Sulfuric Acid-water Nucleation Research Articles

Semiempirical H2SO4-H2O Cluster Standard Gibbs Reaction Energies from Nucleation Experiments: Improved Temperature Dependence.

The thermodynamics of sulfuric acid-water clusters is important for modeling new particle formation in the atmosphere. Particle number densities obtained at 296 K from a photolytic flow reactor are greatly overpredicted by the Sulfuric Acid Water Nucleation (SAWNUC) model. Empirical, temperature-dependent adjustments to the SAWNUC model allow for better agreement with the data obtained at 296 K, while maintaining reasonable agreement with the data of Hanson and Lovejoy at 242 K. Even though these adjustments result in extensive decreases in the modeled particle number densities at room temperature, the changes in the standard Gibbs reaction energies are all less than 1 kcal/mol.

Inversely modeling homogeneous H<sub>2</sub>SO<sub>4</sub> − H<sub>2</sub>O nucleation rate in exhaust-related conditions

Abstract. The homogeneous sulfuric acid–water nucleation rate in conditions related to vehicle exhaust was measured and modeled. The measurements were performed by evaporating sulfuric acid and water liquids and by diluting and cooling the sample vapor with a sampling system mimicking the dilution process occurring in a real-world driving situation. The nucleation rate inside the measurement system was modeled inversely using CFD (computational fluid dynamics) and the aerosol dynamics code, CFD-TUTMAM (Tampere University of Technology Modal Aerosol Model for CFD). The nucleation exponents for the concentrations of sulfuric acid and water and for the saturation vapor pressure of sulfuric acid were found to be 1.9±0.1, 0.50±0.05, and 0.75±0.05, respectively. These exponents can be used to examine the nucleation mechanisms occurring in exhaust from different combustion sources (internal combustion engines, power plant boilers, etc.) or in the atmosphere. Additionally, the nucleation rate can be expressed with the exponents as a function of the concentrations of sulfuric acid and water and of temperature. The obtained function can be used as a starting point for inverse modeling studies of more complex nucleation mechanisms involving extra compounds in addition to sulfuric acid and water. More complex nucleation mechanisms, such as those involving hydrocarbons, are observed with real vehicle exhaust and are also supported by the results obtained in this study. Furthermore, the function can be used to improve air quality models by using it to model the effect of sulfuric acid-emitting traffic and power generation on the particle concentration in urban air.

Nucleation modeling of the Antarctic stratospheric CN layer and derivation of sulfuric acid profiles

Abstract. Recent analysis of long-term balloon-borne measurements of Antarctic stratospheric condensation nuclei (CN) between July and October showed the formation of a volatile CN layer at 21–27 km altitude in a background of existing particles. We use the nucleation model SAWNUC to simulate these CN in subsiding air parcels and study their nucleation and coagulation characteristics. Our simulations confirm recent analysis that the development of the CN layer can be explained with neutral sulfuric acid–water nucleation and we show that outside the CN layer the measured CN concentrations are well reproduced just considering coagulation and the subsidence of the air parcels. While ion-induced nucleation is expected as the dominating formation process at higher temperatures, it does not play a significant role during the CN layer formation as the charged clusters recombine too fast. Further, we derive sulfuric acid concentrations for the CN layer formation. Our concentrations are about 1 order of magnitude higher than previously presented concentrations as our simulations consider that nucleated clusters have to grow to CN size and can coagulate with preexisting particles. Finally, we calculate threshold sulfuric acid profiles that show which concentration of sulfuric acid is necessary for nucleation and growth to observable size. These threshold profiles should represent upper limits of the actual sulfuric acid outside the CN layer. According to our profiles, sulfuric acid concentrations seem to be below midlatitude average during Antarctic winter but above midlatitude average for the CN layer formation.

Effect of dimethylamine on the gas phase sulfuric acid concentration measured by Chemical Ionization Mass Spectrometry.

Sulfuric acid is widely recognized as a very important substance driving atmospheric aerosol nucleation. Based on quantum chemical calculations it has been suggested that the quantitative detection of gas phase sulfuric acid (H2SO4) by use of Chemical Ionization Mass Spectrometry (CIMS) could be biased in the presence of gas phase amines such as dimethylamine (DMA). An experiment (CLOUD7 campaign) was set up at the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber to investigate the quantitative detection of H2SO4 in the presence of dimethylamine by CIMS at atmospherically relevant concentrations. For the first time in the CLOUD experiment, the monomer sulfuric acid concentration was measured by a CIMS and by two CI‐APi‐TOF (Chemical Ionization‐Atmospheric Pressure interface‐Time Of Flight) mass spectrometers. In addition, neutral sulfuric acid clusters were measured with the CI‐APi‐TOFs. The CLOUD7 measurements show that in the presence of dimethylamine (<5 to 70 pptv) the sulfuric acid monomer measured by the CIMS represents only a fraction of the total H2SO4, contained in the monomer and the clusters that is available for particle growth. Although it was found that the addition of dimethylamine dramatically changes the H2SO4 cluster distribution compared to binary (H2SO4‐H2O) conditions, the CIMS detection efficiency does not seem to depend substantially on whether an individual H2SO4 monomer is clustered with a DMA molecule. The experimental observations are supported by numerical simulations based on A Self‐contained Atmospheric chemistry coDe coupled with a molecular process model (Sulfuric Acid Water NUCleation) operated in the kinetic limit.

Roles of Organic Emissions in the Formation of Near Field Aircraft-Emitted Volatile Particulate Matter: A Kinetic Microphysical Modeling Study

A kinetic microphysical modeling approach that describes the formation of volatile aerosols in the presence of organic emissions in near field aircraft emitted plumes at ground level is presented. Our model suggests that self nucleation of organic species, binary nucleation of water-soluble organic vapors with water, and multicomponent nucleation of water-soluble organic vapors with sulfuric acid and water all have very slow nucleation rates. The formation of new homogeneous particles in near field aircraft plumes is thus considered to be driven by binary nucleation of sulfuric acid and water. Volatile organic vapors emitted from aircraft engines primarily contribute to the nucleation process by condensing on existing homogeneous aerosols and only affect the size and the composition (not the number) of the homogeneous aerosols. Our model also shows that under low ambient relative humidity levels or high ambient temperatures, nucleation mode particles are more organic-rich than soot coatings. Organic mass fraction of nucleation mode particles is more sensitive to organic emissions levels compared to that of soot coatings. Ambient temperature and relative humidity were also predicted to affect the nucleation of sulfuric acid–water cores, where higher ambient relative humidity level and lower ambient temperature strongly favor binary sulfuric acid–water nucleation. The effect of ambient conditions on organic fractions was predicted to be relatively insignificant.

Observation of neutral sulfuric acid-amine containing clusters in laboratory and ambient measurements

Abstract. Recent ab initio calculations showed that amines can enhance atmospheric sulfuric acid-water nucleation more effectively than ammonia, and this prediction has been substantiated in laboratory measurements. Laboratory studies have also shown that amines can effectively displace ammonia in several types of ammonium clusters. However, the roles of amines in cluster formation and growth at a microscopic molecular scale (from molecular sizes up to 2 nm) have not yet been well understood. Processes that must be understood include the incorporation of amines into sulfuric acid clusters and the formation of organic salts in freshly nucleated particles, which contributes significantly to particle growth rates. We report the first laboratory and ambient measurements of neutral sulfuric acid-amine clusters using the Cluster CIMS, a recently-developed mass spectrometer designed for measuring neutral clusters formed in the atmosphere during nucleation. An experimental technique, which we refer to as Semi-Ambient Signal Amplification (SASA), was employed. Sulfuric acid was added to ambient air, and the concentrations and composition of clusters in this mixture were analyzed by the Cluster CIMS. This experimental approach led to significantly higher cluster concentrations than are normally found in ambient air, thereby increasing signal-to-noise levels and allowing us to study reactions between gas phase species in ambient air and sulfuric acid containing clusters. Mass peaks corresponding to clusters containing four H2SO4 molecules and one amine molecule were clearly observed, with the most abundant sulfuric acid-amine clusters being those containing a C2- or C4-amine (i.e. amines with masses of 45 and 73 amu). Evidence for C3- and C5-amines (i.e. amines with masses of 59 and 87 amu) was also found, but their correlation with sulfuric acid tetramer was not as strong as was observed for the C2- and C4-amines. The formation mechanisms for those sulfuric acid-amine clusters were investigated by varying the residence time in the inlet. It was concluded that the amines react directly with neutral clusters and that ion-induced clustering of sulfuric acid cluster ions with amines was not a dominant process. Results from ambient measurements using the Cluster CIMS without addition of sulfuric acid have shown that the sulfuric acid-amine clusters were reasonably well correlated with sulfuric acid tetramer and consistent with the SASA experiments at the same Boulder sampling site. Also, clusters that contain C2- or C4-amines were more abundant and better correlated with sulfuric acid tetramer than other types of amine containing clusters. However, ambient measurements of sulfuric acid-amine clusters remain difficult and highly uncertain because their concentrations are only slightly above background levels, even during nucleation events.

Amines are likely to enhance neutral and ion-induced sulfuric acid-water nucleation in the atmosphere more effectively than ammonia

Abstract. We have studied the structure and formation thermodynamics of dimer clusters containing H2SO4 or HSO4− together with ammonia and seven different amines possibly present in the atmosphere, using the high-level ab initio methods RI-MP2 and RI-CC2. As expected from e.g. proton affinity data, the binding of all studied amine-H2SO4 complexes is significantly stronger than that of NH3•H2SO4, while most amine-HSO4− complexes are only somewhat more strongly bound than NH3•HSO4−. Further calculations on larger cluster structures containing dimethylamine or ammonia together with two H2SO4 molecules or one H2SO4 molecule and one HSO4− ion demonstrate that amines, unlike ammonia, significantly assist the growth of not only neutral but also ionic clusters along the H2SO4 co-ordinate. A sensitivity analysis indicates that the difference in complexation free energies for amine- and ammonia-containing clusters is large enough to overcome the mass-balance effect caused by the fact that the concentration of amines in the atmosphere is probably 2 or 3 orders of magnitude lower than that of ammonia. This implies that amines might be more important than ammonia in enhancing neutral and especially ion-induced sulfuric acid-water nucleation in the atmosphere.

Computational investigation of the possible role of some intermediate products of SO 2 oxidation in sulfuric acid–water nucleation

We used quantum chemical calculations to attempt to explain why sulfuric acid produced in situ via the reaction of OH radicals with SO 2 in the presence of water vapor nucleates much more effectively than sulfuric acid evaporated from a liquid reservoir. We investigated whether or not selected SO 2 oxidation intermediates or alternative reaction products could take part in nucleation. We have calculated binding energies and formation free energies for H 2SO 4·H 2SO 4, HSO 3·H 2SO 4, HSO 5·H 2SO 4, H 2SO 5·H 2SO 4, H 2S 2O 8·H 2SO 4, H 2SO 4·H 2SO 4·H 2O, HSO 3·H 2SO 4·H 2O, HSO 5·H 2SO 4·H 2O, H 2SO 5·H 2SO 4·H 2O, and H 2S 2O 8·H 2SO 4·H 2O clusters at the RI-MP2/QZVPP level. Our calculations indicate that the only dimer cluster that is more strongly bound than H 2SO 4·H 2SO 4 is H 2S 2O 8·H 2SO 4. Hydration as modeled by the addition of one water molecule to the cluster does not change the situation. The binding of these intermediates, with the exception of H 2S 2O 8 (peroxo-disulfuric acid), to sulfuric acid is therefore not likely to explain the observations.

Interpreting activity in H2O–H2SO4 binary nucleation

Sulfuric acid-water nucleation is thought to be a key atmospheric mechanism for forming new condensation nuclei. In earlier literature, measurements of sulfuric acid activity were interpreted as the total (monomer plus hydrate) concentration above solution. Due to recent reinterpretations, most literature values for H(2)SO(4) activity are thought to represent the number density of monomers. Based on this reinterpretation, the current work uses the most recent models of H(2)O-H(2)SO(4) binary nucleation along with perturbation analyses to predict a decrease in critical cluster mole fraction, increase in critical cluster diameter, and orders of magnitude decrease in nucleation rate. Nucleation rate parameterizations available in the literature, however, give opposite trends. To resolve these discrepancies, nucleation rates were calculated for both interpretations of H(2)SO(4) activity and directly compared to the available parameterizations as well as the perturbation analysis. Results were in excellent agreement with older parameterizations that assumed H(2)SO(4) activity represents the total concentration and duplicated the predicted trends from the perturbation analysis, but differed by orders of magnitude from more recent parameterizations that assume H(2)SO(4) activity represents only the monomer. Comparison with experimental measurements available in the literature revealed that the calculations of the current work assuming a(a) represents the total concentration are most frequently in agreement with observations.

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.

An improved model for hydrate formation in sulfuric acid–water nucleation

The formation of sulfuric acid–water hydrates in the vapor phase and nucleation rates of sulfuric acid–water clusters are investigated. The result of ab initio calculations and experimental data related to hydrates are utilized to improve the description of sulfuric acid–water hydration and nucleation in atmospheric conditions. The nucleation rates are obtained using the most rigorous nucleation kinetics and the thermodynamically consistent version of the classical nucleation model. The improvements increase the predicted nucleation rates compared to previous models. The predicted nucleation rates are compared with experimental ones, and they are in most cases within experimental errors. Some experimental evidence suggests that the present model gives a more realistic dependence of nucleation rate on relative humidity and sulfuric acid concentration than the earlier versions of the theory.

Hydrates in binary sulfuric acid‐water vapor: Comparison of CIMS measurements with the Liquid‐Drop Model

We report calculations of the distribution of sulfuric acid‐water hydrates under atmospheric conditions using the liquid‐drop hydrate model. The model‐computed total acid concentrations (free acid plus hydrated acid) are compared with chemical ionization mass spectrometer (CIMS) measurements of total sulfuric acid vapor over solutions of varying concentration [Marti et al, 1997] Results from this first direct comparison with experimental measurement suggest that the liquid‐drop model overestimates the extent of hydrate formation. We explore the consequences of this overestimation on binary sulfuric acid‐water nucleation rates, and on higher‐order multi‐component nucleation rates involving these and additional trace species in the atmosphere. In particular, it is found that over‐estimation of hydrate formation by the model can result in substantial underprediction of sulfuric acid‐water nucleation rates.

The occurrence of sulfuric acid-water nucleation in plumes: urban environment

The occurrence of sulfuric acid-water nucleation in plumes: urban environment

The occurrence of sulfuric acid-water nucleation in plumes: urban environment

The production of new sulfuric acid-water particles via homogeneous nucleation was examined in plumes originating from urban point sources. The problem was approached by modeling the time development of the gas phase sulfuric acid concentration during the plume transport, and thereby identifying the meteorological, source-related, and environmental variables governing the behavior of this system. The development of the plume nucleation probability was shown to be dictated by a complicated and nonlinear interaction between the plume dispersion, chemistry, and condensation of sulfuric acid from the gas phase. For all the simulated plumes, those originating from stronger sources and dispersing at slower rates, or having higher SO 2 to fine particulate matter ratios, or higher SO 2 to NO x concentration ratios, showed greater nucleation probabilities indicative of a higher capability for producing new particles. Nucleation was also favored by cool and humid ambient conditions, greater actinic flux, and higher background hydrocarbon concentrations. The most effective way to decrease the plume nucleation probability was to decrease SO 2 emissions alone. Reducing NO x or particle emissions may decrease the total particulate mass but leads to higher plume nucleation probabilities, which is likely to increase particle number concentrations farther downwind of the source. DOI: 10.1034/j.1600-0889.1996.00007.x