Chemistry and Photochemistry of Pyruvic Acid at the Air-Water Interface.

Interfacial regions are unique chemical reaction environments that can promote chemistry not found elsewhere. The air-water interface is ubiquitous in the natural environment in the form of ocean surfaces and aqueous atmospheric aerosols. Here we investigate the chemistry and photochemistry of pyruvic acid (PA), a common environmental species, at the air-water interface and compare it to its aqueous bulk chemistry using two different experimental setups: (1) a Langmuir-Blodgett trough, which models natural water surfaces and provides a direct comparison between the two reaction environments, and (2) an atmospheric simulation chamber (CESAM) to monitor the chemical processing of nebulized aqueous PA droplets. The results show that surface chemistry leads to substantial oligomer formation. The sequence begins with the condensation of lactic acid (LA), formed at the surface, with itself and with pyruvic acid, and LA + LA - H2O and LA + PA - H2O are prominent among the products in addition to a series of higher-molecular-weight oligomers of mixed units of PA and LA. In addition, we see zymonic acid at the surface. Actinic radiation enhances the production of the oligomers and produces additional surface-active molecules known from the established aqueous photochemical mechanisms. The presence and formation of complex organic molecules at the air-water interface from a simple precursor like PA in the natural environment is relevant to contemporary atmospheric science and is important in the context of prebiotic chemistry, where abiotic production of complex molecules is necessary for abiogenesis.

Conformer-Specific Photolysis of Pyruvic Acid and the Effect of Water.

The conformer-specific reactivity of gas-phase pyruvic acid following the S1(nπ*) ← S0 excitation at λmax = 350 nm (290-380 nm) and the effect of water are investigated for the two lowest energy conformers. Conformer-specific gas-phase pyruvic acid photolysis rate constants and their respective populations are measured by monitoring their distinct vibrational OH-stretching frequencies. The geometry, relative energies, fundamental vibrational frequencies, and electronic transitions of the pyruvic acid conformers and their monohydrated complexes are calculated with density functional theory and ab initio methods. Results from experiment and theory show that the more stable conformer with an intramolecular hydrogen bond dominates the gas-phase photolysis of pyruvic acid. Water greatly affects the gas-phase pyruvic acid conformer population and photochemistry through hydrogen bonding interactions. The addition of water decreases the gas-phase relative population of the more stable conformer and decreases the molecule's gas-phase photolysis rate constants. The theoretical results show that even a single water molecule interrupts the intramolecular hydrogen bond, which is essential for the efficient photodissociation of gas-phase pyruvic acid. Results of this study suggest that the aqueous-phase photochemistry of pyruvic acid proceeds through hydrogen-bonded conformers lacking an intramolecular hydrogen bond.

Efficient Formation of Light-Absorbing Polymeric Nanoparticles from the Reaction of Soluble Fe(III) with C4 and C6 Dicarboxylic Acids.

The role of transition metals in the formation and aging of secondary organic aerosol (SOA) from aliphatic and aromatic precursors in heterogeneous/multiphase reactions is not well understood. The reactivity of soluble Fe(III) toward known benzene photooxidation products that include fumaric (trans-butenedioic) and muconic (trans,trans-2,4-hexadienedioic) acids was investigated. Efficient formation of brightly colored nanoparticles was observed that are mostly rod- or irregular-shaped depending on the structure of the organic precursor. The particles were characterized for their optical properties, growth rate, elemental composition, iron content, and oxidation state. Results indicate that these particles have mass absorption coefficients on the same order as black carbon and larger than that of biomass burning aerosols. The particles are also amorphous in nature and consist of polymeric chains of Fe centers complexed to carboxylate groups. The oxidation state of Fe was found to be in between Fe(III) and Fe(II) in standard compounds. The organic reactant to iron molar ratio and pH were found to affect the particle growth rate. Control experiments using maleic acid (cis-butenedioic acid) and succinic acid (butanedioic acid) produced no particles. The formation of particles reported herein could account for new pathways that lead to SOA and brown carbon formation mediated by transition metals. In addition, the multiple chemically active components in these particles (iron, organics, and acidic groups) may have an effect on their chemical reactivity (enhanced uptake of trace gases, catalysis, and production of reactive oxygen species) and their likely poor cloud/ice nucleation properties.

Molecular Characterization of Organosulfur Compounds in Biodiesel and Diesel Fuel Secondary Organic Aerosol.

Secondary organic aerosol (SOA), formed in the photooxidation of diesel fuel, biodiesel fuel, and 20% biodiesel fuel/80% diesel fuel mixture, are prepared under high-NOx conditions in the presence and absence of sulfur dioxide (SO2), ammonia (NH3), and relative humidity (RH). The composition of condensed-phase organic compounds in SOA is measured using several complementary techniques including aerosol mass spectrometry (AMS), high-resolution nanospray desorption electrospray ionization mass spectrometry (nano-DESI/HRMS), and ultrahigh resolution and mass accuracy 21T Fourier transform ion cyclotron resonance mass spectrometry (21T FT-ICR MS). Results demonstrate that sulfuric acid and condensed organosulfur species formed in photooxidation experiments with SO2 are present in the SOA particles. Fewer organosulfur species are formed in the high humidity experiments, performed at RH 90%, in comparison with experiments done under dry conditions. There is a strong overlap of organosulfur species observed in this study with previous field and chamber studies of SOA. Many MS peaks of organosulfates (R-OS(O)2OH) previously designated as biogenic or of unknown origin in field studies might have originated from anthropogenic sources, such as photooxidation of hydrocarbons present in diesel and biodiesel fuel.

Formation of Light Absorbing Soluble Secondary Organics and Insoluble Polymeric Particles from the Dark Reaction of Catechol and Guaiacol with Fe(III).

Transition metals such as iron are reactive components of environmentally relevant surfaces. Here, dark reaction of Fe(III) with catechol and guaiacol was investigated in an aqueous solution at pH 3 under experimental conditions that mimic reactions in the adsorbed phase of water. Using UV-vis spectroscopy, liquid chromatography, mass spectrometry, elemental analysis, dynamic light scattering, and electron microscopy techniques, we characterized the reactants, intermediates, and products as a function of reaction time. The reactions of Fe(III) with catechol and guaiacol produced significant changes in the optical spectra of the solutions due to the formation of light absorbing secondary organics and colloidal organic particles. The primary steps in the reaction mechanism were shown to include oxidation of catechol and guaiacol to hydroxy- and methoxy-quinones. The particles formed within a few minutes of reaction and grew to micron-size aggregates after half an hour reaction. The mass-normalized absorption coefficients of the particles were comparable to those of strongly absorbing brown carbon compounds produced by biomass burning. These results could account for new pathways that lead to atmospheric secondary organic aerosol formation and abiotic polymer formation on environmental surfaces mediated by transition metals.

On surface order and disorder of α-pinene-derived secondary organic material.

The surfaces of secondary organic aerosol particles are notoriously difficult to access experimentally, even though they are the key location where exchange between the aerosol particle phase and its gas phase occurs. Here, we overcome this difficulty by applying standard and sub- 1 cm(-1) resolution vibrational sum frequency generation (SFG) spectroscopy to detect C-H oscillators at the surfaces of secondary organic material (SOM) prepared from the ozonolysis of α-pinene at Harvard University and at the University of California, Irvine, that were subsequently collected on Teflon filters as well as CaF2 windows using electrostatic deposition. We find both samples yield comparable SFG spectra featuring an intense peak at 2940 cm(-1) that are independent of spectral resolution and location or method of preparation. We hypothesize that the SFG spectra are due to surface-active C-H oscillators associated with the four-membered ring motif of α-pinene, which produces an unresolvable spectral continuum of approximately 50 cm(-1) width reminiscent of the similar, albeit much broader, O-H stretching continuum observed in the SFG spectra of aqueous surfaces. Upon subjecting the SOM samples to cycles in relative humidity (RH) between <2% RH and ∼95% RH, we observe reversible changes in the SFG signal intensity across the entire spectral range surveyed for a polarization combination probing components of the vibrational transition dipole moments that are oriented parallel to the plane of incidence, but no signal intensity changes for any other polarization combination investigated. These results support the notion that the C-H oscillators at the surfaces of α-pinene-derived SOM deposited on CaF2 windows shift back and forth between two different molecular orientation distributions as the RH is lowered (more ordered) or raised (less ordered). The findings thus point toward the presence of a reversible surface switch for hindering (more ordered, <2%RH) and promoting (less ordered, ∼95%RH) exchange between the aerosol particle phase and its gas phase.

Direct photolysis of α-pinene ozonolysis secondary organic aerosol: effect on particle mass and peroxide content.

Primary and secondary organic aerosols (POA and SOA) contain a complex mixture of multifunctional chemicals, many of which are photolabile. Much of the previous work that aimed to understand the chemical evolution (aging) of POA and SOA has focused on the reactive uptake of gas-phase oxidants by particles. By stripping volatile compounds and ozone from α-pinene ozonolysis SOA with three 1-m-long denuders, and exposing the residual particles in a flow cell to near-ultraviolet (λ>300 nm) radiation, we find that condensed-phase photochemistry can induce significant changes in SOA particle size and chemical composition. The particle-bound organic peroxides, which are highly abundant in α-pinene ozonolysis SOA (22 ± 5% by weight), have an atmospheric photolysis lifetime of about 6 days at a 24-h average solar zenith angle (SZA) of 65° experienced at 34° latitude (Los Angeles) in the summer. In addition, the particle diameter shrinks 0.56% per day under these irradiation conditions as a result of the loss of volatile photolysis products. Experiments with and without the denuders show similar results, suggesting that condensed-phase processes dominate over heterogeneous reactions of particles with organic vapors, excess ozone, and gas-phase free radicals. These condensed-phase photochemical processes occur on atmospherically relevant time scales and should be considered when modeling the evolution of organic aerosol in the atmosphere.

Photochemistry of aldehyde clusters: cross-molecular versus unimolecular reaction dynamics.

The unimolecular photochemistry of aldehydes has been extensively studied, both experimentally and computationally. However, less is known about the role of cross-molecular photochemical processes in the condensed-phase photolysis of aldehydes. The triplet-state photochemistry of pentanal in its pentameric (n = 5) cluster was investigated as a model for photochemical reactions of aliphatic aldehydes in atmospheric aerosols. This study employs "on the fly" dynamics simulations using a semi-empirical MRCI electronic code for the singlet and triplet states involved. Previous studies have shown that the triplet-state photochemistry of an isolated pentanal molecule is dominated by Norrish I and II reactions. The main findings for the cluster are: (1) 55% of the trajectories lead to a unimolecular or cross-molecular reaction within a timescale of 100 ps; (2) cross-molecular reactions occur in over 70% of the reactive trajectories; (3) the main cross-molecular processes involve an H-atom transfer from the CHO group of the excited pentanal to an O atom of a nearby pentanal; and (4) the unimolecular Norrish II reaction is suppressed by the cluster environment. The predictions are qualitatively supported by experimental results on the condensed-phase photolysis of an aliphatic aldehyde, undecanal. The computational approach should be useful for predicting the mechanisms of other condensed-phase organic photochemical reactions. These results demonstrate a major role of cross-molecular processes in the condensed-phase photolysis of carbonyls. The cross-molecular reactions discussed in this work are relevant to photolysis-driven processes in atmospheric organic aerosols. It is expected that the condensed-phase environment of an organic aerosol particle should support a multitude of similar cross-molecular photochemical processes.

Aerosol Vacuum-Assisted Plasma Ionization (Aero-VaPI) Coupled to Ion Mobility-Mass Spectrometry.

In this communication, we report on the real-time analysis of organic aerosol particles by Vacuum-assisted Plasma Ionization-Mass Spectrometry (Aero-VaPI-MS) using a home-built VaPI ion source coupled to a Synapt G2-S HDMS ion mobility-mass spectrometry (IM-MS) system. Standards of organic molecules of interest in prebiotic chemistry were used to generate aerosols. Monocaprin and decanoic acid aerosol particles were successfully detected in both the positive and negative ion modes, respectively. A complex aerosol mixture of different sizes of polymers of L-malic acid was also examined through ion mobility (IM) separations, resulting in the detection of polymers of up to eight monomeric units. This noncommercial plasma ion source is proposed as a low cost alternative to other plasma ionization platforms used for aerosol analysis, and a higher-performance alternative to more traditional aerosol mass spectrometers. VaPI provides robust online ionization of organics in aerosols without extensive ion activation, with the coupling to IM-MS providing higher peak capacity and excellent mass accuracy. Graphical Abstract ᅟ.

A Real-Time Fast-Flow Tube Study of VOC and Particulate Emissions from Electronic, Potentially Reduced-Harm, Conventional, and Reference Cigarettes.

Tobacco-free electronic cigarettes (e-cigarettes), which are currently not regulated by the FDA, have become widespread as a "safe" form of smoking. One approach to evaluate the potential toxicity of e-cigarettes and other types of potentially "reduced-harm" cigarettes is to compare their emissions of volatile organic compounds (VOCs), including reactive organic electrophillic compounds such as acrolein, and particulate matter to those of conventional and reference cigarettes. Our newly designed fast-flow tube system enabled us to analyze VOC composition and particle number concentration in real-time by promptly diluting puffs of mainstream smoke obtained from different brands of combustion cigarettes and e-cigarettes. A proton transfer reaction time-of-flight mass spectrometer (PTRMS) was used to analyze real-time cigarette VOC emissions with a 1 s time resolution. Particles were detected with a condensation particle counter (CPC). This technique offers real-time analysis of VOCs and particles in each puff without sample aging and does not require any sample pretreatment or extra handling. Several important determining factors in VOC and particle concentration were investigated: (1) puff frequency; (2) puff number; (3) tar content; (4) filter type. Results indicate that electronic cigarettes are not free from acrolein and acetaldehyde emissions and produce comparable particle number concentrations to those of combustion cigarettes, more specifically to the 1R5F reference cigarette. Unlike conventional cigarettes, which emit different amounts of particles and VOCs each puff, there was no significant puff dependence in the e-cigarette emissions. Charcoal filter cigarettes did not fully prevent the emission of acrolein and other VOCs.