Effect of Hydration and Base Contaminants on Sulfuric Acid Diffusion Measurement: A Computational Study

<div><p>We used quantum chemical formation free energies of hydrated sulfuric acid-containing molecular clusters and a dynamic model to simulate a flow tube measurement, and determined the effective diffusion coefficient of sulfuric acid as a function of relative humidity. This type of measurement was performed by Hanson and Eisele, who presented and applied a fitting method to obtain equilibrium constants <i>K</i>₁ and <i>K</i>₂ for the formation of sulfuric acid mono- and dihydrates, respectively, from the experimentally determined diffusion coefficients. The fit is derived assuming that only H₂SO₄ molecules hydrated by up to two water molecules are present. To study the sensitivity of the results to this assumption, we implemented the same fit to the modeled diffusion coefficient data, computed including also larger H₂SO₄ hydrates with more than two waters. We show that according to quantum chemical equilibrium constants, the larger hydrates are likely to be present in nonnegligible amounts, which affects the effective diffusion coefficient. This results in the fitted value obtained for <i>K</i>₁ being lower and for <i>K</i>₂ being higher than the actual values. The results are further altered if contaminant base molecules, such as amines, capable of binding to H₂SO₄ molecules, are able to enter the system, for example, with the water vapor. The magnitude and direction of the effect of the contaminants depends not only on the contaminant concentration, but also on the H₂SO₄ concentration and on the hygroscopicity of the H₂SO₄–base clusters.</p><p>Copyright 2014 American Association for Aerosol Research</p></div

Can Highly Oxidized Organics Contribute to Atmospheric New Particle Formation?

Highly oxidized organic molecules may play a critical role in new-particle formation within Earth’s atmosphere along with sulfuric acid, which has long been considered as a key compound in this process. Here we explore the interactions of these two partners, using quantum chemistry to find the formation free energies of heterodimers and trimers as well as the fastest evaporation rates of (2,2) tetramers. We find that the heterodimers are more strongly bound than pure sulfuric acid dimers. Their stability correlates well with the oxygen to carbon ratio of the organics, their volatility, and the number of hydrogen bonds formed. Most of the stable trimers contain one sulfuric acid and two organics (1,2), whereas many (2,2) tetramers evaporate quickly, probably due to the stability of (1,2) clusters. This finding agrees with recent experimental studies that show how new-particle formation involving oxidized organics and sulfuric acid may be rate-limited by activation of (1,2) trimers, confirming the importance of this process in the atmosphere

Structural Rearrangements and Magic Numbers in Reactions between Pyridine-Containing Water Clusters and Ammonia

Molecular cluster ions H⁺(H₂O)_<i>n</i>, H⁺(pyridine)­(H₂O)_<i>n</i>, H⁺(pyridine)₂(H₂O)_<i>n</i>, and H⁺(NH₃)­(pyridine)­(H₂O)_<i>n</i> (<i>n</i> = 16–27) and their reactions with ammonia have been studied experimentally using a quadrupole-time-of-flight mass spectrometer. Abundance spectra, evaporation spectra, and reaction branching ratios display magic numbers for H⁺(NH₃)­(pyridine)­(H₂O)_<i>n</i> and H⁺(NH₃)­(pyridine)₂(H₂O)_<i>n</i> at <i>n</i> = 18, 20, and 27. The reactions between H⁺(pyridine)_<i>m</i>(H₂O)_<i>n</i> and ammonia all seem to involve intracluster proton transfer to ammonia, thus giving clusters of high stability as evident from the loss of several water molecules from the reacting cluster. The pattern of the observed magic numbers suggest that H⁺(NH₃)­(pyridine)­(H₂O)_<i>n</i> have structures consisting of a NH₄⁺(H₂O)_<i>n</i> core with the pyridine molecule hydrogen-bonded to the surface of the core. This is consistent with the results of high-level ab initio calculations of small protonated pyridine/ammonia/water clusters

Insight into Acid–Base Nucleation Experiments by Comparison of the Chemical Composition of Positive, Negative, and Neutral Clusters

We investigated the nucleation of sulfuric acid together with two bases (ammonia and dimethylamine), at the CLOUD chamber at CERN. The chemical composition of positive, negative, and neutral clusters was studied using three Atmospheric Pressure interface-Time Of Flight (APi-TOF) mass spectrometers: two were operated in positive and negative mode to detect the chamber ions, while the third was equipped with a nitrate ion chemical ionization source allowing detection of neutral clusters. Taking into account the possible fragmentation that can happen during the charging of the ions or within the first stage of the mass spectrometer, the cluster formation proceeded via essentially one-to-one acid–base addition for all of the clusters, independent of the type of the base. For the positive clusters, the charge is carried by one excess protonated base, while for the negative clusters it is carried by a deprotonated acid; the same is true for the neutral clusters after these have been ionized. During the experiments involving sulfuric acid and dimethylamine, it was possible to study the appearance time for all the clusters (positive, negative, and neutral). It appeared that, after the formation of the clusters containing three molecules of sulfuric acid, the clusters grow at a similar speed, independent of their charge. The growth rate is then probably limited by the arrival rate of sulfuric acid or cluster–cluster collision