Investigation of pyruvic acid photolysis at the air-liquid interface as a source of aqueous secondary organic aerosols

Pyruvic acid (PA) is a ubiquitous 2-oxocarboxylic acid in the atmosphere. Its photochemical process at the air-liquid (a-l) interface has been suggested as an important source of aqueous secondary organic aerosols. We investigated the photochemical reaction pathways of PA at the a-l interface using synchrotron-based vacuum ultraviolet single-photon ionization mass spectrometry (VUV SPI-MS) coupled with the System for Analysis at the Liquid Vacuum Interface (SALVI) microreactor. Results from mass spectral analysis and the determination of appearance energies (AEs) indicate that photolysis of PA can generate radicals, then they recombine with carboxylic acids and simple molecular oligomers. Furthermore, the preliminary products could form larger oligomers via radical reaction or esterification in the presence of hydroxyl and carboxyl functional groups. Mass spectral comparison shows that most photochemical reactions would complete within 4 h. The expanded photochemistry-driven reaction flowchart of PA is proposed based on the newly discovered products. Our results reveal that the interfacial PA photochemical reactions have different mechanisms from the bulk liquid due to the interfacial properties, such as molecular density, composition, and ion concentration. Our findings show that in situ mass spectral analysis with bright photon ionization is useful to elucidate the contribution of a-l interfacial reactions leading to aqSOA formation.

Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation

Carbene species play an integral role in high-energy chemistry, transition-metal-carbene chemistry, catalysis, photolytic formation of carbohydrates, and possibly even the formation of interstellar sugars. In 1921, "reactive formaldehyde"─now known as hydroxymethylene (HCOH)─was first implicated as an intermediate in photocatalytic processes. However, due to its transient nature, direct observation of HCOH has predominantly been attained using cryogenic isolation methods. As a result, HCOH gas-phase reactivity measurements have been limited. We directly observed HCOH using photoionization spectroscopy following UV photodissociation of methanol. Our measurements show it reacts slowly with O2 at room temperature. This work provides evidence for the formation mechanism of HCOH from CH3OH and its subsequent reactivity under gas-phase reaction conditions.

Annihilating Actinic Photochemistry of the Pyruvate Anion by One and Two Water Molecules

Photochemical behaviors of pyruvic acid in multiple phases have been extensively studied, while those of its conjugate base, the pyruvate anion (CH₃COCOO^-, PA^-) are less understood and remain contradictory in gaseous versus aqueous phases. Here in this article, we report a joint experimental and theoretical study combining cryogenic, wavelength-resolved negative ion photoelectron spectroscopy (NIPES) and high-level quantum chemical computations to investigate PA^- actinic photochemistry and its dependence on microsolvation in the gas phase. PA^-·nH₂O (n = 0-5) clusters were generated and characterized, with their low-lying isomers identified. NIPES conducted at multiple wavelengths across the PA^- actinic regime revealed the PA^- photochemistry extremely sensitive to its hydration extent. While bare PA^- anions exhibit active photoinduced dissociations that generate the acetyl (CH₃CO^-), methide (CH₃^-) anions, their corresponding radicals, and slow electrons, one single attached water molecule results in significant suppression with a subsequent second water being able to completely block all dissociation pathways, effectively annihilating all PA^- photochemical reactivities. The underlying dissociation mechanisms of PA^-·nH₂O (n = 0-2) clusters are proposed involving nπ* excitation, dehydration, decarboxylation, and further CO loss. Since the photoexcited dihydrate does not have sufficient energy to overcome the full dehydration barrier before PA^- could fragmentate, the PA^- dissociation pathway is completely blocked, with the energy most likely released via loss of one water and internal electronic and vibrational relaxations. The insight unraveled in this work provides a much-needed critical link to connect the seemingly conflicting PA^- actinic chemistry between the gas and condensed phases.

Thermal Electron Attachment to Pyruvic Acid, Thermal Detachment from the Parent Anion, and the Electron Affinity of Pyruvic Acid

The kinetics of electron attachment to pyruvic acid (CH₃COCOOH) and thermal detachment from the resulting parent anion were measured from 300-515 K using a flowing afterglow─Langmuir probe apparatus. An adiabatic electron affinity (EA) for pyruvic acid was derived, 0.84 ± 0.02 eV. Electron attachment rate constants to pyruvic acid of 2.1 × 10^-8 and 1.2 × 10^-8 were measured at 300 and 400 K, respectively. Rate constants at higher temperatures are less well-defined due to possible contributions from attachment to zymonic open ketone, an endemic impurity in pyruvic acid. Similarly, unimolecular detachment rates are complicated by secondary proton transfer reaction of the pyruvic acid anion with pyruvic acid to yield an 87 Da anion. The possible contributions from these chemistries are considered, and in all cases the equilibrium constant between attachment and detachment remains well-defined, allowing for determination of the EA.

Photochemistry of the pyruvate anion produces CO2, CO, CH3-, CH3, and a low energy electron

The photochemistry of pyruvic acid has attracted much scientific interest because it is believed to play critical roles in atmospheric chemistry. However, under most atmospherically relevant conditions, pyruvic acid deprotonates to form its conjugate base, the photochemistry of which is essentially unknown. Here, we present a detailed study of the photochemistry of the isolated pyruvate anion and uncover that it is extremely rich. Using photoelectron imaging and computational chemistry, we show that photoexcitation by UVA light leads to the formation of CO₂, CO, and CH₃⁻. The observation of the unusual methide anion formation and its subsequent decomposition into methyl radical and a free electron may hold important consequences for atmospheric chemistry. From a mechanistic perspective, the initial decarboxylation of pyruvate necessarily differs from that in pyruvic acid, due to the missing proton in the anion.

Pyruvic acid and its conjugate base, the pyruvate anion, are largely present in the atmosphere. Here the authors, using photoelectron imaging and quantum chemistry calculations, investigate the photochemistry of isolated pyruvate anions initiated by UVA radiation and report the formation of CO₂, CO, and CH₃⁻ further decomposing into CH₃ and a free electron.

Role of hydrogen bonding in bulk aqueous phase decomposition, complexation, and covalent hydration of pyruvic acid

The behavior of hydrogen bonding changes between the gas and aqueous phase, altering the mechanisms of various pyruvic acid processes and consequently affecting the aerosol formation in different environments.

Probing the dynamics of the photo-induced decarboxylation of neutral and ionic pyruvic acid

The dynamics of the electronically excited pyruvic acid (PA) and of its unimolecular decomposition upon single photon ionization are investigated by means of a table top fs laser and VUV...

Aqueous Photochemistry of 2-Oxocarboxylic Acids: Evidence, Mechanisms, and Atmospheric Impact

Atmospheric organic aerosols play a major role in climate, demanding a better understanding of their formation mechanisms by contributing multiphase chemical reactions with the participation of water. The sunlight driven aqueous photochemistry of small 2-oxocarboxylic acids is a potential major source of organic aerosol, which prompted the investigations into the mechanisms of glyoxylic acid and pyruvic acid photochemistry reviewed here. While 2-oxocarboxylic acids can be contained or directly created in the particles, the majorities of these abundant and available molecules are in the gas phase and must first undergo the surface uptake process to react in, and on the surface, of aqueous particles. Thus, the work also reviews the acid-base reaction that occurs when gaseous pyruvic acid meets the interface of aqueous microdroplets, which is contrasted with the same process for acetic acid. This work classifies relevant information needed to understand the photochemistry of aqueous pyruvic acid and glyoxylic acid and motivates future studies based on reports that use novel strategies and methodologies to advance this field.

Valence Photoionization and Autoionization of the Formyl Radical

We have used 308 nm photolysis of acetaldehyde to measure a photoionization spectrum of the formyl (HCO) radical between 8 and 11.5 eV using an 11 meV FWHM photoionization energy resolution. We have confirmed that the formyl radical is the carrier of the spectrum by generating an identical spectrum of the HCO product in the Cl + H₂CO reaction. The spectrum of HCO and its deuterated isotopologue (DCO) have several resolved autoionizing resonances above the Franck-Condon envelope, which we assign to autoionization after initial excitation into neutral 3sσ and 3p Rydberg states converging to the first triplet excited state of HCO⁺(ã ³A'). The quantum defects for these states are δ_3sσ = 1.06 ± 0.02 and δ_3p = 0.821 ± 0.019. We report absolute photoionization cross-section measurements of σ_HCO^PI(9.907 eV) = 4.5 ± 0.9 Mb, σ_HCO^PI(10.007 eV) = 4.8 ± 1.0 Mb, σ_HCO^PI(10.107 eV) = 6.0 ± 1.2 Mb, σ_HCO^PI(10.107 eV) = 5.7 ± 1.2 Mb, and σ_HCO^PI(10.304 eV) = 10.6 ± 2.2 Mb relative to the photoionization cross section of the methyl radical. The absolute cross-section measurements are a factor of ∼1.5 larger than those determined in past studies, although the presence of strong autoionizing features supports a dependence on photoionization energy resolution. We propose that the semiempirical model of Xu and Pratt for estimation of free radical photoionization cross sections is more accurate when applied with a reference species containing the same atoms as the free radical rather than isoelectronic species with different atoms.

Spectroscopic characterization and photochemistry of the vinylsulfinyl radical

The simplest α,β-unsaturated sulfinyl radical CH2[double bond, length as m-dash]C(H)SO˙ has been generated in the gas phase by high-vacuum flash pyrolysis (HVFP) of sulfoxide CH2[double bond, length as m-dash]C(H)S(O)CF3 at ca. 800 °C. Two planar cis and trans conformers of CH2[double bond, length as m-dash]C(H)SO˙ were isolated in cryogenic matrixes (N2, Ne, and Ar) and characterized with IR and UV/Vis spectroscopy. In addition to the photo-induced cis ⇋ trans conformational interconversion, CH2[double bond, length as m-dash]C(H)SO˙ displays complex photochemistry. Upon irradiation with a purple light LED (400 nm), CH2[double bond, length as m-dash]C(H)SO˙ isomerizes to novel radicals CH3SCO˙, ˙CH2SC(O)H, and ˙CH2C(O)SH with concomitant dissociation to a caged molecular complex CH3S˙CO. Subsequent UV-laser (266 nm) irradiation causes fragmentation to ˙CH3/OCS and additional formation of an elusive carbonyl radical CH3C(O)S˙, which rearranges to ˙CH2C(O)SH upon further UV-light irradiation (365 nm). The vibrational data and bonding analysis of the two conformers of CH2[double bond, length as m-dash]C(H)SO˙ suggest that both are floppy radicals in which the unpaired electron conjugates with the vicinal π(C[double bond, length as m-dash]C) bond, leading to significant contribution of the canonical resonance form of ˙CH2-C(H)SO. The mechanism for the isomerization of CH2[double bond, length as m-dash]C(H)SO˙ is discussed based on the observed intermediates along with a computed potential energy profile at the CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level of theory.