Diesel engines emit chain-agglomerate particles that can serve as cloud condensation nuclei. This research uses an Environment Scanning Electron Microscope (ESEM) to study the effect of cloud processing on morphologies of individual diesel chain-agglomerates. Particles produced using a Caterpillar 3304 diesel engine from fuels with sulfur contents of 0.84%, 0.32% and 0.034% by weight were collected on silicon wafer substrates. They were subjected to one to three water condensation-evaporation cycles in the ESEM. This process was recorded on video tape, and digitized images of individual particles were used to find the particle's fractal dimension before and after each condensation-evaporation cycle. Significant collapse occurred in particles generated from both the mid-range sulfur fuel (0.32% S) and the low sulfur fuel (0.034% S). The average fractal dimension of the particle images increased from 1.56 to 1.76 and from 1.40 to 1.54 for particles from low and mid-range sulfur fuel, respectively. We observed no significant morphological change in particles from high sulfur fuels. The experiments reflect lower limits for the degree of collapse that diesel chain-agglomerate particles undergo during atmospheric cloud processing.
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Acknowledyements~--We gratefully acknowledge the assistance of the staff of the University of Minnesota High Resolution Microscopy Center, the support of the Electric Power Research Institute (EPRI Contract # RP2023-11), and the University of Minnesota Consortium for the Measurement of Engine Generated Particulate Matter (Exxon, John Deere).