Nanoparticles surrounded by gas agglomerate in a hierarchical fashion. From production until powder processing in the gas phase, nanoparticles go from individual particles to aggregates, simple agglomerates, and complex agglomerates. Even though the structures at each level have unique properties, they are commonly assessed as a whole. Additionally, the effect of external factors on the morphology of these structures during gas processing is not well understood and challenging to study due to the limited techniques for in situ analysis of the dynamic phenomenon. Here, we study three materials in their hydrophobic and hydrophilic version. We describe the structural characteristics of each hierarchical level of complex agglomerate formation obtained from two in situ techniques. The first scale, namely aggregates, are open structures with a fractal dimension of about 1.5, which then form simple agglomerates with a fractal dimension close to 3, that later cluster into complex agglomerates that present a fractal dimension of about 2. Furthermore, gas dynamics were found to densify the simple agglomerates, increasing their fractal dimension by more than 0.1.@en