Heterogeneous Interactions between Gas-Phase Pyruvic Acid and Hydroxylated Silica Surfaces: A Combined Experimental and Theoretical Study

Heterogeneous Reactions of Phenol on Different Components of Mineral Dust Aerosol: Formation of Oxidized Organic and Nitro-Phenolic Compounds

Phenol, a common semi-volatile compound associated with different emissions including from plants and biomass burning, as well as anthropogenic emissions and its derivatives, are important components of secondary organic aerosols (SOAs). Gas and aqueous phase reactions of phenol, in the presence of photochemical drivers, are fairly well understood. However, despite observations showing aromatic content within SOA size and mass increases during dust episodes, the heterogeneous reactions of phenol with mineral dusts are poorly understood. In the current study, surface reactions of phenol at the gas/solid interface with different components of mineral dust including SiO2, α-Fe2O3, and TiO2 have been investigated. Whereas reversible surface adsorption of phenol occurs on SiO2 surfaces, for both α-Fe2O3 and TiO2 surfaces, phenol reacts to form a wide range of OH substituted aromatic products. For α-Fe2O3 surfaces that have been nitrated by gas-phase reactions of nitric acid prior to exposure to phenol, unique compounds form on the surface including nitro-phenolic compounds. Moreover, additional surface chemistry was observed when adsorbed nitro-phenolic products were exposed to gas-phase SO2 as a result of the formation of adsorbed nitrite from nitrate redox chemistry with adsorbed SO2. Overall, this study reveals the extensive chemistry as well as the complexity of reactions of prevalent organic compounds leading to the formation of SOA on mineral surfaces.

Adsorption of 6-MHO on two indoor relevant surface materials: SiO2 and TiO2

The compound 6-methyl-5-hepten-2-one (6-MHO) is a product of skin oil ozonolysis and is of significance in understanding the role of human occupants in the indoor environment. We present a joint computational and experimental study investigating the adsorption of 6-MHO on two model indoor relevant surfaces, SiO₂, a model for a glass window, and TiO₂, a component of paint and self-cleaning surfaces. Our classical force field-based molecular dynamics, ab initio molecular dynamics simulations, and FTIR absorption spectra indicate 6-MHO can adsorb on to both of these surfaces via hydrogen and π-hydrogen bonds and is quite stable due to the linear geometry of 6-MHO. Detailed analysis of 6-MHO on the SiO₂ surface shows that relative humidity does not impact surface adsorption and adsorbed water does not displace 6-MHO from the hydroxylated SiO₂ surface. Additionally, the desorption kinetics of 6-MHO from the hydroxylated SiO₂ surface is compared to other compounds found in indoor environments and 6-MHO is shown to desorb with a first order rate constant that is approximately four times slower than that of limonene, but six times faster than that of carvone. In addition, our joint results indicate 6-MHO forms a stronger interaction with the TiO₂ surface compared to the SiO₂ surface. This study suggests that skin oil ozonolysis products can partition to indoor surfaces leading to the formation of organic films.

Heterogeneous Formation of Organonitrates (ON) and Nitroxy-Organosulfates (NOS) from Adsorbed α-Pinene-Derived Organosulfates (OS) on Mineral Surfaces

Organonitrates (ON) and nitroxy-organosulfates (NOS) are important components of secondary organic aerosols (SOAs). Gas-phase reactions of α-pinene (C₁₀H₁₆), a primary precursor for several ON compounds, are fairly well understood although formation pathways for NOS largely remain unknown. NOS formation may occur via reactions of ON and organic peroxides with sulfates as well as through radical-initiated photochemical processes. Despite the fact that organosulfates (OS) represent a significant portion of the organic aerosol mass, ON and NOS formation from OS is less understood, especially through nighttime heterogeneous and multiphase chemistry pathways. In the current study, surface reactions of adsorbed α-pinene-derived OS with nitrogen oxides on hematite and kaolinite surfaces, common components of mineral dust, have been investigated. α-Pinene reacts with sulfated mineral surfaces, forming a range of OS compounds on the surface. These OS compounds when adsorbed on mineral surfaces can further react with HNO₃ and NO₂, producing several ON and NOS compounds as well as several oxidation products. Overall, this study reveals the complexity of reactions of prevalent organic compounds leading to the formation of OS, ON, and NOS via heterogeneous and multiphase reaction pathways on mineral surfaces. It is also shown that this chemistry is mineralogy-specific.

Heterogeneous Reactions of α-Pinene on Mineral Surfaces: Formation of Organonitrates and α-Pinene Oxidation Products

Organonitrates (ON) are important components of secondary organic aerosols (SOAs). α-Pinene (C₁₀H₁₆), the most abundant monoterpene in the troposphere, is a precursor for the formation of several of these compounds. ON from α-pinene can be produced in the gas phase via photochemical processes and/or following reactions with oxidizers including hydroxyl radical and ozone. Gas-phase nitrogen oxides (NO₂, NO₃) are N sources for ON formation. Although gas-phase reactions of α-pinene that yield ON are fairly well understood, little is known about their formation through heterogeneous and multiphase pathways. In the current study, surface reactions of α-pinene with nitrogen oxides on hematite (α-Fe₂O₃) and kaolinite (SiO₂Al₂O₃(OH)₄) surfaces, common components of mineral dust, have been investigated. α-Pinene oxidizes upon adsorption on kaolinite, forming pinonaldehyde, which then dimerizes on the surface. Furthermore, α-pinene is shown to react with adsorbed nitrate species on these mineral surfaces producing multiple ON and other oxidation products. Additionally, gas-phase oxidation products of α-pinene on mineral surfaces are shown to more strongly adsorb on the surface compared to α-pinene. Overall, this study reveals the complexity of reactions of prevalent organic compounds such as α-pinene with adsorbed nitrate and nitrogen dioxide, revealing new heterogeneous reaction pathways for SOA formation that is mineralogy specific.

Lower degree of dissociation of pyruvic acid at water surfaces than in bulk

Understanding the acid/base behavior of environmentally relevant organic acids is of key relevance for accurate climate modelling. Here we investigate the effect of pH on the (de)protonation state of pyruvic acid at the air-water interface and in bulk by using the analytical techniques surface-specific vibrational sum frequency generation and attenuated total reflection spectroscopy. To provide a molecular interpretation of the observed behavior, simulations are carried out using a free energy perturbation approach in combination with electronic structure-based molecular dynamics. In both the experimental and theoretical results we observe that the protonated form of pyruvic acid is preferred at the air-water interface. The increased proton affinity is the result of the specific microsolvation at the interface.

Nanoscopic Study of Water Uptake on Glass Surfaces with Organic Thin Films and Particles from Exposure to Indoor Cooking Activities: Comparison to Model Systems

Water uptake by thin organic films and organic particles on glass substrates at 80% relative humidity was investigated using atomic force microscopy-infrared (AFM-IR) spectroscopy. Glass surfaces exposed to kitchen cooking activities show a wide variability of coverages from organic particles and organic thin films. Water uptake, as measured by changes in the volume of the films and particles, was also quite variable. A comparison of glass surfaces exposed to kitchen activities to model systems shows that they can be largely represented by oxidized oleic acid and carboxylate groups on long and medium hydrocarbon chains (i.e., fatty acids). Overall, we demonstrate that organic particles and thin films that cover glass surfaces can take up water under indoor-relevant conditions but that the water content is not uniform. The spatial heterogeneity of the changes in these aged glass surfaces under dry (5%) and wet (80%) conditions is quite marked, highlighting the need for studies at the nano- and microscale.

Uptake and hydration of sulfur dioxide on dry and wet hydroxylated silica surfaces: a computational study

We present a first-principles molecular dynamics study on the uptake and hydration of sulfur dioxide on the dry and wet fully hydroxylated surfaces of (0001) α-quartz, which are a proxy for suspended silica dust in the atmosphere. The average adsorption energy for SO₂ is about -10 kcal mol^-1 on both dry and wet surfaces. The adsorption is driven by hydrogen bond formation between SO₂ and the interfacial hydroxyl groups (on dry silica), or with water molecules (in the wet case). In the dry system, we report an additional electrostatic interaction between the interfacial hydroxyl oxygen and the sulfur atom, which further stabilizes the adsorbate. On dry silica, the interfacial hydroxyl group coordinates to SO₂ yielding a surface bound bisulfite (Si-SO₃H) complex. On the wet surface, SO₂ reacts with water forming bisulfite (HSO₃^-), and the latter remains solvated inside the adsorbed water layer. The hydration barrier for sulfur dioxide is 1 kcal mol^-1 and 3 kcal mol^-1 on dry and wet silica, respectively, while for the backward reaction (i.e., bisulfite to SO₂) the barrier is 6 kcal mol^-1 on both surfaces. The modest backward barrier rationalizes earlier experimental findings showing no SO₂ uptake on silica. These results underline the importance of the surface hydroxylation and/or adsorbed water layers for the SO₂ uptake and its hydration on silica. Moreover, the hydration to bisulfite may prevent direct SO₂ photochemistry and be an additional source of sulfate; this is especially relevant in atmospheres subject to a high level of suspended mineral dust, intense solar radiation and atmospheric oxidizers.

Lactic Acid Spectroscopy: Intra- and Intermolecular Interactions

Lactic acid, a relevant molecule in biology and the environment, is an α-hydroxy acid with a high propensity to form hydrogen bonds, both internally and to other hydrogen-bond-accepting molecules. This work includes the novel recording of infrared spectra of gas-phase lactic acid using Fourier transform infrared spectroscopy, and the vibrational absorption features of lactic acid are assigned with the aid of computationally simulated vibrational spectra with anharmonic corrections. Theoretical chemistry methods are used to relate intramolecular hydrogen-bond strengths to the relative stability of lactic acid conformers. The formation of hydrogen-bonded lactic acid dimers and 1:1 water complexes is investigated by simulated vibrational spectra and calculated thermodynamic parameters for the lactic acid monomer and dimer and its water complex in the gas phase. The results of this study are discussed in the context of environmental chemistry with an emphasis on indoor environments.

Impact of Adsorbed Water on the Interaction of Limonene with Hydroxylated SiO2: Implications of π-Hydrogen Bonding for Surfaces in Humid Environments

The indoor environment is a dynamic one with many variables impacting indoor air quality and indoor air chemistry. These include relative humidity (RH) and the presence of different surfaces. Although it has been suggested that the indoor concentrations of gas-phase compounds increase at higher relative humidity, because of displacement of these compounds from indoor surfaces, little is known from a molecular perspective about how RH and adsorbed water impact the adsorption of indoor relevant organic compounds such as limonene with indoor relevant surfaces. Herein, we investigate the effects of RH on the adsorption of limonene, a hydrophobic molecule, on hydroxylated SiO₂ surfaces, a model for glass surfaces. Experimental data using infrared spectroscopy to directly measure limonene adsorption are combined with both force field-based molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulations to understand the competitive interactions between limonene, water, and the SiO₂ surface. The spectroscopic data provide evidence that adsorbed limonene is not completely displaced by adsorbed water, even at high RH (∼80%) when the water layer coverage is close to three monolayers (MLs). These experimental data are supported by AIMD and MD simulations, which indicate that limonene is present at the adsorbed water interface but displaced from direct interactions with SiO₂. This study shows that although some limonene can desorb from the surface, even at the highest RH, more than half the limonene remains adsorbed on the surface that can undergo continued surface reactivity. A complex network of π-hydrogen bonds, water-water hydrogen bonds, and SiO₂-water hydrogen bonds explains these interactions at the air/adsorbed water/SiO₂ interface that hold the hydrophobic limonene molecule at the interface. Importantly, these interactions are most likely present for a range of other systems involving organic compounds and solid surfaces at ambient relative humidity and may be important in a range of scientific areas, from sensor development to cultural heritage science.

Glass surface evolution following gas adsorption and particle deposition from indoor cooking events as probed by microspectroscopic analysis

Indoor surfaces are extremely diverse and their interactions with airborne compounds and aerosols influence the lifetime and reactivity of indoor emissions. Direct measurements of the physical and chemical state of these surfaces provide insights into the underlying physical and chemical processes involving surface adsorption, surface partitioning and particle deposition. Window glass, a ubiquitous indoor surface, was placed vertically during indoor activities throughout the House Observations of Microbial and Environmental Chemistry (HOMEChem) campaign and then analyzed to measure changes in surface morphology and surface composition. Atomic force microscopy-infrared (AFM-IR) spectroscopic analyses reveal that deposition of submicron particles from cooking events is a contributor to modifying the chemical and physical state of glass surfaces. These results demonstrate that the deposition of glass surfaces can be an important sink for organic rich particles material indoors. These findings also show that particle deposition contributes enough organic matter from a single day of exposure equivalent to a uniform film up to two nanometers in thickness, and that the chemical distinctness of different indoor activities is reflective of the chemical and morphological changes seen in these indoor surfaces. Comparison of the experimental results to physical deposition models shows variable agreement, suggesting that processes not captured in physical deposition models may play a role in the sticking of particles on indoor surfaces.

Adsorption and isomerization of glyoxal and methylglyoxal at the air/hydroxylated silica surface

We present results from molecular dynamics simulations coupled with enhanced sampling techniques on the adsorption and isomerization of glyoxal (GL) and methylglyoxal (MG) at the air/hydroxylated silica (α-Quartz) interface. GL and MG are two organic compounds present in the atmosphere as oxidation products of both biogenic and anthropogenic precursors. By adsorption and hydration on liquid droplets or wetted dust particles, they can enable aerosol growth in the atmosphere. Moreover, thanks to the different polar characters of their trans and cis conformers, GL and MG have been suggested as possible molecular switches capable of responding to changes in solvent polarity. Here, we show that the hydroxylated silica surface does not significantly catalyze the trans-to-cis isomerization, but it stabilizes the cis-isomers, indicating a higher interfacial cis/trans relative concentration compared to the gas phase. Moreover, adsorbed GL prefers to lie parallel on the silica surface, while adsorbed MG shows a tilted orientation. In particular, we report the aldehyde group pointing upward (downward) to the gas phase (to the silica surface) in trans-MG (cis-MG). These results will help in the rationalization of upcoming experimental and modeling work on the adsorption of ketonic compounds on dust aerosols, while it clarifies the catalytic role of the solid substrate surface in promoting conformational changes.

Absorption spectra of pyruvic acid in water: insights from calculations for small hydrates and comparison to experiment

This study shows that small hydrate models including the roles of both neutral and deprotonated speciated forms provide a good quantitative description and a microscopic interpretation of the experimental spectrum of pyruvic acid in aqueous solution.

The Case of Formic Acid on Anatase TiO2 (101): Where is the Acid Proton?

Carboxylic-acid adsorption on anatase TiO₂ is a relevant process in many technological applications. Yet, despite several decades of investigations, the acid-proton localization-either on the molecule or on the surface-is still an open issue. By modeling the adsorption of formic acid on top of anatase(101) surfaces, we highlight the formation of a short strong hydrogen bond. In the 0 K limit, the acid-proton behavior is ruled by quantum delocalization effects in a single potential well, while at ambient conditions, the proton undergoes a rapid classical shuttling in a shallow two-well free-energy profile. This picture, supported by agreement with available experiments, shows that the anatase surface acts like a protecting group for the carboxylic acid functionality. Such a new conceptual insight might help rationalize chemical processes involving carboxylic acids on oxide surfaces.