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Comprehensive Investigations of Aerosol Droplet Surfaces and Their Climate Impacts

Periodic Reporting for period 2 - AeroSurf (Comprehensive Investigations of Aerosol Droplet Surfaces and Their Climate Impacts)

Periodo di rendicontazione: 2022-08-01 al 2024-01-31

Aerosols are key components of our atmosphere. They serve as the seeds for cloud droplets and represent the largest negative (cooling) and most uncertain climate forcing. Aerosols are also a major contributor to air pollution, which is responsible for ~7 million deaths worldwide annually. This proposal aims to better characterise the surface properties of aerosols. Aerosol surfaces are emerging as key sources of uncertainty in understanding atmospheric aerosol chemistry and aerosol-climate impacts. For instance, surfactants are recognized as important components of aerosol chemical composition. Aerosol surface tension strongly influences the fraction of atmospheric particles that activate into cloud droplets and affect climate. Chemical reactions in aerosols and droplets have been shown to be accelerated by more than 10^6 times compared to macroscopic solutions, and many explanations for such observations rely on the unique aspects of droplet surfaces. To identify and quantify the significance of aerosol surfaces on climate and health, we require detailed knowledge about aerosol surface composition and reactivity. However, few approaches directly interrogate droplet surfaces, hindering incorporation of surface-mediated processes into climate and air quality models. This project explores directly the droplet-air interface of picolitre droplets with an aim to develop a comprehensive understanding of these microscopic interfaces, leading to an evaluation of their potential impacts on climate and health. This project has three main objectives. The first is to quantify how surfactants partition in microscopic aerosol droplets, exploring how surface-bulk partitioning depends on surfactant properties and droplet size, as well as the timescales for such partitioning. The second objective is to construct completely novel mass spectrometry-based approaches for the chemical analysis of aerosols and single picoliter droplets, with an aim to ultimately develop a surface-selective approach. The third objective is to investigate chemistry (particularly light-induced) at the droplet-air interface. Together, this research will pioneer new and highly sensitive experimental methods, test thermodynamic and kinetic models of aerosol chemistry, and explore how chemical reaction dynamics can be altered by confinement in microscopic compartments.
We have made significant advances towards quantifying the surface properties of picoliter droplets and towards developing new single droplet chemical analysis approaches using mass spectrometry. Using a custom-built single droplet levitation device, we performed comprehensive measurements of the surface tensions of surfactant-containing picoliter droplets near equilibrium conditions, systematically varying droplet size, surfactant identity, and surfactant concentration. We found that surfactant-containing droplets can have surface tensions that are higher than the corresponding macroscopic solution’s surface tension. Moreover, we found that there is a clear droplet size-dependence to surface tension. Consequently, we conclude that, for strong surfactants, a droplet’s surface-area-to-volume ratio becomes more important than differences in surfactant properties in determining droplet surface tension because the high surface-area-to-volume ratio in aerosol droplets alters surface-bulk partitioning. These observations indicate explicit consideration of surface-area-to-volume ratio is required when investigating chemical reactivity in aerosol droplets or estimating aerosol activation to cloud droplets. We also investigated the dynamics of surfactant partitioning to picoliter droplets using a stroboscopic imaging technique that allows surface tension to be quantified on timescales of tens of microseconds. We observe the droplet surface tension change from a value close to that of the solvent (water) immediately after droplet formation to a value closer to the equilibrium value for the surfactant when the droplet surface age is much longer (hundreds of microseconds). These results provide key information about the dynamics of droplet surface properties, enabling testing of kinetic models that describe molecular transport within a microscopic compartment. Lastly, we developed two mass spectrometry approaches to characterize the chemical composition of aerosols. Both approaches use a novel ionization approach called droplet assisted ionization, which requires no high voltage or laser to charge the analyte molecules in the droplets. One approach analyzes plumes of submicron aerosol; the other utilizes a quadrupole to focus a beam of picoliter droplets into a custom-built mass spectrometer inlet. Initial studies are focussed on characterizing the method (e.g. exploring how different parameters alter the ion signal observed by the mass spectrometer).
The finding that, as droplet size decreases, surface-area-to-volume ratio becomes more important than the surfactant’s chemical properties in determining the droplet surface tension (and surface composition), is an advance beyond the state of the art. These results are applicable to any field where high surface-area-to-volume systems are important. Moreover, because our measurements provided a robust set of data against which to test thermodynamic models describing this effect, one may now confidently predict the surface properties of aerosols with only knowledge of the macroscopic solution properties. The mass spectrometry approach we developed is beyond the state of the art, as it has proven thus far to be an extremely sensitive method for chemical analysis of individual microscopic droplets. This approach in principle could be applied to any mass spectrometer and may have broad utility extending to any field where only small amounts of sample are available for chemical analysis (e.g. pharmaceutical synthesis or biological mass spectrometry). Future work will further advance the droplet surface tension studies to more chemically complex systems and begin a collaboration with climate modellers to create new parameterizations for inclusion of surface tension effects in climate models. The mass spectrometry tools will be further refined and ultimately used to study chemical reactions in aerosol droplets. We will also continue to develop these mass spectrometry approaches to create a surface-selective chemical analysis on single droplets. Lastly, we will begin to explore how light can promote unique chemistry in aerosol droplets.
ERC Grant Summary Figure