A surface-promoted redox reaction occurs spontaneously on solvating inorganic aerosol surfaces

The Structure of Carbon Dioxide at the Air-Water Interface and its Chemical Implications

The efficient reduction of CO2 into valuable products is a challenging task in an international context marked by the climate change crisis and the need to move away from fossil fuels. Recently, the use of water microdroplets has emerged as an interesting reaction media where many redox processes which do not occur in conventional solutions take place spontaneously. Indeed, several experimental studies in microdroplets have already been devoted to study the reduction of CO2 with promising results. The increased reactivity in microdroplets is thought to be linked to unique electrostatic solvation effects at the air-water interface. In the present work, we report a theoretical investigation on this issue for CO2 using first-principles molecular dynamics simulations. We show that CO2 is stabilized at the interface, where it can accumulate, and that compared to bulk water solution, its electron capture ability is larger. Our results suggest that reduction of CO2 might be easier in interface-rich systems such as water microdroplets, which is in line with early experimental data and indicate directions for future laboratory studies. The effect of other relevant factors which could play a role in CO2 reduction potential is discussed.

Water-Catalyzed Formation of Reactive Oxygen Species from NO2 on a Weakly Hydrated Calcite Surface

The interfaces of weakly hydrated mineral substrates have been shown to serve as catalytic sites for chemical reactions that may not be accessible in the gas phase or under bulk conditions. Currently known mechanisms for the formation of reactive oxygen species (ROS) from nitrogen dioxide (NO2) involve NO2 dimerization. Here, we report the formation of the ROS HONO via a mechanism involving simple adsorption of a single NO2 molecule on a weakly hydrated calcite substrate. First-principles molecular dynamics simulations coupled with enhanced sampling techniques show how an adsorbed water sublayer can enhance NO2 adsorption on calcite compared to adsorption on a bare dry substrate. On the weakly hydrated calcite surface, an interfacial electric field facilitates proton extraction from water, thus allowing HONO formation from a single adsorbed NO2, i.e., without the need for the formation of a NO2 dimer precomplex. HONO formation on calcite is kinetically more favorable than that in the gas phase, with a reaction barrier of 14 kcal/mol on the weakly hydrated calcite surface compared to 27 kcal/mol in the gas phase. Further photocatalysed HONO production by visible light and HONO dissociation are hampered on calcite, unlike the process on silica. NO2 is a significant anthropogenic pollutant, and understanding its chemistry is crucial for explaining the high ROS levels and haze formation in polluted areas or prebiotic ROS generation. These findings emphasize how mineral substrates under water-restricted hydration conditions can trigger chemical pathways that are unexpected in the gas phase or under bulk conditions.

Revealing the Contribution of Interfacial Processes to Atmospheric Oxidizing Capacity in Haze Chemistry

The atmospheric oxidizing capacity is the most important driving force for the chemical transformation of pollutants in the atmosphere. Traditionally, the atmospheric oxidizing capacity mainly depends on the concentration of O3 and other gaseous oxidants. However, the atmospheric oxidizing capacity based on gas-phase oxidation cannot accurately describe the explosive growth of secondary particulate matter under complex air pollution. From the chemical perspective, the atmospheric oxidizing capacity mainly comes from the activation of O2, which can be achieved in both gas-phase and interfacial processes. In the heterogeneous or multiphase formation pathways of secondary particulate matter, the enhancement of oxidizing capacity ascribed to the O2/H2O-involved interfacial oxidation and hydrolysis processes is an unrecognized source of atmospheric oxidizing capacity. Revealing the enhanced oxidizing capacity due to interfacial processes in high-concentration particulate matter environments and its contribution to the formation of secondary pollution are critical in understanding haze chemistry. The accurate evaluation of atmospheric oxidizing capacity ascribed to interfacial processes is also an important scientific basis for the implementation of PM2.5 and O3 collaborative control in China and around the world.

Insights into the photocatalytic degradation of hydrophobic organic contaminants on the surface of nitrogen doped silica: New findings of the formation of silicon-based substitution products

In this work, nitrogen-doped SiO2 (N-SiO2) was successfully synthesized to develop an "adsorption-photocatalytic degradation" water purification technology to remove hydrophobic organic contaminants (HOCs). As a representative of HOCs, decabromodiphenylethane (DBDPE) could be efficiently degraded under simulated sunlight after adsorption on the surface of N-SiO2. Due to the generation of reactive oxygen species (ROS) and silicon-based radicals, the photodegradation rate of DBDPE on water-SiO2 interface was 1.5-fold higher than that in water. Furthermore, the transformation pathways of DBDPE on N-SiO2 surface were compared with that in water. Bond breaking and debromination reactions were the common pathways, while hydroxylation and silicon-based substitution reactions were the specific transformation pathways for DBDPE on the surface of N-SiO2. Density functional theory (DFT) calculation was used to reveal the generation mechanism of silicon-based radicals and determine the rationality of the involvement of silicon-based radicals in DBDPE transformation. The energy barriers of silicon-based substitution reaction were comparable to that of hydroxylation and debromination reactions, which confirmed the plausibility of the generation of silicon-based substitution products. This study provides an efficient method for the disposal of HOCs, which also gives some new insights into the conversion mechanism of organic pollutants mediated by silicon-based radicals.

Liquid-Vapor Interface of Aqueous Ethylene Glycol Solutions: A Molecular Dynamics Study

While the organic constituent in an aqueous binary solution enriches its liquid-vapor (l-v) interface, the extent of enrichment can depend nonlinearly on its mole fraction. A microscopic quantification and rationalization of this behavior are crucial to understand the dependence of properties such as surface tension and evaporation rate of the solution on its composition. Extensive all-atom molecular dynamics simulations of aqueous ethylene glycol (EG) solutions show that the composition of the solution at the l-v interface deviates the most from that in the bulk solution at an EG mole fraction of 0.3. The population of EG molecules with their central C-C dihedral in the gauche conformation was found to be higher at the l-v interface than that in the bulk solution to facilitate the orientation of its hydrophobic methyl groups toward the vapor phase. Free energy calculations reveal that in dilute EG solutions, an EG molecule is most stable at the l-v interface. The behavior of vapor pressure in aqueous EG solutions is ideal and follows Raoult's law, while in contrast, the aqueous solution of dimethyl sulfoxide does not. A rationale for the same is provided through the orientational distribution of interfacial water molecules in the respective solutions.

Uptake and reactivity of NO2 on the hydroxylated silica surface: A source of reactive oxygen species

We report state-of-the-art first-principles molecular dynamics results on the heterogeneous chemical uptake of NO2, a major anthropogenic pollutant, on the dry and wet hydroxylated surface of α-quartz, which is a significant component of silica-based catalysts and atmospheric dust aerosols. Our investigation spotlights an unexpected chemical pathway by which NO2 (i) can be adsorbed as HONO by deprotonation of interfacial silanols (i.e., -Si-OH group) on silica, (ii) can be barrierless converted to nitric acid, and (iii) can finally dissociated to surface bounded NO and hydroxyl gas phase radicals. This chemical pathway does not invoke any previously experimentally postulated NO2 dimerization, dimerization that is less likely to occur at low NO2 concentrations. Moreover, water significantly catalyzes the HONO formation and the dissociation of nitric acid into surface-bounded NO and OH radicals, while visible light adsorption can further promote these chemical transformations. This work highlights how water-restricted solvation regimes on common mineral substrates are likely to be a source of reactive oxygen species, and it offers a theoretical framework for further and desirable experimental efforts, aiming to better constrain trace gases/mineral interactions at different relative humidity conditions.

Probing Complex Chemical Processes at the Molecular Level with Vibrational Spectroscopy and X-ray Tools

Understanding the origins of structure and bonding at the molecular level in complex chemical systems spanning magnitudes in length and time is of paramount interest in physical chemistry. We have coupled vibrational spectroscopy and X-ray based techniques with a series of microreactors and aerosol beams to tease out intricate and sometimes subtle interactions, such as hydrogen bonding, proton transfer, and noncovalent interactions. This allows for unraveling the self-assembly of arginine-oleic acid complexes in an aqueous solution and growth processes in a metal-organic framework. Terahertz and infrared spectroscopy provide an intimate view of the hydrogen-bond network and associated phase changes with temperature in neopentyl glycol. The hydrogen-bond network in aqueous glycerol aerosols and levels of protonation of nicotine in aqueous aerosols are visualized. Future directions in probing the hydrogen-bond networks in deep eutectic solvents and organic frameworks are described, and we suggest how X-ray scattering coupled to X-ray spectroscopy can offer insight into the reactivity of organic aerosols.

Spontaneous Iodide Activation at the Air-Water Interface of Aqueous Droplets

We present experimental evidence that atomic and molecular iodine, I and I2, are produced spontaneously in the dark at the air-water interface of iodide-containing droplets without any added catalysts, oxidants, or irradiation. Specifically, we observe I3- formation within droplets, and I2 emission into the gas phase from NaI-containing droplets over a range of droplet sizes. The formation of both products is enhanced in the presence of electron scavengers, either in the gas phase or in solution, and it clearly follows a Langmuir-Hinshelwood mechanism, suggesting an interfacial process. These observations are consistent with iodide oxidation at the interface, possibly initiated by the strong intrinsic electric field present there, followed by well-known solution-phase reactions of the iodine atom. This interfacial chemistry could be important in many contexts, including atmospheric aerosols.

Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Concurrent dominant pathways of multifunctional products formed from nocturnal isoprene oxidation

Determination of dominant chemical pathways toward the formation of nocturnal secondary organic aerosols (SOA) remains ambiguous by which nitrogen oxides (NO_x) always affect oxidation of volatile alkenes. Here, comprehensive chamber simulations on dark isoprene ozonolysis were conducted under different nitrogen dioxides (NO₂) mixing ratios to exam multiple functionalized isoprene oxidation products. Aside from that the oxidation processes were concurrently driven by nitrogen radical (NO₃) and small hydroxyl radicals (OH), ozone (O₃) cycloaddition at isoprene was launched initially regardless of NO₂ to rapidly form first-generation oxidation products, i.e., carbonyls and Criegee intermediates (CI) referred to carbonyl oxides. They could further undergo complicated self- and cross-reactions to produce alkylperoxy radicals (RO₂). Corresponding to yields of the C₅H₁₀O₃ tracer, weak OH pathway at night was credited to ozonolysis of isoprene but suppressed by unique NO₃ chemistry. Following the ozonolysis of isoprene, NO₃ played a crucial supplementary role in nighttime SOA formation. The ensuing production of gas-phase nitrooxy carbonyls (the first-generation nitrates) became dominant in the production of a sizeable pool of organic nitrates (RO₂NO₂). By contrast, isoprene dihydroxy dinitrates (C₅H₁₀N₂O₈) were outstanding with the elevated NO₂, related to typical second-generation nitrates. As such, the yielding number concentrations of dark SOA were promoted to approximately 1.8 × 10⁴ cm^-3 but presented a nonlinear relation with excess high-NO₂ condition. This study provides valuable insights into importance of multifunctional organic compounds from alkene oxidation to constitute nighttime SOA.

A critical review of sulfate aerosol formation mechanisms during winter polluted periods

Sulfate aerosol contributes to particulate matter pollution and plays a key role in aerosol radiative forcing, impacting human health and climate change. Atmospheric models tend to substantially underestimate sulfate concentrations during haze episodes, indicating that there are still missing mechanisms not considered by the models. Despite recent good progress in understanding the missing sulfate sources, knowledge on different sulfate formation pathways during polluted periods still involves large uncertainties and the dominant mechanism is under heated debate, calling for more field, laboratory, and modeling work. Here, we review the traditional sulfate formation mechanisms in cloud water and also discuss the potential factors affecting multiphase S(Ⅳ) oxidation. Then recent progress in multiphase S(Ⅳ) oxidation mechanisms is summarized. Sulfate formation rates by different prevailing oxidation pathways under typical winter-haze conditions are also calculated and compared. Based on the literature reviewed, we put forward control of the atmospheric oxidation capacity as a means to abate sulfate aerosol pollution. Finally, we conclude with a concise set of research priorities for improving our understanding of sulfate formation mechanisms during polluted periods.

Unexpected hygroscopic behaviors of individual sub-50 nm NaNO3 nanoparticles observed by in situ atomic force microscopy

Hygroscopicity is one of the most important physicochemical properties of salt nanoparticles, greatly influencing the environment, climate and human health. However, the hygroscopic properties of salt nanoparticles are poorly understood owing to the great challenges of the preparation, preservation and in situ characterization. Here we show the unexpected shape- and size-dependent hygroscopic behaviors of NaNO₃ nanoparticles prepared from molten salts using in situ environment-controlled atomic force microscopy. During the humidifying process, the angular and round sub-50 nm NaNO₃ particles display anisotropic and isotropic water adsorption behaviors, respectively. The sub-10 nm NaNO₃ nanoparticles abnormally shrink and disappear. The growth factors of the NaNO₃ nanoparticles are highly sensitive to their sizes and shapes, and quite different from those of NaNO₃ microparticles. These findings show that the hygroscopic behaviors of salt nanoparticles may not be comprehensively described by the traditional growth factors, and open up a new pathway to study the hygroscopic behaviors of salt nanoparticles.

Second-Order Kinetic Rate Coefficients for the Aqueous-Phase Sulfate Radical (SO4•-) Oxidation of Some Atmospherically Relevant Organic Compounds

The sulfate anion radical (SO₄^•–) is a reactive oxidant formed in the autoxidation chain of sulfur dioxide, among other sources. Recently, new formation pathways toward SO₄^•– and other reactive sulfur species have been reported. This work investigated the second-order rate coefficients for the aqueous SO₄^•– oxidation of the following important organic aerosol compounds (k_SO4): 2-methyltetrol, 2-methyl-1,2,3-trihydroxy-4-sulfate, 2-methyl-1,2-dihydroxy-3-sulfate, 1,2-dihydroxyisoprene, 2-methyl-2,3-dihydroxy-1,4-dinitrate, 2-methyl-1,2,4-trihydroxy-3-nitrate, 2-methylglyceric acid, 2-methylglycerate, lactic acid, lactate, pyruvic acid, pyruvate. The rate coefficients of the unknowns were determined against that of a reference in pure water in a temperature range of 298–322 K. The decays of each reagent were measured with nuclear magnetic resonance (NMR) and high-performance liquid chromatography–high-resolution mass spectrometry (HPLC-HRMS). Incorporating additional SO₄^•– reactions into models may aid in the understanding of organosulfate formation, radical propagation, and aerosol mass sinks.

Sulfur radical formation from the tropospheric irradiation of aqueous sulfate aerosols

It was found that shining natural or artificial sunlight on concentrated solutions of sulfate ions mixed with organics, a mixture commonly found in atmospheric aerosol particles, can generate sulfur-containing radicals under a variety of conditions. This reaction has not previously been characterized in atmospheric chemistry. These reactive radicals can subsequently degrade organic compounds in atmospheric particles, forming a variety of products that stay in the particle water and small molecules that are volatile enough to partition to the gas phase. In particular, this source of sulfur radicals can produce surface-active organosulfates and organic acids.

The sulfate anion radical (SO₄^•–) is known to be formed in the autoxidation chain of sulfur dioxide and from minor reactions when sulfate or bisulfate ions are activated by OH radicals, NO₃ radicals, or iron. Here, we report a source of SO₄^•–, from the irradiation of the liquid water of sulfate-containing organic aerosol particles under natural sunlight and laboratory UV radiation. Irradiation of aqueous sulfate mixed with a variety of atmospherically relevant organic compounds degrades the organics well within the typical lifetime of aerosols in the atmosphere. Products of the SO₄^•– + organic reaction include surface-active organosulfates and small organic acids, alongside other products. Scavenging and deoxygenated experiments indicate that SO₄^•– radicals, instead of OH, drive the reaction. Ion substitution experiments confirm that sulfate ions are necessary for organic reactivity, while the cation identity is of low importance. The reaction proceeds at pH 1–6, implicating both bisulfate and sulfate in the formation of photoinduced SO₄^•–. Certain aromatic species may further accelerate the reaction through synergy. This reaction may impact our understanding of atmospheric sulfur reactions, aerosol properties, and organic aerosol lifetimes when inserted into aqueous chemistry model mechanisms.

Secondary organic aerosol formation from photooxidation of C3H6 under the presence of NH3: Effects of seed particles

Frequently-occurred secondary organic aerosols (SOAs) under low-NO_x conditions contribute to the winter haze episodes and remain unclear in the abundant presence of NH₃. Here, the effects of CaCl₂ seed particles on the photooxidation of low-molecular-weight C₃H₆ with co-existing NO₂ and NH₃ were highlighted and investigated through a chamber-simulation study equipped with high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS). The influences of NH₃ are often overestimated to exclusively enhance SOA yields under a low-[NO₂]₀ condition. Instead, the seeds played a central role in the heterogeneous formation of SOAs in this reaction with two orders of magnitudes higher than that in the absence of seeds at relative humidity (RH) of 82%. Interestedly, the O₃ production was unchanged whether the seeds existed or not, small changes in the production of O₃ were observed whether the seeds existed or not, indicating that the gas-phase conversions of C₃H₆ and NO_x into C1-C3 oxygenated volatile organic compounds (OVOCs) and nitrogen-containing compounds (NOCs) were not affected by seed particles. Given that the ensuing formation of these low-volatile compounds was condensed into nucleation on the seeds, the explosive growth of C₃H₆ SOAs was then stimulated in the addition of NH₃. Besides NO₂ photolysis, the producing O₃ was related to the formation of secondary carbonyls such as formaldehyde and then was consumed in the ·OH generation of approximately 3.40 × 10^-12 molecules cm^-3. This study provides a new insight to better understand the new gas-to-particle formation mechanisms when the haze pollution outbreaks in the complex air mixture.

Solvation and Hydrolysis Reaction of Isocyanic Acid at the Air-Water Interface: A Computational Study

Isocyanic acid (HNCO) is known to be inert to strong oxidants and photolysis in the atmosphere but often appears in different forms of smoke; therefore, it is linked to various smoke-related illnesses due to tobacco usage or wildfire events. To date, the major loss pathway of HNCO is believed to be through its uptake on aerosol droplets. However, the molecular mechanisms underlying such an uptake process are still incompletely understood. Herein, we use the Born-Oppenheimer molecular dynamics (BOMD) simulations to study solvation and hydrolysis reactions of HNCO on water droplets at ambient temperature. The BOMD simulations indicate that the scavenging of HNCO by water droplets is largely attributed to the preferential adsorption of HNCO at the air-water interface, rather than inside bulk water. Specifically, the H atom of HNCO interacts with the O atom of interfacial water, leading to the formation of a hydrogen bond (H-bond) of (HNCO)H···O(H₂O), which prevents HNCO from evaporating. Moreover, the interfacial water can act as H-bond acceptors/donors to promote the proton transfer during the HNCO hydrolysis reaction. Compared to the gas phase, the activation barrier is lowered from 45 to 14 kcal·mol^-1 on the water surface, which facilitates the formation of the key intermediate of NH₂COOH. This intermediate eventually decomposes into NH₃ and CO₂, consistent with the previous study [ Atmos. Chem. Phys. 2016, 16, 703-714]. The new molecular insight into HNCO solvation and reaction on the water surface improves our understanding of the uptake of HNCO on aerosols.

Surface solvation of Martian salt analogues at low relative humidities

Salt aerosols play important roles in many processes related to atmospheric chemistry and the climate systems on both Earth and Mars. Complicated and still poorly understood processes occur on the salt surfaces when interacting with water vapor. In this study, ambient pressure X-ray photoelectron spectroscopy (APXPS) is used to characterize the surface chemical environment of Martian salt analogues originating from saline lakes and playas, as well as their responses to varying relative humidities. Generally, APXPS shows similar ionic compositions to those observed by ion chromatography (IC). However, XPS is a surface-sensitive method while IC is bulk-sensitive and differences are observed for species that preferentially partition to the surface or the bulk. Element-selective surface enhancement of Cl⁻ is observed, likely caused by the presence of SO₄²⁻. In addition, Mg²⁺ is concentrated on the surface while Na⁺ is relatively depleted in the surface layer. Hence, the cations (Na⁺ and Mg²⁺) and the anions (Cl⁻ and SO₄²⁻) show competitive correlations. At elevated relative humidity (RH), no major spectral changes were observed in the XPS results, except for the growth of an oxygen component originating from condensed H₂O. Near-edge X-ray absorption fine structure (NEXAFS) measurements show that the magnesium and sodium spectra are sensitive to the presence of water, and the results imply that the surface is fully solvated already at RH = 5%. The surface solvation is also fully reversible as the RH is reduced. No major differences are observed between sample types and sample locations, indicating that the salts originated from saline lakes commonly have solvated surfaces under the environmental conditions on Earth.

Salt aerosols play important roles in many processes related to atmospheric chemistry and the climate systems on both Earth and Mars.

Midair transformations of aerosols

[Figure: see text].