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