Atmospheric Nucleation Potential Model for Chemically Diverse Systems

Atmospheric aerosol particles play an important role in Earth's weather and climate. Aerosol particles influence Earth's weather patterns and radiation balance by acting as "seed particles" or cloud condensation nuclei (CCN) for cloud droplet formation. A significant fraction of CCN originate from gas phase compounds reacting to form aerosol particles in the atmosphere, in a process known as nucleation. These particles are stable at approximately 1 nm in diameter as at this size, the particle has a higher probability of continuing to grow and coagulate with other particles, than it does to evaporate back to the gas-phase. Sulfuric acid is major contributor to the nucleation process in the lower troposphere and has been shown to readily react with ammonia, amines, oxidized organics, and numerous other compounds to form stable particles. While having accurate nucleation rates at a high spatial and temporal scale would greatly reduce uncertainty in global climate models, there is currently only sparse measurements of atmospheric nucleation rates. This is partially due to instrumentation limitations, as well as the use of only very simplified nucleation reactions in global climate models. For this thesis, I have developed a new nucleation model that can accurately predict particle nucleation rates by estimating the concentration of precursor gases. This model could be used in conjunction with a new measurement technique that is much more portable and complements measurements taken from mass spectrometers. Additionally, this thesis explores the formation of molecular clusters with increasingly complex mixtures of atmospherically relevant compounds, including methanesulfonic acid. 

Nucleation experiments were conducted using a glass flow reactor that was continuously purged with nitrogen, water vapor, and sulfuric acid vapor. The conditions of the reactor are kept clean with continuous flow of nitrogen, water vapor and sulfuric acid vapor. These flow reactors have been used previously to measure and characterize the first steps of nucleation using chemical ionization mass spectrometers, and condensation particle counters. This work includes the use of a custom-built chemical ionization inlet with an atmospheric pressure interface time-of-flight mass spectrometer, known as the Pittsburgh Cluster CIMS (PCC) to measure gas-phase as well as small molecular clusters. Additionally, another custom-built chemical ionization mass spectrometer, known as the Minnesota Cluster CIMS (MCC) was also used for measurements of gas phase and molecular clusters. Both the MCC and the PCC use soft ionization techniques with nitrate, acetate, or hydronium to charge molecular clusters as well as gas-phase compounds. This chemical ionization technique is used to minimize the cluster fragmentation during the ionization process. In addition to the mass spectrometers, a versatile water CPC (vwCPC) was used to measure particles that are greater than 1nm in size. These measurements give a greater insight as to what is happening at size ranges that are too large for the mass spectrometer to measure. Combined, the Cluster CIMS, the vwCPC, and the sulfuric acid flow reactor allowed for the in-depth study of sulfuric acid nucleation in a pristine environment. 

Modeling sulfuric acid nucleation was another important aspect of this thesis. The nucleation potential model (NPM) was built as a part of this thesis with the goal of capturing the process of sulfuric acid nucleation. NPM is a simplified acid-base model, that assumes sulfuric acid react with amines in a 1:1 ratio within the molecular cluster. NPM has the benefits of still modeling sulfuric acid nucleation reactions close to what computational chemistry models predict, but without the computational intensity that is required for these models. Results for the NPM showed that the model can capture sulfuric acid nucleation and can estimate the concentration of stabilizing molecules in each sample. In addition, NPM could also capture the enhancement effects of mixtures of stabilizing molecules, which has previously been too computationally intensive to model. 

Sulfuric acid-amine nucleation has been studied intensively over the past few decades. It has been shown that sulfuric acid readily reacts with many atmospherically relevant amines, amides, oxidized organics, ions, and water. However, nucleation events have only been measured in scattered locations across the world as they can be difficult to measure due to the instrumentation required. Because of the difficulty in measuring sulfuric acid nucleation rates, there are very few measurements and limited understanding of nucleation events in the open ocean. Dimethyl sulfide is a highly prevalent molecule in the marine atmosphere and is largely emitted from phytoplankton. Dimethyl sulfide oxides to form sulfuric acid as well as methanesulfonic acid. Methanesulfonic acid has recently been shown to also have the potential to react with amines to form particles. However, it is unclear how methanesulfonic acid impacts sulfuric acid-amine nucleation. This thesis explored the sulfuric acid-methanesulfonic acid-amine nucleation pathways by looking at the first steps of particle formation with a cluster CIMS. In addition, particle concentrations were measured using a vwCPC to determine MSA's effect on nucleation rates for SA-amine nucleation. Results showed that MSA is involved with the first steps of nucleation, and likely is impacting particle nucleation rates in a marine atmosphere. In addition, MSA is enhancing particle formation for some amines with sulfuric acid, while suppressing particle formation for others. The addition of MSA to global climate models is likely necessary in a marine atmosphere to accurately capture particle formation rates in those regions.