Clouds influence climate and cool the planet by reflecting incoming sunlight away from Earth’s surface. The extent and brightness of these clouds depend on the composition and concentration of atmospheric particles, the starting point for cloud droplet formation. The majority of atmospheric particles originate from a process known as nucleation, whereby low volatility, gaseous precursors react to form stable clusters (~1 nm diameter). Sulfuric acid (H2SO4 or SA) is essential for atmospheric nucleation and in continental regions originates primarily from anthropogenic activities. However, atmospheric nucleation remains difficult to study due to the challenges of measuring incipient particles <1 nm (cluster containing ~<4 SA molecules plus water and other stabilizing compounds) and at atmospherically relevant concentrations of 1 part per quadrillion. The purpose of my thesis work is (1) to better understand the technique of chemical ionization mass spectrometry (CIMS) for measuring SA and its clusters and (2) to measure and model how various atmospherically relevant basic gases, such as ammonia and amines, affect SA nucleation. The main instrument, the Cluster CIMS, uses nitrate (NO3-) to chemically ionize SA vapor and clusters at atmospheric pressure. My measurements and subsequent modeling of the chemical ionization processes show that nitrate is unable to ionize chemically neutral SA clusters, i.e. clusters that contain near equal numbers of acid and base molecules. In contrast, the acetate ion (CH3CO2¬-) is capable of ionizing more cluster types, thereby providing more accurate measurements of cluster concentrations. These findings imply that a large fraction of clusters may have gone unobserved in ambient measurements due to inefficient nitrate chemical ionization. To ensure that the Cluster CIMS accurately measures concentrations of larger SA clusters using acetate chemical ionization, I developed a method for comparing cluster concentrations measured by mass spectrometry with particle number concentrations measured by mobility classification/vapor condensation particle counter in the size range around 1 nm where they overlap. I used computational chemistry, particle dynamics, and the diffusing transfer function of a mobility classifier to reconcile these two very different measurement techniques. Comparisons show good agreement between the instruments for ~1 nm clusters produced in a sulfuric acid/dimethylamine (DMA) environment. The Cluster CIMS and a diethylene glycol scanning mobility particle sizer were also used to study how ammonia and amines react with SA vapor to form clusters and particles in a very clean flow reactor. Results show that DMA and diamines produce higher cluster and particle concentrations than methylamine and ammonia, which implies that they form less volatile clusters leading to higher nucleation rates. Diamines, including compounds such as cadaverine and putrescine, are a previously unstudied class of atmospherically relevant compounds that originate from industrial and natural sources. I observed that diamines+SA can produce 10 times more particles than DMA. Field measurements of diamines in Lamont, OK from spring, 2013 show gas phase diamine concentrations are equal to or greater than DMA concentrations. Thus, diamines are potent nucleating agents that likely contribute significantly to atmospheric nucleation. Overall, my thesis work improved nucleation instrumentation and increased knowledge on the chemical mechanisms behind atmospheric nucleation.
University of Minnesota Ph.D. dissertation. June 2015. Major: Mechanical Engineering. Advisor: Peter McMurry. 1 computer file (PDF); xi, 130 pages.
Chemical Ionization Mass Spectrometry Study of Sulfuric Acid and Amine Chemical Nucleation Processes Pertinent to the Atmosphere.
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