Direct Determination of Aerosol pH: Size-Resolved Measurements of Submicrometer and Supermicrometer Aqueous Particles

Impact of Molecular Chlorine Production from Aerosol Iron Photochemistry on Atmospheric Oxidative Capacity in North China

Elevated levels of atmospheric molecular chlorine (Cl2) have been observed during the daytime in recent field studies in China but could not be explained by the current chlorine chemistry mechanisms in models. Here, we propose a Cl2 formation mechanism initiated by aerosol iron photochemistry to explain daytime Cl2 formation. We implement this mechanism into the GEOS-Chem chemical transport model and investigate its impacts on the atmospheric composition in wintertime North China where high levels of Cl2 as well as aerosol chloride and iron were observed. The new mechanism accounts for more than 90% of surface air Cl2 production in North China and consequently increases the surface air Cl2 abundances by an order of magnitude, improving the model's agreement with observed Cl2. The presence of high Cl2 significantly alters the oxidative capacity of the atmosphere, with a factor of 20-40 increase in the chlorine radical concentration and a 20-40% increase in the hydroxyl radical concentration in regions with high aerosol chloride and iron loadings. This results in an increase in surface air ozone by about 10%. This new Cl2 formation mechanism will improve the model simulation capability for reactive chlorine abundances in the regions with high emissions of chlorine and iron.

Decreases in Epoxide-Driven Secondary Organic Aerosol Production under Highly Acidic Conditions: The Importance of Acid-Base Equilibria

Isoprene has the highest atmospheric emissions of any nonmethane hydrocarbon, and isoprene epoxydiols (IEPOX) are well-established oxidation products and the primary contributors forming isoprene-derived secondary organic aerosol (SOA). Highly acidic particles (pH 0-3) widespread across the lower troposphere enable acid-driven multiphase chemistry of IEPOX, such as epoxide ring-opening reactions forming methyltetrol sulfates through nucleophilic attack of sulfate (SO42-). Herein, we systematically demonstrate an unexpected decrease in SOA formation from IEPOX on highly acidic particles (pH < 1). While IEPOX-SOA formation is commonly assumed to increase at low pH when more [H+] is available to protonate epoxides, we observe maximum SOA formation at pH 1 and less SOA formation at pH 0.0 and 0.4. This is attributed to limited availability of SO42- at pH values below the acid dissociation constant (pKa) of SO42- and bisulfate (HSO4-). The nucleophilicity of HSO4- is 100× lower than SO42-, decreasing SOA formation and shifting particulate products from low-volatility organosulfates to higher-volatility polyols. Current model parameterizations predicting SOA yields for IEPOX-SOA do not properly account for the SO42-/HSO4- equilibrium, leading to overpredictions of SOA formation at low pH. Accounting for this underexplored acidity-dependent behavior is critical for accurately predicting SOA concentrations and resolving SOA impacts on air quality.

Chemical Kinetics in Microdroplets

Micrometer-sized compartments play significant roles in driving heterogeneous transformations within atmospheric and biochemical systems as well as providing vehicles for drug delivery and novel reaction environments for the synthesis of industrial chemicals. Many reports now indicate that reaction kinetics are accelerated under microconfinement, for example, in sprays, thin films, droplets, aerosols, and emulsions. These observations are dramatic, posing a challenge to our understanding of chemical reaction mechanisms with potentially significant practical consequences for predicting the complex chemistry in natural systems. Here we introduce the idea of kinetic confinement, which is intended to provide a conceptual backdrop for understanding when and why microdroplet reaction kinetics differ from their macroscale analogs.

Direct Observation of Particle-To-Particle Variability in Ambient Aerosol pH Using a Novel Analytical Approach Based on Surface-Enhanced Raman Spectroscopy

The pH of atmospheric aerosols is a key characteristic that profoundly influences their impacts on climate change, human health, and ecosystems. Despite widely performed aerosol pH research, determining the pH levels of individual atmospheric aerosol particles has been a challenge. This study presents a novel analytical technique that utilizes surface-enhanced Raman spectroscopy to assess the pH of individual ambient PM2.5-10 aerosol particles in conjunction with examining their hygroscopic behavior, morphology, and elemental compositions. The results revealed a substantial pH variation among simultaneously collected aerosol particles, ranging from 3.3 to 5.7. This variability is likely related to each particle's unique reaction and aging states. The extensive particle-to-particle pH variability suggests that atmospheric aerosols present at the same time and location can exhibit diverse reactivities, reaction pathways, phase equilibria, and phase separation properties. This pioneering study paves the way for in-depth investigations into particle-to-particle variability, size dependency, and detailed spatial and temporal variations of aerosol pH, thus deepening our understanding of atmospheric chemistry and its environmental implications.

Modeling Novel Aqueous Particle and Cloud Chemistry Processes of Biomass Burning Phenols and Their Potential to Form Secondary Organic Aerosols

Phenols emitted from biomass burning contribute significantly to secondary organic aerosol (SOA) formation through the partitioning of semivolatile products formed from gas-phase chemistry and multiphase chemistry in aerosol liquid water and clouds. The aqueous-phase SOA (aqSOA) formed via hydroxyl radical (•OH), singlet molecular oxygen (1O2*), and triplet excited states of organic compounds (3C*), which oxidize dissolved phenols in the aqueous phase, might play a significant role in the evolution of organic aerosol (OA). However, a quantitative and predictive understanding of aqSOA has been challenging. Here, we develop a stand-alone box model to investigate the formation of SOA from gas-phase •OH chemistry and aqSOA formed by the dissolution of phenols followed by their aqueous-phase reactions with •OH, 1O2*, and 3C* in cloud droplets and aerosol liquid water. We investigate four phenolic compounds, i.e., phenol, guaiacol, syringol, and guaiacyl acetone (GA), which represent some of the key potential sources of aqSOA from biomass burning in clouds. For the same initial precursor organic gas that dissolves in aerosol/cloud liquid water and subsequently reacts with aqueous phase oxidants, we predict that the aqSOA formation potential (defined as aqSOA formed per unit dissolved organic gas concentration) of these phenols is higher than that of isoprene-epoxydiol (IEPOX), a well-known aqSOA precursor. Cloud droplets can dissolve a broader range of soluble phenols compared to aqueous aerosols, since the liquid water contents of aerosols are orders of magnitude smaller than cloud droplets. Our simulations suggest that highly soluble and reactive multifunctional phenols like GA would predominantly undergo cloud chemistry within cloud layers, while gas-phase chemistry is likely to be more important for less soluble phenols. But in the absence of clouds, the condensation of low-volatility products from gas-phase oxidation followed by their reversible partitioning to organic aerosols dominates SOA formation, while the SOA formed through aqueous aerosol chemistry increases with relative humidity (RH), approaching 40% of the sum of gas and aqueous aerosol chemistry at 95% RH for GA. Our model developments of biomass-burning phenols and their aqueous chemistry can be readily implemented in regional and global atmospheric chemistry models to investigate the aqueous aerosol and cloud chemistry of biomass-burning organic gases in the atmosphere.

Spiers Memorial Lecture: Water at interfaces

In this article we discuss current issues in the context of the four chosen subtopics for the meeting: dynamics and nano-rheology of interfacial water, electrified/charged aqueous interfaces, ice interfaces, and soft matter/water interfaces. We emphasize current advances in both theory and experiment, as well as important practical manifestations and areas of unresolved controversy.

Liquid-Liquid Phase Separation in Single Suspended Aerosol Microdroplets

Liquid-liquid phase separation (LLPS) is ubiquitous in ambient aerosols. This specific morphology exerts substantial impacts on the physicochemical properties and atmospheric processes of aerosols, particularly on the gas-particle mass transfer, the interfacial heterogeneous reaction, and the surface albedo. Although there are many studies on the LLPS of aerosols, a clear picture of LLPS in individual aerosols is scarce due to the experimental difficulties of trapping a single particle and mimicking the suspended state of real aerosols. Here, we investigate the phase separation in individual contactless microdroplets by a self-constructed laser tweezer/Raman spectroscopy system. The dynamic transformation of the morphology of optically trapped droplets over the course of humidity cycles is detected by the time-resolved cavity-enhanced Raman spectra. The impacts of pH and inorganic components on LLPS in aerosols are discussed. The results show that the increasing acidity can enhance the miscibility between the hydrophilic and hydrophobic phases and decrease the separation relative humidity of aerosols. Moreover, the inorganic components also have various impacts on the aerosol phase state, whose influence depends on their different salting-out capabilities. It brings possible implications on the morphology of actual atmospheric particles, particularly for those dominated by internal mixtures of inorganic and organic components.

Abiotic synthesis with plausible emergence for primitive phospholipid in aqueous microdroplets

Phospholipids are the protective layer of modern cells, but it is challenging for the formation of phospholipids that require a simple abiotic synthesis before the advent of primitive cells. Here, we reported the abiotic synthesis for lysophosphatidic acids (LPAs) with prebiotically plausible reactants in aqueous microdroplets under ambient conditions. The LPAs formation is carried out by fusing two microdroplets streams: one contains glycerol and pyrophosphate in water and the other one contains fatty acids in acetonitrile. Compared with the bulk solution, LPAs were generated in microdroplets without the addition of catalyst and heating. Conditions of reactant concentrations and microdroplet size varied and suggested that LPAs formation occurred near or at the microdroplet surface. The LPAs formation also showed chemoselective toward on chain-length of fatty acids. Finally, the formation of LPAs underwent two-step reactions with glycerol phosphorylation eliminating one water molecule followed by esterification with fatty acids. These results also implicated that pyrophosphate functioned as both catalysts and precursors in prebiotic LPAs synthesis. The approach using fusion aqueous microdroplets has desirable features in studying the substance exchange and interaction in atmosphere.

Chemometric-Assisted Litmus Test: One Single Sensing Platform Adapted from 1-13 to Narrow pH Ranges

A novel 3 × 4 colorimetric sensing platform, named the chemometric-assisted litmus test (CLT), has been developed by covalently anchoring commercial pH indicators to ethylene vinyl alcohol (EVOH). The proposed device can be exploited for pH determinations in a wide range from 1 to 13 and in specific narrow ranges, achieving sufficient accuracy and errors below 0.5 pH units. The experimental procedure is simple, quick and reliable; equilibration is reached in less than 2 h, CLT pictures are acquired by a camera, and data treatment is performed applying chemometric techniques such as principal component analysis (PCA) and partial least square regression (PLS) to RGB indices.

Highly Altered State of Proton Transport in Acid Pools in Charged Reverse Micelles

Transport mechanisms of solvated protons of 1 M HCl acid pools, confined within reverse micelles (RMs) containing the negatively charged surfactant sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) or the positively charged cetyltrimethylammonium bromide (CTABr), are analyzed with reactive force field simulations to interpret dynamical signatures from TeraHertz absorption and dielectric relaxation spectroscopy. We find that the forward proton hopping events for NaAOT are further suppressed compared to a nonionic RM, while the Grotthuss mechanism ceases altogether for CTABr. We attribute the sluggish proton dynamics for both charged RMs as due to headgroup and counterion charges that expel hydronium and chloride ions from the interface and into the bulk interior, thereby increasing the pH of the acid pools relative to the nonionic RM. For charged NaAOT and CTABr RMs, the localization of hydronium near a counterion or conjugate base reduces the Eigen and Zundel configurations that enable forward hopping. Thus, localized oscillatory hopping dominates, an effect that is most extreme for CTABr in which the proton residence time increases dramatically such that even oscillatory hopping is slow.

Raman spectroscopy for profiling physical and chemical properties of atmospheric aerosol particles: A review

Atmosphere aerosols have significant impact on human health and the environment. Aerosol particles have a number of characteristics that influence their health and environmental effects, including their size, shape, and chemical composition. A great deal of difficulty is associated with quantifying and identifying atmospheric aerosols because these parameters are highly variable on a spatial and temporal scale. An important component of understanding aerosol fate is Raman Spectroscopy (RS), which is capable of resolving chemical compositions of individual particles. This review presented strategic techniques, especially RS methods for characterizing atmospheric aerosols. The nature and properties of atmospheric aerosols and their influence on environment and human health were briefly described. Analytical methodologies that offer insight into the chemistry and multidimensional properties of aerosols were discussed. In addition, perspectives for practical applications of atmospheric aerosols using RS are featured.

Spatiotemporally Resolved pH Measurement in Aerosol Microdroplets Undergoing Chloride Depletion: An Application of In Situ Raman Microspectrometry

Acidity is a defining property of atmospheric aerosols that profoundly affects environmental systems, human health, and climate. However, directly measuring the pH of aerosol microdroplets remains a challenge, especially when the microdroplets' composition is nonhomogeneous or dynamically evolving or both. As a result, a pH measurement technique with high spatiotemporal resolution is needed. Here, we report a spatiotemporally resolved pH measurement technique in microdroplets using spontaneous Raman spectroscopy. Our target sample was the microdroplets comprising sodium chloride and oxalic acid─laboratory surrogates of sea spray aerosols and water-soluble organic compounds, respectively. Our measurements show that the chloride depletion from the microdroplets caused a continuous increase in pH by ∼0.5 units in 2 hours. Meanwhile, the surface propensity of chloride anions triggers a stable pH gradient inside a single droplet, with the pH at the droplet surface lower than that at the core by ∼ 0.4 units. The uncertainties arising from the Raman detection limit (±0.08 pH units) and from the nonideal solution conditions (-0.06 pH units) are constrained. Our findings indicate that spontaneous Raman spectroscopy is a simple yet robust technique for precise pH measurement in aerosols with high spatiotemporal resolution.

Aerosol pH and Ion Activities of HSO4- and SO42- in Supersaturated Single Droplets

Accurate determination of acidity (pH) and ion activities in aqueous droplets is a major experimental and theoretical challenge for understanding and simulating atmospheric multiphase chemistry. Here, we develop a ratiometric Raman spectroscopy method to measure the equilibrium concentration of sulfate (SO₄^2-) and bisulfate (HSO₄^-) in single microdroplets levitated by aerosol optical tweezers. This approach enables determination of ion activities and pH in aqueous sodium bisulfate droplets under highly supersaturated conditions. The experimental results were compared against aerosol thermodynamic model calculations in terms of simulating aerosol ion concentrations, ion activity coefficients, and pH. We found that the Extended Aerosol Inorganics Model (E-AIM) can well reproduce the experimental results. The alternative model ISORROPIA, however, exhibits substantial deviations in SO₄^2- and HSO₄^- concentrations and up to a full unit of aerosol pH under acidic conditions, mainly due to discrepancies in simulating ion activity coefficients of SO₄^2--HSO₄^- equilibrium. Globally, this may cause an average deviation of ISORROPIA from E-AIM by 25 and 65% in predicting SO₄^2- and HSO₄^- concentrations, respectively. Our results show that it is important to determine aerosol pH and ion activities in the investigation of sulfate formation and related aqueous phase chemistry.

Acidity-Dependent Atmospheric Organosulfate Structures and Spectra: Exploration of Protonation State Effects via Raman and Infrared Spectroscopies Combined with Density Functional Theory

Organosulfates formed from heterogeneous reactions of organic-derived oxidation products with sulfate ions can account for >15% of secondary organic aerosol (SOA) mass, primarily in submicron particles with long atmospheric lifetimes. However, fundamental understanding of organosulfate molecular structures is limited, particularly at atmospherically relevant acidities (pH = 0-6). Herein, for 2-methyltetrol sulfates (2-MTSs), an important group of isoprene-derived organosulfates, protonation state and vibrational modes were studied using Raman and infrared spectroscopy, as well as density functional theory (DFT) calculations of vibrational spectra for neutral (RO-SO₃H) and anionic/deprotonated (RO-SO₃^-) structures. The calculated sulfate group vibrations differ for the two protonation states due to their different sulfur-oxygen bond orders (1 or 2 versus 1²/₃ for the neutral and deprotonated forms, respectively). Only vibrations at 1060 and 1041 cm^-1, which are associated with symmetric S-O stretches of the 2-MTS anion, were observed experimentally with Raman, while sulfate group vibrations for the neutral form (∼900, 1200, and 1400 cm^-1) were not observed. Additional calculations of organosulfates formed from other SOA-precursor gases (α-pinene, β-caryophyllene, and toluene) identified similar symmetric vibrations between 1000 and 1100 cm^-1 for RO-SO₃^-, consistent with corresponding organosulfates formed during laboratory experiments. These results suggest that organosulfates are primarily deprotonated at atmospheric pH values, which have further implications for aerosol acidity, heterogeneous reactions, and continuing chemistry in atmospheric aerosols.

Direct measurement of the pH of aerosol particles using carbon quantum dots

The pH of aerosol particles remains challenging to measure because of their small size, complex composition, and high acidity. Acidity in aqueous aerosol particles, which are found abundantly in the atmosphere, impacts many chemical processes from reaction rates to cloud formation. Only one technique - pH paper - currently exists for directly determining the pH of aerosol particles, and this is restricted to measuring average acidity for entire particle populations. Other methods for evaluating aerosol pH include filter samples, particle-into-liquid sampling, Raman spectroscopy, organic dyes, and thermodynamic models, but these either operate in a higher pH range or are unable to assess certain chemical species or complexity. Here, we present a new method for determining acidity of individual particles and particle phases using carbon quantum dots as a novel in situ fluorophore. Carbon quantum dots are easily synthesized, shelf stable, and sensitive to pH in the highly acidic regime from pH 0 to pH 3 relevant to ambient aerosol particles. To establish the method, a calibration curve was formed from the ratiometric fluorescence intensity of aerosolized standard solutions with a correlation coefficient (R²) of 0.99. Additionally, the pH of aerosol particles containing a complex organic mixture (COM) representative of environmental aerosols was also determined, proving the efficacy of using carbon quantum dots as pH-sensitive fluorophores for complex systems. The ability to directly measure aerosol particle and phase acidity in the correct pH range can help parametrize atmospheric models and improve projections for other aerosol properties and their influence on health and climate.

Initial pH Governs Secondary Organic Aerosol Phase State and Morphology after Uptake of Isoprene Epoxydiols (IEPOX)

Aerosol acidity increases secondary organic aerosol (SOA) formed from the reactive uptake of isoprene-derived epoxydiols (IEPOX) by enhancing condensed-phase reactions within sulfate-containing submicron particles, leading to low-volatility organic products. However, the link between the initial aerosol acidity and the resulting physicochemical properties of IEPOX-derived SOA remains uncertain. Herein, we show distinct differences in the morphology, phase state, and chemical composition of individual organic-inorganic mixed particles after IEPOX uptake to ammonium sulfate particles with different initial atmospherically relevant acidities (pH = 1, 3, and 5). Physicochemical properties were characterized via atomic force microscopy coupled with photothermal infrared spectroscopy (AFM-PTIR) and Raman microspectroscopy. Compared to less acidic particles (pH 3 and 5), reactive uptake of IEPOX to the most acidic particles (pH 1) resulted in 50% more organosulfate formation, clearer phase separation (core-shell), and more irregularly shaped morphologies, suggesting that the organic phase transitioned to semisolid or solid. This study highlights that initial aerosol acidity may govern the subsequent aerosol physicochemical properties, such as viscosity and morphology, following the multiphase chemical reactions of IEPOX. These results can be used in future studies to improve model parameterizations of SOA formation from IEPOX and its properties, toward the goal of bridging predictions and atmospheric observations.

A SERS pH sensor for highly alkaline conditions and its application for pH sensing in aerosol droplets

Surface-Enhanced Raman Scattering (SERS) has been widely used in pH sensing. However, SERS sensors capable of stably analysing pH under highly alkaline conditions are still scarce. In this work, a SERS pH sensor employing Alizarin Yellow R as the molecular probe was carefully developed for strong alkaline solutions. The results showed that the probe presented excellent sensing performance in the pH range of 10.04-14.04, including desirable stability and reversibility. Raman band assignments of the probe molecules with the protonated and deprotonated forms were calculated using Gaussian 09. To demonstrate the application, we measured the centroid pH of the phosphate buffer (PB) droplet and compared it to the value obtained with 4-mercaptobenzoic acid (4-MBA) as a probe.

Direct Measurement of pH Evolution in Aerosol Microdroplets Undergoing Ammonium Depletion: A Surface-Enhanced Raman Spectroscopy Approach

Accurately measuring the pH of atmospheric aerosols is a prerequisite for understanding the multiphase chemistry that profoundly affects the environment and climate systems. Despite the advancements of experimental techniques for in situ pH measurements in aerosols, current studies are limited to measuring the static pH of aerosol microdroplets with an unperturbed composition. This steady-state scenario, however, deviates from the real-world aerosols undergoing atmospheric aging reactions, specifically, those characterized with a spontaneous displacement of strong bases (or acids) with high volatility. Here, we introduce a continuous and in situ measurement of aerosol pH by using a 4-mercaptopyridine-functionalized silver nanoparticle probe and surface-enhanced Raman spectroscopy. We find that the ammonium depletion─a spontaneous displacement of ammonium by dicarboxylic acid salts─continuously acidifies aerosol water over time. The decaying trends of pH in the aerosols under various humidity conditions can be unified with a universal exponential function. Such an exponentially decaying function further indicates that the ammonium depletion reaction is a self-limiting process. Our technique can be applied to study the dynamic change of aerosol acidity during the complex atmospheric aging processes, toward elucidating their implications on atmospheric chloride, nitrate, and ammonium cycles.

Photochemical Reactions of Glyoxal during Particulate Ammonium Nitrate Photolysis: Brown Carbon Formation, Enhanced Glyoxal Decay, and Organic Phase Formation

Glyoxal is an important precursor of aqueous secondary organic aerosol (aqSOA). Its photooxidation to form organic acids and oligomers and reactions with reduced nitrogen compounds to form brown carbon (BrC) have been extensively investigated separately, although these two types of reactions can occur simultaneously during the daytime. Here, we examine the reactions of glyoxal during photooxidation and BrC formation in premixed NH₄NO₃ + Glyoxal droplets. We find that nitrate photolysis and photosensitization can enhance the decay rates of glyoxal by a factor of ∼5 and ∼6 compared to those under dark, respectively. A significantly enhanced glyoxal decay rate by a factor of ∼12 was observed in the presence of both nitrate photolysis and photosensitization. Furthermore, a new organic phase was formed in irradiated NH₄NO₃ + Glyoxal droplets, which had no noticeable degradation under prolonged photooxidation. It was attributed to the imidazole oxidation mediated by nitrate photolysis and/or photosensitization. The persistent organic phase suggests the potential to contribute to SOA formation in ambient fine particles. This study highlights that glyoxal photooxidation mediated by nitrate photolysis and photosensitization can significantly enhance the atmospheric sink of glyoxal, which may partially narrow the gap between model predictions and field measurements of ambient glyoxal concentrations.

Water as a probe for pH measurement in individual particles using micro-Raman spectroscopy

Atmospheric aerosol acidity impacts numerous physicochemical processes, but the determination of particle pH remains a significant challenge due to the nonconservative nature of the H⁺ concentration ([H⁺]). Traditional measurements have difficulty in describing the practical state of an aerosol because they comprise chemical components or hypotheses that change the nature of the particles. In this work, we present a direct pH measurement that uses water as a general probe to detect [H⁺] in individual particles by micro-Raman spectroscopy. Containing the vibrational bands of ions and water influenced by ions, the spectra of hydrated ion were decomposed from the solution spectra as standard spectra by multivariate curve resolution analysis. Meanwhile, ratios of hydrated ions were calculated between the Raman spectra and standard spectra to evaluate concentration profiles of each ion. It demonstrated that good quantitative models between the ratio and concentration for all ions including H⁺ can be built with correlation coefficients (R²) higher than 0.95 for the solutions. The method was further applied to individual particle pH measurement. The pH value of sulfate aerosol particles was calculated, and the standard error was 0.09 using pH values calculated from the [HSO₄^-]/[SO₄^2-] as a reference. Furthermore, the applicability of the method was proven by detecting the pH value of chloride particles. Therefore, utilizing water, the most common substance, as the spectroscopic probe to measure [H⁺] without restriction of the ion system, this method has potential to measure the pH value of atmospheric particles with various compounds, although more work needs to be done to improve the sensitivity of the method.

Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism

The properties of the water network in concentrated HCl acid pools in nanometer-sized reverse non-ionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms. We identify that only at a critical micelle size of W0=9 do solvated proton complexes form in the water pool, accompanied by a change in mechanism from Grotthuss forward shuttling to one that favors local oscillatory hopping. This is due to a preference for H+ and Cl- ions to adsorb to the micelle interface, together with an acid concentration effect that causes a "traffic jam" in which the short-circuiting of the hydrogen-bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle size increases. These findings have implications for atmospheric chemistry, biochemical and biophysical environments, and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration.

Enhanced Rates of Transition-Metal-Ion-Catalyzed Oxidation of S(IV) in Aqueous Aerosols: Insights into Sulfate Aerosol Formation in the Atmosphere

The oxidation of S(IV) is a critical step in the fate of sulfur dioxide emissions that determines the amount of sulfate aerosol in the atmosphere. Herein, we measured accelerated S(IV) oxidation rates in micron-sized aqueous aerosols compared to bulk solutions. We have investigated both buffered and unbuffered systems across a range of pH values in the presence of atmospherically relevant transition-metal ions and salts and consistently found the oxidation rate to be accelerated by ca. 1-2 orders of magnitude in the aerosol. This enhancement is greater than can be explained by the enrichment of species in the aerosol compared to the bulk and indicates that surface effects and potentially aerosol pH gradients play important roles in the S(IV) oxidation process in the aqueous aerosol. In addition, our experiments were performed with dissolved S(IV) ions (SO₃^2-/HSO₃^-), allowing us to demonstrate that acceleration occurs in the condensed phase showing that enhanced sulfate formation is not exclusively due to gas-aerosol partitioning or interfacial SO₂ oxidation. Our findings are an important step forward in understanding larger than expected sulfate concentrations observed in the atmosphere and show that inorganic oxidation processes can be accelerated in micron-sized aqueous droplets compared to the bulk solution.

Micro-droplet Trapping and Manipulation: Understanding Aerosol Better for a Healthier Environment

Understanding the physicochemical properties and heterogeneous processes of aerosols is key not only to elucidate the impacts of aerosols on the atmosphere and humans but also to exploit their further applications, especially for a healthier environment. Experiments that allow for spatially control of single aerosol particles and investigations on the fundamental properties and heterogeneous chemistry at the single-particle level have flourished during the last few decades, and significant breakthroughs in recent years promise better control and novel applications aimed at resolving key issues in aerosol science. Here we propose graphene oxide (GO) aerosols as prototype aerosols containing polycyclic aromatic hydrocarbons, and GO can behave as two-dimensional surfactants which could modify the interfacial properties of aerosols. We describe the techniques of trapping single particles and furthermore the current status of the optical spectroscopy and chemistry of GO. The current applications of these single-particle trapping techniques are summarized and interesting future applications of GO aerosols are discussed.

Changing atmospheric acidity as a modulator of nutrient deposition and ocean biogeochemistry

Anthropogenic emissions to the atmosphere have increased the flux of nutrients, especially nitrogen, to the ocean, but they have also altered the acidity of aerosol, cloud water, and precipitation over much of the marine atmosphere. For nitrogen, acidity-driven changes in chemical speciation result in altered partitioning between the gas and particulate phases that subsequently affect long-range transport. Other important nutrients, notably iron and phosphorus, are affected, because their soluble fractions increase upon exposure to acidic environments during atmospheric transport. These changes affect the magnitude, distribution, and deposition mode of individual nutrients supplied to the ocean, the extent to which nutrient deposition interacts with the sea surface microlayer during its passage into bulk seawater, and the relative abundances of soluble nutrients in atmospheric deposition. Atmospheric acidity change therefore affects ecosystem composition, in addition to overall marine productivity, and these effects will continue to evolve with changing anthropogenic emissions in the future.

Acidity across the interface from the ocean surface to sea spray aerosol

Sea spray aerosol, produced through breaking waves, is one of the largest sources of environmental particles. Once in the atmosphere, sea spray aerosol influences cloud formation, serves as microenvironments for multiphase atmospheric chemical reactions, and impacts human health. All of these impacts are affected by aerosol acidity. Here we show that freshly emitted sea spray aerosol particles become highly acidic within minutes as they are transferred across the ocean−air interface. These results have important implications for atmospheric chemistry and climate, including aerosol/gas partitioning, heterogeneous reactions, and chemical speciation at the surface and within sea spray aerosol.

Aerosols impact climate, human health, and the chemistry of the atmosphere, and aerosol pH plays a major role in the physicochemical properties of the aerosol. However, there remains uncertainty as to whether aerosols are acidic, neutral, or basic. In this research, we show that the pH of freshly emitted (nascent) sea spray aerosols is significantly lower than that of sea water (approximately four pH units, with pH being a log scale value) and that smaller aerosol particles below 1 μm in diameter have pH values that are even lower. These measurements of nascent sea spray aerosol pH, performed in a unique ocean−atmosphere facility, provide convincing data to show that acidification occurs “across the interface” within minutes, when aerosols formed from ocean surface waters become airborne. We also show there is a correlation between aerosol acidity and dissolved carbon dioxide but no correlation with marine biology within the seawater. We discuss the mechanisms and contributing factors to this acidity and its implications on atmospheric chemistry.

Surface-Enhanced Raman Spectroscopy for Environmental Monitoring of Aerosols

Surface-enhanced Raman spectroscopy (SERS) is conducted from single aerosol particles held in a linear electrodynamic quadrupole trap. SERS measurements from two representative types of ambient aerosol particles, semi-liquid and solid aerosols, are demonstrated; aerosol composed of adenine where the metallic nanoparticles (MNPs) are volume distributed throughout the particle and aerosol composed of polystyrene latex (PSL) beads where the MNPs are surface coated. An enhancement factor > 10⁶ is demonstrated from 5 μm aerosols containing trace amounts of adenine (0.1% by mass), with a detection limit of 10^-8 M corresponding to 5 × 10⁵ molecules (equivalent to 100 ag in mass or a 50 nm diameter sphere), and a ratio of 100 adenine molecules per Ag NP. SERS signal intensities are linear with particle adenine concentration up to a saturation point. Both the linearity and enhancement factor were confirmed by SERS measurements of adenine as bulk suspensions. The SERS spectra of adenine as bulk suspensions were explored as a function of excitation wavelength ranging from 400 to 800 nm. The two main Raman peaks of adenine at 738 and 1336 cm^-1 exhibit SERS maxima for excitation in the 450-500 nm range for commercially available 40 nm spherical Ag nanoparticles (NPs) used in this study, which shifts to longer wavelengths with the addition of NaCl. Shifts in SERS and spontaneous Raman shifts were observed between aqueous and dry adenine, in agreement with the literature, demonstrating the utility of SERS to possibly study water uptake of aerosols. SERS is measured from MNP surface-coated PSL beads with an enhancement factor of 30 for 5 μm PSLs. Theoretical extrapolation demonstrates that the enhancement factor will increase for decreasing particle size with an estimated enhancement factor of 140 for 1 μm PSLs.

Direct Quantification of the Effect of Ammonium on Aerosol Droplet pH

Ammonium is an important atmospheric constituent that dictates many environmental processes. The impact of the ammonium ion concentration on 10-50 μm aerosol droplet pH was quantified using pH nanoprobes and surface-enhanced Raman spectroscopy (SERS). Sample solutions were prepared by mixing 1 M ammonium sulfate (AS), ammonium nitrate (AN), sodium sulfate (SS), or sodium nitrate (SN) solutions with 1 M phosphate buffer (PB) at different volume ratios. Stable pH values were measured for pure PB, AS, and AN droplets at different concentrations. The centroid pH of 1 M PB droplets was ∼11, but when PB was systematically replaced with ammonium (AS- or AN-PB), the centroid pH within the droplets decreased from ≈11 to 5.5. Such a decrease was not observed in sodium (SS- or SN-PB) droplets, and no pH differences were observed between sulfate and nitrate salts. Ammonia partitioning to the gas phase in ammonium-containing droplets was evaluated to be negligible. Raman sulfate peak (∼980 cm^-1) intensity measurements and surface tension measurements were conducted to investigate changes in ion distribution. The pH difference between ammonium-containing droplets and ammonium-free droplets is attributed to the alteration of the ion distribution in the presence of ammonium.

New Multiphase Chemical Processes Influencing Atmospheric Aerosols, Air Quality, and Climate in the Anthropocene

Atmospheric aerosols and fine particulate matter (PM_2.5) are strongly affecting human health and climate in the Anthropocene, that is, in the current era of globally pervasive and rapidly increasing human influence on planet Earth. Poor air quality associated with high aerosol concentrations is among the leading health risks worldwide, causing millions of attributable excess deaths and years of life lost every year. Besides their health impact, aerosols are also influencing climate through interactions with clouds and solar radiation with an estimated negative total effective radiative forcing that may compensate about half of the positive radiative forcing of carbon dioxide but exhibits a much larger uncertainty. Heterogeneous and multiphase chemical reactions on the surface and in the bulk of solid, semisolid, and liquid aerosol particles have been recognized to influence aerosol formation and transformation and thus their environmental effects. However, atmospheric multiphase chemistry is not well understood because of its intrinsic complexity of dealing with the matter in multiple phases and the difficulties of distinguishing its effect from that of gas phase reactions.

Recently, research on atmospheric multiphase chemistry received a boost from the growing interest in understanding severe haze formation of very high PM_2.5 concentrations in polluted megacities and densely populated regions. State-of-the-art models suggest that the gas phase reactions, however, are not capturing the high concentrations and rapid increase of PM_2.5 observed during haze events, suggesting a gap in our understanding of the chemical mechanisms of aerosol formation. These haze events are characterized by high concentrations of aerosol particles and high humidity, especially favoring multiphase chemistry. In this Account, we review recent advances that we have made, as well as current challenges and future perspectives for research on multiphase chemical processes involved in atmospheric aerosol formation and transformation. We focus on the following questions: what are the key reaction pathways leading to aerosol formation under polluted conditions, what is the relative importance of multiphase chemistry versus gas-phase chemistry, and what are the implications for the development of efficient and reliable air quality control strategies? In particular, we discuss advances and challenges related to different chemical regimes of sulfate, nitrate, and secondary organic aerosols (SOAs) under haze conditions, and we synthesize new insights into the influence of aerosol water content, aerosol pH, phase state, and nanoparticle size effects. Overall, there is increasing evidence that multiphase chemistry plays an important role in aerosol formation during haze events. In contrast to the gas phase photochemical reactions, which are self-buffered against heavy pollution, multiphase reactions have a positive feedback mechanism, where higher particle matter levels accelerate multiphase production, which further increases the aerosol concentration resulting in a series of record-breaking pollution events. We discuss perspectives to fill the gap of the current understanding of atmospheric multiphase reactions that involve multiple physical and chemical processes from bulk to nanoscale and from regional to global scales. A synthetic approach combining laboratory experiments, field measurements, instrument development, and model simulations is suggested as a roadmap to advance future research.

Aerosol Acidity: Novel Measurements and Implications for Atmospheric Chemistry

The pH of a solution is one of its most fundamental chemical properties, impacting reaction pathways and kinetics across every area of chemistry. The atmosphere is no different, with the pH of the condensed phase driving key chemical reactions that ultimately impact global climate in numerous ways. The condensed phase in the atmosphere is comprised of suspended liquid or solid particles, known as the atmospheric aerosol, which are differentiated from cloud droplets by their much smaller size (primarily <10 μm). The pH of the atmospheric aerosol can enhance certain chemical reactions leading to the formation of additional condensed phase mass from lower volatility species (secondary aerosol), alter the optical and water uptake properties of particles, and solubilize metals that can act as key nutrients in nutrient-limited ecosystems or cause oxidative stress after inhalation. However, despite the importance of aerosol acidity for climate and health, our fundamental understanding of pH has been limited due to aerosol size (by number >99% of particles are <1 μm) and complexity. Within a single atmospheric particle, there can be hundreds to thousands of distinct chemical species, varying water content, high ionic strengths, and different phases (liquid, semisolid, and solid). Making aerosol analysis even more challenging, atmospheric particles are constantly evolving through heterogeneous reactions with gases and multiphase chemistry within the condensed phase. Based on these challenges, traditional pH measurements are not feasible, and, for years, indirect and proxy methods were the most common way to estimate aerosol pH, with mixed results. However, aerosol pH needs to be incorporated into climate models to accurately determine which chemical reactions are dominant in the atmosphere. Consequently, experimental measurements that probe pH in atmospherically relevant particles are sorely needed to advance our understanding of aerosol acidity.This Account describes recent advances in measurements of aerosol particle acidity, specifically three distinct methods we developed for experimentally determining particle pH. Our acid-conjugate base method uses Raman microspectroscopy to probe an acid (e.g., HSO₄^-) and its conjugate base (e.g., SO₄^2-) in individual micrometer-sized particles. Our second approach is a field-deployable colorimetric method based on pH indicators (e.g., thymol blue) and cell phone imaging to provide a simple, low-cost approach to ensemble average (or bulk) pH for particles in distinct size ranges down to a few hundred nanometers in diameter. In our third method, we monitor acid-catalyzed polymer degradation of a thin film (∼23 nm) of poly(ε-caprolactone) (PCL) on silicon by individual particles with atomic force microscopy (AFM) after inertially impacting particles of different pH. These measurements are improving our understanding of aerosol pH from a fundamental physical chemistry perspective and have led to initial atmospheric measurements. The impact of aerosol pH on key atmospheric processes, such as secondary organic aerosol (SOA) formation, is discussed. Some unique findings, such as an unexpected size dependence to aerosol pH and kinetic limitations, illustrate that particles are not always in thermodynamic equilibrium with the surrounding gas. The implications of our limited, but improving, understanding of the fundamental chemical concept of pH in the atmospheric aerosol are critical for connecting chemistry and climate.

Fe3+ and intracellular pH determination based on orange fluorescence carbon dots co-doped with boron, nitrogen and sulfur

The fluorescent boron, nitrogen and sulfur co-doped carbon dots (BNSCDs) were prepared by simple hydrothermal reaction of 4-carboxyphenylboronic acid and 2,5-diaminobenzenesulfonic acid at 200 °C for 8 h. The fluorescence of the BNSCDs could be quenched by Fe³⁺ based on the electron transfer between Fe³⁺ and BNSCDs, so a label-free, good selectivity and high sensitivity method for Fe³⁺determination was established with linear range and LOD of 1.5-692 μmol/L and 87 nmol/L, respectively. And then the fluorescent probe was employed for detection of Fe³⁺ in tap water, coal gangue, fly ash and food samples successfully. Moreover, the as-prepared BNSCDs could serve as a novel pH fluorescent probe in the range of pH 1.60-7.00, which could be attributed to the proton transfer of carboxyl groups on the surface of BNSCDs. More importantly, the pH fluorescent probe possesses fast, real-time and low toxicity, applying for intracellular pH fluorescence imaging in HIC, HIEC, LO2 and SMMC7721 cells. In view of its simplicity, timely response and outstanding compatibility, the as-fabricated BNSCDs show the potential applications in water quality and solid waste monitoring, food detection, real-time measuring of intracellular pH change in vitro.

Iron solubility in fine particles associated with secondary acidic aerosols in east China

Soluble iron (Fe_S) in aerosols contributes to free oxygen radical generation with implications for human health, and potentially catalyzes sulfur dioxide oxidation. It is also an important external source of micronutrients for ocean ecosystems. However, factors controlling Fe_S concentration and its contribution to total iron (Fe_T) in aerosols remain poorly understand. Here, Fe_S and Fe_T in PM_2.5 was studied at four urban sites in eastern China from 21 to 31 December, 2017. Average Fe_T (869-1490 ng m^-3) and Fe_S (24-68 ng m^-3) concentrations were higher in northern than southern China cities, but Fe solubility (%Fe_S, 2.7-5.0%) showed no spatial pattern. Correlation analyses suggested %Fe_S was strongly correlated with Fe_S and PM_2.5 instead of Fe_T concentrations. Individual particle observations confirmed that more than 65% of nano-sized Fe-containing particles were internally mixed with sulfates and nitrates. Furthermore, there was a high correlation between sulfates or nitrates/Fe_T molar ratio and %Fe_S. We also found that the sulfates/nitrates had weaker effects on %Fe_S at RH < 50% than at RH > 50%, suggesting RH as indirect factor can influence %Fe_S in PM_2.5. These results suggest an important role of chemical processing in enhancing %Fe_S in the polluted atmosphere.

Multiphase Photochemistry of Iron-Chloride Containing Particles as a Source of Aqueous Chlorine Radicals and Its Effect on Sulfate Production

Photolysis of iron chlorides is a well-known photolytic source of Cl^• in environmental waters. However, the role of particulate chlorine radicals (Cl^• and Cl₂^•-) in their multiphase oxidative potential has been much less explored. Herein, we examine the effect of Cl^•/Cl₂^•- produced from photolysis of particulate iron chlorides on atmospheric multiphase oxidation. As a model system, experiments on multiphase oxidation of SO₂ by Cl^•/Cl₂^•- were performed. Fast sulfate production from SO₂ oxidation was observed with reactive uptake coefficients of ∼10^-5, comparable to the values necessary for explaining the observations in the haze events in China. The experimental and modeling results found a good positive correlation between the uptake coefficient, γ_SO2, and the Cl^• production rate, d[Cl^•]/dt, as γ_SO2 = 5.3 × 10^-6 × log(d[Cl^•]/dt) + 4.9 × 10^-5. When commonly found particulate dicarboxylic acids (oxalic acid or malonic acid) were added, sulfate production was delayed due to the competition of Fe³⁺ between chloride and the dicarboxylic acid for its complexation at the initial stage. After the delay, comparable sulfate production was observed. The present study highlights the importance of photochemistry of particulate iron chlorides in multiphase oxidation processes in the atmosphere.

Liquid-Liquid Phase Separation in Supermicrometer and Submicrometer Aerosol Particles

ConspectusThe interactions of aerosol particles with light and clouds are among the most uncertain aspects of anthropogenic climate forcings. The effects of aerosol particles on climate depend on their optical properties, heterogeneous chemistry, water uptake behavior, and ice nucleation activity. These properties in turn depend on aerosol physics and chemistry including composition, size, shape, internal structure (morphology), and phase state. The greatest numbers of particles are found at small, submicrometer sizes, and the properties of aerosol particles can differ on the nanoscale compared with measurements of bulk materials. As a result, our focus has been on characterizing the phase transitions of aerosol particles in both supermicrometer and submicrometer particles. The phase transition of particular interest for us has been liquid-liquid phase separation (LLPS), which occurs when components of a solution phase separate due to a difference in solubilities. For example, organic compounds can have limited solubility in salt solutions especially as the water content decreases, increasing the concentration of the salt solution, and causing phase separation between organic-rich and inorganic-rich phases. To characterize the systems of interest, we primarily use optical microscopy for supermicrometer particles and cryogenic-transmission microscopy for submicrometer particles.This Account details our main results to date for the phase transitions of supermicrometer particles and the morphology of submicrometer aerosol. We have found that the relative humidity (RH) at which LLPS occurs (separation RH; SRH) is highly sensitive to the composition of the particles. For supermicrometer particles, SRH decreases as the pH is lowered to atmospherically relevant values. SRH also decreases when non-phase-separating organic compounds are added to the particles. For submicrometer particles, a size dependence of morphology is observed in systems that undergo LLPS in supermicrometer particles. In the limit of slow drying rates, particles <30 nm are homogeneous and larger particles are phase-separated. This size dependence of aerosol morphology arises because small particles cannot overcome the activation barrier needed to form a new phase when phase separation occurs by a nucleation and growth mechanism. The inhibition of LLPS in small particles is observed for mixtures of ammonium sulfate with single organic compounds as well as complex organics like α-pinene secondary organic matter. The morphology of particles affects activation diameters for the formation of cloud condensation nuclei. These results more generally have implications for aerosol properties that affect the climate system. In addition, LLPS is also widely studied in materials and biological chemistry, and our results could potentially translate to implications for these fields.

Water-in-oil microcompartments for the study of biomimetic drug metabolism

Microcompartments in the form of water-in-oil droplets have been utilized to construct artificial cells and simulate human body environment. However, the performance of subcellular structure involved metabolism in emulsion droplets has not been explored, and the underlying mechanism is still being elucidated. In this work, drug metabolism is presented on the basis of great amounts of microcompartments formed of picoliter-volume droplets with different radius (R), using a commercial four-way valve as a droplet generator. A model substrate, phenacetin, and its metabolite, paracetamol, are quantitatively analyzed by liquid-chromatography (LC) tandem mass spectrometry (MS/MS), and the reaction kinetics is characterized. In microdroplets of varying size (R = 18, 27, 42, and 51 μm, respectively), both conversion ratio and reaction rate constant of the metabolism are influenced in different degree. For instance, the substrate conversion ratio after 60 min of incubation in R = 27 μm droplets improves from 15% to 42%, and the reaction rate constant improves nearly five-fold, compared to that in bulk phase. The influence of microcompartment size on metabolism rate is further explored by simulation using a diffusion-reaction model. The droplet-based strategy is rapid, accurate and cost-efficient, fitting especially into biomimetic metabolism studies.

Aerosol Acidity Sensing via Polymer Degradation

The acidity of atmospheric aerosols is a critical property that affects the chemistry and composition of the atmosphere. Many key multiphase chemical reactions are pH-dependent, impacting processes like secondary organic aerosol formation, and need to be understood at a single particle level due to differences in particle-to-particle composition that impact both climate and health. However, the analytical challenge of measuring aerosol acidity in individual particles has limited pH measurements for fine (<2.5 μm) and coarse (2.5-10 μm) particles. This has led to a reliance on indirect methods or thermodynamic modeling, which focus on average, not individual, particle pH. Thus, new approaches are needed to probe single particle pH. In this study, a novel method for pH measurement was explored using degradation of a pH-sensitive polymer, poly(ε-caprolactone), to determine the acidity of individual submicron particles. Submicron particles of known pH (0 or 6) were deposited on a polymer film (21-25 nm thick) and allowed to react. Particles were then rinsed off, and the degradation of the polymer was characterized using atomic force microscopy and Raman microspectroscopy. After degradation, holes in the PCL films exposed to pH 0 were observed, and the loss of the carbonyl stretch was monitored at 1723 cm^-1. As particle size decreased, polymer degradation increased, indicating an increase in aerosol acidity at smaller particle diameters. This study describes a new approach to determine individual particle acidity and is a step toward addressing a key measurement gap related to our understanding of atmospheric aerosol impacts on climate and health.

The Acidity of Atmospheric Particles and Clouds

Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semi-volatile gases such as HNO3, NH3, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally-constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicates acidity may be relatively constant due to the semi-volatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.

Enhanced Sulfate Production by Nitrate Photolysis in the Presence of Halide Ions in Atmospheric Particles

Heterogeneous oxidation of SO₂ is an effective production pathway of sulfate in the atmosphere. We recently reported a novel pathway for the heterogeneous oxidation of SO₂ by in-particle oxidants (OH, NO₂, and NO₂^-/HNO₂) produced from particulate nitrate photolysis (Environ. Sci. Technol. 2019, 53, 8757-8766). Particulate nitrate is often found to coexist with chloride and other halide ions, especially in aged sea-salt aerosols and combustion aerosols. Reactive uptake experiments of SO₂ with UV-irradiated nitrate particles showed that sulfate production rates were enhanced by a factor of 1.4, 1.3, and 2.0 in the presence of Cl^-, Br^-, and I^-, respectively, compared to those in the absence of halide ions. The larger sulfate production was attributed to enhanced nitrate photolysis promoted by the increased incomplete solvation of nitrate at the air-particle interface due to the presence of surface-active halide ions. Modeling results based on the experimental data showed that the nitrate photolysis rate constants increased by a factor of 2.0, 1.7, and 3.7 in the presence of Cl^-, Br^-, and I^-, respectively. A linear relation was found between the nitrate photolysis rate constant, j_NO3-, and the initial molar ratio of Cl^- to NO₃^-, [Cl^-]₀/[NO₃^-]₀, as j_NO3- = 9.7 × 10^-5[Cl^-]₀/[NO₃^-]₀ + 1.9 × 10^-5 at [Cl^-]₀/[NO₃^-]₀ below 0.2. The present study demonstrates that the presence of halide ions enhances sulfate production produced during particulate nitrate photolysis and provides insights into the enhanced formation of in-particle oxidants that may increase atmospheric oxidative capacity.

Direct measurement of aerosol pH in individual malonic acid and citric acid droplets under different relative humidity conditions via Raman spectroscopy

Acidity of aerosol particles plays important roles in atmospheric chemistry, in turn, impacting climate system and public health. Current knowledge of acidity in atmosphere aerosols remains fairly scarce largely because of difficulty in direct measurement. On the other hand, indirect methods for estimating aerosol pH are often inconsistent with pH values predicted by thermodynamic models. Recently, a direct Raman spectroscopy method has been reported to determine pH values of acid-conjugate base equilibria systems based on Raman intensity of distinct characteristic peaks of conjugate acid-base pair. Nevertheless, for pure carboxylic acid aerosol particles, such as malonic acid (MA), characteristic peak of its conjugate base cannot be clearly observed in Raman spectra owing to small K_a value (weak acid dissociation constant), which leads to little dissociation of weak acid and low concentration of its conjugated base. As a result, pH of carboxylic acid particles cannot be directly determined by calibrating concentrations of acid and its conjugated base using the above-mentioned method. To address such an issue, we demonstrate a new approach for determining pH values of malonic acid (MA) and citric acid (CA) droplets under different relative humidity (RH) based on calibration curves. We measure Raman intensity ratios of acid solutions at different concentrations and their pH values to establish a calibration curve, and then using the intensity ratio of MA and CA droplets under different RH to determine aerosol particle pH based on calibration curves. Results have shown that aerosol pH of MA droplet decreases with a decreasing RH and pH values ranges from 1.03 to -0.12, when RH value is reduced from 90% to 26%, in good agreement with model prediction values. In addition, we also, for the first time, report pH values of CA droplets under different RH conditions and its pH values range from 1.13 to -0.74 when RH is reduced from 91% to 28%.

In Situ pH Measurements of Individual Levitated Microdroplets Using Aerosol Optical Tweezers

The pH of microscale reaction environments controls numerous physicochemical processes, requiring a real-time pH microprobe. We present highly accurate real-time pH measurements of microdroplets using aerosol optical tweezers (AOT) and analysis of the whispering gallery modes (WGMs) contained in the cavity-enhanced Raman spectra. Uncertainties ranging from ±0.03 to 0.06 in pH for picoliter droplets are obtained through averaging Raman frames acquired at 0.5 Hz over 3.3 min. The high accuracy in pH determination is achieved by combining two independent measurements uniquely provided by the AOT approach: the anion concentration ratio from the spontaneous Raman spectra, and the total solute concentration from the refractive index retrieved from WGM analysis of the stimulated cavity-enhanced Raman spectra. pH can be determined over a range of -0.36 to 0.76 using the aqueous sodium bisulfate system. This technique enables direct measurements of pH-dependent chemical and physical changes experienced by individual microparticles and exploration of the role of pH in the chemical behavior of confined microenvironments.

Current and Future Responses of Aerosol pH and Composition in the U.S. to Declining SO2 Emissions and Increasing NH3 Emissions

Aerosol pH can affect gas-particle partitioning of semivolatile species, secondary aerosol formation, aerosol water uptake and growth, acid deposition, and, potentially, aerosol toxicity. Despite its importance, aerosol pH projected in the near future has not been addressed explicitly while investigating the response of aerosol concentrations to emission regulations. In this study, we apply CMAQ to simulate aerosol pH in 2011 and 2050 across the continental U.S. We also assess the influence of two major emission trends, declining SO₂ emissions and rising NH₃ emissions, with a set of sensitivity simulations. Our results show that the aerosols will remain acidic with average pH typically ranging from 0.5 to 3.5 in 2050. Further reducing domestic SO₂ emissions does not significantly decrease aerosol acidity, even if SO₂ emissions were reduced to preindustrial levels because of the nonlinear response of SO₄^2- concentration to SO₂ emissions, and the semivolatile NH₃-NH₄⁺ buffering effect. Aerosol pH response to NH₃ emission increase will remain minor. Consequently, future fine particulate matter control efficiency will not be undercut by additional nitrate aerosol formation even if SO₂ emissions from industry and electricity generation are aggressively controlled, although areas will see some substitution leading to nitrate increases if NO_x emissions are not reduced.

Heterogeneous Oxidation of SO2 in Sulfate Production during Nitrate Photolysis at 300 nm: Effect of pH, Relative Humidity, Irradiation Intensity, and the Presence of Organic Compounds

Heterogeneous oxidation of SO₂ is one of the promising mechanisms to account for high loading of sulfate during severe haze periods in China. Our earlier work reported on the SO₂ oxidation by OH and NO₂ produced during 250 nm nitrate photolysis (Environ. Sci. Technol. Lett. 2019, 6, 86-91). Here, we extend that work to examine sulfate production during nitrate photolysis at 300 nm irradiation, which can additionally generate NO₂^- or HNO₂, N(III). Flow cell/in situ Raman experiments showed that the reactive uptake coefficient of SO₂, γ_SO2, can be expressed as γ_SO2 = 1.64 × p_NO3-, where p_NO3- is the nitrate photolysis rate in the range of (1.0-8.0) × 10^-5 M s^-1. Our kinetic model with the p_NO3- predicts that N(III) is the main contributor to the SO₂ oxidation, followed by NO₂ contribution. Furthermore, the addition of OH scavengers (e.g., glyoxal or oxalic acid) does not suppress the sulfate production because of the reduced N(III)-consuming reactions and the high particle pH sustained by their presence. Our calculations illustrate that under characteristic haze conditions, the nitrate photolysis mechanism can produce sulfate at ∼1 μg m^-3 h^-1 at pH 4-6 and p_NO3- = 10^-5 M s^-1. The present study highlights the importance of in-particle nitrate photolysis in heterogeneous oxidation of SO₂ by reactive nitrogen (NO₂^-/HNO₂ and NO₂) under atmospherically relevant actinic irradiation. However, the nitrate photolysis rate constant needs to be better constrained for ambient aerosols.

Titration of Aerosol pH through Droplet Coalescence

The pH of aqueous aerosols, as well as cloud and fog droplets, has an important influence on the chemistry that takes place within these unique microenvironments. Utilizing conjugate acid/base pairs to infer pH changes, we investigate, for the first time, changes in aerosol pH upon coalescence. In particular, we show that the pH within individual aqueous aerosols that are ∼8 μm in diameter can be titrated via droplet coalescence in an aerosol optical tweezer. Using sulfate/bisulfate and carbonate/bicarbonate as model systems, the pH of trapped aerosols is determined before and after introduction of smaller aerosols containing a strong acid. The pH change upon coalescence with the smaller, acidic aerosol is calculated using specific ion interaction theory. Furthermore, we show that the pH of an individual aerosol can be altered along a fairly wide range of pH values, paving the way for future studies requiring rigorous pH control of an aqueous aerosol.

Swine-Origin H1 Influenza Viruses Isolated from Humans Exhibit Sustained Infectivity in an Aerosol State

The relative importance of influenza virus transmission via aerosols is not fully understood, but experimental data suggest that aerosol transmission may represent a critical mode of influenza virus spread among humans. Decades ago, prototypical laboratory strains of influenza were shown to persist in aerosols; however, there is a paucity of data available covering currently circulating influenza viruses, which differ significantly from their predecessors. In this study, we evaluated the longevity of influenza viruses in aerosols generated in the laboratory. We selected a panel of H1 viruses that exhibit diverse transmission profiles in the ferret model, including four human isolates of swine origin (referred to as variant) and a seasonal strain. By measuring the ratio of viral RNA to infectious virus maintained in aerosols over time, we show that influenza viruses known to transmit efficiently through the air display enhanced stability in an aerosol state for prolonged periods compared to those viruses that do not transmit as efficiently. We then assessed whether H1 influenza virus was still capable of infecting and causing disease in ferrets after being aged in suspended aerosols. Ferrets exposed to very low levels of influenza virus (≤17 PFU) in aerosols aged for 15 or 30 min became infected, with five of six ferrets shedding virus in nasal washes at titers on par with ferrets who inhaled higher doses of unaged influenza virus. We describe here an underreported characteristic of influenza viruses, stability in aerosols, and make a direct connection to the role this characteristic plays in influenza transmission.IMPORTANCE Each time a swine influenza virus transmits to a human, it provides an opportunity for the virus to acquire adaptations needed for sustained human-to-human transmission. Here, we use aerobiology techniques to test the stability of swine-origin H1 subtype viruses in aerosols and evaluate their infectivity in ferrets. Our results show that highly transmissible influenza viruses display enhanced stability in an aerosol state compared to viruses that do not transmit as efficiently. Similar to human-adapted strains, swine-origin influenza viruses are infectious in ferrets at low doses even after prolonged suspension in the air. These data underscore the risk of airborne swine-origin influenza viruses and support the need for continued surveillance and refinement of innovative laboratory methods to investigate mammalian exposure to inhaled pathogens. Determination of the molecular markers that affect the longevity of airborne influenza viruses will improve our ability to quickly identify emerging strains that present the greatest threat to public health.

Role of pH in Aerosol Processes and Measurement Challenges

pH is one of the most basic chemical properties of aqueous solution, but its measurement in nanoscale aerosol particles presents many challenges. The pH of aerosol particles is of growing interest in the atmospheric chemistry community because of its demonstrated effects on heterogeneous chemistry and human health, as well as potential effects on climate. The authors have shown that phase transitions of aerosol particles are sensitive to pH, focusing on systems that undergo liquid-liquid phase separation. Currently, aerosol pH is calculated indirectly from knowledge of species present in the gas and aerosol phases through the use of thermodynamic models. From these models, ambient aerosol is expected to be highly acidic (pH ∼ 0-3). Direct measurements have focused on model systems due to the difficulty of this measurement. This area is one in which physical chemists should be encouraged to contribute because of the potential consequences for aerosol processes in the environment.