## 3 Results and Discussion
### Proposed Growth Mechanism
This is evident from the openings on the surfaces of the microspheres (Figure 8(a9-a12)). Some microspheres have various openings (Figure 8(a10-a12)). Large, smooth, and perfectly spherical structures without openings and condensate materials on their surfaces are frequently encountered in analysis of the airborne samples (Figure 8(a13)). Figure 8(a14) is an SEM image of a fractured microsphere. Close inspection of the fracture region indicates that this microsphere is solid. However, scanning its surface shows initial openings (Figure 8(a15)). This indicates that the core-shell morphology of the microspheres follows after the coalescence process. Pyrotechnics are highly energetic compounds, and during the explosion, these materials burn to generate volatile gasses that expand and create pressure within the particles. Temperature, phase transformation, and heat transfer in and around a particle are crucial factors influencing the growth process. [PERSON] et al. (2013) were able to successfully measure the temperatures within an explosive fireball based on the light emissions from the fireball itself [38]. It was reported that the temperatures inside the fireball are in the range of 1600 K to 1900 K with a peak of 1850 K occurring 12 milliseconds after the explosion, with a uniform temperature distribution throughout the fireball. Additionally, [PERSON] et al. (2012) demonstrated that the physical (spheroidal, solid, and hollow) and chemical morphology of firework residue is similar to that found in gunshot residue [27]. It is reasonable to suggest that during the explosion of the fireworks, as nucleation and growth occur (depending on the explosion's location), there is a uniform particle volume temperature. Nucleation and growth begin at the surface of the sphere. However, the temperature at the center can be significantly higher than at the surface, which is continuously cooled by convection and radiation. As the spheroidal particles are carried away from the reaction zone by the plume toward cooler regions, volatile gasses become trapped, resulting in the formation of hollow or core-shell morphology. Other surrounding vapors can act as the source material for the finer particles. In the airborne samples, crystallization of these finer particles on the surface of the microspheres  
Figure 8: Various firework schematics (**a1–a4**) mortar-style firework function process; (**a4**) various effects produced in mortar-style fireworks; (**a5**) illustrates the process of using the drone for airborne particle collection; (**b**) schematic illustrating the growth of microspheres resulted from coalesce; (**46–48**) SEM images of the various morphologies of the microspheres supporting the growth mechanism; (**c1,c2**) sparkler and fountain function process; (**d1–d4**) schematic illustrating the growth process; (**c3,c4**) SEM images of the various morphologies of the structures at 2 m and 20 m from the reaction zone, respectively. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Low Altitude Samples** & **High Altitude Samples** \\ \hline Average Diameter & 12 μm & 39 μm \\ Standard Deviation & 16 μm & 33 μm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: LAS and HAS sizing comparison. is evident (Figure 8(a16-a18)). The larger surface areas of these structures provide ideal sites for the recrystallization of \"leftover\" vapors from the explosion as they condense. A similar mechanism has been proposed for the formation of finer materials on the surfaces of larger structures found in ashes formed during volcanic explosions ([PERSON] et al., 2023) ([PERSON] et al., 2023). [PERSON], H et al. (2016) conducted laser-induced explosions on the surface of transition metal specimens revealing that vapors or sources of material for recrystallization of particulates were formed through sublimation ([PERSON] et al., 2016).