)SO(4) and H(2)O for most conditions found in the middle and upper troposphere. These laboratory-based upper limits to the nucleation rates are also much lower than the predictions of the liquid drop model/classical nucleation theory.

Negative atmospheric ions and their potential role in ion‐induced nucleation

Mass identified ion cluster distributions were measured under ambient atmospheric conditions and compared with model predictions based on laboratory ion cluster thermodynamics data. The results are shown from several days where atmospheric sulfur concentrations were high and thus ion‐induced cluster growth was anticipated. Atmospheric gas phase sulfuric acid, temperature, relative humidity, SO2, mobility distributions of ions and small charged particles, and aerosol size distributions were also measured in support of the model calculations. The relative agreement of measurement and model for the first and second sulfuric acid clusters (HSO4−(H2SO4)m) for m = 1 and 2 is quite good but suggests that sulfuric acid clustering may not occur at the collision rate. Clusters for higher m values were not observed, which is also consistent with model predictions for the conditions under which measurements were performed. The lack of both observed and predicted large ion clusters is also consistent with the independent measurements of ion mobility distributions and particle size distributions, which showed similar numbers of positively and negatively charged ultrafine particles, suggesting that neither positive nor negative ion‐induced nucleation processes were likely to have contributed significantly to observed new particle formation rates during this study. The relatively low observed concentrations of the bisulfate ion also suggest that the processes leading to the first sulfuric acid/bisulfate cluster (HSO4−H2SO4) may be more complicated than simple sulfuric acid clustering or exchange reactions. While nucleation was observed on some days, measurements suggest that ion‐induced nucleation did not contribute significantly to new particle production or growth during these events. This does not rule out the possibility that ion‐induced nucleation could contribute significantly to atmospheric new particle formation under very different atmosphere conditions such as in areas with much lower temperatures and higher ion concentrations.

Quasi-unary homogeneous nucleation of H2SO4-H2O

We show that the binary homogeneous nucleation (BHN) of H2SO4-H2O can be treated as quasi-unary nucleation of H2SO4 in equilibrium with H2O vapor. A scheme to calculate the evaporation coefficient of H2SO4 molecules from H2SO4-H2O clusters is presented and a kinetic model to simulate the quasi-unary nucleation of H2SO4-H2O is developed. In the kinetic model, the growth and evaporation of sulfuric acid clusters of various sizes are explicitly simulated. The kinetic quasi-unary nucleation model does not have two well-recognized problems associated with the classical BHN theory (violation of the mass action law and mismatch of the cluster distribution for monomers) and is appropriate for the situations where the assumption of equilibrium cluster distribution is invalid. The nucleation rates predicted with our quasi-unary kinetic model are consistent with recent experimental nucleation experiments in all the cases studied, while the most recent version of the classical BHN model systematically overpredicts the nucleation rates. The hydration of sulfuric acid clusters, which is not considered in the classical model but is accounted for implicitly in our kinetic quasi-unary model, is likely to be one of physical mechanisms that lead to lower nucleation rates. Further investigation is needed to understand exactly what cause the difference between the kinetic quasi-unary model and the classical BHN model.

Reliable potential for small sulfuric acid–water clusters

Abstract We have constructed a reliable potential model for clusters of sulfuric acid and water. Such model requires potentials for sulfuric acid (H 2 SO 4 ), protonated sulfuric acid (HSO 4 − ), hydronium ion (H 3 O + ) and water (H 2 O). To develop this model, we have used the available ab initio data for small clusters containing one sulfuric acid. The ab initio data are well reproduced with our model. We computed the number of waters around a sulfuric acid at atmospheric conditions (300 K and 50% relative humidity) to be ca. 1.3. This is in good agreement with experiments which predict 1.3 waters around the acid. We have also tested our model with small clusters containing two sulfuric acids and find the results to be in good agreement with ab initio calculations. Larger systems with up to 50 water molecules were studied as well. Here, the hydronium ions were found to be on the surface of the cluster and the bisulfate ions inside the cluster. The clusters are very flexible and large fluctuations were observed in the sulfur–sulfur and sulfur–hydronium distance.

An improved parameterization for sulfuric acid–water nucleation rates for tropospheric and stratospheric conditions

In this paper we present parameterized equations for calculation of sulfuric acid–water critical nucleus compositions, critical cluster radii and homogeneous nucleation rates for tropospheric and stratospheric conditions. The parameterizations are based on a classical nucleation model. We used an improved model for the hydrate formation relying on ab initio calculations of small sulfuric acid clusters and on experimental data for vapor pressures and equilibrium constants for hydrate formation. The most rigorous nucleation kinetics and the thermodynamically consistent version of the classical binary homogeneous nucleation theory were used. The parameterized nucleation rates are compared with experimental ones, and at room temperature and relative humidities above 30% they are within experimental error. At lower temperatures and lower humidities the agreement is somewhat poorer. Overall, the values of nucleation rates are increased compared to a previous parameterization and are within an order of magnitude compared with theoretical values for all conditions studied. The parameterized equations will reduce the computing time by a factor 1/500 compared to nonparameterized nucleation rate calculations and therefore are in particular useful for large‐scale models. The parameterized formulas are valid at temperatures between 230.15 K and 305.15 K, relative humidities between 0.01% and 100%, and sulfuric acid concentrations from 104 to 1011 cm−3. They can be used to extrapolate the classical results down to 190 K. The parametrization is limited to cases where nucleation rates are between 10−7 and 1010 cm−3s−1, and the critical cluster contains at least four molecules.

Unambiguous identification and measurement of sulfuric acid cluster chemiions in aircraft jet engine exhaust

Measurements of negative chemiions (CI) emitted by a jet engine at the ground were made with an ion trap mass spectrometer. The new instrument offered a high-mass resolution, which led to a first unambiguous identification of negative CI formed by a jet engine. The observed ions are HSO 4 −(H 2SO 4) a clusters proved by an isotope study. From the mass spectra an efficiency ε for fuel sulfur conversion to S VI of 2%±0.8 could be inferred. In addition thermodynamic properties of the observed cluster ions were inferred from measured ion abundance ratios. An effective free energy Δ G a−1, a 0=−14 kcal/mol was calculated (for a=3) and an enthalpy of Δ H a−1, a 0=−24 (for a=3) kcal/mol was estimated. This indicates a low stability of HSO 4 −(H 2SO 4) a ( a⩾3) cluster ions against thermal detachment of H 2SO 4 at the high temperatures of our experiment. However the low temperatures at cruise altitudes around 10–12 km lead to high H 2SO 4/H 2O supersaturation and therefore a rapid growth of HSO 4 −(H 2SO 4) a cluster ions seems to be possible which is not hindered by thermal H 2SO 4 detachment.

Ion trap studies of H +(H 2SO 4) m(H 2O) n reactions with water, ammonia, and a variety of organic compounds

Rate coefficients and products are reported for the reactions of protonated sulfuric acid and water clusters H +(H 2SO 4) m (H 2O) n with water, ammonia, and a variety of organic compounds. The cluster ions were generated in an external ion source and the reactions were studied in a quadrupole ion trap. These data yield the stabilities of H +H 2SO 4(H 2O) n=1–3 . The implications of these results to the understanding of the role of H 2SO 4 in atmospheric ion chemistry are discussed. Effective internal temperatures of trapped H +(H 2O) n=3,4 and H +(NH 3) 3 are also derived from kinetics of the unimolecular decomposition in the ion trap.

Coexistence of Neutral and Ion-Pair Clusters of Hydrated Sulfuric Acid H2SO4(H2O)n (n = 1−5)A Molecular Orbital Study

Various isomeric structures of the hydrated clusters of sulfuric acid, H2SO4(H2O)n (n = 1−5), are examined using a density functional molecular orbital method. Due to the small energy difference between trans and cis conformations about two OH groups of sulfuric acid, there are three types of isomeric forms of the hydrated clusters of sulfuric acid which involve the proton nontransferred trans conformer, the proton transferred trans conformer, and the proton nontransferred cis conformer of sulfuric acid. In the case of transoid H2SO4, the proton transferred ion-pair structures become more stable than the proton nontransferred structures as the number of water molecules increases. The hydrated clusters of the cis conformation remain neutral hydrogen-bonded structures even if the number of water molecules increases. All stable clusters tend to form multi-cyclic structures. While both protons of sulfuric acid participate in cyclic hydrogen bonding in the neutral structures, the OH group of HSO4- in the ion-p...

H2SO4 vapor pressure of sulfuric acid and ammonium sulfate solutions

Few measurements of H2SO4 vapor pressure have been made for sulfuric acid in the temperature and concentration ranges of atmospheric interest because of the very low pressures involved (below 10−4 Pa, or 10−6 torr); no such measurements appear to have been made for sulfuric acid solutions neutralized with ammonia. This work presents measurements of H2SO4 vapor pressure for aqueous sulfuric acid solutions between 55 and 77 wt% H2SO4 (corresponding to about 5–25% relative humidity), ammonium sulfate solids at low humidities, and partially neutralized sulfate solutions with [NH4+]: [SO4=] ratios between 0.13 and 1.0. The vapor pressure data collected over sulfuric acid solutions generally agree with the predictions of Ayers, et al. [1980], although positive deviation was observed for the more dilute solutions. The good agreement between this measurement and previous efforts by absolute techniques suggests that the evaporative coefficient for the H2SO4‐H2O system is near unity. H2SO4 vapor pressures over solid ammonium sulfate were measured between 27°C and 60°C; the data were fitted to In p = A/T + B, with A = −5928 and B = −3.77. The H2SO4 vapor pressures of mixed H2SO4‐H2O‐(NH4)2SO4 solutions dropped significantly as the [NH4+]:[SO4=] ratio exceeded 0.5. The results suggest that ammonia could very effectively stabilize molecular clusters of sulfuric acid and water in the atmosphere against evaporation, leading to rates of new particle formation higher than those predicted by binary H2SO4‐H2O theory.

Implications for atmospheric negative ion composition measurements of laboratory ECA studies of sulfuric acid cluster ions

Laboratory studies of electric field-induced collisional activation (ECA) of HSO 4 −(H 2SO 4) n cluster ions were made revealing fragmentation via two channels: HSO 4 − + H 2SO 4 and HSO 4 −SO 3 + H 2O. This result suggests that mixed cluster ions of the type HSO 4 −(H 2SO 4) 1 X with X having a mass of 81±1 amu which were recently detected in the stratosphere may not represent true ambient ions but are fragments formed by ECA of ambient HSO 4 −(H 2SO 4) n clusters during ion sampling into the mass spectrometer. Implications for stratospheric negative ion composition measurements are discussed.