Temporal Evolution and Vertical Distribution of Size-Resolved Aerosol Composition and Mixing State within the Boundary Layer Using an Unmanned Aerial Vehicle

Abstract Characteristics of individual particles and aerosol mixing state strongly influence atmospheric chemistry, radiation, and cloud formation yet remain poorly constrained. Here, we investigated the spatiotemporal evolution of size-resolved particle composition in a coastal environment by applying a new capability using a microscopy-based particle sampler coupled to an unmanned aerial vehicle (UAV). With sampling resolutions up to 30 m and 1 min, over 100 000 particles were analyzed by computer-controlled scanning electron microscopy with energy-dispersive X-ray spectroscopy. The observations revealed a pronounced temporal transition in particle chemistry from sea-breeze-influenced aerosol dominated by aged sea salt and dust (46–83%) to continentally impacted aerosol dominated by organic-sulfur-containing particles (57–89%). Fine-resolution vertical profiles further showed that this transition was governed by the complex interactions among boundary layer dynamics, regional transport, and secondary aerosol formation in the coastal atmosphere. The aerosol mixing state showed pronounced diurnal variability and size dependence but weak vertical variation. Formation of organic-sulfur-containing particles shifts the aerosol population toward external mixing, decreasing the mixing state index from 53% to 27%. These findings provide new insights into aerosol transformation and demonstrate the value of high spatiotemporal single-particle characterizations for understanding aerosol environmental and climatic impacts.