Aerosol Mixing States and Electron Microscopy (Invited Presentation)

Atmospheric production and processing on surfaces or within individual aerosol particles can lead to multi-component grains with complex nano-scale structures and mixing states. Knowledge of these details is important. Particles evolve as they age and react with gaseous species in the atmosphere. Their mixing states are governed by source emissions, transport, and atmospheric transformation and provide important climate-relevant properties because of their influence on radiative behavior, hygroscopic properties, and, ultimately, aerosol health and climate-related effects. Although the importance of the mixing state is clear, its specific characteristics and interpretations differ depending on the definition and measurement methods.

Single-particle analysis using transmission electron microscopy (TEM) facilitates observing particle complexity and provides an essential perspective for the interpretation of internal mixing as well as for the shapes and coatings that are produced under various environmental conditions. Using TEM, one can identify – by composition and, when appropriate, crystallographic structure – all inorganic species in individual internally mixed particles and recognize the areas containing organic material (OM) and the physical ways in which it is intergrown with or coats the inorganic phases.

Morphological measurements, including views of internal intergrowths, yield perhaps the simplest model for mixing states since they provide visual images and thus almost intuitive understanding. These are in distinction to idealized conditions such as volume mixing that can refer to particles containing widely dissimilar phases (e.g., combinations of NaCl, ns-soot, silicate, or OM) as being uniformly and homogeneously mixed even though such mixtures are geochemically improbable. TEM images also indicate issues and inconsistencies between the widely used core-shell model and actual particles. We recently introduced the concept of embedding for many observed particles as more realistic.

Although the morphological mixing state is intuitive, the concept of aerosol population mixing state, which is defined as the distribution of per-particle aerosol species (e.g., ns-soot, sulfate and OM) in each sample, has received recent attention because it is concerned with the results of ensemble measurements. By using population mixing states one can utilize the TEM measurements of large numbers of particles to describe how the bulk samples are mixed. Variables include a) proportion of internally vs. externally mixed particles; b) fraction of organic particles in our three viscosity categories (low, medium, high); c) proportions of coated, embedded, or core-shell particles; d) coating thickness and roughness of internally mixed ns-soot; and e) fraction of internally mixed ns-soot in the centers or exteriors of particles, for direct comparison to refractory black carbon (rBC) data measured by complementary methods.