Guzman Lab Publications

Abstract. Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO 3 ) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO 3 -BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO 3 -BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO 3 radical, the difficulty of characterizing ...

Abstract: Aerosols affect climate change, the energy balance of the atmosphere, and public health due to their variable chemical composition, size, and shape. While the formation of secondary organic aerosols (SOA) from gas phase precursors is relatively well understood, studying aqueous chemical reactions contributing to the total SOA budget is the current focus of major attention. Field measurements have revealed that mono-, di-, and oxo-carboxylic acids are abundant species present in SOA and atmospheric waters. This work explores the fate of one of these 2-oxocarboxylic acids, glyoxylic acid, which can photogenerate reactive species under solar irradiation. Additionally, the dark thermal aging of photoproducts is studied by UV–visible and fluorescence spectroscopies to reveal that the optical properties are altered by the glyoxal produced. The optical properties display periodicity in the time domain of the UV–visible spectrum of chromophores with absorption enhanceme...

  Abstract: The reductive tricarboxylic acid (rTCA) cycle is an important central biosynthetic pathway that fixes CO 2 into carboxylic acids. Among the five reductive steps in the rTCA cycle, the two-electron reduction of fumarate to succinate proceeds nonenzymatically on the surface of photoexcited sphalerite (ZnS) colloids suspended in water. This model reaction is chosen to systematically study the surface photoprocess occurring on ZnS in the presence of [Na 2 S] (1–10 mM) hole scavenger at 15 °C. Experiments at variable pH (5–10) indicate that monodissociated fumaric acid is the primary electron acceptor forming the monoprotic form of succinic acid. The following reaction scheme is proposed: (1) photoexcitation of ZnS generates conduction band electrons and valence band holes, (2) the hole scavenger donates electrons while producing sulfur-containing intermediates en route to sulfate formation, (3) a first electron transfer occurs at the conduction band converting...

  Abstract: Natural and anthropogenic emissions of aromatic hydrocarbons from biomass burning, agro-industrial settings, and fossil fuel combustion contribute precursors to secondary aerosol formation (SOA). How these compounds are processed under humid tropospheric conditions is the focus of current attention to understand their environmental fate. This work shows how catechol thin films, a model for oxygenated aromatic hydrocarbons present in biomass burning and combustion aerosols, undergo heterogeneous oxidation at the air–solid interface under variable relative humidity (RH = 0–90%). The maximum reactive uptake coefficient of O 3 (g) by catechol γ O 3 = (7.49 ± 0.35) × 10 –6 occurs for 90% RH. Upon exposure of ca. 104-μm thick catechol films to O 3 (g) mixing ratios between 230 ppbv and 25 ppmv, three main reaction pathways are observed. (1) The cleavage of the 1,2 carbon–carbon bond at the air–solid interface resulting in the formation of cis , cis -muconic aci...

  Abstract. The formation of secondary organic aerosol (SOA) from the oxidation of β-pinene via nitrate radicals is investigated in the Georgia Tech Environmental Chamber (GTEC) facility. Aerosol yields are determined for experiments performed under both dry (relative humidity (RH) < 2 %) and humid (RH = 50 % and RH = 70 %) conditions. To probe the effects of peroxy radical (RO 2 ) fate on aerosol formation, "RO 2 + NO 3 dominant" and "RO 2 + HO 2 dominant" experiments are performed. Gas-phase organic nitrate species (with molecular weights of 215, 229, 231, and 245 amu, which likely correspond to molecular formulas of C 10 H 17 NO 4 , C 10 H 15 NO 5 , C 10 H 17 NO 5 , and C 10 H 15 NO 6 , respectively) are detected by chemical ionization mass spectrometry (CIMS) and their formation mechanisms are proposed. The NO + (at m/z 30) and NO 2 + (at m/z 46) ions contribute about 11 % to the combined organics and nitrate signals in the typical aerosol m...

  Abstract: The photoreduction of CO 2 to formate (HCOO – ) in sphalerite (ZnS) aqueous suspensions is systematically studied in the presence of Na 2 S hole scavenger. A series of cut-on filters at λ cut-on ≥ 280, 295, 305, 320, and 400 nm are used to measure the reaction rate of formate production. The dependence of the measured reaction rates on λ cut-on indicates that a wavelength of λ = 345 nm is associated with the actual bandgap of the semiconductor nanocrystallites suspended in water. The results from apparent quantum yield measurements during periodic illumination experiments suggest that (1) valence-band holes on the surface of ZnS disappear within deciseconds due to the oxidation of the scavenger while simultaneously pumping electrons to the conduction band, (2) excited electrons in the conduction band of ZnS are transferred to CO 2 to produce the intermediate CO 2 •– , and (3) CO 2 •– abstracts a proton from water and undergoes further photoreduction on ...

  Abstract: Anthropogenic emissions of aromatic hydrocarbons promptly react with hydroxyl radicals undergoing oxidation to form phenols and polyphenols (e.g., catechol) typically identified in the complex mixture of humic-like substances (HULIS). Because further processing of polyphenols in secondary organic aerosols (SOA) can continue mediated by a mechanism of ozonolysis at interfaces, a better understanding about how these reactions proceed at the air–water interface is needed. This work shows how catechol, a molecular probe of the oxygenated aromatic hydrocarbons present in SOA, can contribute interfacial reactive species that enhance the production of HULIS under atmospheric conditions. Reactive semiquinone radicals are quickly produced upon the encounter of 40 ppbv–6.0 ppmv O 3 (g) with microdroplets containing [catechol] = 1–150 μM. While the previous pathway results in the instantaneous formation of mono- and polyhydroxylated aromatic rings (PHA) and chromoph...

  Abstract: Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air–ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale. Although new experimental techniques have extended our knowledge of the surface ...

Summary: Recently, Griffith et al. reported that acetoin is a major product of the photolysis of aqueous pyruvic acid (PA), and proposed that its mechanism of formation involves the thermal decarboxylation of α-acetolactic acid (AL) intermediate. This finding is in contrast with a previous study by Guzman et al., based on experiments performed under similar conditions, which reported the formation of two major products: 2,3-dimethyl tartaric acid (A) and 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic acid (B). Both A and B result after triplet radical pairs are formed via long-range (proton-coupled) electron transfer between carbonyl groups. Product B can thermally decompose into acetoin, but its carbonyl chromophore absorbs at λmax ∼ 285 nm vs. 276 nm for acetoin . In fact, Guzman et al. had shown that the thermal decarboxylation of aqueous AL proceeds at significantly slower rates than postphotolysis CO2(g) emissions, thereby excluding its participation in the mechanism of ...

Abstract: Halides are incorporated into aerosol sea spray, where they start the catalytic destruction of ozone (O 3 ) over the oceans and affect the global troposphere. Two intriguing environmental problems undergoing continuous research are (1) to understand how reactive gas phase molecular halogens are directly produced from inorganic halides exposed to O 3 and (2) to constrain the environmental factors that control this interfacial process. This paper presents a laboratory study of the reaction of O 3 at variable iodide (I – ) concentration (0.010–100 μM) for solutions aerosolized at 25 °C, which reveal remarkable differences in the reaction intermediates and products expected in sea spray for low tropospheric [O 3 ]. The ultrafast oxidation of I – by O 3 at the air–water interface of microdroplets is evidenced by the appearance of hypoiodous acid (HIO), iodite (IO 2 – ), iodate (IO 3 – ), triiodide (I 3 – ), and molecular iodine (I 2 ). Mass spectrometry measurements r...

Abstract: The physical, chemical, and biological processes involving organics in ice in the environment impact a number of atmospheric and biogeochemical cycles. Organic material in snow or ice may be biological in origin, deposited from aerosols or atmospheric gases, or formed chemically in situ. In this manuscript, we review the current state of knowledge regarding the sources, properties, and chemistry of organic materials in environmental ices. Several outstanding questions remain to be resolved and fundamental data gathered before an accurate model of transformations and transport of organic species in the cryosphere will be possible. For example, more information is needed regarding the quantitative impacts of chemical and biological processes, ice morphology, and snow formation on the fate of organic material in cold regions. Interdisciplinary work at the interfaces of chemistry, physics and biology is needed in order to fully characterize the nature and evolutio...

Abstract: The chemisorption of O 2 on nanoscale n-doped CdS semiconductors is computed in terms of a Wolkenstein isotherm coupled to the Schrödinger Poisson equation. Present numerical results show the dependence of the chemisorbed charge and the differential capacitance on oxygen partial pressure. A comparison against the classical Poisson-Boltzmann approach shows a higher chemisorbed charge in the quantum model, but a greater differential capacitance in the classical case. Francesco Ciucci , Carlo de Falco , Marcelo I. Guzman , Sara Lee , Tomonori Honda; Chemisorption on semiconductors: The role of quantum corrections on the space charge regions in multiple dimensions. Appl. Phys. Lett. 30 April 2012; 100 (18): 183106. https://doi.org/10.1063/1.4709483

Abstract: During the aerosolization process at the sea surface, halides are incorporated into aerosol droplets, where they may play an important role in tropospheric ozone chemistry. Although this process may significantly contribute to the formation of reactive gas phase molecular halogens, little is known about the environmental factors that control how halides selectively accumulate at the air–water interface. In this study, the production of sea spray aerosol is simulated using electrospray ionization (ESI) of 100 nM equimolar solutions of NaCl, NaBr, NaI, NaNO 2 , NaNO 3 , NaClO 4 , and NaIO 4 . The microdroplets generated are analyzed by mass spectrometry to study the comparative enrichment of anions ( f X – ) and their correlation with ion properties. Although no correlation exists between f X – and the limiting equivalent ionic conductivity, the correlation coefficient of the linear fit with the size of the anions R X – , dehydration free-energy Δ G dehyd , and polariz...

Abstract: The basic concepts necessary to understand the origin of life are presented. The existing models conducting to the emergence of biochemistry from geochemistry on Earth are introduced. The chapter is focused on abiotic photosynthesis as a way to capture energy and generate the core of central metabolism. Universality in intermediate metabolism, mineral catalysis, hydrothermal vents, and the iron-sulfur proposals are discussed. Special emphasis is given to the model of non-enzymatic prebiotic metabolism that can potentially connect the RNA world and the compartmentalization in protocells approach. Catalysis in mineral surfaces is suggested as the main mechanism for the possible origin of the central metabolic pathways. Candidate semiconductor minerals such as zinc sulfide could have driven this prebiotic chemistry in the young planet. A shallow-water hydrothermal vent system is presented as a model environment where the first microorganisms on Earth used the sugg...

Abstract. The production of secondary organic aerosol (SOA) by the dark ozonolysis of gas-phase β-caryophyllene was studied. The experiments were conducted in a continuous-flow environmental chamber for organic particle mass concentrations of 0.5 to 30 μg m −3 and with ozone in excess, thereby allowing the study of second-generation particle-phase products under atmospherically relevant conditions. The particle-phase products were characterized by an ultra-performance liquid chromatograph equipped with an electrospray ionization time-of-flight mass spectrometer (UPLC-ESI-ToF-MS). Fragmentation mass spectra were used for the structural elucidation of each product, and the structures were confirmed as consistent with the accurate m / z values of the parent ions. In total, fifteen products were identified. Of these, three are reported for the first time. The structures showed that 9 out of 15 particle-phase products were second generation, including all three of ...

Abstract: The reaction of glyoxylate with carbon dioxide to produce lactate is promoted when zinc sulfide is irradiated by ultraviolet light. These results, representing a model for the action of colloidal mineral semiconductors on early Earth, complete a consecutive series that culminates in entry-point molecules of the reductive tricarboxylic acid cycle.   M.I. Guzman and S.T. Martin. Photo-Production of Lactate from Glyoxylate: How Minerals Can Facilitate Energy Storage in a Prebiotic World . Chemical Communications (2010), 46, 2265-2267, DOI:10.1039/b924179e. https://doi.org/10.1039/B924179E

Abstract: Laboratory experiments show that the optical absorptivity of model organic matter is not an intrinsic property, but a strong function of relative humidity, temperature, and insolation. Suites of representative polyfunctional C x H y O z oligomers in water develop intense visible absorptions upon addition of inert electrolytes. The resulting mixtures reach mass absorption cross sections σ(532 nm) ∼ 0.1 m 2 /gC in a few hours, absorb up to 9 times more solar radiation than the starting material, can be half-bleached by noon sunlight in ∼1 h, and can be repeatedly recycled without carbon loss. Visible absorptions red-shift and evolve increasingly faster in subsequent thermal aging cycles. Thermochromism and its strong direct dependences on ionic strength and temperature are ascribed to the dehydration of >CH−C(OH)< groups into >C═C< unsaturations by a polar E1 mechanism, and bleaching to photoinduced retrohydration. These transformations are deemed to...

, , Abstract: Aerosol particles affect the Earth’s energy balance by absorbing and scattering radiation according to their chemical composition, size, and shape. It is generally believed that their optical properties could be deduced from the molecular composition of the complex organic matter contained in these particles, a goal pursued by many groups via high-resolution mass spectrometry, although: (1) absorptivity is associated with structural chromophores rather than with molecular formulas, (2) compositional space is a small projection of structural space, and (3) mixtures of polar polyfunctional species usually exhibit supramolecular interactions. Here we report a suite of experiments showing that the photolysis of aqueous pyruvic acid (a proxy for aerosol α-dicarbonyls absorbing at λ > 300 nm) generates mixtures of identifiable aliphatic polyfunctional oligomers that develop absorptions in the visible upon standing in the dark. These absorptions and their induc...

Abstract: A reductive tricarboxylic acid (rTCA) cycle could have fixed carbon dioxide as biochemically useful energy-storage molecules on early Earth. Nonenzymatic chemical pathways for some steps of the rTCA cycle, however, such as the production of the α -ketocarboxylic acids pyruvate and α -ketoglutarate, remain a challenging problem for the viability of the proposed prebiotic cycle. As a class of compounds, α -ketocarboxylic acids have high free energies of formation that disfavor their production. We report herein the production of pyruvate from lactate and of α -ketoglutarate from pyruvate in the millimolar concentration range as promoted by ZnS mineral photoelectrochemistry. Pyruvate is produced from the photooxidation of lactate with 70% yield and a quantum efficiency of 0.009 at 15°C across the wavelength range of 200–400 nm. The produced pyruvate undergoes photoreductive back reaction to lactate at a 30% yield and with a quantum efficiency of 0.0024. Pyruvate alt...

Abstract: From urea to nucleobases : Freeze–thaw cycles in urea ( 1 ) solutions under methane/nitrogen atmospheres lead, with application of an energy source, to the synthesis of pyrimidines (mainly cytosine ( 2 ) and uracil ( 3 )), triazines (such as cyanuric acid ( 4 )), and adenine. This synthesis appears to be dependent on the atmosphere and the freezing conditions. At room temperature, hydantoin ( 5 ) is obtained. However, a freezing urea/water system subjected to an energy source under an inert atmosphere generates s -triazines. Herein, we report the efficient synthesis of RNA bases and functionalized s -triazines from 0.1  M urea solutions in water after subjection to freeze–thaw cycles for three weeks. The icy solution was under a reductive, methane-based atmosphere, which was subjected to spark discharges as an energy source for the first 72 h of the experiment. Analysis of the products indi...

Abstract: The carboxylic acids produced by the reductive tricarboxylic acid (rTCA) cycle are possibly a biosynthetic core of initial life, although several steps such as the reductive kinetics of oxaloacetate (OAA) to malate (MA) are problematic by conventional chemical routes. In this context, we studied the kinetics of this reaction as promoted by ZnS mineral photoelectrochemistry. The quantum efficiency φ MA of MA production from the photoelectrochemical reduction of OAA followed φ MA =0.13 [OAA] (2.1×10 −3 +[OAA]) −1 and was independent of temperature (5 to 50°C). To evaluate the importance of this forward rate under a prebiotic scenario, we also studied the temperature-dependent rate of the backward thermal decarboxylation of OAA to pyruvate (PA), which followed an Arrhenius behavior as log ( k −2 )=11.74–4956/ T , where k −2 is in units of s −1 . These measured rates were employed in conjunction with the indirectly estimated carboxylation rate of PA to OAA to asse...

Abstract. It has been shown that sunlit snow and ice plays an important role in processing atmospheric species. Photochemical production of a variety of chemicals has recently been reported to occur in snow/ice and the release of these photochemically generated species may significantly impact the chemistry of the overlying atmosphere. Nitrogen oxide and oxidant precursor fluxes have been measured in a number of snow covered environments, where in some cases the emissions significantly impact the overlying boundary layer. For example, photochemical ozone production (such as that occurring in polluted mid-latitudes) of 3–4 ppbv/day has been observed at South Pole, due to high OH and NO levels present in a relatively shallow boundary layer. Field and laboratory experiments have determined that the origin of the observed NO x flux is the photochemistry of nitrate within the snowpack, however some details of the mechanism have not yet been elucidated. A variety of low molec...

  Abstract: The abnormal spikes detected in some CO and CO 2 polar ice core records indicate persistent chemical activity in glacial ice. Since CO and CO 2 spikes are correlated, and their amplitudes scale with reported CO/CO 2 yields for the photolysis of dissolved natural organic matter, a common photochemical source is implicated. Given that sufficient actinic radiation is constantly generated throughout ice by cosmic muons (Colussi and Hoffmann, 2003), it remains to be shown that the photolyses of typical organic contaminants proceed by similar mechanisms in water and ice. Here we report that the photodecarboxylation of pyruvic acid (PA, an ubiquitous ice contaminant) indeed leads to the same products nearly as efficiently in both media. CO 2 is promptly released from frozen PA/H 2 O films upon illumination and continues to evolve after photolysis. By analogy with our studies in water (Guzmán et al., 2006b), we infer that 3 PA* reacts with PA in ice producing CH ...

  Abstract: About 3.5 ± 0.3 water molecules are still involved in the exothermic hydration of 2-oxopropanoic acid (PA) into its monohydrate (2,2-dihydroxypropanoic acid, PAH) in ice at 230 K. This is borne out by thermodynamic analysis of the fact that Q H ( T ) = [PAH]/[PA] becomes temperature independent below ∼250 K (in chemically and thermally equilibrated frozen 0.1 ≤ [PA]/ M ≤ 4.6 solutions in D 2 O), which requires that the enthalpy of PA hydration (Δ H H ∼ −22 kJ mol - 1 ) be balanced by a multiple of the enthalpy of ice melting (Δ H M = 6.3 kJ mol - 1 ). Considering that:  (1) thermograms of frozen PA solutions display a single endotherm, at the onset of ice melting, (2) the sum of the integral intensities of the 1 δ PAH and 1 δ PA methyl proton NMR resonances is nearly constant while, (3) line widths increase exponentially with decreasing temperature before diverging below ∼230 K, we infer that PA in ice remains cooperatively hydrated within interstitial m...

  Abstract: Ice is selectively intolerant to impurities. A preponderance of implanted anions or cations generates electrical imbalances in ice grown from electrolyte solutions. Since the excess charges are ultimately neutralized via interfacial (H + /HO - ) transport, the acidity of the unfrozen portion can change significantly and permanently. This insufficiently recognized phenomenon should critically affect rates and equilibria in frozen media. Here we report the effective 19 F NMR chemical shift of 3-fluorobenzoic acid as in situ probe of the acidity of extensively frozen electrolyte solutions. The sign and magnitude of the acidity changes associated with freezing are largely determined by specific ion combinations, but depend also on solute concentration and/or the extent of supercooling. NaCl solutions become more basic, those of (NH 4 ) 2 SO 4 or Na 2 SO 4 become more acidic, while solutions of the 2-( N -morpholino)ethanesulfonic acid zwitterion barely chang...

Abstract: The 320 nm-band photodecarboxylation of aqueous pyruvic acid (PA), a representative of the α-oxocarboxylic acids widely found in the atmospheric aerosol, yields 2,3-dimethyltartaric ( A ) and 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic ( B ) acids, rather than 3-hydroxy-2-oxobutanone as previously reported. A and B are identified by liquid chromatography with UV and ESI-MS detection, complemented by collisionally induced dissociation and 2 H and 13 C isotope labeling experiments. The multifunctional ether B gives rise to characteristic δ ∼ 80 ppm 13 C NMR resonances. Product quantum yields are proportional to [PA]( a + [PA]) -1 in the range [PA] = 5−100 mM. CO 2 (g) release rates are halved, while A and B are suppressed by the addition of >1.5 mM TEMPO. A and B are only partially quenched in air-saturated solutions. These observations are shown to be consistent with an oligomerization process initiated by a bimolecular reaction between 3 PA* and PA producing k...

  Abstract: The λ > 300 nm photolysis of h 4 - or d 4 -pyruvic acid aqueous glasses at 77 K yields identical electron magnetic resonance (EMR) spectra arising from distant ( r ≳ 0.5 nm) triplet radical pairs. Spectra comprise:  (1) well-resolved quartets, X , at g ∼ g e , that closely match the powder spectra of spin pairs interacting across r ∼ 1.0 nm with D ∼ 3.0 mT, E ∼ 0 mT zero field splittings (ZFS), and (2) broad signals, Y, centered at g ∼ 2.07 that display marked g -anisotropy and g -strain, exclude D ≳ 20.0 mT values (i.e., r ≲ 0.5 spin nm separations), and track the temperature dependence of related g ∼ 4 features. These results imply that the n − π * excitation of pyruvic acid, PA, induces long-range electron transfer from the promoted carbonyl chromophore into neighboring carbonyl acceptors, rather than homolysis into contact radical pairs or concerted decarboxylation into a carbene. Since PA is associated into hydrogen-bonded dimers prior to vitrifi...

Abstract: Cracking, flaking, and separation of paint films containing lead white from the substrate on medieval parchment manuscripts and other works of art have been frequently reported by conservators. In this study, Fourier transform infrared spectroscopy (FTIR) was used to follow up the changes induced by the pigment on some properties of proteinaceous binding media commonly mentioned in medieval treatises. Evidence of the removal of water molecules from the vicinity of the peptide linkage groups C=O and CNH, with a small change in the protein's secondary structure due to aggregation, was obtained. The properties of the lead white-binder samples were compared to those of massicot-containing mixtures. Massicot was also found to promote aggregation of the binders, though it induces a smaller change than does lead white in the hydrogen-bonding characteristics of the peptide CNH group.   Silvia A. Centeno, Marcelo I. Guzman, Akiko Yamazakikleps and Carlos O. Della Vedo...

  Ab stract: The stereochemistry and absolute configuration of the four deodarols and two deodarones derived from R-(+)-limonene is described. The stereochemistry and absolute configuration of the four deodarols 3 and two deodarones 4 derived from (R)-(+)-limonene 1 is described. Margarita B. Villecco, Luis R. Hernández, Marcelo I. Guzman, Cesar A.N. Catalan, Marı́a A. Bucio, Pedro Joseph-Nathan. Synthesis, stereochemistry and absolute configuration of deodarols and deodar ones.Tetrahedron: Asymmetry, Volume 12, Issue 21, 26 November 2001, Pages 2947-2953, https://doi.org/10.1016/S0957-4166(01)00521-3