Comprehensive Investigations of Aerosol Droplet Surfaces and Their Climate Impacts

By serving as cloud droplet seeds, aerosols represent the largest negative (cooling) and most uncertain climate forcing. Particulate matter is also a major contributor to air pollution, attributed to ~7 million annual deaths. Aerosol surfaces hold the greatest source of uncertainty for atmospheric chemistry and climate impacts. Surfactants are now routinely identified within atmospheric aerosol samples, and surface tension governs the fraction of particles that activate into cloud droplets, significantly impacting aerosol-cloud climate effects. Sunlight-driven interfacial reactions have recently emerged as important modifiers of atmospheric composition and proceed via unique pathways relative to bulk solutions. A complete understanding of aerosol climate and health impacts requires detailed knowledge of 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 will study directly the droplet-air interface of picolitre droplets in size ranges relevant to growing cloud droplets to develop a comprehensive, molecular level understanding of interfacial composition, reactivity, and climate and health impacts. Aerosol droplet surfaces will be studied with novel, sensitive approaches. The dynamic and equilibrium partitioning of surfactants to aerosol droplet surfaces will be investigated directly for the first time, providing information required for accurate cloud droplet activation predictions. Entirely new approaches to selectively analyse the surface and bulk molecular composition of a levitated micron-sized droplet by mass spectrometry will allow direct investigation of chemistry on aerosol surfaces. Together, these approaches will address outstanding questions in interfacial photochemistry, link directly droplet surface tension to climate impacts, and resolve a poorly understood aspect of aerosol chemistry.