The Acidity of Atmospheric Particles and Clouds

Microplastic aging and adsorption in the atmosphere, and their associated impacts on various spheres of the earth: A review

Microplastic (MPs, size <5 mm) is an emerging category of contaminants with detrimental effects on human health, climate, and ecology. The atmospheric pathway is a crucial transport route for the migration of MPs from source to receptor locations. This long-range transport leads to the ubiquitous presence of MPs across all environmental matrices and constrains the source-transport pathway-sink interaction. During atmospheric transport, MPs experience aging and adsorption as a result of interactions with winds, solar radiation, moisture, pH, and atmospheric pollutants, which alters their hydrophilicity, structure, surface area, size, color, and the capacity for adsorption, often resulting in elevated toxicity and associated risks. However, the multifaceted dynamics of atmospheric aging of MPs and consequent impacts are poorly understood. This review presents a critical assessment of three major factors that determine the nature and degree of MP aging and adsorption in the atmosphere, namely: intrinsic MP properties such as the degree of unsaturation, crystallinity, presence of functional groups, charge, specific surface area, and structural defects; environmental factors such as temperature, pH, moisture, and the presence of chemical species; and pollutant characteristics such as charge and hydrophilicity/hydrophobicity that influence adsorption, with an emphasis on potential mechanisms. Additionally, the review presents a comparative assessment of the critical factors and mechanisms responsible for aging and adsorption in atmosphere with those in other environmental media. Further, the potential impacts of atmospherically aged MPs on climate, the biosphere, cryosphere, pedosphere, and hydrosphere are summarized. The review finally identifies key knowledge gaps and outlines perspectives for future research.

Quantitative analysis of influencing factors to aerosol pH and its responses to PM2.5 and O3 pollution in a coastal city

Aerosol acidity (pH) plays an important role in the multiphase chemical processes of atmospheric particles. In this study, we demonstrated the seasonal trends of aerosol pH calculated with the ISORROPIA-II model in a coastal city of southeast China. We performed quantitative analysis on the various influencing factors on aerosol pH, and explored the responses of aerosol pH to different PM2.5 and O3 pollution levels. The results showed that the average aerosol pH was 2.92 ± 0.61, following the order of winter > spring > summer > autumn. Sensitivity tests revealed that SO42-, NHx, T and RH triggered the variations of aerosol pH. Quantitative analysis results showed that T (37.9%-51.2%) was the main factors affecting pH variations in four seasons, followed by SO42- (6.1%-23.7%), NHx (7.2%-22.2%) and RH (0-14.2%). Totally, annual mean meteorological factors (52.9%) and chemical compositions (41.3%) commonly contributed the aerosol ΔpH in the coastal city. The concentrations of PM2.5 was positively correlated with aerosol liquid water content (R2 = 0.53) and aerosol pH (R2 = 0.26), indicating that the increase in pH was related with the elevated NH4NO3 and decreased SO42-, and also the changes of T and RH. The Ox (O3 + NO2) was moderately correlated with aerosol pH (R2 = -0.48), attributable to the fact that the proportion of SO42- increased under high T and low RH conditions. The study strengthened our understanding of the contributions of influencing factors to aerosol pH, and also provided scientific evidences for chemical processes of atmospheric particles in coastal areas.

Key Roles of Bulk Viscosity and Acidity on Liquid-Liquid Phase Separation of Atmospheric Organic-Inorganic Mixed Aerosols

Liquid-liquid phase separation (LLPS) and the resulting particle morphologies in atmospheric organic-inorganic mixed aerosols are key regulators of aerosol chemistry and climate forcing. However, the influence of coexisting viscous water-soluble organic compounds (WSOCs) on the LLPS behavior in complex multicomponent aerosol systems remains poorly understood. In this study, we introduced three representative WSOCs, i.e., sucrose (SUC), glycerol (GLY), and citric acid (CA), to increase the bulk viscosity of a model LLPS system composed of 1,2,6-hexanetriol (HXT) and ammonium sulfate (AS). Using microscopic imaging techniques and viscosity model predictions, we examined the impact of mass transfer limitations on LLPS. As WSOC fractions increased, both the phase separation relative humidity (SRH) and the efflorescence relative humidity (ERH) progressively decreased. For the HXT/AS/SUC and HXT/AS/CA mixed systems with molar ratios of 1:1:0.5 and 1:1:0.75, LLPS was completely suppressed, although efflorescence still occurred. In the 1:1:1 mixtures, neither LLPS nor efflorescence was observed. In contrast, the addition of GLY caused minimal changes to phase transitions due to its minor effect on the aqueous-phase viscosity. Additionally, reducing bulk acidity, along with the transformation of CA into its salts, alleviated molecular transport limitations, leading to increased SRH and ERH values for the HXT/AS/CA mixtures. These findings are critical for advancing high-resolution phase state modeling of multicomponent aerosols and assessing the atmospheric implications of particle morphologies in the presence or absence of LLPS.

Transition metals and chemical compositions determine the oxidation capacity of atmospheric particulate matters

The knowledge of the causal relationship between exposure to airborne particulate matter (PM) and respiratory-related health issues remains unsatisfactory, owing to the complexities of physical and chemical characteristics in PM. One measure that greatly lifts the complexity is oxidative potential (OP), the overall production capacity of reactive oxygen species. We analyzed PM at different size fractions from three localities, exhibiting different source emission properties and photochemical aging states. We also investigated possible causes for their OPs, which were assessed using cellular and acellular assays. We found that higher PM mass did not always yield higher OP. Instead, chemical composition, modified by photochemical alteration (particle oxidation), played a critical role in the PM's reactivity. From a pollution hot spot to a downwind country town, the PM2.5 levels (mean ± SD) were 9.3 ± 4.5, 9.7 ± 4.9, and 6.6 ± 4.7 μg/m3, respectively. In contrast, the PM mass-normalized OP values in the downwind region were approximately 20 % higher than those in the upwind region based on the cellular assay and about three times higher from the acellular assay. Enhanced PM OP is associated with atmospheric oxidation, approximated by sulfur and nitrogen oxidation ratios. We further identified transition metals, particularly copper, a single most important species group, the primary determinant to the values of OP measured, contributing directly to OP and indirectly through metal-oxides enhanced photochemical alterations to PM.

Multiphase Buffering: A Mechanistic Regulator of Aerosol Sulfate Formation and Its Dominant Pathways

Sulfate formation in the aerosol aqueous phase represents a pH-sensitive atmospheric chemical process, with the formation pathways significantly influenced by the fluctuations in aerosol acidity. Buffer capacity, stemming from conjugate acid-base pairs, can resist pH changes in aerosol multiphase systems under external perturbations. However, the regulating role of multiphase buffering in pH-dependent aqueous sulfate formation mechanisms remains unexplored. Here, we propose that multiphase buffering can stabilize aerosol pH and further regulate dominant sulfate formation pathways. In this work, we delve into the instantaneous buffer capacity β and sulfate formation pathways based on field observation and theoretical calculation and further introduce the total buffer capacity α in the aerosol multiphase system to quantify the buffer-constrained pH change after the external acid/base variation during the entire buffering process. The NH4+/NH3 agent (average β 30.8 mol kg-1) shows a superior buffering effect in stabilizing aerosol pH and regulating sulfate formation pathway transition compared with the HNO3/NO3- agent (average β 15.1 mol kg-1). Geos-Chem simulation and machine learning results also validate the buffer capacity as a pivotal factor in sulfate formation. In addition to reactants, the buffer agents and acid/base can also be factors of concern for the sulfate formation mechanism. The diverse sensitivities to acid/base variation and the region-specific responses to pH change can provide insight into regulating acid and base emission measures, modulating regional aerosol acidity, and understanding pH-related atmospheric chemical processes.

Aerosol Toxicokinetics: A Framework for Unraveling Toxicological Dynamics from Air to the Body

Exposure to atmospheric aerosols threatens human health and is yet to be effectively addressed globally. Aerosol toxicity strongly depends upon components whose chemical profiles and concentrations can constantly evolve throughout atmospheric transformation, inhalation, distribution, metabolism, and excretion. Despite the abundant studies on aerosol components and their toxic effects, the dynamics in component concentrations and related biological effects from air to the body remain unclear. Here, we propose a conceptual toxicokinetic framework to mathematically deduce the bioavailable concentration from the changing bulk concentration of aerosol constituents in the atmosphere. The biological effects of single or multiple components are further predicted via toxicodynamic modeling according to their bioavailable concentrations. Atmospheric concentrations of toxic composition can in turn be regulated by risk-based guidelines, aiming to alleviate in vivo toxic effects. This perspective demonstrates how serial toxicokinetic-toxicodynamic equations bridge the knowledge gap between ambient aerosols and associated toxic effects in human bodies. The prediction from an inhalation perspective also allows connecting with the exposomes from aggregate exposure pathways. We call for the development of the model validity and integrate quantitative adverse outcome pathways to apply for exposure-disease modeling, providing novel insights into air quality policymaking and public health management.

Photosensitizers Regulate Nitrate Photoproduct Yields in Bulk Aqueous Matrices

Photolysis of nitrate (NO3-) in the presence of photosensitizers is thought to promote the release of atmospherically important reactive nitrogen species (NOy), such as HONO, via mechanisms that are poorly understood. To address this knowledge gap, we conducted photochemistry studies on mixtures of NO3-, a model photosensitizer [4-benzoylbenzoic acid (4-BBA)], and various aliphatic organic matter proxies. We found that aliphatic organic matter enhances the production of NOy and superoxide (O2-) from bulk aqueous nitrate photolysis, while the addition of 4-BBA decreases NOy and O2- yields in most cases. This effect was not observed in heterogeneous systems, where 4-BBA enhanced photochemical NOy production when coadsorbed with NO3- on silica surfaces. This demonstrates that photosensitizers act as both oxidants and reductants, regulating the yields of NOy and reactive oxygen species from nitrate photochemistry. Given sufficient residence time in the bulk aqueous phase, nitrate photoproducts can be scavenged via secondary reactions with photosensitizers. In heterogeneous and aerosol systems with high surface area-to-volume ratios, however, volatilization of NOy photoproducts is favored, and NOy production is less affected by chromophoric organic matter sinks in the bulk phase. This work demonstrates the intricate role of aliphatic and chromophoric moieties in natural organic matter in the photochemical conversion of NO3- to NOy (i.e., renoxification) in the atmosphere.

How Acid Iron Dissolution in Aged Dust Particles Responds to the Buffering Capacity of Carbonate Minerals during Asian Dust Storms

Aerosol deposition significantly impacts ocean ecosystems by providing bioavailable iron (Fe). Acid uptake during the transport of Fe-containing particles has been shown to cause Fe dissolution. However, carbonate in dust particles affects the Fe acidification process, influencing Fe dissolution. Here, we carried out atmospheric observations and modeling to show that Fe solubility substantially increased from locations near dust sources to downwind regions in aged dust particles with pH > 3, driven by proton-promoted dissolution. We found that Fe solubility remained low when Ca solubility was under 45 ± 5%, but increased with Ca solubility when it was above 45 ± 5%. Moreover, we found that Fe dissolved in aqueous Ca-nitrate coatings on Fe-containing dust particles. Our results suggest that the mixing state and buffering capacity of carbonate and Fe minerals should be represented in atmospheric biogeochemical models to more accurately simulate acid Fe dissolution processes.

Nuclear Quantum Effects and the Grotthuss Mechanism Dictate the pH of Liquid Water

Water's ability to autoionize into hydronium (H3O+) and hydroxide (OH-) ions dictates the acidity or basicity of aqueous solutions, influencing the reaction pathways of many chemical and biochemical processes. In this study, we determine the molecular mechanism of the autoionization process by leveraging both the computational efficiency of a deep neural network potential trained on highly accurate data calculated within density-corrected density functional theory and the ability of enhanced sampling techniques to ensure a comprehensive exploration of the underlying multidimensional free-energy landscape. By properly accounting for nuclear quantum effects, our simulations provide an accurate estimate of the autoionization constant of liquid water (pKw = 13.71 ± 0.16), offering a realistic molecular-level picture of the autoionization process and emphasizing its quantum-mechanical nature. Importantly, our simulations highlight the central role played by the Grotthuss mechanism in stabilizing solvent-separated ion pair configurations, revealing its profound impact on acid-base equilibria in aqueous environments.

Simulating Droplet-Resolved Haze and Cloud Chemistry Forming Secondary Organic Aerosols in Turbulent Conditions within Laboratory and Cloud Parcels

Most models do not explicitly simulate droplet-resolved cloud chemistry and the interactions between turbulence and cloud chemistry due to large associated computational costs. Here, we incorporate the formation of isoprene epoxydiol secondary organic aerosol (IEPOX-SOA) in individual droplets within a one-dimensional explicit mixing parcel model (EMPM-Chem). We apply EMPM-Chem to simulate turbulence and droplet-resolved IEPOX-SOA formation using a laboratory cloud chamber configuration. We find that the dissolution of IEPOX gases is weighted more toward larger cloud droplets due to their large liquid water content (compared to smaller droplets), while the conversion of dissolved IEPOX to IEPOX-SOA is much greater within smaller deliquesced haze particles due to their higher acidity and ionic strengths compared to cloud droplets. We also apply the EMPM-Chem model to simulate how IEPOX-SOA formation evolves in individual cloud droplets within rising cloudy parcels in the atmosphere. We find that as subsaturated air is entrained into and turbulently mixed with the cloud parcel, evaporation causes a reduction in droplet sizes, which leads to corresponding increases in per droplet ionic strength and acidity. Increased droplet acidity, in turn, greatly accelerates the kinetics of IEPOX-SOA formation. Our results provide key insights into single cloud-droplet chemistry, suggesting that entrainment mixing may be an important process that increases SOA formation in the real atmosphere.

The role of the droplet interface in controlling the multiphase oxidation of thiosulfate by ozone

Predicting reaction kinetics in aqueous microdroplets, including aerosols and cloud droplets, is challenging due to the probability that the underlying reaction mechanism can occur both at the surface and in the interior of the droplet. Additionally, few studies directly measure the surface activities of doubly charged anions, despite their prevalence in the atmosphere. Here, deep-UV second harmonic generation spectroscopy is used to probe surface affinities of the doubly charged anions thiosulfate, sulfate, and sulfite, key species in the thiosulfate ozonation reaction mechanism. Thiosulfate has an appreciable surface affinity with a measured Gibbs free energy of adsorption of -7.3 ± 2.5 kJ mol-1 in neutral solution, while sulfate and sulfite exhibit negligible surface propensity. The Gibbs free energy is combined with data from liquid flat jet ambient pressure X-ray photoelectron spectroscopy to constrain the concentration of thiosulfate at the surface in our model. Stochastic kinetic simulations leveraging these novel measurements show that the primary reaction between thiosulfate and ozone occurs at the interface and in the bulk, with the contribution of the interface decreasing from ∼65% at pH 5 to ∼45% at pH 13. Additionally, sulfate, the major product of thiosulfate ozonation and an important species in atmospheric processes, can be produced by two different pathways at pH 5, one with a contribution from the interface of >70% and the other occurring predominantly in the bulk (>98%). The observations in this work have implications for mining wastewater remediation, atmospheric chemistry, and understanding other complex reaction mechanisms in multiphase environments. Future interfacial or microdroplet/aerosol chemistry studies should carefully consider the role of both surface and bulk chemistry.

Unveiling secondary inorganic PM2.5 pollution in northern Taiwan: The role of aerosol acidity and transformation processes

Efforts are being made to manage air pollution in Taiwan. To address the concern of anthropogenic aerosol loadings, it is imperative to comprehensively monitor and analyze the physicochemical and thermodynamic properties of secondary inorganic PM2.5 constituents. This study focuses on the high-resolution inorganic aerosol chemistry in an urban area of northern Taiwan, using hourly measurements of trace gases and water-soluble inorganic ions (WSIIs), integrated with the ISORROPIA II thermodynamic model. Our findings indicate that aerosol acidity (pH) and aerosol liquid water content (ALWC) are crucial in controlling gas-to-particle phase partitioning of fine aerosols and PM2.5 mass loadings in the ambient air. The relationship between ALWC and the conversion ratios, including sulfur oxidation ratio (SOR), nitrogen oxidation ratio (NOR), and ammonia conversion ratio (NHR), indicates that secondary ion formation increases with higher ALWC, thereby directly affecting air quality. The diurnal pattern of NOR highlights the significant role of photochemical reactions in NO3- formation during the day, while the nighttime increase in SOR levels, coupled with their daytime decrease, supports the occurrence of nighttime aqueous-phase oxidation driven by elevated ALWC. Peak neutralization of secondary anions by NH4+ was observed during winter under NH3-rich conditions, leading to a hygroscopic increase in PM2.5 mass concentrations. The pH sensitivity analysis of regional atmospheric secondary PM2.5 formation identified NO3- as the limiting factor. This suggests that reducing HNO3 and precursor NOx levels, rather than NH3, would more effectively and quickly lower PM2.5 concentrations. Implementing such a strategy could offer a cost-effective and impactful solution for managing high aerosol loadings in northern Taiwan.

Intermolecular Interactions Enhance the Light Absorption of a Methoxyphenol Constituent of Biomass Burning Emissions

Brown carbon (BrC) components of biomass burning organic aerosol (BBOA) absorb sunlight at visible wavelengths. However, it is not clear whether the total light absorption of this BrC is simply the sum of the contributions of the individual components or whether the components can bind noncovalently to give additional absorption through charge transfer. Here, intermolecular interactions between guaiacol and quinones (1,4-benzoquinone and 1,4-naphthoquinone) were identified in proxies of the nonpolar, water-insoluble phase of BBOA, using UV-vis spectroscopy. Guaiacol and its derivatives are some of the most abundant emissions of smoldering coniferous species. Enhanced light absorption occurred instantaneously upon mixing colorless guaiacol with either quinone in n-heptane and did not increase with time, in contrast to the absorbance changes that would be expected for a covalent product. This enhancement decreased by about 25% as the temperature increased from 303 to 323 K, consistent with exothermic association to give complexes, yielding enthalpies of complexation of -13.3 ± 0.6 and -12.3 ± 0.4 kJ mol-1 for guaiacol with benzoquinone and naphthoquinone, respectively. Enhancement was also observed upon gas-liquid partitioning of benzoquinone into thin films of guaiacol, for example, with a thickness of 20 μm. This multiphase processing, mimicking partitioning of quinones into liquid BBOA, produced absorption comparable to moderately absorbing BrC from other sources, suggestive of the atmospheric relevance of these interactions.

A water probe for direct pH measurement of individual particles via micro-Raman spectroscopy

The acidity of atmospheric aerosols influences fundamental physicochemical processes that affect climate and human health. We recently developed a novel and facile water-probe-based method for directly measuring of the pH for micrometer-size droplets, providing a promising technique to better understand aerosol acidity in the atmosphere. The complex chemical composition of fine particles in the ambient air, however, poses certain challenges to using a water-probe for pH measurement, including interference from interactions between compositions and the influence of similar compositions on water structure. To explore the universality of our method, it was employed to measure the pH of ammonium, nitrate, carbonate, sulfate, and chloride particles. The pH of particles covering a broad range (0-14) were accurately determined, thereby demonstrating that our method can be generally applied, even to alkaline particles. Furthermore, a standard spectral library was developed by integrating the standard spectra of common hydrated ions extracted through the water-probe. The library can be employed to identify particle composition and overcome the spectral overlap problem resulting from similar effects. Using the spectral library, all ions were identified and their concentrations were determined, in turn allowing successful pH measurement of multicomponent (ammonium-sulfate-nitrate-chloride) particles. Insights into the synergistic effect of Cl-, NO3-, and NH4+ depletion obtained with our approach revealed the interplay between pH and volatile partitioning. Given the ubiquity of component partitioning and pH variation in particles, the water probe may provide a new perspective on the underlying mechanisms of aerosol aging and aerosol-cloud interaction.

Cooperative and competitive effects in pH-dependent surface composition of atmospherically relevant organic ions in water

The molecular surface compositions of aerosols can differ considerably from their bulk counterparts, an aspect often overlooked in climate models. This oversight can potentially affect our understanding of the complex interplay between composition, optical characteristics, and their influence on cloud formation and properties. A substantial portion of aerosol mass often includes organic compounds, such as carboxylic acids and alkyl amines. These organic compounds are surface-active and influence surface tension, an important aspect of cloud droplet activation. To better understand the impact of complex organic mixtures on aerosol surfaces, we report a liquid-jet X-ray photoelectron spectroscopy (XPS) study exploring the pH-dependent surface composition of aqueous solutions of butyric acid and butylamine, both isolated and mixed. Our findings reveal that the surface composition is highly influenced by the ratio between these solutes and their total surface concentration. Around pH 7, where both solutes are charged, the mixed solution demonstrates cooperative surface behavior, leading to an increased presence of organics at the surface. At extreme pH values, where one solute is charged and the other remains neutral, the solutes act independently, with the neutral species dominating the surface enrichment. We also discuss the molecular mechanisms driving these interactions and their broader implications for aerosol behavior in climate models.

T- and pH-Dependent Hydroxyl-Radical Reaction Kinetics of Lactic Acid, Glyceric Acid, and Methylmalonic Acid in the Aqueous Phase

Carboxylic acids are a common class of compounds found in atmospheric aerosols and cloud droplets. In this study, the oxidation kinetics of several carboxylic acids in the aqueous phase by the atmospherically relevant •OH radical were investigated to better understand the loss processes for this class of compounds. The rate constants for the reactions of the •OH radical were determined using the thiocyanate competition kinetics method for lactic acid, glyceric acid, and methylmalonic acid as a function of temperature and pH. The Arrhenius equations for oxidation by the •OH radical are as follows (unit in L mol-1 s-1): For lactic acid: k(T, HA) = (1.3 ± 0.1) × 1010 × exp[(-910 ± 160 K)/T] and k(T, A-) = (1.3 ± 0.1) × 1010 × exp[(-800 ± 80 K)/T]; for glyceric acid: k(T, HA) = (6.0 ± 0.2) × 1010 × exp[(-1100 ± 170 K)/T] and k(T, HA±) = (3.6 ± 0.1) × 1010 × exp[(-1500 ± 100 K)/T]; and for methylmalonic acid: k(T, H2A) = (5.5 ± 0.1) × 1010 × exp[(-1760 ± 100 K)/T], k(T, HA-) = (1.4 ± 0.1) × 109 × exp[(-530 ± 80 K)/T] and k(T, A2-) = (9.6 ± 0.4) × 1010 × exp[(-1530 ± 270 K)/T]. The general trend of the •OH rate constant was observed kA2- > kHA- > kH2A. The energy barriers of the •OH radical reaction and thus the most probable site of H atom abstraction were calculated using density functional theory simulations in Gaussian with the M06-2X method and the 6-311++G(3df,2p) basis set. Kinetic data predicted from structure-activity relationships were compared to the measured •OH radical rate constants. •OH radical oxidation in the aqueous phase could serve as an important sink for carboxylic acids, and the pH- and T-dependent rate constants of •OH radical reactions provide a better description of the aqueous-phase sink processes. Hence, the atmospheric lifetime as well as the partitioning of the investigated carboxylic acids was calculated.

Spatiotemporal distribution of marine aerosols and gaseous species over the North Pacific Ocean

Observational studies of marine aerosols are essential for understanding the global aerosol budget and its environmental impacts. This study presents simultaneous in-situ measurements of major ionic components (Cl-, NO3-, SO42-, NH4+, K+, Ca2+, Na+, and Mg2+) in aerosols and gaseous species (HCl, HNO3, HONO, SO2, and NH3) over the North Pacific Ocean from July 4 to 15 and September 19 to October 3, 2022. Using high temporal resolution instruments aboard the Republic of Korea's icebreaker research vessel Araon, this study aimed to (1) report the spatial and temporal distributions of aerosols and gaseous species, (2) estimate the source contributions of continental anthropogenic pollutants, and (3) assess the influence of aerosol chemical composition and gaseous species on aerosol acidity and water content. Our results revealed a significant decline in anthropogenic contributions, from 72.4 ± 11.2 % in the Yellow Sea and East Sea to 32.0 ± 10.8 % in the remote Pacific, accompanied by an increase in natural aerosols from 27.6 ± 11.2 % to 68.0 ± 10.8 %. Elevated concentrations of ammonia (NH3) and nitrous acid (HONO) were observed in the remote ocean, likely associated with the marine environment and biological activities. This increase in NH3, along with a decrease in sulfate concentrations, contributed to the formation of more alkaline aerosols in the remote ocean. Despite the limited availability of previous studies for direct comparisons, our findings align with observed trends and highlight the unique physicochemical properties of marine aerosols. These results enhance our understanding for the interactions between continental pollutants and marine environments, emphasizing the distinct characteristics of marine aerosols and their potential role in modifying atmospheric processes and influencing climate change.

Understanding of enhanced nitrate in fine particles at agricultural sites in summer with high ammonia level

Nitrate is one of the major constituents of fine particles and has not been effectively alleviated in Northeast Asia. Field measurements of various gases and the chemical composition of fine particles were conducted at two agricultural sites (cropland and livestock) in ammonia-rich environments to understand the effect of ammonia on nitric acid-nitrate partitioning using a thermodynamic model and to suggest a possible strategy to control total nitrate (i.e., nitric acid formation). High nitrate levels were observed at the agricultural sites in summer compared to those at the urban sites. It was found that high level of ammonia in summer led to increased aerosol pH and nitrate fraction. At the cropland site in summer, the daily nitrate fraction was particularly sensitive to aerosol pH, suggesting that ammonia reduction should be effective in decreasing nitrate formation via nitric acid-nitrate partitioning (with a 50% reduction in ammonia, nitrate concentration can decrease by 34%). Aerosol water content also played a significant role in determining nitrate fraction in the aerosol pH range of 2.5-3.0. It was found that the sites were under high NOx conditions, and that the reduction of OH production (daytime) and O3 (nighttime) was important for controlling total nitrate, but this is challenging due to the high contributions of background O3. It was concluded that the reduction of ammonia emissions for the control of the nitrate fraction via gas-to-particle partitioning should be important to mitigate nitrate in fine particles at agricultural sites in summer.

Molecular insights into the composition, sources, and aging of atmospheric brown carbon

The light-absorbing chemical components of atmospheric organic aerosols are commonly referred to as Brown Carbon (BrC), reflecting the characteristic yellowish to brown appearance of aerosol. BrC is a highly complex mixture of organic compounds with diverse compositions and variable optical properties of its individual chromophores. BrC significantly influences the radiative budget of the climate and contributes to adverse air pollution effects such as reduced visibility and the presence of inhalable pollutants and irritants. However, a fundamental understanding of the sources, formation, and transformation (aging effects) of BrC remains incomplete. This gap in knowledge necessitates advanced chemical characterization of individual aerosol components and the correlation of their composition with optical properties. Over the past decade, a multi-modal analytical platform composed of high-performance liquid chromatography with a photodiode array UV-vis detector and high-resolution mass spectrometry has been extensively used for the untargeted analysis of BrC components in complex mixtures of atmospheric organic aerosols and their laboratory proxies. This method separates solvent-extractable BrC compounds into distinct fractions, each characterized by specific retention times, UV-vis absorption spectra, and elemental compositions, offering comprehensive molecular insights into BrC components. In this review, we highlight the application of this platform in analyzing both real-world aerosol samples and laboratory-generated proxies. These studies have identified composition-specific sources and transformations of BrC, advancing our understanding of these complex atmospheric mixtures. Atmospheric humic-like substances (HULIS), formed through cloud processing of wildfire smoke and the oligomerization of water-soluble organics, are key contributors to BrC. Additional HULIS originate from fossil fuel combustion, biogenic, and marine emissions. Key BrC chromophores include nitroaromatics, imidazoles, N-heterocycles, polyaromatic hydrocarbons, quinones, and others. Aging processes, including photolysis and multiphase reactions, can significantly alter BrC optical properties by generating new chromophores or degrading existing ones. The fundamental knowledge gained from these investigations is essential for assessing BrC optical properties. Additionally, it provides practical composition metrics necessary to inform and improve future atmospheric models, enabling more accurate predictions of BrC behavior and its impact on climate and air quality.

On using an aerosol thermodynamic model to calculate aerosol acidity of coarse particles

Thermodynamic modeling is still the most widely used method to characterize aerosol acidity, a critical physicochemical property of atmospheric aerosols. However, it remains unclear whether gas-aerosol partitioning should be incorporated when thermodynamic models are employed to estimate the acidity of coarse particles. In this work, field measurements were conducted at a coastal city in northern China across three seasons, and covered wide ranges of temperature, relative humidity and NH3 concentrations. We examined the performance of different modes of ISORROPIA-II (a widely used aerosol thermodynamic model) in estimating aerosol acidity of coarse and fine particles. The M0 mode, which incorporates gas-phase data and runs the model in the forward mode, provided reasonable estimation of aerosol acidity for coarse and fine particles. Compared to M0, the M1 mode, which runs the model in the forward mode but does not include gas-phase data, may capture the general trend of aerosol acidity but underestimates pH for both coarse and fine particles; M2, which runs the model in the reverse mode, results in large errors in estimated aerosol pH for both coarse and fine particles and should not be used for aerosol acidity calculations. However, M1 significantly underestimates liquid water contents for both fine and coarse particles, while M2 provides reliable estimation of liquid water contents. In summary, our work highlights the importance of incorporating gas-aerosol partitioning when estimating coarse particle acidity, and thus may help improve our understanding of acidity of coarse particles.

Atmospheric Hydroxyl Radical Route Revealed: Interface-Mediated Effects of Mineral-Bearing Microdroplet Aerosol

Hydroxyl radical (·OH) plays a crucial role in atmospheric chemistry, regulating the oxidative potential and aerosol composition. This study reveals an unprecedented source of ·OH in the atmosphere: mineral dust-bearing microdroplet aerosols. We demonstrate that Kaolin clay particles in microdroplet aerosols trigger rapid ·OH production upon solar irradiation, with rates reaching an order of at least 10-3 M s-1. This production rate is several orders of magnitude higher than that of the bulk phase (2.4 × 10-11 M s-1) and previously known pathways. On this basis, the surface-based interfacial ·OH production rate is estimated to be 8.9 × 10-5 mol m-2 s-1 at the air-water-solid interface of 1 μm sized aerosol particles. The enhanced ·OH formation is attributed to the unique features of air-water-solid interfaces, where the lifespan of photoinduced holes was significantly increased due to the presence of strong electric fields at the air-water interface. We further investigated the impacts of various environmental factors and aerosol properties on ·OH production, including light intensity, relative humidity, particle size, and pH. Our findings provide new insights into atmospheric photochemical processes mediated by mineral dust-bearing microdroplet aerosols, which are important contributors to ·OH source in the atmosphere. This work advances our understanding of atmospheric interfacial chemistry and its profound and lasting implications for air quality and climate.

Wavelength-dependent intersystem crossing dynamics of phenolic carbonyls in wildfire emissions

Quantum chemical calculations were employed to construct Jablonski diagrams for a series of phenolic carbonyls, including vanillin, iso-vanillin, 4-hydroxybenzaldehyde, syringaldehyde, and coniferyl aldehyde. These molecules can enter the Earth's atmosphere from forest fire emissions and participate in photochemical reactions within the atmospheric condensed phase, including cloud and fog droplets and aqueous aerosol particles. This photochemistry alters the composition of light-absorbing organic content, or brown carbon, in droplets and particles through the formation and destruction of key chromophores. This study demonstrates that following photon absorption, phenolic carbonyls efficiently transition to triplet states via intersystem crossings (ISC), with rate coefficients ranging from 109 to 1010 s-1. Despite the presence of multiple potential ISC pathways due to several lower-lying triplet states, a single channel is found to dominate for each system. We further investigated the dependence of the ISC rate constant (kISC) on the vibrational excitation energy of the first accessible (ππ*) singlet excited state (S1 or S2, depending on the molecule), and compared it with the measured wavelength dependence of the photochemical quantum yield (Φloss). Although our model only accounts for intramolecular nonradiative electronic transitions, it successfully captures the overall trends. All studied molecules, except coniferyl aldehyde, exhibit saturation in the dependence of both kISC and Φloss on the wavelength (or vibrational excitation energy). In contrast, coniferyl aldehyde displays a single maximum, followed by a monotonic decrease as the excitation energy increases (wavelength decreases). This distinct behavior in coniferyl aldehyde may be attributed to the presence of a double-bonded substituent, which enhances π-electron conjugation, and reduces the exchange energy and thus the adiabatic energy gap between the S1(ππ*) state and the target triplet state. For small energy gaps, the classical acceptor modes of the ISC process are less effective, leading to a low effective density of final states. Larger gaps enhance the effective density of states, making the wavelength dependence of the ISC more pronounced. Our calculations show that while all the studied phenolic carbonyls have similar acceptor modes, coniferyl aldehyde has a substantially smaller adiabatic gap (1700 cm-1) than the other molecules. The magnitude of the adiabatic energy gap is identified as the primary factor determining the energy/wavelength dependence of the ISC rate and thus Φloss.

Biomass-burning organic aerosols as a pool of atmospheric reactive triplets to drive multiphase sulfate formation

Biomass-burning organic aerosol(s) (BBOA) are rich in brown carbon, which significantly absorbs solar irradiation and potentially accelerates global warming. Despite its importance, the multiphase photochemistry of BBOA after light absorption remains poorly understood due to challenges in determining the oxidant concentrations and the reaction kinetics within aerosol particles. In this study, we explored the photochemical reactivity of BBOA particles in multiphase S(IV) oxidation to sulfate. We found that sulfate formation in BBOA particles under light is predominantly driven by photosensitization involving the triplet excited states (3BBOA*) instead of iron, nitrate, and S(IV) photochemistry. Rates in BBOA particles are three orders of magnitude higher than those observed in the bulk solution, primarily due to the fast interfacial reactions. Our results highlight that the chemistry of 3BBOA* in particles can greatly contribute to the formation of sulfate, as an example of the secondary pollutants. Photosensitization of BBOA will likely become increasingly crucial due to the intensified global wildfires.

Aerosol Fluorescent Labeling via Probe Molecule Volatilization

The physicochemical properties of aerosols, including hygroscopicity, phase state, pH, and viscosity, influence important processes ranging from virus transmission and pulmonary drug delivery to atmospheric light scattering and chemical reactivity. Despite their importance, measurements of these key properties in aerosols remain experimentally challenging due to small particle sizes and low mass densities in air. Fluorescence probe spectroscopy is one of the only analytical techniques that is capable of experimentally determining these properties in situ in a nondestructive and minimally perturbative manner. However, the application of fluorescence probe spectroscopy to important classes of aerosols including exhaled respiratory and ambient atmospheric aerosols has been limited due to a typical reliance on premixing the probe molecule with particle constituents prior to particle generation, which is not always possible. Here, a method for aerosol fluorescent labeling based on probe molecule volatilization is developed. The method is first applied to label model polyethylene glycol (PEG) aerosols with two different polarity-sensitive probes, Nile red and Prodan. The similarity of the relative humidity-dependent fluorescent emission of each probe between prelabeled and volatilized-probe PEG particles validated the methodology. A preliminary application of the technique to indicate the hygroscopicity of artificial saliva respiratory particles and model atmospheric secondary organic aerosol particles is demonstrated. The methodology developed here paves the way for future studies applying powerful fluorescent probe-based analytical techniques to study exhaled or natural aerosols for which fluorescent prelabeling is not possible.

Review of health effects driven by aerosol acidity: Occurrence and implications for air pollution control

Acidity, generally expressed as pH, plays a crucial role in atmospheric processes and ecosystem evolution. Atmospheric acidic aerosol, triggering severe air pollution in the industrialization process (e.g., London Great Smoke in 1952), has detrimental effects on human health. Despite global endeavors to mitigate air pollution, the variation of aerosol acidity remains unclear and further restricts the knowledge of the acidity-driven toxicity of fine particles (PM2.5) in the atmosphere. Here, we summarize the toxicological effects and mechanisms of inhalable acidic aerosol and its response to air pollution control. The acidity could adjust toxic components (e.g., metals, quinones, and organic peroxides) bonded in aerosol and synergize with oxidant gaseous pollutants (e.g., O3 and NO2) in epithelial lining fluid to induce oxidative stress and inflammation. The inhaled aerosol from the ambient air with higher acidity might elevate airway responsiveness and cause worse pulmonary dysfunction. Furthermore, historical observation data and model simulation indicate that PM2.5 can retain its acidic property despite considerable reductions in acidifying gaseous pollutants (e.g., SO2 and NOx) from anthropogenic emissions, suggesting its continuing adverse impacts on human health. The study highlights that aerosol acidity could partially offset the health benefits of emission reduction, indicating that acidity-related health effects should be considered for future air pollution control policies.

Exploring the use of ground-based remote sensing to identify new particle formation events: A case study in the Beijing area

New Particle Formation (NPF) is an important process of secondary aerosol production in the atmosphere, which has significant impacts on the Earth's radiation balance, air quality, and climate change. In this study, we develop a method to identify NPF events based on ground-based remote sensing. We propose a proxy to characterize NPF events utilizing ground-based remote sensing of gaseous precursors and aerosol optical depth (AOD). This proxy is applied to identify the NPF events in Beijing in the winter of 2022 and tested by comparison with in-situ observations of aerosol particle number size distributions (PNSD) from SMPS. The comparison shows that the NPF events for regional nucleation can be identified effectively when the threshold for sulfur dioxide and organic gases (i.e. formaldehyde) are determined as 0.44 × 10-4 and 1.07 × 10-4. Based on these thresholds, the NPF events can be identified at a high percentage (84 %) compared with in-situ observations. The relationship between identification of NPF events and meteorological conditions shows that NPF events in Beijing winter occurred more frequently under weather conditions with north-west wind direction, high wind speed and low relative humidity.

Wavelength- and pH-Dependent Optical Properties of Aqueous Aerosol Particles Containing 4-Nitrocatechol

The radiative forcing caused by atmospheric aerosol represents one of the largest uncertainties in climate models. In part, these uncertainties derive from poor characterizations of the optical properties of light-absorbing brown carbon (BrC) containing aerosols. Here, single particle cavity ring-down spectroscopy (SP-CRDS) is used to determine the complex refractive index at the optical wavelength of 405 nm for aqueous particles composed of an abundant BrC species, 4-nitrocatechol. Moreover, the effect of acidity on the complex refractive index of 4-nitrocatechol is explored. UV/visible spectroscopy allows measurement of the wavelength-dependent (from 200 to 800 nm) imaginary refractive index for bulk aqueous solutions of 4-nitrocatechol, for which the pH is adjusted between ∼1 and 13. Applying a physically based refractive index mixing rule, wavelength-dependent imaginary refractive index values are estimated for the fully protonated, singly deprotonated and doubly deprotonated forms of 4-nitrocatechol. A Kramers-Kronig analysis constrained by the 405 nm SP-CRDS and 632.8 nm elastic light scattering measurements gives the wavelength-dependent real refractive index values. The real and imaginary refractive indices are essential for computing the radiative properties of these abundant BrC chromophores in aerosol plumes and cloudwater.

Online Aerosol pH Detection Using 3D-Printed Microfluidic Devices with a Novel Magnetic SERS Sensor

Accurate measurement of aerosol pH is crucial for understanding atmospheric processes and mitigating haze pollution. However, online detection of aerosol pH is challenging due to the complex composition of single-particle matter and trace components. This study develops a sensitive and selective sensor for the online detection of aerosol pH using surface-enhanced Raman spectroscopy (SERS). A novel Fe3O4@SiO2@Au-p-aminothiophenol (FA-pATP) sensor was fabricated using a layer-by-layer self-assembly method, achieving enhanced uniformity and increased density of SERS-active hotspots. Magnetic aggregation was employed to further amplify the Raman signal. This sensor was integrated into a 3D-printed microfluidic device to facilitate online monitoring of aerosol pH. The FA-pATP sensor exhibited a significant increase in peak intensity ratio with rising pH, demonstrating high sensitivity and responsiveness due to structural changes in the -NH2 groups of pATP under different pH conditions. The sensor demonstrated a linear pH response ranging from 5 to 11. The 3D-printed microfluidic device, coupled with the FA-pATP sensor, demonstrated notable performance in various environmental media, indicating strong anti-interference capabilities. The proposed sensor shows great promise for real-time online monitoring of aerosol pH, with broad applications in environmental monitoring.

Observational study of microphysical and chemical characteristics of size-resolved fog in different regional backgrounds in China

To investigate the relationship between microphysical and chemical characteristics of size-resolved fog droplets in different regional backgrounds, we conducted observational experiments in urban, mountainous, rainforest, and rural areas of China. Fog water samples across different diameter ranges (4-16 μm, 16-22 μm, and >22 μm) were collected, alongside fog droplet spectra data. Our findings reveal a close relationship between pH value, electrical conductivity (EC), total ion concentration (TIC) of droplets, and droplet sizes, with smaller droplets exhibiting stronger acidity and higher ion concentrations. Significant differences in chemical composition are observed across size ranges and regional backgrounds. Droplet number concentration (N) and liquid water content (LWC) distributions in different regional backgrounds are skewed, with peak diameters of LWC spectra similar to those of N spectra, yet overall spectral distributions varied significantly. Droplet number concentrations are highest in urban area, while large droplets contribute more to overall LWC in mountainous, rainforest, and rural areas. No direct evidence linked LWC or surface area (S) to LWC ratio to water-soluble ion concentrations of size-resolved fog droplets in different regional backgrounds. However, by adjusting the contributions of S and LWC proportions of different-sized droplets to the ion concentration proportions, we find that expanding the LWC proportion to 2.43 times and decreasing the S proportion to 0.2 times for large droplets, while decreasing the LWC ratio to 0.76 times for small droplets, provided a better explanation for the distribution of ion concentrations. This study advances our understanding of the intricate relationship between the microphysical and chemical characteristics of fog, helping to develop more robust and comprehensive models for fog prediction and management.

Aerosolized algal bloom toxins are not inert

Harmful algal blooms (HABs) are projected to become increasingly prevalent, extending over longer periods and wider geographic regions due to the warming surface ocean water and other environmental factors, including but not limited to nutrient concentrations and runoff for marine and freshwater environments. Incidents of respiratory distress linked to the inhalation of marine aerosols containing HAB toxins have been documented, though the risk is typically associated with the original toxins. However, aerosolized toxins in micrometer and submicrometer particles are vulnerable to atmospheric processing. This processing can potentially degrade HAB toxins and produce byproducts with varying potencies compared to the parent toxins. The inhalation of aerosolized HAB toxins, especially in conjunction with co-morbid factors such as exposure to air pollutants from increased commercial activities in ports, may represent a significant exposure pathway for a considerable portion of the global population. Understanding the chemistry behind the transformation of these toxins can enhance public protection by improving the existing HAB alert systems.

Atmospheric reactive nitrogen conversion kicks off the co-directional and contra-directional effects on PM2.5-O3 pollution

As the two important ambient air pollutants, particulate matter (PM2.5) and ozone (O3) can both originate from gas nitrogen oxides. In this study, applied by theoretical analysis and machine learning method, we examined the effects of atmospheric reactive nitrogen on PM2.5-O3 pollution, in which nitric oxide (NO), nitrogen dioxide (NO2), gaseous nitric acid (HNO3) and particle nitrate (pNO3-) conversion process has the co-directional and contra-directional effects on PM2.5-O3 pollution. Of which, HNO3 and SO2 are the co-directional driving factors resulting in PM2.5 and O3 growing or decreasing simultaneously; while NO, NO2, and temperature represent the contra-directional factors, which can promote the growth of one pollutant and reduce another one. Our findings suggest that designing the suitable co-controlling strategies for PM2.5-O3 sustainable reduction should target at driving factors by considering the contra-directional and co-directional effects under suitable sensitivity regions. For co-directional driving factors, the design of suitable mitigation strategies will jointly achieve effective reduction in PM2.5 and O3; while for contra-directional driving factors, it should be more patient, otherwise, it is possible to reduce one item but increase another one at the same time.

Indoor air concentrations of PM2.5 quartz fiber filter-collected ionic PFAS and emissions to outdoor air: findings from the IPA campaign

Per- and polyfluoroalkyl substances (PFAS) are prevalent in consumer products used indoors. However, few measurements of ionic PFAS exist for indoor air. We analyzed samples collected on PM2.5 quartz fiber filters (QFFs) in 11 North Carolina homes 1-3 times in living rooms (two QFFs in series), and immediately outside each home (single QFF), for 26 ionic PFAS as part of the 9 months Indoor PFAS Assessment (IPA) Campaign. All targeted PFAS, except for PFDS and 8:2 monoPAP, were detected indoors. PFBA, PFHpA, PFHxA, PFOA, PFOS, and 6:2 diPAP were detected in >50% of indoor samples. PFHxA, PFOA, and PFOS had the highest detection frequency (DF = 80%; medians = 0.5-0.7 pg m-3), while median PFBA concentrations (3.6 pg m-3; DF = 67%) were highest indoors. Residential indoor air concentrations (sum of measured PFAS) were, on average, 3.4 times higher than residential outdoor air concentrations, and an order of magnitude higher than regional background concentrations. Indoor-to-outdoor emission rate estimates suggest that emissions from single unit homes could be a meaningful contributor to PFBA, PFOA, and PFOS emissions in populated areas far from major point sources. Backup QFFs were observed to adsorb some targeted PFAS from the gas-phase, making reported values upper-bounds for particle-phase and lower-bounds for total air (gas plus particle) concentrations. We found that higher concentrations of carbonaceous aerosol were associated with a shift in partitioning of short chain PFCAs and long chain PFSAs toward the particle phase.

Stark seasonal contrast of fine aerosol levels, composition, formation mechanism, and characteristics in a polluted megacity

In this study, we investigated the temporal variation of organic and inorganic aerosol with its optical properties in Mumbai (India), an urban coastal region. Mean PM2.5 concentrations during the sampling period were 175 μg/m3 (winter) and 90 μg/m3 (summer). During winter, the average concentrations of organic (OC), elemental (EC), and water-soluble organic carbon (WSOC) were three times higher than in summer. Secondary organic carbon (SOC) contribution in OC was higher in summer (78%) than in winter (53%), and strong solar radiation in summer likely caused this outcome. Aerosols were slightly acidic in both seasons, with an average pH of 5.7 (winter) and 6.0 (summer). A correlation was observed between SOC and the acidity of particles in summer (R2 = 0.6), indicating some amount of acid-catalysed SOC formation. In both seasons, the sulphate oxidation ratio (SOR) was higher than the nitrate oxidation ratio (NOR), which may reflect a preference for SO2 oxidation over NO2 or the difference in partitioning ammonium nitrate into ammonium sulphate under high RH. The dominant mechanism of SOC formation (gas vs aqueous phase oxidation) also showed seasonal variation. In winter, a relatively steep reduced major axis (RMA) slope of O3/CO suggests gas phase oxidation was the dominant mechanism of SOC production. Winter has more BrC fraction than summer, indicating higher absorbing aerosols, though the efficiency of absorbing the light was higher in summer. To assess the radiative forcing of PM2.5 on a local scale, an effective carbon ratio (ECR) was computed. The findings pointed to a local radiative heating impact caused by PM2.5. The spectral slope ratio and MAE at 250 to 300 nm ratio (E2/E3) revealed a higher abundance of high molecular weight species in WSOC during summer than in winter.

Depletion of iodide in ageing aerosols and the role of humidity: A case study of mixed sodium iodide-malonic acid aerosol

Global sea to air iodine emissions, along with organic emissions and their oxidation products, have increased tremendously. This work presents a comprehensive analysis of the humidity mediated changes in ageing aerosols comprising iodide and water soluble dicarboxylic acid using aerosol micro-Raman spectroscopy. The studies in the model system, sodium iodide-malonic acid mixed aerosols, unveiled the depletion in iodide. Mechanistic insights gleaned through comparative studies conducted under inert (nitrogen) and oxidative (air) atmospheres reveal the iodide depletion occurs possibly via oxidation to molecular iodine. The reaction involves gaseous components, diffusion of which across the particles will be impacted by the physical state of the particles, such as viscosity, which in turn is intricately linked to ambient humidity levels. To this end, studies on the temporal evolution of the reaction at three distinct RHs covering 30-80% revealed the enhanced progression of the reaction with increasing humidity. Given that geographical locations serving as major sources for atmospheric iodine typically experience high humidity, these reactions could emerge as an additional process controlling iodine speciation in ageing aerosols.

Quantifying the Effect of Basic Minerals on Acid- and Ligand-Promoted Dissolution Kinetics of Iron in Simulated Dark Atmospheric Aging of Dust and Coal Fly Ash Particles

The content and multiphase chemistry of iron (Fe) in multicomponent atmospheric aerosols are important to global climate and oceanic models. To date, reported dissolution rates of Fe span orders of magnitude with no quantifiable dependency on the content of basic minerals that coexist with Fe. Here, we report dissolution rates of Fe in simulated dark atmospheric aging of fully characterized multielement particles under acidic conditions (bulk pH 1 or 3) with and without oxalic acid and pyrocatechol. Our main findings are (a) the total amount of Ca and Mg was higher in coal fly ash than in Arizona test dust, (b) Fe dissolution initial rates increased exponentially with %Ca/Al and %Mg/Al below 50%, (c) a reduction in the Fe dissolution initial rate was observed with %Ca/Al higher than 50%, (d) reactive Ca and Mg minerals increased the calculated initial pH at the liquid/solid interface to values higher by only 1.5-2 units than the measured bulk pH, yet interfacial water remained acidic for Fe dissolution to take place, and (e) reactive Ca and Mg minerals enhanced the deprotonation of organics at the interface, aiding in ligand-promoted dissolution of Fe. The impact of these results is discussed within the context of constraining Fe dissolution kinetic models.

Effects of copper on chemical kinetics and brown carbon formation in the aqueous ˙OH oxidation of phenolic compounds

Many phenolic compounds (PhCs) in biomass burning and fossil fuel combustion emissions can partition into atmospheric aqueous phases (e.g., cloud/fog water and aqueous aerosols) and undergo reactions to form secondary organic aerosols (SOAs) and brown carbon (BrC). Redox-active transition metals, particularly Fe and Cu, are ubiquitous species in atmospheric aqueous phases known to participate in Fenton/Fenton-like chemistry as a source of aqueous ˙OH. However, even though the concentrations of water-soluble Cu are close to those of water-soluble Fe in atmospheric aqueous phases in some areas, unlike Fe, the effects that Cu have on SOA and BrC formation in atmospheric aqueous phases have scarcely been studied and remain poorly understood. We investigated the effects of Cu(II) on PhC reaction rates and BrC formation during the aqueous oxidation of four PhCs (guaiacol, catechol, syringol, and vanillin) by ˙OH generated from Fenton-like chemistry under different pH conditions. While the PhCs reacted when both H2O2 and Cu(II) were present in the absence (i.e., dark oxidation) and presence (i.e., photooxidation) of light, the reaction rates were at least one order of magnitude higher during photooxidation. Higher PhC reaction rates were measured at higher pH during both dark oxidation and photooxidation as a result of higher ˙OH concentrations produced by Fenton-like chemistry. Only water-soluble BrC was formed during dark oxidation and photooxidation when Cu(II) was present. Mass absorption coefficients (103 to 104 cm2 g-1) comparable to those of biomass burning BrC were measured during dark oxidation and photooxidation when Cu(II) was present. Light absorption was enhanced at higher pH during dark oxidation and photooxidation, which indicated that higher quantities and/or more absorbing BrC chromophores were formed at higher pH. The effects that Cu(II) had on the PhC reaction rates and the composition of SOAs and BrC formed depended on the PhC base structure (i.e., benzenediol vs. methoxyphenol). Overall, these results show how aqueous reactions involving Cu(II), H2O2, and PhCs can be an efficient source of daytime and nighttime water-soluble BrC and SOAs, which can have significant implications for how the atmospheric fates of PhCs are modeled for areas with substantial concentrations of water-soluble Cu in highly to moderately acidic cloud/fog water and aqueous aerosols.

Rising Alkali-to-Acid Ratios in the Atmosphere May Correspond to Increased Aerosol Acidity

Aerosol acidity (or pH) is one central parameter in determining the health, climate, and ecological effects of aerosols. While it is traditionally assumed that the long-term aerosol pH levels are determined by the relative abundances of atmospheric alkaline to acidic substances (referred to as RC/A hereinafter), we observed contrasting pH─RC/A trends at different sites globally, i.e., rising alkali-to-acid ratios in the atmosphere may unexpectedly lead to increased aerosol acidity. Here, we examined this apparently counterintuitive phenomenon using the multiphase buffer theory. We show that the aerosol water content (AWC) set a pH "baseline" as the peak buffer pH, while the RC/A and particle-phase chemical compositions determine the deviation of pH from this baseline within the buffer ranges. Therefore, contrasting long-term pH trends may emerge when RC/A increases while the AWC or nitrate fraction decreases, or vice versa. Our results provided a theoretical framework for a quantitative understanding of the response of aerosol pH to variations in SO2, NOx versus NH3, and dust emissions, offering broad applications in studies on aerosol pH and the associated environmental and health effects.

Modeling Indoor Inorganic Aerosol Concentrations During the ATHLETIC Campaign with IMAGES

In 2018, the ATHLETIC campaign was conducted at the University of Colorado Dal Ward Athletic Center and characterized dynamic indoor air composition in a gym environment. Among other parameters, inorganic particle and gas-phase species were alternatingly measured in the gym's supply duct and weight room. The Indoor Model of Aerosols, Gases, Emissions, and Surfaces (IMAGES) uses the inorganic aerosol thermodynamic equilibrium model, ISORROPIA, to estimate the partitioning of inorganic aerosols and corresponding gases. In this study herein, measurements from the ATHLETIC campaign were used to evaluate IMAGES' performance. Ammonia emission rates, nitric acid deposition, and particle deposition velocities were related to observed occupancy, which informed these rates in IMAGES runs. Initially, modeled indoor inorganic aerosol concentrations were not in good agreement with measurements. A parametric investigation revealed that lowering the temperature or raising the relative humidity used in the ISORROPIA model drove the semivolatile species more toward the particle phase, substantially improving modeled-measured agreement. One speculated reason for these solutions is that aerosol water was enhanced by increasing the RH or decreasing the temperature. Another is that thermodynamic equilibrium was not established in this indoor setting or that the thermodynamic parametrizations in ISORROPIA are less accurate for typical indoor settings. This result suggests that applying ISORROPIA indoors requires further careful experimental validation.

Biogenic and Anthropogenic Secondary Organic Aerosols Become Fluorescent after Highly Acidic Aging

Primary biological aerosol particles (PBAPs) and secondary organic aerosol (SOA) both contain organic compounds that share similar chemical and optical properties. Fluorescence is often used to characterize PBAPs; however, this may be hindered due to interferences from fluorophores in SOA. Despite extensive efforts to understand the aging of SOA under elevated particle acidity conditions, little is known about how these processes affect the fluorescence of SOA and thereby their interference with the measurements of PBAPs. The objective of this study is to investigate the fluorescence of SOA and understand the influence of acidity on the optical properties of organic aerosols and potential interference for the analysis of bioaerosols. The SOA was generated by O3- or OH-initiated oxidation of d-limonene or α-pinene, as well as by OH-initiated oxidation of toluene or xylene. The SOA compounds were then aged by exposure to varying concentrations of aqueous H2SO4 for 2 days. Absorption and fluorescence spectrophotometry were used to examine the changes in the optical properties before and after aging. The key observation was the appearance of strongly light-absorbing and fluorescent compounds at pH = ∼-1, suggesting that acidity is a major driver of SOA aging. The aged SOA from biogenic precursors (d-limonene and α-pinene) resulted in stronger fluorescence than the aged SOA from toluene and xylene. The absorption spectra of the aged SOA changed drastically in shape upon dilution, whereas the shapes of the fluorescence spectra remained the same, suggesting that the fluorophores and chromophores in SOA are separate sets of species. The fluorescence spectra of aged SOA overlapped with the fluorescence spectra of PBAPs, suggesting that SOA exposed to highly acidic conditions can be confused with PBAPs detected by fluorescence-based methods. These processes are likely to play a role in the atmospheric regions where high concentrations of H2SO4 persist, such as the upper troposphere and lower stratosphere.

Competing Nonadiabatic Relaxation Pathways for Near-UV Excited ortho-Nitrophenol in Aqueous Solution

Nitrophenols are atmospheric pollutants found in brown carbon aerosols produced by biomass burning. Absorption of solar radiation by these nitrophenols contributes to atmospheric radiative forcing, but quantifying this climate impact requires better understanding of their photochemical pathways. Here, the photochemistry of near-UV (λ = 350 nm) excited ortho-nitrophenol in aqueous solution is investigated using transient absorption spectroscopy and time-resolved infrared spectroscopy over the fs to μs time scale to characterize the excited states, intermediates, and photoproducts. Interpretation of the transient spectroscopy data is supported by quantum chemical calculations using linear-response time-dependent density functional theory (LR-TDDFT). Our results indicate efficient nonradiative decay via an S1(ππ*)/S0 conical intersection leading to hot ground state ortho-nitrophenol which vibrationally cools in solution. A previously unreported minor pathway involves intersystem crossing near an S1(nπ*) minimum, with decay of the resulting triplet ortho-nitrophenol facilitated by deprotonation. These efficient relaxation pathways account for the low quantum yields of photodegradation.

Size-dependent acidity of aqueous nano-aerosols

Understanding the accurate acidity of nano-aerosols is important for the research on atmospheric chemistry. Herein, we propose the contributions from both the aerosol size and multiphase buffer effect to the steady-state acidity of nano-aerosols at a constant aerosol water content (AWC) through molecular simulations. As increasing of the aerosol size, the solvation free energy (SFE, ΔGs) became more negative (decreasing by 3-130 kcal mol-1 for different types of species) and Henry's law constant (H) apparently increased (from e6 to e16 mol m-3 Pa-1) in the nano-aerosols compared to that in bulk solutions. The lower SFE led to lower solute pKa and pKb values; thus, the acidity of HSO4- and HNO3 and the alkalinity of NH3 showed positive relations with the aerosol size. The lower H also increased the pKa of gaseous solutes, leading to a decrease in the acidity of HNO3 and a shift from alkaline to acidic for the NH4+/NH3 buffer pair in the nano-aerosols. The present study revealed the relationship between aerosol acidity and solvent size from a microscopic perspective. Specifically, the acidity of aerosols containing HSO4-/SO42- and HNO3/NO3- decreased with an increase in their radii, whereas aerosols containing NH4+/NH3 exhibited an opposite trend. This phenomenon can be attributed to the disappearance of the interfacial effect with an increase in the size of the aerosols. The above conclusions are of great significance for studying the pH-dependent multi-phase chemical processes in aerosols.

Enhanced aqueous formation and neutralization of fine atmospheric particles driven by extreme cold

The prevailing view for aqueous secondary aerosol formation is that it occurs in clouds and fogs, owing to the large liquid water content compared to minute levels in fine particles. Our research indicates that this view may need reevaluation due to enhancements in aqueous reactions in highly concentrated small particles. Here, we show that low temperature can play a role through a unique effect on particle pH that can substantially modulate secondary aerosol formation. Marked increases in hydroxymethanesulfonate observed under extreme cold in Fairbanks, Alaska, demonstrate the effect. These findings provide insight on aqueous chemistry in fine particles under cold conditions expanding possible regions of secondary aerosol formation that are pH dependent beyond conditions of high liquid water.

Rapid aqueous-phase dark reaction of phenols with nitrosonium ions: Novel mechanism for atmospheric nitrosation and nitration at low pH

Dark aqueous-phase reactions involving the nitrosation and nitration of aromatic organic compounds play a significant role in the production of light-absorbing organic carbon in the atmosphere. This process constitutes a crucial aspect of tropospheric chemistry and has attracted growing research interest, particularly in understanding the mechanisms governing nighttime reactions between phenols and nitrogen oxides. In this study, we present new findings concerning the rapid dark reactions between phenols containing electron-donating groups and inorganic nitrite in acidic aqueous solutions with pH levels <3.5. This reaction generates a substantial amount of nitroso- and nitro-substituted phenolic compounds, known for their light-absorbing properties and toxicity. In experiments utilizing various substituted phenols, we demonstrate that their reaction rates with nitrite depend on the electron cloud density of the benzene ring, indicative of an electrophilic substitution reaction mechanism. Control experiments and theoretical calculations indicate that the nitrosonium ion (NO+) is the reactive nitrogen species responsible for undergoing electrophilic reactions with phenolate anions, leading to the formation of nitroso-substituted phenolic compounds. These compounds then undergo partial oxidation to form nitro-substituted phenols through reactions with nitrous acid (HONO) or other oxidants like oxygen. Our findings unveil a novel mechanism for swift atmospheric nitrosation and nitration reactions that occur within acidic cloud droplets or aerosol water, providing valuable insights into the rapid nocturnal formation of nitrogen-containing organic compounds with significant implications for climate dynamics and human health.

Inverse Phase Transition in Droplet Clusters Levitating over the Locally Heated Water Layer

Self-assembled microdroplet clusters can levitate above a locally heated water surface. Normally, the temperature of droplets is in the range of 50-95 °C. However, it is possible to generate clusters at lower temperatures. Here, we study the structure and behavior of such cold-stabilized droplet clusters with variable temperature. It has been established that as the temperature decreases, the role of aerodynamic forces decreases, while electrostatic forces, on the contrary, increase. We studied the behavior of droplet clusters at relatively low temperatures down to 28 °C. A chaotic motion of droplets and a phase transition were observed at the surface temperature of the water below a critical value of about Tmax = 35 ± 2 °C. The orderliness of the cluster was quantified with the Shannon/Voronoi entropy. Several stages of cluster evolution were observed and analyzed, and a mechanism of this phenomenon is discussed. An inverse phase transition in which cooling of the cluster decreases its orderliness is discussed. Frequencies of the droplets' oscillations coincide qualitatively with the frequency of the plasma oscillations within the cluster.

Photoreduction and origin of dissolved and particulate mercury in cloud water: Insights from stable mercury isotopes

The photoreduction of mercury (Hg) in clouds is crucial for determining global Hg cycling. The recently-developed isotope approach provides new insight into the fate of atmospheric Hg, however, limited data have been reported on the dynamics of Hg isotopes in clouds. This study presented the isotopic compositions of dissolved mercury (DHg) and particulate mercury (PHg) in cloud water collected at Mt. Tai (1545 m a.s.l.) in eastern China during summer 2021. Both DHg and PHg exhibited positive mass-independent fractionation of odd isotopes (odd-MIF, denoted as Δ199Hg), with averaged Δ199Hg values of 0.83 ± 0.34‰ and 0.20 ± 0.11‰, respectively. This high odd-MIF likely resulted from aqueous photoreduction in clouds, with DHg being more susceptible to photolysis than PHg. Our findings indicated that the photoreduction was promoted by sunlight and influenced by the chemical compositions of cloud water that controlled the Hg(II) speciation. The isotope mixing model estimation revealed that particulate-bound Hg and reactive gaseous Hg constituted the principal sources of Hg in cloud water, accounting for 55% to 99% of the total, while gaseous element Hg also made a notable contribution. Additionally, cloud water samples with faster reduction rates of Hg(II) were located outside of the isotope mixing models, which indicated an enhanced photoreduction process in cloud water.

Chemical composition of cloud and rainwater at a high-altitude mountain site in western India: source apportionment and potential factors

This study focuses on the chemical composition of cloud water (CW) and rainwater (RW) collected at Sinhagad, a high-altitude station (1450 m AMSL) located in the western region of India. The samples were collected during the monsoon over two years (2016-2017). The chemical analysis suggests that the concentration of total ionic constituents was three times higher in CW than in RW, except for NH4+ (1.0) and HCO3- (0.6). Compared to RW, high concentrations of SO42- and NO3- were observed in CW. The weighted average RW pH (6.5 ± 0.3) was slightly more alkaline than CW pH (6.1 ± 0.5). This can be attributed to the high concentrations of neutralizing ions such as nss-Ca2+, nss-Mg2+, K+, and NH4+, indicating the greater extent of wet scavenging during rainfall. These ions counteract the acidity generated by SO42- and NO3-. A high correlation between Ca2+, Na+, K+, NO3-, and SO42- makes it difficult to estimate the contribution of SO42- from different sources. Anthropogenic sulfur emissions and soil dust significantly influence the ionic composition of clouds and rain. Positive matrix factorization (PMF) was used to identify the contribution of different sources to the samples. In the CW, the extracted factors were cooking and vehicles, aging sea salt, agriculture, and dust. In RW, the factors were industries, cooking and vehicles, agriculture and dust, and aging sea salt. The findings of this study have significant implications for the monsoon build-up, ecosystems, agriculture, and climate change.

Clair JM, Dibb J, Temime-Roussel B, D’Anna B, Moon A, Alexander B, Yang Y, Nenes A, Mao J, Weber RJ. Hydroxymethanesulfonate and Sulfur(IV) in Fairbanks Winter During the ALPACA Study

Hydroxymethanesulfonate (HMS) in fine aerosol particles has been reported at significant concentrations along with sulfate under extreme cold conditions (-35 °C) in Fairbanks, Alaska, a high latitude city. HMS, a component of S(IV) and an adduct of formaldehyde and sulfur dioxide, forms in liquid water. Previous studies may have overestimated HMS concentrations by grouping it with other S(IV) species. In this work, we further investigate HMS and the speciation of S(IV) through the Alaskan Layered Pollution and Chemical Analysis (ALPACA) intensive study in Fairbanks. We developed a method utilizing hydrogen peroxide to isolate HMS and found that approximately 50% of S(IV) is HMS for total suspended particulates and 70% for PM2.5. The remaining unidentified S(IV) species are closely linked to HMS during cold polluted periods, showing strong increases in concentration relative to sulfate with decreasing temperature, a weak dependence on particle water, and similar particle size distributions, suggesting a common aqueous formation process. A portion of the unidentified S(IV) may originate from additional aldehyde-S(IV) adducts that are unstable in the water-based chemical analysis process, but further chemical characterization is needed. These results show the importance of organic S(IV) species in extreme cold environments that promote unique aqueous chemistry in supercooled liquid particles.

An Electrochemical Perspective on Reaction Acceleration in Microdroplets

Analytical techniques operating at the nanoscale introduce confinement as a tool at our disposal. This review delves into the phenomenon of accelerated reactivity within micro- and nanodroplets. A decade of accelerated reactivity observations was succeeded by several years of fundamental studies aimed at mechanistic enlightenment. Herein, we provide a brief historical context for rate enhancement in and around micro- and nanodroplets and summarize the mechanisms that have been proposed to contribute to such extraordinary reactivity. We highlight recent electrochemical reports that make use of restricted mass transfer to enhance electrochemical reactions and/or quantitatively measure reaction rates within droplet-confined electrochemical cells. A comprehensive approach to nanodroplet reactivity is paramount to understanding how nature takes advantage of these systems to provide life on Earth and, in turn, how to harness the full potential of such systems.

Large contribution of in-cloud production of secondary organic aerosol from biomass burning emissions

Organic compounds released from wildfires and residential biomass burning play a crucial role in shaping the composition of the atmosphere. The solubility and subsequent reactions of these compounds in the aqueous phase of clouds and fog remain poorly understood. Nevertheless, these compounds have the potential to become an important source of secondary organic aerosol (SOA). In this study, we simulated the aqueous SOA (aqSOA) from residential wood burning emissions under atmospherically relevant conditions of gas-liquid phase partitioning, using a wetted-wall flow reactor (WFR). We analyzed and quantified the specific compounds present in these emissions at a molecular level and determined their solubility in clouds. Our findings reveal that while 1% of organic compounds are fully water-soluble, 19% exhibit moderate solubility and can partition into the aqueous phase in a thick cloud. Furthermore, it is found that the aqSOA generated in our laboratory experiments has a substantial fraction being attributed to the formation of oligomers in the aqueous phase. We also determined an aqSOA yield of 20% from residential wood combustion, which surpasses current estimates based on gas-phase oxidation. These results indicate that in-cloud chemistry of organic gases emitted from wood burning can serve as an efficient pathway to produce organic aerosols, thus potentially influencing climate and air quality.