Review of Kuula et al., ar-2025-38

Abstract. This study experimentally investigated the influence of aggregate morphology on soot light absorption. Fresh soot was generated using an inverted-flame burner and compacted through controlled humidification–drying cycles to isolate the effects of structural transformation from those of chemical composition or coating. Size-resolved absorption measurements were performed at three wavelengths (440, 516, and 635 nm) using a cantilever-enhanced photoacoustic spectrometer (CEPAS) coupled with a differential mobility analyzer (DMA) and a centrifugal particle mass analyzer (CPMA). For small particles, the absorption cross-section increased after compaction but decreased for larger particles, with the wet-to-dry absorption ratio transitioning from values above unity to below unity as particle mass increased. The tipping point occurred in the 1–2 fg mass range, corresponding to a volume-equivalent diameter of approximately 102–129 nm for spherical particles at a soot material density of 1.8 g cm-3. This behavior suggests a competition between near-field dipole–dipole coupling, which enhances optical absorption in moderately compact aggregates, and optical shielding, which suppresses absorption in highly compact structures. The findings are consistent with theoretical predictions and numerical studies of aggregate optics, providing experimental evidence for morphology-dependent absorption transitions. These results emphasize the importance of accurately representing soot morphology in optical and climate models and motivate further controlled experiments to disentangle the effects of structure, coating, and composition on soot radiative properties.