Conceptualizing the mixing structure of individual aerosol particles

This study investigated the particle size and age-dependent mixing structure of individual particles in clean and polluted air. Individual particles were classified into eight particle types: sea salt, mineral dust, fly ash, metal, soot, sulfates, nitrates, and organic matter (OM). Based on the particle classification, a particle that consists of two or more types of aerosol particles can be defined as an internally mixed particle. Otherwise, the particle is considered to be an externally mixed particle. Within the internally-mixed particle class, we identified four heterogeneous mixing structures: core-shell, dumbbell, OM-coating, and dispersed-OM, as well as one homogeneous mixing structure. Homogeneous mixing mainly occurred in fine particles (< 1 μm), while the frequency of occurrence of heterogeneously mixed particles increased with particle size. Our study demonstrated that particle mixing structures differ depending on their size and location and that they evolve over time. OM-coating and core-shell structures are important indicators for particle ageing in air as long as they are distant from the emission sources. Long-range transported particles tended to have core-shell and OM-coating structures. We found that secondary aerosol materials (e.g., sulfates, nitrates and organics) determined particle mixing structures because their phases change following particle hydration and dehydration at different relative humidities. Once externally mixed particles are transformed into internally mixed particles, they cannot revert to their former state, except when semi-volatile aerosol materials evaporate. Categorizing mixing structures of individual particles is essential for studying their optical and hygroscopic properties and for tracing development of their physical or chemical properties.