Probing the physicochemical properties of sea spray aerosols using atomic force microscopy
Creators: name: Madawala, Chamika Kethmini
Publisher: name: University of Iowa
Year: 2026
Subjects: subject: Analytical Chemistry
subject: FOS: Chemical sciences

description: Atmospheric aerosols are suspended solid or liquid microscopic particles in the atmosphere that exhibit diverse compositions, sizes and physicochemical properties. Comprehending the extent of their impact on climate and atmospheric processes pose challenges due to their complexity and the inherent size limitations of various techniques used for their studies. These attributes, influenced by environmental conditions like humidity and temperature, necessitate innovative experimental tools capable of measuring dynamic properties while controlling conditions. Moreover, measurements must be conducted on individual particles of varying sizes, as even particles from the same source can exhibit significant particle-to-particle variabilities. This study focuses on utilizing atomic force microscopy (AFM) as a versatile technique to study single aerosol particles of various sizes originating from various sources. Through the development of various AFM-based methodologies to quantify important physicochemical properties, these approaches can then be applied to study atmospheric aerosols and enable better understanding of their climate-related effects. The subsequent sections in this work outline key properties like mixing state, water uptake, phase state, and viscosity, elucidating how AFM was used in devising methods for the direct measurement of these properties on individual submicrometer-sized model aerosol particles. This thesis also explores the application of these methodologies to examine real sea spray aerosols, resulting in noteworthy discoveries. Investigation of water uptake and phase state of nano-dimensional atmospheric aerosols has been a difficult task due to their inherent size limitation. To address this issue, an AFM methodology that was previously established for sub-micrometer aerosols is extended to measure the water uptake and identify the phase state of individual sucrose nanoparticles with varying heights below 100 nm. Data revealed that the quantified growth factors (GFs) of individual sucrose nanoparticles up to 60% RH were lower than expected values. At RH &gt; 60%, sucrose nanoparticles exhibited liquid behavior, where GFs overlap well with the sub-micrometer particles. Despite this observation for water uptake behavior, data showed that measurements of the phase state using AFM are applicable to nanosized particles where the identified phase transitions between solid and semisolid and between semisolid and liquid for sucrose were at ∼18 and 60% RH respectively, even when the substrate alters the shape of semisolid nanoparticles and alters the water uptake behavior. The diffusion timescale of atmospheric gases into an aerosol is largely governed by its viscosity, which in turn influences climate-relevant aerosol effects. AFMs’ ability to quantify the viscosity of semisolid submicrometer aerosols as a function of relative humidity (RH) was investigated using saccharide model systems. This novel methodology is based on AFM force spectroscopy measurements coupled with the Kelvin-Voigt viscoelastic model. The results revealed that the developed AFM viscoelastic model provides accurate viscosity quantification for semisolid sea spray aerosols (SSA), which otherwise cannot be studied with conventional viscosity measurement techniques, largely due to inherent size limitations of existing techniques. Using glucose, sucrose, and raffinose as model systems, we demonstrated the accuracy of the AFM method within the viscosity range of ~104 – 107 Pa s. The combined aerosol effect on the climate and environment remains uncertain in part due to lack of direct characterization of their physicochemical properties and due to the size-dependent particle-to-particle variability in SSA physicochemical properties. Thus, AFM-based characterization of these physicochemical properties for substrate deposited individual submicrometer nascent SSA (nSSA) were performed. Results revealed a significant variability of physicochemical properties such as morphology, composition, water uptake and phase state as a function of particle size, RH, and biological activity in seawater demonstrating the importance of single particle studies to better understand the effect of SSA on the Earth’s climate and atmosphere.
description: Variable wind speeds over the ocean can have a significant impact on the formation mechanism and physical-chemical properties of sea spray aerosols (SSA), which in turn influences their climate-relevant impacts. Our study further extended to investigate the effect of wind speed on physicochemical properties of substrate deposited individual SSA (e.g., morphology, composition, phase state, water uptake ability and viscosity) at two different wind speeds specifically at 10 and 19 m/s due to their atmospheric relevance of 10 m/s, representing the average wind speed over the ocean, and 19 m/s, indicative of the wind speeds encountered in stormy conditions. Collectively, our observations were attributed to the change in the structure and composition of sea surface microlayer at varying wind conditions. In addition, we showed that the
description: observed changes were not uniform across the entire SSA population, but rather showed a size-dependent particle-to-particle variability. The findings of this study emphasize the importance of considering particle mixing state, and other physicochemical properties in relation to wind speed in accurate assessment of aerosol-related climate processes.
description: In conclusion, this thesis underscores the significance of atomic force microscopy as an instrumental tool for unraveling the intricate properties of atmospheric aerosols and other complex systems. By bridging the gap between macro and nanoscale observations, AFM sheds light on the diverse behaviors of these entities, offering valuable insights for understanding their roles in various environmental contexts.