New Particle Formation from Gas-phase Sulfuric Acid, Amine, and Organic Acid Reactions in flow reactors

New particle formation (NPF) from precursor gases is a significant source of cloud condensation nuclei (CCN) in the atmosphere and thus affects the Earth’s radiative balance. NPF comprises nucleation of stable clusters (~ 1 nm in diameter) from gaseous compounds, such as sulfuric acid, water, ammonia, amines, and oxidized organics, followed by subsequent growth. Despite advancements in instrumentation capable of measuring aerosol particles down to molecular clusters composed of two molecules, the mechanisms behind NPF remain understudied due to the sheer diversity of potential particle formation reactions that may occur in the atmosphere. As part of this thesis, I have developed experimental techniques to examine how sulfuric acid interacts with various amines and organic acids in the atmosphere to form particles that can affect the Earth’s radiative budget.

A flow reactor has been designed to generate sulfuric acid-amine nucleated clusters in the presence of organics in order to characterize the nucleation kinetics for the formation of 1 nm particles. This flow reactor enables the study of nucleation reactions which are difficult to examine in the field due to the chemical complexity of the atmosphere. The chemical composition and concentrations of the freshly nucleated clusters from the flow reactor experiments were analyzed using a custombuilt chemical ionization transverse inlet connected to an atmospheric pressure interface long time-of-flight mass spectrometer (Tofwerk AG), called the Pittsburgh Cluster CIMS (PCC). The transverse chemical ionization inlet was designed as part of this thesis and allows the detection of neutral clusters formed in the flow reactor. The PCC uses chemical ionization, a soft ionization method, with nitrate, acetate, or hydronium reagent ions to minimize cluster fragmentation during measurements. This flow reactor was connected in-line with the PCC to study three key aspects of new particle formation: (1) neutral sulfuric acid-alkanolamine nucleation, (2) ion-induced nucleation, and (3) organic acid nucleation and growth.

Sulfuric acid-amine nucleation reactions have been shown to contribute to NPF events in various parts of the world (Chen et al., 2012; Zhao et al., 2011). Alkanolamines, highly reactive compounds with both amino and hydroxy functional groups, have not been studied for their impact on particle formation in the atmosphere. These alkanolamines, such as monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA), are widely used for CO₂ capture in various industrial processes. Emissions of these alkanolamines in industrial cities can severely impact regional air quality and health through particle formation with sulfuric acid. My thesis examined the role of alkanolamines on sulfuric acid nucleation. Experimental results indicate that MEA, DEA, and TEA at the part per trillion by volume (pptv) concentrations can enhance sulfuric acid dimer formation rates but to a lesser extent than dimethylamine (DMA) which has been shown to greatly enhance sulfuric acid nucleation in the atmosphere. Therefore, these amines will likely contribute to sulfuric acid nucleation in industrial areas where they are widely used. Atmospheric ions from galactic cosmic rays are also a key reactant for particle formation, especially in the upper troposphere where ion concentrations can go up to 5000 cm^-3 (Hirsikko et al., 2011). These ions can react with gaseous compounds to form stable clusters in a process referred to as ion-induced nucleation (IIN). The rate at which ions form clusters depends on the ion-molecule rate constant. However, thisrate constant varies based on the ion composition, which is often unknown in the atmosphere. The difference between the reaction rate constants of atmospherically abundant ions, specifically nitrate and acetate, with sulfuric acid was measured. The rate constant of acetate with sulfuric acid was observed to be ~ 2.4 times greater than that of nitrate with sulfuric acid. Knowledge of this ion-molecule rate constant for acetate with sulfuric acid will help evaluate the contribution of acetate ions to atmospheric IIN. 

In addition, carboxylic acids such as malonic acid, which is abundant in the atmosphere, have been measured in ultrafine particles. However, little is known about how these compounds contribute to the formation and growth of 1 nm particles in the atmosphere. The role of malonic acid on sulfuric acid-DMA nucleation was examined through flow reactor experiments with a versatile water condensation particle counter (vWCPC) and the PCC. Malonic acid was shown to have no impact on sulfuric acid-DMA nucleation and particle growth for clusters < 2 nm in diameter. 

My thesis examined nucleation reactions that occur in polluted and pristine environments, which could lead to high particle number concentrations. Through combined theoretical and experimental techniques, amines such as alkanolamines have been shown to enhance the formation of sulfuric acid clusters, which could lead to particle formation events in industrial cities. As many countries around the world work towards reducing their carbon footprint, alkanolamine solvent-based technologies are likely to be increasingly used in industries to reduce CO₂ emissions. Therefore, the results presented in my thesis are essential in understanding the atmospheric fate of these alkanolamines. In addition, sulfuric acid-DMA-malonic acid experiments showed that malonic acid does not contribute to the formation and growth of 1 nm particles. The results presented in my thesis will help provide more knowledge on the chemical processes responsible for particle nucleation and growth in the atmosphere. Ultimately, this thesis will help improve global climate models predicting how aerosols affect the earth’s radiative budget. Also, understanding the fate of air pollutants will aid regulatory agencies and policymakers in drafting comprehensive climate legislation to address the pressing challenges of climate change.