For over 25 years, transmission electron microscopy (TEM) has provided a method for the study of aerosol particles with sizes from below the optical diffraction limit to several microns, resolving the particles as well as smaller features. The wide use of this technique to study aerosol particles has contributed important insights about environmental aerosol particle samples and model atmospheric systems. TEM produces an image that is a 2D projection of aerosol particles that have been impacted onto grids and, through associated techniques and spectroscopies, can contribute additional information such as the determination of elemental composition, crystal structure, and 3D particle structures. Soot, mineral dust, and organic/inorganic particles have all been analyzed using TEM and spectroscopic techniques. TEM, however, has limitations that are important to understand when interpreting data including the ability of the electron beam to damage and thereby change the structure and shape of particles, especially in the case of particles composed of organic compounds and salts. In this paper, we concentrate on the breadth of studies that have used TEM as the primary analysis technique. Another focus is on common issues with TEM and cryogenic-TEM. Insights for new users on best practices for fragile particles, that is, particles that are easily susceptible to damage from the electron beam, with this technique are discussed. Tips for readers on interpreting and evaluating the quality and accuracy of TEM data in the literature are also provided and explained.

Additional details about the experimental configurations and transmission electron microscope (TEM) imaging protocols, further methods and details on particle spreading and additional modes in scanning mobility particle size (SMPS) spectra, additional visual and numerical data pertaining to scanning electron microscopy (SEM) and atomic force microscopy (AFM) experiments, and a description of the assessment of secondary organic-and organic proxycoated samples over time. MethodsSamples of several inorganic and organic/inorganic salt systems were prepared for TEM and SEM analysis. Particles with two components comprised ammonium sulfate as well as succinic acid (>99.0%, TCI America), 2-methylglutaric acid (98%, Alfa Aesar) or PEG400 (BioUltra, Sigma-Aldrich) in a 1:1 ratio for a total of 0.05 -0.1 wt%. Single component salt systems included ammonium bisulfate (99.9%, Alfa Aesar), sodium chloride (>99.9%, VWR Chemicals), potassium chloride (>99.9%, DOT Scientific) and sodium acetate (99%, Mallinckrodt Chemicals). All such were prepared at the Pennsylvania State University using the sample generation methods described in the Methods section using carbon/copper substrates. SEM samples of both ammonium sulfate and ammonium sulfate/2-methylglutaric acid particles were prepared at the Pennsylvania State University in the same manner as TEM samples and using continuous carbon/copper TEM substrates. SEM images were obtained using an Apreo SEM (Thermo Fisher).Additionally, a series of measurements were taken over the course of several weeks to test the volatility of the shell layer of coated particles. Particles composed of 0.1 wt% succinic acid and ammonium sulfate in a 1:1 ratio were produced via the Pennsylvania State University setup described in the Methods section where the solution was aerosolized, rapidly dried using a diffusion drier, size selected, and impacted onto carbon-coated copper TEM grids. Additionally, ammonium sulfate seed particles with secondary organic coatings were generated at the Pennsylvania State University by adding dry ammonium sulfate particles into a 1 m 3 Tedlar bag (Welch Fluorocarbon) which were allowed to equilibrate followed by 150 ppb of a-pinene and 200 ppb ozone from a corona discharge tube ozone generator (Poseidon 200, Ozotech Inc.). These