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

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

Increasing Isoprene Epoxydiol-to-Inorganic Sulfate Aerosol Ratio Results in Extensive Conversion of Inorganic Sulfate to Organosulfur Forms: Implications for Aerosol Physicochemical Properties.

Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX), key isoprene oxidation products, with inorganic sulfate aerosol yields substantial amounts of secondary organic aerosol (SOA) through the formation of organosulfur compounds. The extent and implications of inorganic-to-organic sulfate conversion, however, are unknown. In this article, we demonstrate that extensive consumption of inorganic sulfate occurs, which increases with the IEPOX-to-inorganic sulfate concentration ratio (IEPOX/Sulfinorg), as determined by laboratory measurements. Characterization of the total sulfur aerosol observed at Look Rock, Tennessee, from 2007 to 2016 shows that organosulfur mass fractions will likely continue to increase with ongoing declines in anthropogenic Sulfinorg, consistent with our laboratory findings. We further demonstrate that organosulfur compounds greatly modify critical aerosol properties, such as acidity, morphology, viscosity, and phase state. These new mechanistic insights demonstrate that changes in SO2 emissions, especially in isoprene-dominated environments, will significantly alter biogenic SOA physicochemical properties. Consequently, IEPOX/Sulfinorg will play an important role in understanding the historical climate and determining future impacts of biogenic SOA on the global climate and air quality.

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.

Atmospheric Aqueous Aerosol Surface Tensions: Isotherm-Based Modeling and Biphasic Microfluidic Measurements.

Surface properties of atmospheric aerosol particles are crucial for accurate assessments of the fates of liquid particles in the atmosphere. Surface tension directly influences predictions of particle activation to clouds, as well as indirectly acting as a proxy for chemical surface partitioning. Challenges to accounting for surface effects arise from surface tension dependence on solution concentration and the presence of complex aqueous mixtures in aerosols, including both surface-active organic solutes and inorganic electrolytes. Also, the interface itself is varied, in that it may be a liquid-vapor interface, as in the surface of an aerosol particle with ambient air, or a liquid-liquid interface between two immiscible liquids, as in the interior surfaces that exist in multiphase particles. In this Feature Article, we highlight our previous work entailing thermodynamic modeling of liquid-vapor surfaces to predict surface tension and microscopic examinations of liquid-liquid interfacial phenomena to measure interfacial tension using biphasic microscale flows. New results are presented for binary aqueous organic acids and their ternary solutions with ammonium sulfate. Ultimately, improved understanding of aerosol particle surfaces would enhance treatment of aerosol particle-to-cloud activation states and aerosol effects on climate.

Statistical Thermodynamic Model for Surface Tension of Organic and Inorganic Aqueous Mixtures.

The surface composition and tensions of aqueous aerosols govern a set of processes that largely determine the fate of particles in the atmosphere. Predictive modeling of surface tension can provide significant contributions to studies of atmospheric aerosol effects on climate and human health. A previously derived surface tension model for single solute aqueous solutions used adsorption isotherms and statistical mechanics to enable surface tension predictions across the entire concentration range as a function of solute activity. Here, we extend the model derivation to address multicomponent solutions and demonstrate its accuracy with systems containing mixtures of electrolytes and organic solutes. Binary model parameters are applied to the multicomponent model, requiring no further parametrization for mixtures. Five ternary systems are studied here and represent three types of solute combinations: organic-organic (glycerol-ethanol), electrolyte-organic (NaCl-succinic acid, NaCl-glutaric acid), and electrolyte-electrolyte (NaCl-KCl and NH4NO3-(NH4)2SO4). For the NaCl-glutaric acid system, experimental measurements of picoliter droplet surface tension using aerosol optical tweezers show excellent agreement with the model predictions.

Statistical Thermodynamic Model for Surface Tension of Aqueous Organic Acids with Consideration of Partial Dissociation.

With statistical mechanics, an isotherm-based surface tension model for single solute aqueous solutions was derived previously (Wexler et al. J. Phys. Chem. Lett. 2013) for the entire concentration range, from infinite dilution to pure liquid solute, as a function of solute activity. In recent work (Boyer et al. J. Phys. Chem. Lett. 2015), empirical model parameters were reduced through physicochemical interpretations of both electrolyte and organic solutes, enabling surface tension predictions for systems where there is little or no data. The prior binary model is extended in the current work for the first time to treat multicomponent systems to predict surface tensions of partially dissociating organic acids (acetic, butyric, citric, formic, glutaric, maleic, malic, malonic, oxalic, propionic, and succinic acids). These organic acids are especially applicable to the study of atmospheric aqueous aerosols, due to their abundance in the atmosphere. In the model developed here, surface tension depends explicitly on activities of both the neutral organic and deprotonated components of the acid. The relative concentrations of the nondissociated and dissociated mole fractions are found using known dissociation constants. Model parameters strongly depend on molecular size, number of functional groups, O:C ratio, and number of carbons. For all organic acids in this study, fully predictive modeling of surface tensions is demonstrated.

Interfacial Tensions of Aged Organic Aerosol Particle Mimics Using a Biphasic Microfluidic Platform.

Secondary organic aerosol (SOA) particles are a major component of atmospheric particulate matter, yet their formation processes and ambient properties are not well understood. These complex particles often contain multiple interfaces due to internal aqueous- and organic-phase partitioning. Aerosol interfaces can profoundly affect the fate of condensable organic compounds emitted into the atmosphere by altering the way in which ambient organic vapors interact with suspended particles. To accurately predict the evolution of SOA in the atmosphere, we must improve our understanding of aerosol interfaces. In this work, biphasic microscale flows are used to measure interfacial tension of reacting methylglyoxal, formaldehyde, and ammonium sulfate aqueous mixtures with a surrounding oil phase. Our experiments show a suppression of interfacial tension as a function of organic content that remains constant with reaction time for methylglyoxal-ammonium sulfate systems. We also reveal an unexpected time dependence of interfacial tension over a period of 48 h for ternary solutions of both methylglyoxal and formaldehyde in aqueous ammonium sulfate, indicating a more complicated behavior of surface activity where there is competition among dissolved organics. From these interfacial tension measurements, the morphology of aged atmospheric aerosols with internal liquid-liquid phase separation is inferred.