## 3 Results and Discussion
### Proposed Growth Mechanism
To further support the proposed mechanism for the formation of structures in the HAS, LAS (near ground level) was conducted using passive samplers installed at two different locations relative to the reaction zone, in the direction of the wind. The plumes released from the personal fireworks are very close to the ground, making them ideal for this study. Figure 8(c2) depicts the firing process of personal fireworks (mostly sparklers and fountains) and the collection process. One passive sampler was located about 2 m from the reaction zone (at a height of \(\sim\)1 m), while the other was about 20 m away (at a height of \(\sim\)2 m) (Figure 8(c2)). The smoke plume from the LAS passive samplers appears to be composed of a greater variety of particulates compared to the HAS. SEM analysis of the passive sampler located 1 m from the reaction zone reveals the presence of a heterogeneous microstructure, including spheroidal, irregular or shapeless, and fine materials. The set of images in Figure 8(c3a-c3f) displays SEM images of particulates collected at a distance of approximately 2 m and a height of 1 m from the reaction zone. Figure 8(c3a) shows a hexagonal platelet-like structure (\(\sim\)50 \(\upmu\)m along the axis and 40 \(\upmu\)m across) with a high deposition of condensate particulates on its upper surface. A high-resolution SEM image of the boxed area in Figure 8(c3a) shows that the condensate particles consist of finer spherical shapes and irregular or shapeless structures (Figure 8(c3b)). The samples collected from this close location to the reaction zone also show the presence of very large irregular structures (e.g., those with sharp and pointy edges) without finer materials deposited on their surfaces (Figure 8(c3d)). The size of this irregular structure is approximately 25 \(\upmu\)m along the axis and about 18 \(\upmu\)m across (Figure 8(c3d)). The presence of spherical-shaped particulates with condensate or crystallized fine particles on their surfaces is evident (Figure 8(c3e,c3f)). The set of images of Figure 8(c4a-c4f) represents SEM images of particulates collected at approximately 20 m and a height of 2 m from the reaction zone. Higher-resolution SEM imaging on the condensed structures show that they are composed of spheroidal (white dotted arrow), slender platelets (black solid arrow), and shapeless particles (white solid arrow) (Figure 8(c4b)). Large irregular structures with sharp and pointy edges and minimal condensate particles are also present (Figure 8(c4e,c4f)). At this distance, agglomerated particles forming clusters are evident (Figure 8(c4d)). The cluster in Figure 8(c4d) is mainly composed of spheroidal shapes, each only a few microns in diameter. Analysis of passive samplers placed at 22 m from the reaction zone indicates that their physical morphology is very similar to those collected and analyzed at 2 m (set of images of Figure 8(c3,c4), respectively). Specifically, the presence of platelet-like structures and finer condensate formations is a typical characteristic observed in the samples. It is theorized that these clusters may form from the collision of particulates as they are carried within the plume. Professional fireworks contain more energetic compounds, making them more explosive and resulting in higher temperatures. These results strongly support the presented hypothesis of a two-step formation process: the first step involves the formation of large structures, and the second step is the condensation of volatile gasses on the surfaces of the larger structures, leading to the nucleation of finer particles. The spherical particles in personal fireworks are smaller due to the lack of a coalescence process, which is limited by the nature of personal fireworks (e.g., being less explosive). The HAS samples, however, did not exhibit these characteristics and were instead formed into large spheres that were less concentrated than in the LAS samples.