Surfactant Partitioning Dynamics in Freshly Generated Aerosol Droplets.

Aerosol droplets are unique microcompartments with relevance to areas as diverse as materials and chemical synthesis, atmospheric chemistry, and cloud formation. Observations of highly accelerated and unusual chemistry taking place in such droplets have challenged our understanding of chemical kinetics in these microscopic systems. Due to their large surface-area-to-volume ratios, interfacial processes can play a dominant role in governing chemical reactivity and other processes in droplets. Quantitative knowledge about droplet surface properties is required to explain reaction mechanisms and product yields. However, our understanding of the compositions and properties of these dynamic, microscopic interfaces is poor compared to our understanding of bulk processes. Here, we measure the dynamic surface tensions of 14-25 µm radius (11-65 pL) droplets containing a strong surfactant (either sodium dodecyl sulfate or octyl-ß-D-thioglucopyranoside) using a stroboscopic imaging approach, enabling observation of the dynamics of surfactant partitioning to the droplet-air interface on time scales of 10s to 100s of microseconds after droplet generation. The experimental results are interpreted with a state-of-the-art kinetic model accounting for the unique high surface-area-to-volume ratio inherent to aerosol droplets, providing insights into both the surfactant diffusion and adsorption kinetics as well as the time-dependence of the interfacial surfactant concentration. This study demonstrates that microscopic droplet interfaces can take up to many milliseconds to reach equilibrium. Such time scales should be considered when attempting to explain observations of accelerated chemistry in microcompartments.

Physical properties of short chain aqueous organosulfate aerosol.

Organosulfates comprise up to 30% of the organic fraction of aerosol. Organosulfate aerosol physical properties, such as water activity, density, refractive index, and surface tension, are key to predicting their impact on global climate. However, current understanding of these properties is limited. Here, we measure the physical properties of aqueous solutions containing sodium methyl or ethyl sulfate and parameterise the data as a function of solute concentration. The experimental data are compared to available literature data for organosulfates, as well as salts (sodium sulfate and sodium bisulfate) and organics (short alkyl chain length alcohols and carboxylic acids) to determine if the physical properties of organosulfates can be approximated by molecules of similar functionality. With the exception of water activity, we find that organosulfates have intermediate physical properties between those of the salts and short alkyl chain organics. This work highlights the importance of measuring and developing models for the physical properties of abundant atmospheric organosulfates in order to better describe aerosol's impact on climate.

Surface-Area-to-Volume Ratio Determines Surface Tensions in Microscopic, Surfactant-Containing Droplets.

The surface composition of aerosol droplets is central to predicting cloud droplet number concentrations, understanding atmospheric pollutant transformation, and interpreting observations of accelerated droplet chemistry. Due to the large surface-area-to-volume ratios of aerosol droplets, adsorption of surfactant at the air-liquid interface can deplete the droplet's bulk concentration, leading to droplet surface compositions that do not match those of the solutions that produced them. Through direct measurements of individual surfactant-containing, micrometer-sized droplet surface tensions, and fully independent predictive thermodynamic modeling of droplet surface tension, we demonstrate that, for strong surfactants, the droplet's surface-area-to-volume ratio becomes the key factor in determining droplet surface tension rather than differences in surfactant properties. For the same total surfactant concentration, the surface tension of a droplet can be >40 mN/m higher than that of the macroscopic solution that produced it. These observations indicate that an explicit consideration of surface-area-to-volume ratios is required when investigating heterogeneous chemical reactivity at the surface of aerosol droplets or estimating aerosol activation to cloud droplets.

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.

Hygroscopicity of Microplastic and Mixed Microplastic Aqueous Ammonium Sulfate Systems.

A growing body of research suggests the presence and long-range transport of microplastics in the atmosphere. However, the interactions between these microplastics and atmospheric aerosol are poorly understood. Environmental microplastics vary in color, morphology, and chemical composition and become oxidized over time by UV, mechanical, and biological action. Once introduced to the atmosphere, these microplastics will likely become mixed with atmospheric aerosol. Determining how microplastics interact with aerosol particles and how they may alter aerosol physical properties, including water uptake and loss, is necessary to understand the impact of these microplastics on our environment. Herein, we investigate the effect of microplastics on the water activity of bulk water and ammonium sulfate solutions. We compare a variety of plastic compositions and microplastic morphologies including plastics that have been aged by UV irradiation and mechanical forces in the lab. In addition, we investigate the water uptake and loss in microplastic samples through dynamic vapor sorption. We find an increase in total water sorption for UV-aged plastics compared to pristine plastics. Finally, we investigate the effect of fractional surface coverage on the equilibration time scale.

The wavelength-dependent optical properties of weakly absorbing aqueous aerosol particles.

The refractive index (RI) is a key quantity in calculating many aerosol properties required for climate models. To accurately describe the RI of aerosol, the wavelength and temperature dependence as well as the variation with aerosol water content must be considered. Aside from water, aged ambient aerosol can contain both inorganic salts and a myriad of organic molecules. Determining the optical properties of each organic molecule and their contribution to the aerosol as a whole would be an incredibly time consuming and, in many cases, intractable task. Using single aerosol particle spectroscopy measurements and an effective oscillator model, we are able to measure parameters that can be used to accurately calculate the wavelength-dependent RI of mixed organic-inorganic aqueous aerosol particles. Measured oscillator parameters are presented for a number of atmospherically relevant inorganic ions and surrogate organic species. Finally, the effect of temperature on the oscillator parameters is investigated.

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%.

A nurse-led gynaecology emergency assessment service.

A gynaecology emergency assessment unit was established because an increasing number of women were being referred to the gynaecology ward for assessment. On average 180 women a month are assessed at the unit and there is potential for that number to increase. An audit has revealed that only 7.2% of women seen in the unit required admission, the remainder being suitable for treatment as outpatients. Before the unit was established it was normal practice for any referrals to be automatically admitted until the junior doctor on call could assess them and decide an appropriate care plan.