Heating-Induced Transformations of Atmospheric Particles: Environmental Transmission Electron Microscopy Study.

Environmental transmission electron microscopy was employed to probe transformations in the size, morphology, and composition of individual atmospheric particles as a function of temperature. Two different heating devices were used and calibrated in this work: a furnace heater and a Micro Electro Mechanical System heater. The temperature calibration used sublimation temperatures of NaCl, glucose, and ammonium sulfate particles, and the melting temperature of tin. Volatilization of Suwanee River Fulvic Acid was further used to validate the calibration up to 800 °C. The calibrated furnace holder was used to examine both laboratory-generated secondary organic aerosol particles and field-collected atmospheric particles. Chemical analysis by scanning transmission X-ray microscopy and near-edge fine-structure spectroscopy of the organic particles at different heating steps showed that above 300 °C particle volatilization was accompanied by charring. These methods were then applied to ambient particles collected in the central Amazon region. Distinct categories of particles differed in their volatilization response to heating. Spherical, more-viscous particles lost less volume during heating than particles that spread on the imaging substrate during impaction, due to either being liquid upon impaction or lower viscosity. This methodology illustrates a new analytical approach to accurately measure the volume fraction remaining for individually tracked atmospheric particles at elevated temperatures.