Spray Aerosols From Saltwater to Freshwater Breaking Waves

Abstract

While sea spray aerosols (SSAs) generation by oceanic breaking waves continues to be an active research area, lake spray aerosols (LSAs) production by freshwater breaking waves is an emerging research field. Recent studies have linked LSAs to regional cloud processes and the aerosolization of freshwater pathogens and pollutants. Yet, differences in spray aerosol ejection between freshwater and saltwater and their impact on the water-to-air dispersal of microorganisms and pollutants are poorly understood. The goals of this dissertation work were to understand mechanistic differences between spray aerosol generation in freshwater and saltwater, develop a representation of LSA emissions in atmospheric models and evaluate their impact on regional aerosol loading, and compare the aerosolization of bacteria and microplastics by SSAs and LSAs. Experiments in a breaking-waves analogue tank revealed that the subsurface bubble plume in saltwater is characterized by more submillimeter bubbles than that in freshwater, and hence, saltwater surface foams were more persistent and were comprised of more submillimeter surface bubbles. Consequently, the average number concentration of generated SSAs was eight times higher than that of LSAs. Using these measurements, the developed LSA emission parametrization revealed that freshwater emissions were, at least, an order of magnitude lower than saltwater emissions for the same wave-breaking conditions. When implementing this emission parameterization to simulate LSA emissions from the Laurentian Great Lakes, LSAs did not contribute significantly to regional aerosol loading (< 2%), yet their impact on coarse-mode aerosols was more significant with up to a 19-fold increase in some areas. Furthermore, modeled LSAs reached up to 1000 km inland and were incorporated in the lakes' cloud layer. Despite the generation of more spray aerosols in saltier waters, cumulative salt additions in the freshwater–saltwater continuum (0-35 g/kg) led to a nonmonotonic increase in freshwater bacterial aerosolization abundance, which exhibited a peak at lower oligohaline conditions (0.5-1 g/kg). However, the aerosolization of microplastics by SSAs was one order of magnitude higher than that by LSAs. Overall, this dissertation work showed that LSA emissions are intrinsically different from SSA emissions, which influences their role in transferring microorganisms and pollutants at the air-water interface.