Perspective: Aerosol microphysics: From molecules to the chemical physics of aerosols

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.

Modeling transport and filtration of nanoparticle suspensions in porous media

Recently membrane filters have gained in significance due to the need to provide protection against airborne pollution. A question of importance, and some controversy, is the efficiency of filters for small nanoparticles with diameters below 100 nm as these are considered particularly dangerous due to possible penetration into the lungs. The efficiency is measured by the number of particles blocked by the pore structure after passing though the filter. To study the penetration into pores by nanoparticles suspended in a fluid, a stochastic transport theory based on an atomistic model is used to calculate particle density and flow within the pores, resulting pressure gradient, and filter efficiency. The importance of pore size relative to particle diameter and of the parameters of the pore wall interactions are investigated. The theory is applied to aerosols in fibrous filters and found to reproduce common trends in measurements. As particles enter the initially empty pores on relaxation to the steady state the small penetration measured at the onset of filtration increases faster in time the smaller the nanoparticle diameter. Control of pollution by filtration is achieved by strong repulsion of pore walls for particle diameters greater than twice the effective pore width. For smaller nanoparticles the steady-state efficiency decreases as the pore wall interactions weaken. Effective efficiency is increased when the suspended nanoparticles inside the pores combine into clusters of sizes greater than the filter channel width.

Solvent-Mediated Charge Transfer Dynamics of a Model Brown Carbon Aerosol Chromophore: Photophysics of 1-Phenylpyrrole Induced by Water Solvation

Nitrogen heterocycles are known to be important light-absorbing chromophores in a newly discovered class of aerosols, commonly referred to as "brown carbon" (BrC) aerosols. Due to their significant absorption and spectral overlap with the solar actinic flux, these BrC chromophores steer the physical and optical properties of aerosols. To model the local aqueous solvation environment surrounding BrC chromophores, we generated cold molecular complexes with water and a prototypical BrC chromophore, 1-phenylpyrrole (1PhPy), using supersonic jet-cooling and explored their intermolecular interactions using single-conformation spectroscopy. Herein, we utilized resonant two-photon ionization (R2PI) and UV holeburning (UV HB) double-resonance spectroscopies to obtain a molecular-level understanding of the role of water microsolvation in charge transfer upon photoexcitation of 1PhPy. Quantum chemical calculations and one-dimensional discrete variable representation simulations revealed insights into the charge transfer efficacy of 1PhPy with and without addition of a single water molecule. Taken together, our results indicate that the intermolecular interactions with water guide the geometry of 1PhPy to adopt a more twisted intramolecular charge transfer (TICT) configuration, thus facilitating charge transfer from the pyrrole donor to the phenyl ring acceptor. Furthermore, the water network surrounding 1PhPy reports on the charge transfer such that the H₂O solvent primarily interacts with the pyrrole ring donor in the ground state, whereas it preferentially interacts with the phenyl ring acceptor in the excited state. Large Franck-Condon activity is evident in the 1PhPy + 1H₂O excitation spectrum for the water-migration vibronic bands, supporting H₂O solvent reorganization upon excitation of the 1PhPy chromophore. Fluorescence measurements with increasing H₂O % volume corroborated our gas-phase studies by indicating that a polar water solvation environment stabilizes the TICT configuration of 1PhPy in the excited electronic state, from which emission is observed at a lower energy compared to the locally excited configuration.

Time-dependent photoemission from droplets: influence of size and charge on the photophysics near the surface

Photoemission from submicrometer droplets containing a mixture of dioctyl phthalate and dioctyl sebacate was investigated by femtosecond and nanosecond photoionization. Photoelectron spectra recorded after ionization with single 4.7 eV femtosecond or nanosecond laser pulses showed marked differences between the two cases. These differences were attributed to ionization of long-lived states which only occurred within the duration of the nanosecond pulse. The tentative assignment of the long-lived states to dioctyl phthalate triplet states is discussed. A nanosecond–femtosecond pump–probe scheme using 4.7 eV (pump) and 3.1 eV (probe) pulses was used to investigate the decay dynamics of these long-lived states. The dynamics showed an accelerated decay rate at higher dioctyl phthalate concentrations. Furthermore, the dependence of the decay dynamics on droplet size and charge was investigated. The decay of the long-lived states was found to be faster in smaller droplets as well as in neutral droplets compared with both positively and negatively charged droplets. Possible mechanisms to explain these observations and the dominance of contributions from the droplets surface are discussed.

Time-resolved photoemission spectroscopy is used to study the photophysics of droplets containing dioctyl phthalate. Long-lived excited states survive longer at lower phthalate concentrations, and in larger or electrically charged droplets.

Measuring the Chemical Evolution of Levitated Particles: A Study on the Evaporation of Multicomponent Organic Aerosol

Single-particle levitation methods provide an effective platform for probing the physical properties of atmospheric aerosol via micrometer-sized particles. Until recently, chemical composition measurements on levitated particles were limited to spectroscopy, yielding only basic chemical information. Here, we describe, benchmark, and discuss the applications of an approach for probing the physical properties and chemical composition of single levitated particles using high-resolution mass spectrometry (MS). Using a linear quadrupole electrodynamic balance (LQ-EDB) coupled to paper spray mass spectrometry, we report accurate measurements of the evolving size within 5 nm (using broadband light scattering) and relative composition (using MS) of evaporating multicomponent levitated particles in real time. Measurements of the evaporation dynamics of semivolatile organic particles containing a range of n-ethylene glycols (n = 3, 4, and 6) in various binary and ternary mixtures were made under dry conditions and compared with predictions from a gas-phase diffusion evaporation model. Under assumptions of ideal mixing, excellent agreement for both size and composition evolution between measurements and models were obtained for these mixtures. At increased relative humidity, the presence of water in particles causes the assumption of ideality to break down, and the evaporative mass flux becomes a function of the mole fraction and activity coefficient. Through compositionally resolved evaporation measurements and thermodynamic models, we characterize the activity of organic components in multicomponent particles. Our results demonstrate that the LQ-EDB-MS platform can identify time-dependent size and compositional changes with high precision and reproducibility, yielding an effective methodology for future studies on chemical aging and gas-particle partitioning in suspended particles.

Shining New Light on the Kinetics of Water Uptake by Organic Aerosol Particles

The uptake of water vapor by various organic aerosols is important in a number of applications ranging from medical delivery of pharmaceutical aerosols to cloud formation in the atmosphere. The coefficient that describes the probability that the impinging gas-phase molecule sticks to the surface of interest is called the mass accommodation coefficient, α_M. Despite the importance of this coefficient for the description of water uptake kinetics, accurate values are still lacking for many systems. In this Feature Article, we present various experimental techniques that have been evoked in the literature to study the interfacial transport of water and discuss the corresponding strengths and limitations. This includes our recently developed technique called photothermal single-particle spectroscopy (PSPS). The PSPS technique allows for a retrieval of α_M values from three independent, yet simultaneous measurements operating close to equilibrium, providing a robust assessment of interfacial mass transport. We review the currently available data for α_M for water on various organics and discuss the few studies that address the temperature and relative humidity dependence of α_M for water on organics. The knowledge of the latter, for example, is crucial to assess the water uptake kinetics of organic aerosols in the Earth's atmosphere. Finally, we argue that PSPS might also be a viable method to better restrict the α_M value for water on liquid water.

Aqueous-phase fates of α-alkoxyalkyl-hydroperoxides derived from the reactions of Criegee intermediates with alcohols

In the atmosphere, carbonyl oxides known as Criegee intermediates are produced mainly by ozonolysis of volatile organic compounds containing C[double bond, length as m-dash]C double bonds, such as biogenic terpenoids. Criegee intermediates can react with OH-containing species to produce labile organic hydroperoxides (ROOHs) that are taken up into atmospheric condensed phases. Besides water, alcohols are an important reaction partner of Criegee intermediates and can convert them into α-alkoxyalkyl-hydroperoxides (α-AHs), R1R2C(-OOH)(-OR'). Here, we report a study on the aqueous-phase fates of α-AHs derived from ozonolysis of α-terpineol in the presence of methanol, ethanol, 1-propanol, and 2-propanol. The α-terpineol α-AHs and the decomposition products were detected as their chloride adducts by electrospray mass spectrometry as a function of reaction time. Our discovery that the rate of decomposition of α-AHs increased as the pH decreased from 5.9 to 3.8 implied that the decomposition mechanism was catalyzed by H+. The use of isotope solvent experiments revealed that a primary decomposition product of α-AHs in an acidic aqueous solution was a hemiacetal R1R2C(-OH)(-OR') species that was further transformed into other products such as lactols. The proposed H+-catalyzed decomposition of α-AHs, which provides H2O2 and multifunctional species in ambient aerosol particles, may be faster than other degradation processes (e.g., photolysis by solar radiation).

Hygroscopic growth of simulated lung fluid aerosol particles under ambient environmental conditions

The hygroscopicity of respiratory aerosol determines their particle size distribution and regulates solute concentrations to which entrained microorganisms are exposed. Here, we report the hygroscopicity of simulated lung fluid (SLF) particles. While the response of aqueous particles follow simple mixing rules based on composition, we observe phase hysteresis with increasing and decreasing relative humidity (RH) and clear uptake of water prior to deliquescence. These results indicate that RH history may control the state of respiratory aerosol in the environment and influence the viability of microorganisms.

Relative humidity in droplet and airborne transmission of disease

A large number of infectious diseases are transmitted by respiratory droplets. How long these droplets persist in the air, how far they can travel, and how long the pathogens they might carry survive are all decisive factors for the spread of droplet-borne diseases. The subject is extremely multifaceted and its aspects range across different disciplines, yet most of them have only seldom been considered in the physics community. In this review, we discuss the physical principles that govern the fate of respiratory droplets and any viruses trapped inside them, with a focus on the role of relative humidity. Importantly, low relative humidity-as encountered, for instance, indoors during winter and inside aircraft-facilitates evaporation and keeps even initially large droplets suspended in air as aerosol for extended periods of time. What is more, relative humidity affects the stability of viruses in aerosol through several physical mechanisms such as efflorescence and inactivation at the air-water interface, whose role in virus inactivation nonetheless remains poorly understood. Elucidating the role of relative humidity in the droplet spread of disease would permit us to design preventive measures that could aid in reducing the chance of transmission, particularly in indoor environment.

Aerosol Droplet Surface Measurement Methods

Aerosol droplets play a critical role in the development of weather patterns, yet are notoriously difficult to analyze because of their small size, transient nature and potentially complex composition. As a result, there has been a surge in recent years in the development of analysis techniques aimed at the study of aerosol droplets, namely of their surface tension properties, which are thought to play a great role in aerosol/cloud growth and subsequently having an impact on the resulting weather patterns. To capture the state of the field at this key time, we have collected and described some of the most relevant and influential studies, with a focus on those that have had the most impact. This review will present and describe the most used analytical techniques for studying the surface tension of micrometer-sized aqueous droplets, with a focus on historical trends and how the current techniques are posed to revolutionize the field.

Combining Mass Spectrometry of Picoliter Samples with a Multicompartment Electrodynamic Trap for Probing the Chemistry of Droplet Arrays

Single droplet levitation provides contactless access to the microphysical and chemical properties of micrometer-sized samples. Most applications of droplet levitation to chemical and biological systems use nondestructive optical techniques to probe droplet properties. To provide improved chemical specificity, we coupled a multicompartment quadrupole electrodynamic trap (QET) with single droplet mass spectrometry. Our QET continuously traps a monodisperse droplet population (tens to hundreds of droplets) and allows for the simultaneous sizing of a single droplet using its Mie scattering pattern. Single droplets are subsequently ejected into the ionization region of an ambient pressure inlet mass spectrometer. We optimized two complementary soft ionization techniques for picoliter aqueous droplets: (1) paper spray (PS) ionization and (2) thermal desorption glow discharge (TDGD) ionization. Both techniques detect oxygenated organic acids in single droplets, with signal-to-noise ratios >100 and detection limits on the order of 10 pg. Sensitivity and reproducibility across single droplets are driven by the droplet deposition location and spray stability in PS-MS and the ionization region humidity and analyte evaporation rate in TDGD-MS. Importantly, the analyte evaporation rate can control the TDGD-MS quantitative capability because high evaporation rates result in significant ion suppression. This effect is mitigated by optimizing the vaporization temperature, droplet size range, and analyte volatility. We demonstrate quantitative and reproducible measurements with a droplet internal standard (<10% RSD) and compare the sensitivity of PS-MS and TDGD-MS. Finally, we demonstrate the application of QET-MS to the study of heterogeneous chemical kinetics with the reaction of gas phase O₃ and aqueous maleic acid droplets.

Decomposition of the simplest ketohydroperoxide in the ozonolysis of ethylene

Hydroperoxides from the ozonolysis of alkenes, in addition to Criegee intermediates, have been proposed as an atmospheric source of OH radicals in the absence of sunlight, but have remained largely elusive due to their reactivity. A weak peroxide bond enables facile OH elimination, and subsequent β-scission can lead to a variety of decomposition products depending on the nature of the peroxide. In this paper we explore this process theoretically for the simplest ketohydroperoxide, hydroperoxyacetaldehyde (HPA), which is believed to be formed in the ozonolysis of ethylene. Despite it being the most stable C₂H₄O₃ species in this reaction scheme, lower in energy than the starting materials by around 100 kcal mol^-1, HPA has only been directly observed once in the ozonolysis of ethylene by photoionization mass spectrometry appearance energy. Here we report predictions of the rotational spectrum of HPA conducted in support of microwave spectroscopy experiments. We suggest a new dissociation path from HPA to glyoxal [HOOCH₂CHO → HCOCH₂O + OH → CHOCHO + H], supported by thermochemical calculations. We encourage the search for glyoxal using complementary experimental methods, and suggest possible future experimental directions. Evidence of glyoxal formation from ethylene ozonolysis might provide evidence of this underappreciated path in an important and long studied reaction system.

Paper spray mass spectrometry for the analysis of picoliter droplets

Recent experimental efforts have shown that single particle levitation methods may be effectively coupled with mass spectrometry (MS) using paper spray (PS) ionization for compositional analysis of picoliter droplets. In this work, we characterize the response of PS-MS to analytes delivered in the form of picoliter droplets and explore its potential for identification and quantification of these samples. Using a microdroplet dispenser to generate droplets, we demonstrate sensitivity to a range of oxygenated organic molecules typical of compounds found in atmospheric secondary organic aerosol. We assess experimental factors that influence the reproducibility and sensitivity of the method and explore the linearity of the system response to increasing analyte mass in droplets containing single or multicomponent analytes. We show that the ratio of analyte signal from multicomponent samples may be used to characterize the relative composition of the system. These measurements demonstrate that the droplet PS-MS method is an effective tool for qualitative and quantitative analysis of single picoliter droplets containing picogram levels of analyte. The potential applications of this technique for characterizing the composition of levitated particles will be discussed.

The surface tension of surfactant-containing, finite volume droplets

Atmospheric aerosol particles cool Earth’s climate by serving as cloud droplet seeds. This cooling effect represents both the single most uncertain and the largest negative radiative forcing. Cloud droplet activation is strongly influenced by aerosol particle surface tension, which in climate models is assumed equivalent to that of pure water. We directly measure the surface tensions of surfactant-coated, high surface-to-volume ratio droplets, demonstrating that their surface tensions are significantly lower than pure water but do not match the surface tension of the solution from which they were produced and depend on finite droplet size. These results suggest surfactants could potentially significantly modify radiative forcing and highlight the need for a better understanding of atmospheric surfactant concentrations and properties.

Surface tension influences the fraction of atmospheric particles that become cloud droplets. Although surfactants are an important component of aerosol mass, the surface tension of activating aerosol particles is still unresolved, with most climate models assuming activating particles have a surface tension equal to that of water. By studying picoliter droplet coalescence, we demonstrate that surfactants can significantly reduce the surface tension of finite-sized droplets below the value for water, consistent with recent field measurements. Significantly, this surface tension reduction is droplet size-dependent and does not correspond exactly to the macroscopic solution value. A fully independent monolayer partitioning model confirms the observed finite-size-dependent surface tension arises from the high surface-to-volume ratio in finite-sized droplets and enables predictions of aerosol hygroscopic growth. This model, constrained by the laboratory measurements, is consistent with a reduction in critical supersaturation for activation, potentially substantially increasing cloud droplet number concentration and modifying radiative cooling relative to current estimates assuming a water surface tension. The results highlight the need for improved constraints on the identities, properties, and concentrations of atmospheric aerosol surfactants in multiple environments and are broadly applicable to any discipline where finite volume effects are operative, such as studies of the competition between reaction rates within the bulk and at the surface of confined volumes and explorations of the influence of surfactants on dried particle morphology from spray driers.

Microcanonical Nucleation Theory for Anisotropic Materials Validated on Alumina Clusters

Nucleation kinetics in gas phase remains an open issue with no general model. The derivation of the reaction constants assuming a canonical ensemble fails to describe anisotropic materials such as oxides. We have developed a general and versatile model using activated complex kinetics with a microcanonical approach. This approach handles the kinetics issue in cluster growth when the transient nature of the processes hinders the use of the canonical ensemble. The model efficiently reproduces experimental size distributions of alumina clusters formed by laser ablation with different buffer gas densities, including magic numbers. We show that the thermodynamic equilibrium is not reached during the growth. The bounding energy measured is 10 times lower than the one deduced from DFT calculation, but also the one expected from the bulk cohesive energy.

Simultaneous Retrieval of the Size and Refractive Index of Suspended Droplets in a Linear Quadrupole Electrodynamic Balance

Single-particle trapping is an effective strategy to explore the physical and optical properties of aerosol with high precision. Laser-based methods are commonly used to probe the size, optical properties, and composition of nonlight-absorbing droplets in optical and electrodynamic traps. However, these methods cannot be applied to droplets containing photoactive chromophores, and thus, single-particle methods have been restricted to only a subset of atmospherically relevant particle compositions. In this work, we explore the application of a broadband light scattering approach, Mie resonance spectroscopy, to simultaneously probe the size and the refractive index (RI) of droplets in a linear quadrupole electrodynamic balance. We examine the evaporation of poly(ethylene glycol)s and compare the calculated vapor pressures with literature values to benchmark the size accuracy without prior constraint on the RI. We then explore the hygroscopic growth and deliquescence of sodium chloride droplets, measuring RI at the deliquescence relative humidity and demonstrating agreement to literature values. These data allow the wavelength dependence of the RI of aqueous NaCl to be determined using a first-order Cauchy equation, and we effectively reproduce literature data from multiple techniques. We finally discuss measurements from a light-absorbing aqueous droplet containing humic acid and interpret the spectra via the imaginary component of the RI. The approach described here allows the radius of nonabsorbing droplets to be determined within 0.1%, the refractive index within 0.2%, and the first-order term in the Cauchy dispersion equation within ∼5%.

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

Synchrotron radiation provides insight into spectroscopy and dynamics in clusters and nanoparticles.

Assessment of air purifier on efficient removal of airborne bacteria, Staphylococcus epidermidis, using single-chamber method

We evaluated the efficiency of an air purifier using the single-chamber method for the effective removal of airborne Staphylococcus epidermidis, a nosocomial infection-causing bacterium. In this experiment, the bacterial strain S. epidermidis was injected using a nebulizer into the test chamber, which was similar to a consumer living space (60 m3). The microbial sampling was conducted via the air sampler method, and the reduction in S. epidermidis growth was monitored by performing three consecutive tests. Initially, a blank test was conducted to determine the natural decay rate and calibrate the experimental setup. After injecting the bacterial strain from 1240 to 11180 CFU per unit volume (m3), the natural decay rate showed a maximum deviation of 3.1% with a sampling error of 1.1% p at a confidence level of 95%. In addition, the particle size distribution in the test chamber was found to range from 0.3 to 5.0 μm, and a subsequent decrease in large-sized particles was observed with the operation of the air purifier, which is the size similar to that of suspended airborne bacteria. This can be used to assess the performance of the air purifier by calibrating the natural reduction value to the reduced operation value. Thus, the single-chamber technique is a promising approach for analyzing the removal efficacy of airborne bacteria from indoor air.

Visualizing reaction and diffusion in xanthan gum aerosol particles exposed to ozone

Atmospheric aerosol particles with a high viscosity may become inhomogeneously mixed during chemical processing. Models have predicted gradients in condensed phase reactant concentration throughout particles as the result of diffusion and chemical reaction limitations, termed chemical gradients. However, these have never been directly observed for atmospherically relevant particle diameters. We investigated the reaction between ozone and aerosol particles composed of xanthan gum and FeCl₂ and observed the in situ chemical reaction that oxidized Fe²⁺ to Fe³⁺ using X-ray spectromicroscopy. Iron oxidation state of particles as small as 0.2 μm in diameter were imaged over time with a spatial resolution of tens of nanometers. We found that the loss off Fe²⁺ accelerated with increasing ozone concentration and relative humidity, RH. Concentric 2-D column integrated profiles of the Fe²⁺ fraction, α, out of the total iron were derived and demonstrated that particle surfaces became oxidized while particle cores remained unreacted at RH = 0-20%. At higher RH, chemical gradients evolved over time, extended deeper from the particle surface, and Fe²⁺ became more homogeneously distributed. We used the kinetic multi-layer model for aerosol surface and bulk chemistry (KM-SUB) to simulate ozone reaction constrained with our observations and inferred key parameters as a function of RH including Henry's Law constant for ozone, H_O3, and diffusion coefficients for ozone and iron, D_O3 and D_Fe, respectively. We found that H_O3 is higher in our xanthan gum/FeCl₂ particles than for water and increases when RH decreased from about 80% to dry conditions. This coincided with a decrease in both D_O3 and D_Fe. In order to reproduce observed chemical gradients, our model predicted that ozone could not be present further than a few nanometers from a particle surface indicating near surface reactions were driving changes in iron oxidation state. However, the observed chemical gradients in α observed over hundreds of nanometers must have been the result of iron transport from the particle interior to the surface where ozone oxidation occurred. In the context of our results, we examine the applicability of the reacto-diffusive framework and discuss diffusion limitations for other reactive gas-aerosol systems of atmospheric importance.

Surface Tensions of Picoliter Droplets with Sub-Millisecond Surface Age

Aerosols are key components of the atmosphere and play important roles in many industrial processes. Because aerosol particles have high surface-to-volume ratios, their surface properties are especially important. However, direct measurement of the surface properties of aerosol particles is challenging. In this work, we describe an approach to measure the surface tension of picoliter volume droplets with surface age <1 ms by resolving their dynamic oscillations in shape immediately after ejection from a microdroplet dispenser. Droplet shape oscillations are monitored by highly time-resolved (500 ns) stroboscopic imaging, and droplet surface tension is accurately retrieved across a wide range of droplet sizes (10-25 μm radius) and surface ages (down to ∼100 μs). The approach is validated for droplets containing sodium chloride, glutaric acid, and water, which all show no variation in surface tension with surface age. Experimental results from the microdroplet dispenser approach are compared to complementary surface tension measurements of 5-10 μm radius droplets with aged surfaces using a holographic optical tweezers approach and predictions of surface tension using a statistical thermodynamic model. These approaches combined will allow investigation of droplet surface tension across a wide range of droplet sizes, compositions, and surface ages.

Chemical signatures of surface microheterogeneity on liquid mixtures

Many chemical reactions in Nature, the laboratory, and chemical industry occur in solvent mixtures that bring together species of dissimilar solubilities. Solvent mixtures are visually homogeneous, but are not randomly mixed at the molecular scale. In the all-important binary water-hydrotrope mixtures, small-angle neutron and dynamic light scattering experiments reveal the existence of short-lived (<50 ps), short-ranged (∼1 nm) concentration fluctuations. The presence of hydrophobic solutes stabilizes and extends such fluctuations into persistent, mesoscopic (10-100 nm) inhomogeneities. While the existence of inhomogeneities is well established, their impacts on reactivity are not fully understood. Here, we search for chemical signatures of inhomogeneities on the surfaces of W:X mixtures (W = water; X = acetonitrile, tetrahydrofuran, or 1,4-dioxane) by studying the reactions of Criegee intermediates (CIs) generated in situ from O₃(g) addition to a hydrophobic olefin (OL) solute. Once formed, CIs isomerize to functionalized carboxylic acids (FC) or add water to produce α-hydroxy-hydroperoxides (HH), as detected by surface-specific, online pneumatic ionization mass spectrometry. Since only the formation of HH requires the presence of water, the dependence of the R = HH/FC ratio on water molar fraction x w expresses the accessibility of water to CIs on the surfaces of mixtures. The finding that R increases quasi-exponentially with x w in all solvent mixtures is consistent with CIs being preferentially produced (from their OL hydrophobic precursor) in X-rich, long-lived OL:X _m W _n interfacial clusters, rather than randomly dispersed on W:X surfaces. R vs x w dependences therefore reflect the average ⟨m, n⟩ composition of OL:X _m W _n interfacial clusters, as weighted by cluster reorganization dynamics. Water in large, rigid clusters could be less accessible to CIs than in smaller but more flexible clusters of lower water content. Since mesoscale inhomogeneities are intrinsic to most solvent mixtures, these phenomena should be quite general.

Characterization of the simplest hydroperoxide ester, hydroperoxymethyl formate, a precursor of atmospheric aerosols

Atmospheric aerosols are large clusters of molecules and particulate matter that profoundly affect the Earth's radiation budget and climate.