Chemical mechanisms governing atmospheric new particle formation

Johnston, MurrayThe goal of this dissertation is to understand the chemistry that governs new particle formation, a ubiquitous and important atmospheric process. New particle formation occurs when gas phase precursors condense to create small molecular clusters on the order of 1 nm diameter. Those clusters must then grow rapidly and ultimately may serve as the seeds for cloud droplets. However, modelers have substantial difficulty predicting the frequency and efficiency of new particle formation. This predictive difficulty is an important contributor to the uncertainty in aerosol effects on global climate and therefore also contributes to the large uncertainty in anthropogenic effects on climate. To reduce these uncertainties, a more precise understanding of how particles nucleate and grow in the atmospheric is required. In this dissertation, mass spectrometry is used to determine the chemical processes involved in new particle formation. Gas phase species such as sulfuric acid, ammonia, amines, and organic matter are contributors but exactly how and how much each contributes to the growth of nanoparticles is not well understood. Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and Nano Aerosol Mass Spectrometry (NAMS) are used to study the chemical composition and reactivity of clusters < 3 nm diameter and nanoparticles 10-20 nm diameter, respectively. The FTICR-MS studies are laboratory based, whereas the NAMS studies are field based. Measurements of cluster composition and reactivity using FTICR-MS permit prediction of the composition of ambient molecular clusters. For ambient molecular clusters to become relevant to climate by serving as cloud condensation nuclei, they must grow rapidly. NAMS measurements at 20 nm diameter permit determination of nanoparticle growth pathways. This dissertation shows that sulfuric acid adds to both clusters and nanoparticles in a collision limited manner. On the other hand, ammonia uptake in both size regimes may not necessarily occur at a collision limited rate. Finally, additional nitrogen containing compounds are important to new particle formation in both size regimes, but the molecular form of these species is different for each size regime. Amines are important contributors to the growth of molecular clusters, whereas other organic nitrogen species are important to the growth of nanoparticles.University of Delaware, Department of Chemistry and BiochemistryPh.D