Aerosol Acidity: Novel Measurements and Implications for Atmospheric Chemistry

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.

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.

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.

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.

A review on vulnerable atmospheric aerosol nanoparticles: Sources, impact on the health, ecosystem and management strategies

The Earth's atmosphere contains ultrafine particles known as aerosols, which can be either liquid or solid particles suspended in gas. These aerosols originate from both natural sources and human activities, termed primary and secondary sources respectively. They have significant impacts on the environment, particularly when they transform into ultrafine particles or aerosol nanoparticles, due to their extremely fine atomic structure. With this context in mind, this review aims to elucidate the fundamentals of atmospheric-derived aerosol nanoparticles, covering their various sources, impacts, and methods for control and management. Natural sources such as marine, volcanic, dust, and bioaerosols are discussed, along with anthropogenic sources like the combustion of fossil fuels, biomass, and industrial waste. Aerosol nanoparticles can have several detrimental effects on ecosystems, prompting the exploration and analysis of eco-friendly, sustainable technologies for their removal or mitigation.Despite the adverse effects highlighted in the review, attention is also given to the generation of aerosol-derived atmospheric nanoparticles from biomass sources. This finding provides valuable scientific evidence and background for researchers in fields such as epidemiology, aerobiology, and toxicology, particularly concerning atmospheric nanoparticles.

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.

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.

Relative Humidity-Dependent Phase Transitions in Submicron Respiratory Aerosols

Respiratory viruses, such as influenza and severe acute respiratory syndrome coronavirus 2, represent a substantial public health burden and are largely transmitted through respiratory droplets and aerosols. Environmental factors such as relative humidity (RH) and temperature impact virus transmission rates, and a precise mechanistic understanding of the connection between these environmental factors and virus transmission would improve efforts to mitigate respiratory disease transmission. Previous studies on supermicrometer particles observed RH-dependent phase transitions and linked particle phase state to virus viability. Phase transitions in atmospheric aerosols are dependent on size in the submicrometer range, and actual respiratory particles are expelled over a large size range, including submicrometer aerosols that can transmit diseases over long distances. Here, we directly investigated the phase transitions of submicrometer model respiratory aerosols. A probe molecule, Nile red, was added to particle systems including multiple mucin/salt mixtures, a growth medium, and simulated lung fluid. For each system, the polarity-dependent fluorescence emission was measured following RH conditioning. Notably, the fluorescence measurements of mucin/NaCl and Dulbecco's modified Eagle's medium particles indicated that liquid-liquid phase separation (LLPS) also occurs in submicron particles, suggesting that LLPS can also impact the viability of viruses in submicron particles and thus affect aerosol virus transmission. Furthermore, the utility of fluorescence-based measurements to study submicrometer respiratory particle physicochemical properties in situ is demonstrated.

Comparison of acidity and chemical composition of summertime cloud water and aerosol at an alpine site in Northwest China: Implications for the neutral property of clouds in the free troposphere

Aerosol and cloud acidity are essential to human health, ecosystem health and productivity, as well as climate effects. The main chemical composition of cloud water greatly varies in different regions, resulting in substantial differences in the pH of cloud water. However, the influences of the anthropogenic emissions of acidic gases and substances, alkaline dust components, and dicarboxylic acids (diacids) on the ground concerning the acidity of cloud water in the free troposphere of the Guanzhong Plain, China, remain clear. In this study, cloud water and PM2.5 samples were simultaneously collected in the troposphere (Mt. Hua, 2060 m a.s.l). The results indicated that the cloud water was alkaline (pH = 7.6) and PM2.5 was acidic (pH = 3.2). These results showed the neutral property of clouds collected in the heavily polluted Guanzhong Plain, although most previous studies always considered acidity as a marker of pollution. The sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+) (SNA) of particulate matter and cloud water in the same period were compared. SO42- was dominant in particulate matters (accounting for 63.4 % of the total SNA) but substantially decreased in cloud water (only 30.1 % of the total SNA), whereas NO3- and NH4+ increased from 28.5 % and 8.2 % to 39.8 % and 30.2 %, respectively. This could be attributed to the complex formation mechanism and sources of SO42- and NO3- in the cloud. The results of ion balance indicated that a significant deficit of inorganic anion equivalents was observed in the cloud water samples. The high concentration of diacids in the cloud phase (1237.4 μg L-1) may facilitate the formation of salt complexes with NH4+, thus influencing the acidity of the cloud water. The pH of cloud water has increased in recent decades due to the sustained reduction of sulfur dioxide, which may also affect the acidity of future precipitation.

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.

Acidity in rainwater and airborne suspended particles in the southwestern coast of the East Sea (Sea of Japan): Their potential impact on seawater total alkalinity

Our understanding of the impact of atmospheric acid deposition on marine carbonate system remains limited, largely due to a lack of data regarding acidity present in atmospheric particles and precipitation. Previous research has relied on the electroneutrality-based ion balance method for indirect estimation of atmospheric acidity. In this study, atmospheric samples collected at a coastal site of South Korea were mixed with seawater to measure the change in seawater total alkalinity (ΔTAAPL) associated with atmospheric proton loading. For the precipitation samples, the measured ΔTAAPL and electroneutrality-based estimates showed a significant correlation. However, we did not observe similar results for the atmospheric particle samples. Furthermore, the decrease in oceanic TA due to ΔTAAPL was substantially smaller than that in dissolved inorganic carbon from concurrent nitrogen fertilization. Consequently, the adverse impact of acid deposition on ocean acidification or air-sea exchange of CO2 appears to be insignificant on a short-term scale.

Impact of pH on Gas-Particle Partitioning of Semi-Volatile Organics in Multicomponent Aerosol

The partitioning of semivolatile organic compounds (SVOCs) between the condensed and gas phases can have significant implications for the properties of aerosol particles. In addition to affecting size and composition, this partitioning can alter radiative properties and impact cloud activation processes. We present measurements and model predictions on how activity and pH influence the evaporation of SVOCs from particles to the gas phase, specifically investigating aqueous inorganic particles containing dicarboxylic acids (DCAs). The aerosols are studied at the single-particle level by using optical trapping and cavity-enhanced Raman spectroscopy. Optical resonances in the spectra enable precise size tracking, while vibrational bands allow real-time monitoring of pH. Results are compared to a Maxwell-type model that accounts for volatile and nonvolatile solutes in aqueous droplets that are held at a constant relative humidity. The aerosol inorganic-organic mixture functional group activity coefficients thermodynamic model and Debye-Hückel theory are both used to calculate the activities of the species present in the droplet. For DCAs, we find that the evaporation rate is highly sensitive to the particle pH. For acidity changes of approximately 1.5 pH units, we observe a shift from a volatile system to one that is completely nonvolatile. We also observe that the pH itself is not constant during evaporation; it increases as DCAs evaporate, slowing the rate of evaporation until it eventually ceases. Whether a DCA evaporates or remains a stable component of the droplet is determined by the difference between the lowest pKa of the DCA and the pH of the droplet.

Tracking the Photomineralization Mechanism in Irradiated Lab-Generated and Field-Collected Brown Carbon Samples and Its Effect on Cloud Condensation Nuclei Abilities

Organic aerosols affect the planet's radiative balance by absorbing and scattering light as well as by activating cloud droplets. These organic aerosols contain chromophores, termed brown carbon (BrC), and can undergo indirect photochemistry, affecting their ability to act as cloud condensation nuclei (CCN). Here, we investigated the effect of photochemical aging by tracking the conversion of organic carbon into inorganic carbon, termed the photomineralization mechanism, and its effect on the CCN abilities in four different types of BrC samples: (1) laboratory-generated (NH₄)₂SO₄-methylglyoxal solutions, (2) dissolved organic matter isolate from Suwannee River fulvic acid (SRFA), (3) ambient firewood smoke aerosols, and (4) ambient urban wintertime particulate matter in Padua, Italy. Photomineralization occurred in all BrC samples albeit at different rates, evidenced by photobleaching and by loss of organic carbon up to 23% over a simulated 17.6 h of sunlight exposure. These losses were correlated with the production of CO up to 4% and of CO₂ up to 54% of the initial organic carbon mass, monitored by gas chromatography. Photoproducts of formic, acetic, oxalic and pyruvic acids were also produced during irradiation of the BrC solutions, but at different yields depending on the sample. Despite these chemical changes, CCN abilities did not change substantially for the BrC samples. In fact, the CCN abilities were dictated by the salt content of the BrC solution, trumping a photomineralization effect on the CCN abilities for the hygroscopic BrC samples. Solutions of (NH₄)₂SO₄-methylglyoxal, SRFA, firewood smoke, and ambient Padua samples had hygroscopicity parameters κ of 0.6, 0.1, 0.3, and 0.6, respectively. As expected, the SRFA solution with a κ of 0.1 was most impacted by the photomineralization mechanism. Overall, our results suggest that the photomineralization mechanism is expected in all BrC samples and can drive changes in the optical properties and chemical composition of aging organic aerosols.

Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

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

Rapid acquisition of absorption spectra to monitor proton migration in nanoliter space using the RGB-spectrum-conversion method with a 10 ms interval

Acquisition protocol of absorption spectra at nanoliter spaces from the RGB values preserved in video data at 10 ms intervals was established using the principal-component-analysis-based RGB-conversion method. Proton behavior was monitored using a camera to acquire the video footage to monitor colorimetric change in the nanoliter space. The RGB values observed in the video were converted into a score vector using a conversion matrix. A linear combination of the predetermined loading vectors with the score values was calculated to reproduce the absorption spectra. The reproduced absorption spectra correlated well with those acquired using a conventional spectrophotometer during a short period. This method was applied to monitor the proton diffusion from a single cationic ion-exchange resin into hydrogels at low concentrations. The rapid acquisition and quick response of this method may enable the monitoring of the initial diffusion of protons, which is challenging with conventional spectrophotometry and electrochemical approaches.

Optical trapping and light scattering in atmospheric aerosol science

Aerosol particles are ubiquitous in the atmosphere, and currently contribute a large uncertainty to climate models. Part of the endeavour to reduce this uncertainty takes the form of improving our understanding of aerosol at the microphysical level, thus enabling chemical and physical processes to be more accurately represented in larger scale models. In addition to modeling efforts, there is a need to develop new instruments and methodologies to interrogate the physicochemical properties of aerosol. This perspective presents the development, theory, and application of optical trapping, a powerful tool for single particle investigations of aerosol. After providing an overview of the role of aerosol in Earth's atmosphere and the microphysics of these particles, we present a brief history of optical trapping and a more detailed look at its application to aerosol particles. We also compare optical trapping to other single particle techniques. Understanding the interaction of light with single particles is essential for interpreting experimental measurements. In the final part of this perspective, we provide the relevant formalism for understanding both elastic and inelastic light scattering for single particles. The developments discussed here go beyond Mie theory and include both how particle and beam shape affect spectra. Throughout the entirety of this work, we highlight numerous references and examples, mostly from the last decade, of the application of optical trapping to systems that are relevant to the atmospheric aerosol.

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.

Effects of Acidity on Reactive Oxygen Species Formation from Secondary Organic Aerosols

Reactive oxygen species (ROS) play a critical role in the chemical transformation of atmospheric secondary organic aerosols (SOA) and aerosol health effects by causing oxidative stress in vivo. Acidity is an important physicochemical property of atmospheric aerosols, but its effects on the ROS formation from SOA have been poorly characterized. By applying the electron paramagnetic resonance spin-trapping technique and the Diogenes chemiluminescence assay, we find highly distinct radical yields and composition at different pH values in the range of 1-7.4 from SOA generated by oxidation of isoprene, α-terpineol, α-pinene, β-pinene, toluene, and naphthalene. We observe that isoprene SOA has substantial hydroxyl radical (^•OH) and organic radical yields at neutral pH, which are 1.5-2 times higher compared to acidic conditions in total radical yields. Superoxide (O₂ ^•-) is found to be the dominant species generated by all types of SOAs at lower pH. At neutral pH, α-terpineol SOA exhibits a substantial yield of carbon-centered organic radicals, while no radical formation is observed by aromatic SOA. Further experiments with model compounds show that the decomposition of organic peroxide leading to radical formation may be suppressed at lower pH due to acid-catalyzed rearrangement of peroxides. We also observe 1.5-3 times higher molar yields of hydrogen peroxide (H₂O₂) in acidic conditions compared to neutral pH by biogenic and aromatic SOA, likely due to enhanced decomposition of α-hydroxyhydroperoxides and quinone redox cycling, respectively. These findings are critical to bridge the gap in understanding ROS formation mechanisms and kinetics in atmospheric and physiological environments.

pH-Dependent Chemical Transformations of Humic-Like Substances and Further Cognitions Revealed by Optical Methods

Humic-like substances (HULIS) are macromolecular complex groups in water-soluble organic compounds (WSOC). pH is a crucial factor that influences the chemical transformations of HULIS in atmospheric particles, but this has been rarely investigated, especially under varying pH conditions. This study attempted to unveil the chemical transformation mechanisms of HULIS under a range of pH conditions using optical methods. The pH-dependent light absorption and fluorescence properties of HULIS were comprehensively analyzed; the acidity coefficient (pK_a) of HULIS in relation to chemical structures was determined, and the hypothetical chemical transformation mechanisms of HULIS with increasing pH were analyzed by optical characterizations. The results suggested that pH greatly impacted the light absorption and fluorescence efficiencies of HULIS in both winter and summer seasons, and pK_a was an important inflection point. The pK_a of HULIS ranged from 3.5 to 8.0 in winter and 6.4 to 10.0 in summer. The acidic/basic groups were identified as -OH or -NH₂ substituted quinolines, carboxylic aromatics, and pyridines. The pH-sensitive species accounted for about 6% and 21% of HULIS-C (carbon concentrations of HULIS) in winter and summer, respectively. The varying optical spectra with increasing pH might result from charge transfer or complex reactions with HULIS deprotonation.

Secondary organic aerosol formation from monocyclic aromatic hydrocarbons: insights from laboratory studies

Monocyclic aromatic hydrocarbons (MAHs) are key anthropogenic pollutants and often dominate the volatile organic compound emissions and secondary organic aerosol (SOA) formation especially in the urban atmosphere. To evaluate the environmental impacts of SOA formed from the oxidation of MAHs (aromatic SOA), it is of great importance to elucidate their chemical composition, formation mechanism, and physicochemical properties under various atmospheric conditions. Here we seek to compile a common framework for the current studies on aromatic SOA formation and summarize the knowledge on what has been primarily learned from laboratory studies. This review begins with a brief summary of MAHs' emission characteristics, followed by an overview of atmospheric degradation mechanisms for MAHs as well as gas- and particle-phase reactions involving aromatic SOA formation. SOA formation processes highlighted in this review are complex and depend highly on environmental conditions, posing a substantial challenge for theoretical description of aromatic SOA formation. Therefore, the following issues are further discussed in detail: the response of gas-phase chemistry and aromatic SOA mass yield as well as composition to NO_x levels, particle-phase reactions and molecular characterization of aromatic SOA in the presence of acidic sulfate, and physicochemical processes of SOA formation involving gas- or particle-phase water. Building on this current understanding, available experimental studies on the effects of environmental conditions were explored. A brief description of the atmospheric importance of aromatic SOA including their optical properties and health influences is also presented. Finally, we highlight the current challenges in laboratory studies and outline directions for future aromatic SOA research.

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.

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.

A critical analysis of electrospray techniques for the determination of accelerated rates and mechanisms of chemical reactions in droplets

Electrospray and Electrosonic Spray Ionization Mass Spectrometry (ESI-MS and ESSI-MS) have been widely used to report evidence that many chemical reactions in micro- and nano-droplets are dramatically accelerated by factors of ∼102 to 106 relative to macroscale bulk solutions. Despite electrospray's relative simplicity to both generate and detect reaction products in charged droplets using mass spectrometry, substantial complexity exists in how the electrospray process itself impacts the interpretation of the mechanism of these observed accelerated rates. ESI and ESSI are both coupled multi-phase processes, in which analytes in small charged droplets are transferred and detected as gas-phase ions with a mass spectrometer. As such, quantitative examination is needed to evaluate the impact of multiple experimental factors on the magnitude and mechanisms of reaction acceleration. These include: (1) evaporative concentration of reactants as a function of droplet size and initial concentration, (2) competition from gas-phase chemistry and reactions on experimental surfaces, (3) differences in ionization efficiency and ion transmission and (4) droplet charge. We examine (1-4) using numerical models, new ESI/ESSI-MS experimental data, and prior literature to assess the limitations of these approaches and the experimental best practices required to robustly interpret acceleration factors in micro- and nano-droplets produced by ESI and ESSI.