Kevin R. Wilson

Carbon oxidation state as a metric for describing the chemistry of atmospheric organic aerosol

A detailed understanding of the sources, transformations and fates of organic species in the environment is crucial because of the central roles that they play in human health, biogeochemical cycles and the Earths climate. However, such an understanding is hindered by the immense chemical complexity of environmental mixtures of organics; for example, atmospheric organic aerosol consists of at least thousands of individual compounds, all of which likely evolve chemically over their atmospheric lifetimes. Here, we demonstrate the utility of describing organic aerosol (and other complex organic mixtures) in terms of average carbon oxidation state, a quantity that always increases with oxidation, and is readily measured using state-of-the-art analytical techniques. Field and laboratory measurements of the average carbon oxidation state, using several such techniques, constrain the chemical properties of the organics and demonstrate that the formation and evolution of organic aerosol involves simultaneous changes to both carbon oxidation state and carbon number.

Elucidating secondary organic aerosol from diesel and gasoline vehicles through detailed characterization of organic carbon emissions

Emissions from gasoline and diesel vehicles are predominant anthropogenic sources of reactive gas-phase organic carbon and key precursors to secondary organic aerosol (SOA) in urban areas. Their relative importance for aerosol formation is a controversial issue with implications for air quality control policy and public health. We characterize the chemical composition, mass distribution, and organic aerosol formation potential of emissions from gasoline and diesel vehicles, and find diesel exhaust is seven times more efficient at forming aerosol than gasoline exhaust. However, both sources are important for air quality; depending on a region’s fuel use, diesel is responsible for 65% to 90% of vehicular-derived SOA, with substantial contributions from aromatic and aliphatic hydrocarbons. Including these insights on source characterization and SOA formation will improve regional pollution control policies, fuel regulations, and methodologies for future measurement, laboratory, and modeling studies.

Chemical Sinks of Organic Aerosol: Kinetics and Products of the Heterogeneous Oxidation of Erythritol and Levoglucosan

The heterogeneous oxidation of pure erythritol (C(4)H(10)O(4)) and levoglucosan (C(6)H(10)O(5)) particles was studied in order to evaluate the effects of atmospheric aging on the mass and chemical composition of atmospheric organic aerosol. In contrast to what is generally observed for the heterogeneous oxidation of reduced organics, substantial volatilization is observed in both systems. However, the ratio of the decrease in particle mass to the decrease in the concentration of the parent species is about three times higher for erythritol than for levoglucosan, indicating that details of chemical structure (such as carbon number, cyclic moieties, and oxygen-containing functional groups) play a governing role in the importance of volatilization reactions. The kinetics of the reaction indicate that while both compounds react at approximately the same rate, reactions of their oxidation products appear to be slowed substantially. Estimates of volatilities of organic species based on elemental composition measurements suggest that the heterogeneous oxidation of oxygenated organics may be an important loss mechanism of organic aerosol.

Surface relaxation in liquid water and methanol studied by x-ray absorption spectroscopy

X-ray absorption spectroscopy is a powerful probe of local electronic structure in disordered media. By employing extended x-ray absorption fine structure spectroscopy of liquid microjets, the intermolecular O–O distance has been observed to undergo a 5.9% expansion at the liquid water interface, in contrast to liquid methanol for which there is a 4.6% surface contraction. Despite the similar properties of liquid water and methanol (e.g., abnormal heats of vaporization, boiling points, dipole moments, etc.), this result implies dramatic differences in the surface hydrogen bond structure, which is evidenced by the difference in surface tension of these liquids. This result is consistent with surface vibrational spectroscopy, which indicates both stronger hydrogen bonding and polar ordering at the methanol surface as a consequence of “hydrophobic packing” of the methyl group.

Investigation of volatile liquid surfaces by synchrotron x-ray spectroscopy of liquid microjets

Soft x-ray absorption spectroscopy is a powerful probe of surface electronic and geometric structure in metals, semiconductors, and thin films. Because these techniques generally require ultrahigh vacuum, corresponding studies of volatile liquid surfaces have hitherto been precluded. We describe the design and implementation of an x-ray experiment based on the use of liquid microjets, permitting the study of volatile liquid surfaces under quasi-equilibrium conditions by synchrotron-based spectroscopy. The liquid microjet temperatures are also characterized by Raman spectroscopy, which connects our structural studies with those conducted on liquid samples under equilibrium conditions. In recent experiments, we have observed and quantified the intermolecular surface relaxation of liquid water and methanol and have identified a large population of “acceptor-only” molecules at the liquid water interface.

Characterization of hydrogen bond acceptor molecules at the water surface using near-edge x-ray absorption fine-structure spectroscopy and density functional theory

We present a combined experimental/computational study of the near-edge x-ray absorption fine structure of the liquid water surface which indicates that molecules with acceptor-only hydrogen bonding configurations constitute an important and previously unidentified component of the liquid/vapour interface.

An interfacial mechanism for cloud droplet formation on organic aerosols

Organic contributions to cloud theory Current theories about the formation of cloud droplets from aerosol particles containing organic components assume that the organic molecules are distributed throughout the droplet. Ruehl et al. show that this assumption is not always correct (see the Perspective by Noziere). During droplet nucleation, droplet diameters were 50% larger than predicted by the standard model. This suggests that the organic particles reside in a surface layer rather than in the bulk of the droplet. Models that neglect organic surface activity will thus underestimate how well organic-rich particles seed clouds. Science, this issue p. 1447; see also p. 1396 Organic molecules can form a surface layer around nucleating cloud droplets. [Also see Perspective by Noziere] Accurate predictions of aerosol/cloud interactions require simple, physically accurate parameterizations of the cloud condensation nuclei (CCN) activity of aerosols. Current models assume that organic aerosol species contribute to CCN activity by lowering water activity. We measured droplet diameters at the point of CCN activation for particles composed of dicarboxylic acids or secondary organic aerosol and ammonium sulfate. Droplet activation diameters were 40 to 60% larger than predicted if the organic was assumed to be dissolved within the bulk droplet, suggesting that a new mechanism is needed to explain cloud droplet formation. A compressed film model explains how surface tension depression by interfacial organic molecules can alter the relationship between water vapor supersaturation and droplet size (i.e., the Köhler curve), leading to the larger diameters observed at activation.

Real time in situ detection of organic nitrates in atmospheric aerosols

A novel instrument is described that quantifies total particle-phase organic nitrates in real time with a detection limit of 0.11 microg m(-3) min(-1), 45 ppt min(-1) (-ONO(2)). Aerosol nitrates are separated from gas-phase nitrates with a short residence time activated carbon denuder. Detection of organic molecules containing -ONO(2) subunits is accomplished using thermal dissociation coupled to laser induced fluorescence detection of NO(2). This instrument is capable of high time resolution (seconds) measurements of particle-phase organic nitrates, without interference from inorganic nitrate. Here we use it to quantify organic nitrates in secondary organic aerosol generated from high-NO(x) photooxidation of limonene, alpha-pinene, Delta-3-carene, and tridecane. In these experiments the organic nitrate moiety is observed to be 6-15% of the total SOA mass.

Chemical Dynamics, Molecular Energetics, and Kinetics at the Synchrotron

Scientists at the Chemical Dynamics Beamline of the Advanced Light Source in Berkeley are continuously reinventing synchrotron investigations of physical chemistry and chemical physics with vacuum ultraviolet light. One of the unique aspects of a synchrotron for chemical physics research is the widely tunable vacuum ultraviolet light that permits threshold ionization of large molecules with minimal fragmentation. This provides novel opportunities to assess molecular energetics and reaction mechanisms, even beyond simple gas phase molecules. In this perspective, significant new directions utilizing the capabilities at the Chemical Dynamics Beamline are presented, along with an outlook for future synchrotron and free electron laser science in chemical dynamics. Among the established and emerging fields of investigations are cluster and biological molecule spectroscopy and structure, combustion flame chemistry mechanisms, radical kinetics and product isomer dynamics, aerosol heterogeneous chemistry, planetary and interstellar chemistry, and secondary neutral ion-beam desorption imaging of biological matter and materials chemistry.

Reaction of the C2H Radical with 1-Butyne (C4H6): Low-Temperature Kinetics and Isomer-Specific Product Detection†

The rate coefficient for the reaction of the ethynyl radical (C(2)H) with 1-butyne (H-C[triple bond]C-CH(2)-CH(3)) is measured in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by laser photolysis of acetylene (C(2)H(2)) at 193 nm and detected via chemiluminescence (C(2)H + O(2) --> CH (A(2)Delta) + CO(2)). The rate coefficients are measured over the temperature range of 74-295 K. The C(2)H + 1-butyne reaction exhibits no barrier and occurs with rate constants close to the collision limit. The temperature-dependent rate coefficients can be fit within experimental uncertainties by the expression k = (2.4 +/- 0.5) x 10(-10)(T/295 K)(-(0.04+/-0.03)) cm(3) molecule(-1) s(-1). Reaction products are detected at room temperature (295 K) and 533 Pa using a multiplexed photoionization mass spectrometer (MPIMS) coupled to the tunable vacuum ultraviolet synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory. Two product channels are identified for this reaction: m/z = 64 (C(5)H(4)) and m/z = 78 (C(6)H(6)) corresponding to the CH(3)-loss and H-loss channels, respectively. Photoionization efficiency (PIE) curves are used to analyze the isomeric composition of both product channels. The C(5)H(4) products are found to be exclusively linear isomers composed of ethynylallene and methyldiacetylene in a 4:1 ratio. In contrast, the C(6)H(6) product channel includes two cyclic isomers, fulvene 18(+/-5)% and 3,4-dimethylenecyclobut-1-ene (DMCB) 32(+/-8)%, as well as three linear isomers, 2-ethynyl-1,3-butadiene 8(+/-5)%, 3,4-hexadiene-1-yne 28(+/-8)%, and 1,3-hexadiyne 14(+/-5)%. Within experimental uncertainties, we do not see appreciable amounts of benzene and an upper limit of 10% is estimated. Diacetylene (C(4)H(2)) formation via the C(2)H(5)-loss channel is also thermodynamically possible but cannot be observed due to experimental limitations. The implications of these results for modeling of planetary atmospheres, especially of Saturns largest moon Titan and the relationships to combustion reactions, are discussed.