Variability in morphology of soot particles during non-cloud and in-cloud processes

The morphology of soot particle influences their optical properties and their ability to act as cloud condensation nuclei (CCN). However, the morphology of soot particles has not been thoroughly examined from non-cloud to in-cloud processes. This study analyzed the morphology and mixing structures of cloud interstitial (INT) and cloud residual (RES) soot particles collected at three mountain sites. Soot particles were classified into four categories based on their mixing structures: INT-Bare, INT-Thinly coated, INT-Thickly coated, and RES soot particles. The fractal dimensions (D_f) of INT-Thinly coated (1.93) and INT-Thickly coated (1.99) are larger than those of INT-Bare (1.86) soot particles, indicating significant soot compaction due to non-cloud atmospheric aging processes. Interestingly, we found that the D_f of RES soot particles (1.95) is close to that of non-cloud internally mixed soot particles in this study (1.97) and previous literature (1.96). Our results indicate that cloud processing may not further compact the soot particles. This discovery has significant implications for the accurate portrayal of soot morphology within cloud droplets in models, ultimately impacting the modeling of radiative and climate effects. Additionally, our study highlights the potential significance of coating density as a factor influencing soot compaction.