Ion formation mechanism of cortisone molecules and clusters in charged nanodroplets.

Mass spectrometry measurements coupled with classical molecular dynamics (MD) simulations have been conducted in recent years to understand the final stage of ion formation in electrospray ionization (ESI). Here, to characterize the ion formation mechanism in the recently developed droplet-assisted ionization (DAI) source, MD simulations with various conditions (solute number, temperature, ions, composition) were performed to help explain DAI-based measurements. The specific binding ability of cortisone with preformed ions (ions of sodium, cesium and iodide) in evaporating nanodroplets makes the ion formation process characteristic of both the ion evaporation and charge residue models (IEM and CRM, respectively). Most preformed ions are ejected with dozens of solvent molecules to form gas-phase ions by IEM, while clusters of one or more cortisone molecules with one or more preformed ions remain in the evaporating droplet to form gas-phase ions by CRM. As the ratio of cortisone molecules to preformed ions increases, the number of preformed ions held in the droplet without ejection by the IEM increases. In other words, increasing the molecular solute to preformed ion ratio in the droplet increases the fraction of gas-phase ions formed by CRM relative to IEM. The increase in CRM relative to IEM is accompanied by an increase in the calculated activation energy barrier, which can explain the activation energy measurements by DAI, where droplets without preformed ions exhibit higher activation energies for gas-phase ion formation than droplets containing large numbers of preformed ions.

Online Characterization of Organic Aerosol by Condensational Growth into Aqueous Droplets Coupled with Droplet-Assisted Ionization.

Online analysis of ultrafine (<100 nm diameter) particles was performed by sending the aerosol through a condensation growth chamber (CGC) to create micrometer-size aqueous droplets that were subsequently analyzed by mass spectrometry with droplet-assisted ionization (DAI). Three experiments are reported which illustrate key performance characteristics of the method and give insight into the ion formation process: size-selected cortisone particles, size-selected secondary organic aerosol (SOA) particles, and freshly nucleated SOA under atmospherically relevant conditions. In each case, SOA was produced by α-pinene ozonolysis. For size-selected cortisone particles between 30 and 90 nm diameter and SOA particles between 30 and 70 nm, the ion signal intensity was found to be approximately independent of particle size. This observation is attributed to the formation of aqueous droplets in the CGC whose size distribution is independent of the original particle size. A consequence of this behavior is that the sensitivity of molecular detection increases as the particle size decreases, and the method is particularly well suited for new particle formation studies under atmospherically relevant conditions. This aspect of the CGC-DAI method was illustrated by the online analysis of freshly nucleated SOA samples with median diameters, number concentrations, and mass concentrations on the order of 25 nm, 104 cm-3, 0.2 µg m-3, respectively. Mass spectra of freshly nucleated SOA could be explained by condensation of highly oxidized molecules (HOMs) that subsequently reacted in the particle phase. Size-selected SOA showed increasing oligomerization with increasing particle size, which is consistent with established particle growth mechanisms.

Ion formation in droplet-assisted ionization.

RATIONALE: In droplet-assisted ionization (DAI), intact molecular ions are generated from molecules in aerosol droplets by passing the droplets through a temperature-controlled capillary inlet. Ion formation is explored through the effects of analyte mass flow, droplet solvent composition, and capillary temperature on ion signal intensity. METHODS: A Waters SYNAPT G2-S is adapted for DAI by reconfiguring the inlet with a temperature-controlled capillary. Droplets are generated by atomization of a solution containing analyte and then sampled through the inlet. If desired, solvent can be removed from the droplets prior to analysis by sending the aerosol through a series of diffusion dryers. Size distributions of the dried aerosols allow the mass flow of analyte into the inlet to be determined. RESULTS: Analyte signal intensities are orders of magnitude higher from droplets containing a protic solvent (water) than an aprotic solvent (acetonitrile). The highest signal intensities for DAI are obtained with inlet temperatures above 500°C, though the optimum temperature is analyte dependent. At elevated temperatures, droplets are thought to undergo rapid solvent evaporation and bursting to produce ions. The lowest signal intensities are generally obtained in the 100-350°C range, where slow solvent evaporation is thought to inhibit ion formation. As the temperature decreases from 100°C down to 25°C, the signal intensity increases significantly. When 3-nitrobenzonitrile, a common matrix for solid-state matrix-assisted ionization (MAI), is added to droplets consisting of 50/50 v/v water and acetonitrile, the matrix enhances ion formation to produce a signal intensity comparable to DAI in 100% water. CONCLUSIONS: The results are consistent with other inlet ionization techniques, suggesting that similar ion formation mechanisms are operative. Optimized ion yields (the combined effects of ionization probability and ion transmission) for DAI are currently in the 10-5 to 10-6 range, which is sufficient for many aerosol applications.

Reaction Kinetics of Organic Aerosol Studied by Droplet Assisted Ionization: Enhanced Reactivity in Droplets Relative to Bulk Solution.

Droplet Assisted Ionization (DAI) is a relatively new method for online analysis of aerosol droplets that enables measurement of the rate of an aerosol reaction. Here, we used DAI to study the reaction of carbonyl functionalities in secondary organic aerosol (SOA) with Girard's T (GT) reagent, a reaction that can potentially be used to enhance the detection of SOA in online measurements. SOA was produced by α-pinene ozonolysis. Particulate matter was collected on a filter, extracted, and mixed with GT reagent in water. While the reaction hardly proceeded at all in bulk solution, products were readily observed with DAI when the solution was atomized to produce micron-size droplets. Varying the droplet transit time between the atomizer and mass spectrometer allowed the reaction rate constant to be determined, which was found to be 4 orders of magnitude faster than what would be expected from bulk solution kinetics. Decreasing the water content of the droplets, either by heating the capillary inlet to the mass spectrometer or by decreasing the relative humidity of the air surrounding the droplets in the transit line from the atomizer to the mass spectrometer, enhanced product formation. The results suggest that reaction enhancement occurs at the droplet surface, which is consistent with previous reports of reaction acceleration during mass spectrometric analysis, where a bulk solution is analyzed with an ionization method that produces aerosol droplets.

Ion Formation from Rapidly Heated Aqueous Droplets by Droplet-Assisted Ionization.

When aqueous droplets travel through a temperature-controlled capillary from atmospheric pressure into a vacuum, they undergo aerodynamic and/or thermal breakup to give charged progeny droplets that subsequently produce gas-phase molecular ions from solutes that were in the original droplets. This phenomenon is the basis of droplet-assisted ionization, a method that was recently developed for online characterization of aerosols by mass spectrometry. The conditions allowing initial droplets to break up into progeny droplets were studied by computational fluid dynamics (CFD) with a droplet evaporation model. The CFD results were then used to interpret experimental measurements of ion current vs capillary wall temperature. For capillary wall temperatures below about 150 °C, the abilities of droplets to undergo either aerodynamic or thermal breakup are strongly temperature dependent. Above this temperature, the mode of initial droplet breakup becomes temperature independent, and the temperature dependence of the ion signal intensity can be explained in relation to ion formation from charged progeny droplets. Activation energies for ion formation fall into two main categories: ∼41 kJ mol-1 for droplets containing predominantly nonionic solutes, which matches the enthalpy of vaporization for water and suggests a charge residue process for ion formation, and ∼24 kJ mol-1 for droplets containing salts, which suggests an ion evaporation process where the ion is ejected from the droplet surface within a cluster of solvent molecules.

Molecular Characterization of Atmospheric Organic Aerosol by Mass Spectrometry.

Atmospheric aerosol, particulate matter suspended in the air we breathe, exerts a strong impact on our health and the environment. Controlling the amount of particulate matter in air is difficult, as there are many ways particles can form by both natural and anthropogenic processes. We gain insight into the sources of particulate matter through chemical composition measurements. A substantial portion of atmospheric aerosol is organic, and this organic matter is exceedingly complex on a molecular scale, encompassing hundreds to thousands of individual compounds that distribute between the gas and particle phases. Because of this complexity, no single analytical technique is sufficient. However, mass spectrometry plays a crucial role owing to its combination of high sensitivity and molecular specificity. This review surveys the various ways mass spectrometry is used to characterize atmospheric organic aerosol at a molecular level, tracing these methods from inception to current practice, with emphasis on current and emerging areas of research. Both offline and online approaches are covered, and molecular measurements with them are discussed in the context of identifying sources and elucidating the underlying chemical mechanisms of particle formation. There is an ongoing need to improve existing techniques and develop new ones if we are to further advance our knowledge of how to mitigate the unwanted health and environmental impacts of particles.

Mechanisms of Atmospherically Relevant Cluster Growth.

Atmospheric aerosols impact global climate either directly by scattering solar radiation or indirectly by serving as cloud condensation nuclei, which influence cloud albedo and precipitation patterns. Our scientific understanding of these impacts is poor relative to that of, for instance, greenhouse gases, in part because it is difficult to predict particle number concentrations. One important pathway by which particles are added to the atmosphere is new particle formation, where gas phase precursors form molecular clusters that subsequently grow to the climatically relevant size range (50-100 nm diameter). It is predicted that up to 50% of atmospheric particles arise from this process, but the key initial chemical processes are poorly resolved. In general, a combination of inorganic and organic molecules are thought to contribute to new particle formation, but the chemical composition of molecular clusters and pathways by which they grow to larger sizes is unclear. Cluster growth is a key component of new particle formation, as it governs whether molecular clusters will become climatically relevant. This Account discusses our recent work to understand the mechanisms underlying new particle growth. Atmospherically relevant molecular clusters containing the likely key contributors to new particle formation (sulfuric acid, ammonia, amines, and water) were investigated experimentally by Fourier transform mass spectrometry as well as computationally by density functional theory. Our laboratory experiments investigated the molecular composition of charged clusters, the molecular pathways by which these clusters may grow, and the kinetics of base incorporation into them. Computational chemistry allowed confirmation and rationalization of the experimental results for charged clusters and extension of these principles to uncharged and hydrated clusters that are difficult to study by mass spectrometry. This combination of approaches enabled us to establish a framework for cluster growth involving sulfuric acid, ammonia, amines, and water. Charged or uncharged, cluster growth occurs primarily through an ammonium (or aminium) bisulfate coordinate. In these clusters, proton transfer is maximized between acids and bases to produce cations (ammonium, aminium) and anions (bisulfate), whereas additional molecules (water and unneutralized sulfuric acid) remain un-ionized. Experimental measurements suggest the growth of positively charged clusters occurs by successive acidification and neutralization steps. The acidification step is nearly barrierless, whereas the neutralization step exhibits a significant activation barrier in the case of ammonia. Bases are also incorporated into these clusters by displacement of one base for another. Base displacement is barrierless on the cluster surface but not within the cluster core. The favorability of amines relative to ammonia in charged clusters is governed by the trade-off between gas phase basicity and binding energetics. Computational studies indicate that water has a relatively small effect on cluster energetics. In short, amines are effective at assisting the formation and initial growth of clusters but become less important as cluster size increases, especially when hydration is considered. More generally, this work shows how experiment and computation can provide important, complementary information to address problems of environmental interest.

Aqueous Reaction of Dicarbonyls with Ammonia as a Potential Source of Organic Nitrogen in Airborne Nanoparticles.

Nitrogen-containing organic species such as imines and imidazoles can be formed by aqueous reactions of carbonyl-containing compounds in the presence of ammonia. In the work described here, these reactions are studied in airborne aqueous nanodroplets containing ammonium sulfate and glyoxal, methylglyoxal, or glycolaldehyde using a combination of online and offline mass spectrometry. N/C ratios attributed to the organic fraction of the particles (N/Corg) produced from glyoxal and methylglyoxal were quantified across a wide relative humidity (RH) range. As the RH was lowered, glyoxal was found to increase N/Corg, attributed to "salting-in" with increasing solute concentration, while methylglyoxal led to a decrease in N/Corg, attributed to "salting-out". Glycolaldehyde was found to evaporate from the droplets rather than react in the aqueous phase and did not form particulate-phase organic matter from aerosol drying under any of the conditions studied. The results are discussed in the context of ambient nanoparticle composition measurements and suggest that aqueous chemistry may significantly impact nanoparticle composition and growth during new particle formation in locations where emissions of water-soluble dicarbonyls are high, such as the eastern United States.

Looking for engineered nanoparticles (ENPs) in wastewater treatment systems: Qualification and quantification aspects.

The current study developed a rationalized method for the quantification and identification of engineered nanoparticles (ENPs) in wastewaters. A review of current literature revealed that overall, presently available methods focused on single ENP mostly and were applicable mainly to samples of low organic loadings or under well-controlled laboratory conditions. In the present research, procedures including dialysis for desalting and low-temperature oxidation for organic removal were used to pretreat samples of high organic loadings, specifically, municipal wastewater and sludge. SEM mapping technique identified the presence of nanoparticles, which was followed by ICP-OES quantification of different engineering nanoparticles in wastewater and sludge samples collected from two major regional municipal wastewater treatment plants. Results showed successful identification and quantification of nano-size titanium and zinc oxides from wastewater treatment plants studied. Concentration profile was mapped out for the wastewater treatment plants (WWTPs) using the method developed in this research. Results also showed an overall 80% and 68% removal of titanium and zinc by primary and secondary sludge particulates, respectively. Mass flux of engineered nanoparticles (ENPs) was also calculated to estimate the daily flow of engineered nanoparticles in the system.

Aerosol Formation from OH Oxidation of the Volatile Cyclic Methyl Siloxane (cVMS) Decamethylcyclopentasiloxane.

Aerosol formation from OH oxidation of decamethylcyclopentasiloxane (D5, C10H30O5Si5), a cyclic volatile methyl siloxane (cVMS) found in consumer products, was studied in a flow-through photo-oxidation chamber with and without the presence of ammonium sulfate seed aerosol. For the unseeded experiments, chemical characterization with high-performance mass spectrometry showed that the molecular composition changed substantially with aerosol mass loading in the 1-12 µg/m3 range. Monomers (5 Si atoms/molecule) and dimers (10 Si atoms/molecule) dominated the mass spectra of aerosols at higher mass loadings, while ring-opened species (neither 5 nor 10 Si atoms/molecule) dominated the mass spectra of aerosols at lower mass loadings. Molecular signal intensity dependencies upon the aerosol volume/surface area ratio suggest that non-volatile ring-opened species are formed in the gas phase and assist particle formation through condensation, while dimers are formed by accretion reactions within the particle phase as the particles grow. These conclusions are supported by experiments in the presence of seed aerosol with a similar siloxane aerosol mass loading but higher volume/surface area ratio, where ring-opened species are much less prevalent than monomers or dimers and the aerosol yield is higher. Because of the importance of accretion chemistry, the aerosol yield from D5 oxidation is likely to be strongly dependent upon the particle size and morphology.

Droplet Assisted Inlet Ionization for Online Analysis of Airborne Nanoparticles.

Airborne nanoparticles play a key role in climate effects as well as impacting human health. Their small mass and complex chemical composition represent significant challenges for analysis. This work introduces a new ionization method, droplet assisted inlet ionization (DAII), where aqueous droplets are produced from airborne nanoparticles. When these droplets enter the mass spectrometer through a heated inlet, rapid vaporization leads to the formation of molecular ions. The method is demonstrated with test aerosols consisting of polypropylene glycol (PPG), angiotensin II, bovine serum albumin, and the "thermometer" compound p-methoxybenzylpyridinium chloride. High-quality spectra were obtained from PPG particles down to 13 nm in diameter and sampled masses in the low pictogram range. These correspond to aerosol number and mass concentrations smaller than 1000 particles/cm3 and 100 ng/m3, respectively, and a time resolution on the order of seconds. Fragmentation of the thermometer ion using DAII was inlet temperature dependent and similar in magnitude to that observed with a conventional ESI source on the same instrument. DAII should be applicable to other types of aerosols including workplace aerosols and those produced for drug delivery by inhalation.

Ion mobility spectrometry-mass spectrometry examination of the structures, stabilities, and extents of hydration of dimethylamine-sulfuric acid clusters.

We applied an atmospheric pressure differential mobility analyzer (DMA) coupled to a time-of-flight mass spectrometer to examine the stability, mass-mobility relationship, and extent of hydration of dimethylamine-sulfuric acid cluster ions, which are of relevance to nucleation in ambient air. Cluster ions were generated by electrospray ionization and were of the form: [H((CH3)2NH)x(H2SO4)y](+) and [(HSO4)((CH3)2NH)x(H2SO4)y](-), where 4 ≤ x ≤ 8, and 5 ≤ y ≤ 12. Under dry conditions, we find that positively charged cluster ions dissociated via loss of both multiple dimethylamine and sulfuric acid molecules after mobility analysis but prior to mass analysis, and few parent ions were detected in the mass spectrometer. Dissociation also occurred for negative ions, but to a lesser extent than for positive ions for the same mass spectrometer inlet conditions. Under humidified conditions (relative humidities up to 30% in the DMA), positively charged cluster ion dissociation in the mass spectrometer inlet was mitigated and occurred primarily by H2SO4 loss from ions containing excess acid molecules. DMA measurements were used to infer collision cross sections (CCSs) for all identifiable cluster ions. Stokes-Millikan equation and diffuse/inelastic gas molecule scattering predicted CCSs overestimate measured CCSs by more than 15%, while elastic-specular collision model predictions are in good agreement with measurements. Finally, cluster ion hydration was examined by monitoring changes in CCSs with increasing relative humidity. All examined cluster ions showed a modest amount of water molecule adsorption, with percentage increases in CCS smaller than 10%. The extent of hydration correlates directly with cluster ion acidity for positive ions.

High Sensitivity Analysis of Nanoliter Volumes of Volatile and Nonvolatile Compounds using Matrix Assisted Ionization (MAI) Mass Spectrometry.

First results are reported using a simple, fast, and reproducible matrix-assisted ionization (MAI) sample introduction method that provides substantial improvements relative to previously published MAI methods. The sensitivity of the new MAI methods, which requires no laser, high voltage, or nebulizing gas, is comparable to those reported for MALDI-TOF and n-ESI. High resolution full acquisition mass spectra having low chemical background are acquired from low nanoliters of solution using only a few femtomoles of analyte. The limit-of-detection for angiotensin II is less than 50 amol on an Orbitrap Exactive mass spectrometer. Analysis of peptides, including a bovine serum albumin digest, and drugs, including drugs in urine without a purification step, are reported using a 1 µL zero dead volume syringe in which only the analyte solution wetting the walls of the syringe needle is used in the analysis.

Characterization of Highly Oxidized Molecules in Fresh and Aged Biogenic Secondary Organic Aerosol.

In this work, highly oxidized multifunctional molecules (HOMs) in fresh and aged secondary organic aerosol (SOA) derived from biogenic precursors are characterized with high-resolution mass spectrometry. Fresh SOA was generated by mixing ozone with a biogenic precursor (ß-pinene, limonene, α-pinene) in a flow tube reactor. Aging was performed by passing the fresh SOA through a photochemical reactor where it reacted with hydroxyl radicals. Although these aerosols were as a whole not highly oxidized, molecular analysis identified a significant number of HOMs embedded within it. HOMs in fresh SOA consisted mostly of monomers and dimers, which is consistent with condensation of extremely low-volatility organic compounds (ELVOCs) that have been detected in the gas phase in previous studies and linked to SOA particle formation. Aging caused an increase in the average number of carbon atoms per molecule of the HOMs, which is consistent with particle phase oxidation of (less oxidized) oligomers already existing in fresh SOA. HOMs having different combinations of oxygen-to-carbon ratio, hydrogen-to-carbon ratio and average carbon oxidation state are discussed and compared to low volatility oxygenated organic aerosol (LVOOA), which has been identified in ambient aerosol based on average elemental composition but not fully understood at a molecular level. For the biogenic precursors and experimental conditions studied, HOMs in fresh biogenic SOA have molecular formulas more closely resembling LVOOA than HOMs in aged SOA, suggesting that aging of biogenic SOA is not a good surrogate for ambient LVOOA.

Molecular Characterization of Secondary Aerosol from Oxidation of Cyclic Methylsiloxanes.

Cyclic volatile methylsiloxanes (cVMS) have been identified as important gas-phase atmospheric contaminants, but knowledge of the molecular composition of secondary aerosol derived from cVMS oxidation is incomplete. Here, the chemical composition of secondary aerosol produced from the OH-initiated oxidation of decamethylcyclopentasiloxane (D5, C10H30O5Si5) is characterized by high performance mass spectrometry. ESI-MS reveals a large number of monomeric (300 < m/z < 470) and dimeric (700 < m/z < 870) oxidation products. With the aid of high resolution and MS/MS, it is shown that oxidation leads mainly to the substitution of a CH3 group by OH or CH2OH, and that a single molecule can undergo many CH3 group substitutions. Dimers also exhibit OH and CH2OH substitutions and can be linked by O, CH2, and CH2CH2 groups. GC-MS confirms the ESI-MS results. Oxidation of D4 (C8H24O4Si4) exhibits similar substitutions and oligomerizations to D5, though the degree of oxidation is greater under the same conditions and there is direct evidence for the formation of peroxy groups (CH2OOH) in addition to OH and CH2OH.

Growth of Ammonium Bisulfate Clusters by Adsorption of Oxygenated Organic Molecules.

Quantum chemical calculations were employed to model the interactions of the [(NH4(+))4(HSO4(-))4] ammonium bisulfate cluster with one or more molecular products of monoterpene oxidation. A strong interaction was found between the bisulfate ion of this cluster and a carboxylic acid, aldehyde, or ketone functionality of the organic molecule. Free energies of adsorption for carboxylic acids were in the -70 to -73 kJ/mol range, while those for aldehydes and ketones were in the -46 to -50 kJ/mol range. These values suggest that a small ambient [(NH4(+))4(SO4(-))4]cluster is able to adsorb an oxygenated organic molecule. While adsorption of the first molecule is highly favorable, adsorption of subsequent molecules is less so, suggesting that sustained uptake of organic molecules does not occur, and thus is not a pathway for continuing growth of the cluster. This result is consistent with ambient measurements showing that particles below ∼1 nm grow slowly, while those above 1 nm grow at an increasing rate presumably due to a lower surface energy barrier enabling the uptake of organic molecules. This work provides insight into the molecular level interactions which affect sustained cluster growth by uptake of organic molecules.

Activation barriers in the growth of molecular clusters derived from sulfuric acid and ammonia.

Unraveling the chemical mechanism of atmospheric new particle formation (NPF) has important implications for the broader understanding of the role of aerosols in global climate. We present computational results of the transition states and activation barriers for growth of atmospherically relevant positively charged molecular clusters containing ammonia and sulfuric acid. Sulfuric acid uptake onto the investigated clusters has a small activation free-energy barrier, consistent with nearly collision-limited uptake. Ammonia uptake requires significant reorganization of ions in the preexisting cluster, which yields an activation barrier on the order of 29-53 kJ/mol for the investigated clusters. For this reason, ammonia uptake onto positively charged clusters may be too slow for cluster growth to proceed by the currently accepted mechanism of stepwise addition of sulfuric acid followed by ammonia. The results presented here may have important implications for modeling atmospheric NPF and nanoparticle growth, which typically does not consider an activation barrier along the growth pathway and usually assumes collision-limited molecular uptake.

Silicon is a frequent component of atmospheric nanoparticles.

Nanoparticles are the largest fraction of aerosol loading by number. Knowledge of the chemical components present in nanoparticulate matter is needed to understand nanoparticle health and climatic impacts. In this work, we present field measurements using the Nano Aerosol Mass Spectrometer (NAMS), which provides quantitative elemental composition of nanoparticles around 20 nm diameter. NAMS measurements indicate that the element silicon (Si) is a frequent component of nanoparticles. Nanoparticulate Si is most abundant in locations heavily impacted by anthropogenic activities. Wind direction correlations suggest the sources of Si are diffuse, and diurnal trends suggest nanoparticulate Si may result from photochemical processing of gas phase Si-containing compounds, such as cyclic siloxanes. Atmospheric modeling of oxidized cyclic siloxanes is consistent with a diffuse photochemical source of aerosol Si. More broadly, these observations indicate a previously overlooked anthropogenic source of nanoaerosol mass. Further investigation is needed to fully resolve its atmospheric role.

Formation and growth of molecular clusters containing sulfuric acid, water, ammonia, and dimethylamine.

The structures and thermochemistry of molecular clusters containing sulfuric acid, water, ammonia, and/or dimethylamine ((CH3)2NH or DMA) are explored using a combination of Monte Carlo configuration sampling, semiempirical calculations, and density functional theory (DFT) calculations. Clusters are of the general form [(BH(+))n(HSO4(-))n(H2O)y], where B = NH3 or DMA, 2 ≤ n ≤ 8, and 0 ≤ y ≤ 10. Cluster formulas are written based on the computed structures, which uniformly show proton transfer from each sulfuric acid molecule to a base molecule while the water molecules remain un-ionized. Cluster formation is energetically favorable, owing to strong electrostatic attraction among the ions. Water has a minor effect on the energetics of cluster formation, lowering the free energy of formation by ∼ 10% depending on the cluster size and number of water molecules. Cluster growth (addition of one base molecule and one sulfuric acid molecule to a pre-existing cluster) and base substitution (substituting DMA for ammonia) are also energetically favorable processes for both anhydrous and hydrated clusters. However, the effect of water is different for different bases. Hydrated ammonium bisulfate clusters have a more favorable free energy for growth (i.e., incrementing n with fixed y) than anhydrous clusters, while the reverse is observed for dimethylammonium bisulfate clusters, where the free energy for growth is more favorable for anhydrous clusters. The substitution of DMA for ammonia in bisulfate clusters is favorable but exhibits a complex water dependence. Base substitution in smaller bisulfate clusters is enhanced by the presence of water, while base substitution in larger bisulfate clusters is less favorable for hydrated clusters than that for anhydrous clusters. While DMA substitution can stabilize small clusters containing one or a few sulfuric acid molecules, the free energy advantage of forming amine clusters relative to ammonia clusters becomes less pronounced at larger sizes, especially when the effect of water is considered.