UC Berkeley

This dissertation presents a series of research projects designed and carried out to elucidate the physical chemistry and assess the atmospheric relevance of (1) carbonyl oxide radicals (i.e., Criegee intermediates) produced in alkene ozonolysis and (2) nitrous oxide (N2O) produced in lightning-induced corona discharges. The results provide UV absorption spectra and reaction rate coefficients for Criegee intermediates that will help constrain the formation and loss pathways of aerosol nucleation precursors such as H2SO4 and oxidized volatile organic compounds, and the isotopic signature of N2O formed in lightning that can help distinguish various N2O sources in atmospheric measurements.

Criegee intermediates are byproducts of the reaction of alkenes with ozone. Bimolecular reactions of Criegee intermediates can lead to the production of low-volatility organic compounds and acids in the atmosphere, which in turn play a role in determining the concentration, size, and optical properties of aerosols. Recently, a novel method for producing measurable quantities of stabilized Criegee intermediates in the laboratory paved the way for the development of new experimental techniques to study their chemical properties and predict their importance in the atmosphere. For this dissertation, a unique apparatus combining time-resolved UV absorption in a flow cell with laser depletion in a molecular beam was adapted to obtain the absolute absorption spectrum of CH3CHOO with high resolution and accuracy relative to previous spectral measurements by other groups. The resulting absorption cross sections imply a photolysis lifetime of about seven seconds in the atmosphere, long enough for CH3CHOO to participate in unimolecular and bimolecular reactions. The broad absorption band with weak structure in the long-wavelength region of the spectrum represents a “spectral fingerprint” for identifying CH3CHOO in future studies, and the cross sections provide valuable benchmarks for theory to characterize electronically excited states of CH3CHOO.

The fast reaction of CH2OO with water dimer is thought to dominate CH2OO removal in the atmosphere. However, reaction rates can vary considerably under different conditions of temperature, humidity, and pressure. A temperature-controlled flow cell was designed to measure the transient absorption of CH2OO and obtain rate coefficients for its reaction with water dimer from 283 to 324 K. The rate of the reaction of CH2OO with water dimer was found to exhibit a strong negative temperature dependence, pointing to the participation of a hydrogen-bonded pre-reactive complex between CH2OO and two water molecules. Due to the strong temperature dependence, and shifting competition between water dimer and water monomer (which has a positive temperature dependence), the effective loss rate of CH2OO by reaction with water vapor is highly sensitive to atmospheric conditions. The role played by the stable pre-reactive complex suggests that similar complexes could form between water dimer and other larger Criegee intermediates, and that the stability and relative energy of these complexes control the reaction rate with water and its temperature dependence.

Effective loss rates of Criegee intermediates due to bimolecular reactions in the atmosphere are limited by their rates of unimolecular decomposition. The rates of decomposition depend strongly on the molecular geometry, which affects the accessible isomerization pathways and dissociation products. (CH3)2COO is the main product of tetramethylethylene ozonolysis, and has been found to react slowly with water dimer and rapidly with SO2. While CH2OO decomposes slowly via isomerization to dioxirane, (CH3)2COO may decompose faster via intramolecular hydrogen transfer to form vinyl hydroperoxide. Fast (CH3)2COO decomposition could affect the significance of the Criegee intermediate H2SO4 source, as well as the non-photolytic production of OH radicals. In this dissertation, measurements of the transient absorption of (CH3)2COO to obtain thermal decomposition rate coefficients from 283 to 323 K by extrapolating the observed loss rate to zero concentration are reported. The rate of unimolecular decomposition is ~400 s-1 at 298 K and varies by nearly an order of magnitude within the studied temperature range. The effective loss rate of (CH3)2COO in the atmosphere due to thermal decomposition is thus competitive with its loss due to reaction with water vapor and with SO2, suggesting that the unimolecular decomposition pathway is a significant sink for (CH3)2COO and possibly other di-substituted Criegee intermediates, and should be included in models of Criegee chemistry in the atmosphere as well as in kinetic models of tetramethylethylene ozonolysis.

N2O is the third most important greenhouse gas after CO2 and methane, and is mainly emitted to the atmosphere as a byproduct of microbial activity in soils. The expanding use of nitrogen-containing fertilizers in agriculture has led to an increase in N2O atmospheric concentrations since preindustrial times. Isotopic measurements are a valuable tool to distinguish the influence of different sources of N2O, but the isotopic composition of N2O formed from corona discharge in lightning has not previously been measured. Here, a corona discharge cell apparatus was used to generate a corona discharge in flowing or static zero air, and the N2O formed at discharge cell pressures from ~0.1 to 10 Torr and discharge voltages from 0.25 to 5 kV was collected and measured with isotope ratio mass spectrometry to determine its isotopic composition. The results show enrichments in 15N of N2O up to 32‰ relative to the reactant N2, and even larger enrichments in 15N of up to 77‰ at the central nitrogen atom. Large depletions in 18O as large as -71‰ relative to reactant O2 were also measured. The isotopic composition measured here may help to elucidate the chemical mechanisms leading to N2O formation and destruction in a corona discharge. Furthermore, the isotope-isotope relationships of the N2O produced in the corona discharge experiments are distinct from those of N2O from other sources, implying that isotopic measurements can be used to determine whether local variations in the atmospheric concentration of N2O – e.g., the enhanced N2O levels recently measured in the upper tropical and subtropical troposphere – are due to lightning activity, soil emissions, or biomass burning.