UC Irvine

Accurately representing the sources of important precursor gases for new particle formation and their chemical reactions in the atmosphere are critical for developing a robust understanding of the impacts of ultrafine particles on climate and health. Many types of compounds contribute to these nucleation and growth processes, including inorganic acids, bases and low-volatility organic compounds. Both formation of these compounds in the atmosphere and the processes by which they are converted to ultrafine particles depend greatly on their composition. Therefore, in this dissertation, these sources and processes were investigated using measurements of chemical composition, specifically chemical ionization mass spectrometry (CIMS). CIMS was used to measure important precursor gases for ultrafine particle formation and growth in controlled laboratory chamber studies for three separate chemical systems.

In Chapter 2, the chemical composition of gasses emitted from vehicle brake wear were measured using both nitrate reagent ion (nitrate-CIMS) and iodide reagent ion (iodide-CIMS). Gases, coarse-mode particles and ultrafine particles were generated from a home-built brake dynamometer system, which simulated moderate and heavy braking conditions. The brake pad formulation (ceramic vs semi-metallic), degree of wear (new, gently worn, heavily worn), and temperature of the brake rotor influenced measured gaseous emissions. A critical temperature range was observed for ultrafine particle formation. These critical temperatures were found to be lower for ceramic brake pads (100-140 C) than semi-metallic brake pads (140-200 C), and increase as the brake pads became worn. High concentrations of sulfuric acid and sulfur trioxide were generated during heavy braking conditions at high temperatures, and were more abundantly emitted by the semi-metallic pads than the ceramic. Moderate braking conditions produced NOx, CO, inorganic nitrogen compounds HONO, N2O5, oxidized organics and organic acids. Emissions from different brake pad formulations and degrees of wear were highly variable, emphasizing the importance of representing brake-wear emissions over the lifetime of a brake pad for future control strategies.

In Chapter 3, the effects of monoterpene structure on nitrate radical-induced oxidation mechanism of four monoterpenes (a-pinene, b-pinene, d-carene, a-thujene) was investigated by conducting laboratory chamber experiments. Oxidized reaction products were measured with nitrate-CIMS. The major gas-phase species produced in each system were distinctly different, showing the effect of monoterpene structure on the oxidation mechanism and further elucidated the contributions of these species to particle formation and growth. Measured effective O:C ratio of reaction products were anti-correlated to new particle formation intensity and number concentration for each system; however, monomer:dimer ratio of products had a small positive trend. Gas phase yields of oxidation products correlated with particle number concentrations for each monoterpene system, with the exception of a-thujene, which produced a considerable amount of low volatility products but no particles. The lack of measurable new particle formation in spite of the presence of these dimers indicates a more complex relationship between highly oxidized organic compounds and new particle formation.

In Chapter 4, the formation of low-volatility gasses and new particle formation from ozonolysis of a-thujene was observed in laboratory chamber studies. This system was found to produce lower yields of particles at all mass loadings compared to s-pinene ozonolysis systems. The unique structure of a-thujene led to the competitive production of semi-volatile fragment compounds that reduced the overall volatility of the oxidized products. Theoretical growth rates (on the order of nm per min) calculated from volatility-parameterized organics underpredicted measured growth rates. However, the simple condensational growth model that was used did not account for the irreversible partitioning of semi-volatile organics, which were abundantly produced by this system.