Understanding the Mechanisms of Salt Formation in New Particle Formation and Growth

Atmospheric nanoparticle formation and growth processes are major sources of uncertainty in our understanding of global climate. However, nanoparticle composition is notoriously difficult to measure below 30~nm due to their incredibly low mass, and so a full understanding of the compounds that contribute to nanoparticle growth still remains elusive. In addition, nanoparticle physical and chemical properties are continuously changing at different sizes. To date, nanoparticles have been known to form from either the condensation of extremely low volatility organic compounds or the reactions of acids and bases in the atmosphere. For the latter, sulfuric acid has been observed to be the predictor of new particle formation events and, recently, amines and ammonia have been shown to stabilize sulfuric acid-containing clusters and contribute to nanoparticle formation and growth. However, size-resolved composition of nanoparticles resulting from these acid-base pairs, as well as the extent to which other atmospherically relevant acids and bases contribute to new particle formation, are both still unknown. This dissertation uses both modeling and experimental methods to understand different acid-base systems’ nanoparticle formation and growth processes. In Chapter 2, we generated particles made from sulfuric acid with either dimethylamine and ammonia under dry and humid conditions to measure their size-resolved composition using Thermal Desorption Chemical Ionization Mass Spectrometry. Sulfuric acid-ammonia experiments were conducted with 100x higher concentrations of ammonia compared to sulfuric acid to mimic atmospheric conditions, while sulfuric acid-dimethylamine experiments were conducted with concentrations on the same order of magnitude. In all cases, the particles that were formed deviated from stoichiometric neutrality. In both humid and dry cases, sulfuric acid and ammonia formed particles that contained more acid than base, despite the starting concentration differences. In humid conditions, sulfuric acid and dimethylamine formed particles that also had a higher concentration of sulfuric acid compared to dimethylamine for particles smaller than 12 nm. However, under dry conditions, these particles seemed to have more dimethylamine than sulfuric acid. Thermodynamic models using different pK\textsubscript{a} values to describe the acid and base behavior in these systems are discussed. Next, in Chapter 3, we probed the composition of particles formed from nitric acid and dimethylamine using the same methods. In this case, under all conditions, nitric acid and dimethylamine formed particles in a 1$:$1 acid:base ratio at all sizes measured (9--30 nm). Here we also applied quantum chemical methods to calculate cluster stabilities for clusters containing up to 4 acid and 4 base molecules to discern if the initial growth pathways were reflected in composition measurements. Results show that cluster simulations predicted the behavior of nitric acid and dimethylamine particles, because both nitric acid and dimethylamine are too volatile to remain in the particle phase without salt formation. In order to better understand the very first steps of new particle formation, we used the same computational theory to model and calculate the stabilities of acid-base heterodimers (1 acid and 1 base clusters) formed from 3 different acid and 9 different base molecules in Chapter 4. We compared these values to aqueous-phase acidity, gas-phase acidity, base vapor pressure, dipole moment, and polarizability to find out which was the strongest predictor of heterodimer stability, the first step in forming a cluster. Then, we compared heterodimer stability of just the sulfuric acid salts with calculated new particle formation rates. From this, we developed a model that parametrizes heterodimer stability to predict new particle formation rates, which were found to be in good agreement with experimental values for the sulfuric acid-ammonia system