## 3 Results and Discussion
### Proposed Growth Mechanism  
A possible growth mechanism for the formation of particulates from fireworks is proposed in this study. It can be hypothesized that all structures (e.g., microspheres, irregular, and finer particles) are formed instantaneously during the reaction zone or burning of the fireworks, regardless of whether they are generated from professional or personal fireworks. Although the temperature and chemical species present in professional fireworks are generally much stronger due to the potent and energetic chemical compounds, they can contribute to higher combustion temperatures and more intensive reactions. A comparison of the SEM analysis conducted on collected samples from professional fireworks (HAS) and  
Figure 7: Energy dispersive X-ray (EDX) maps and EDX spectra collected on selected airborne particulate matter of low-altitude fireworks. **(a1)** Secondary electron SEM image of a region with 5 grouped particles; **(a2–a5)** EDX maps of Al-K, O-K, K-K, Si-K in the grouped structure; **(a6)** associated EDX spectra; **(b1)** secondary electron SEM image of a grouped “splatter” region **(b2,b3)** EDX maps of Cl-K and Na-K; **(b4)** associated EDX spectra; **(c1)** secondary electron SEM image of compound granular particle; **(c2–c4)** EDX maps of elements present including Al-K, O-K, K-K; **(c5)** associated EDX Spectra. personal fireworks (LAS) shows that, overall, (i) particles from the professional fireworks are much larger in size and more spherical compared to those from the personal fireworks (Table 2); (ii) both types of fireworks exhibit a common trend where some of the larger particles have finer particulates deposited on their surfaces; and (iii) there is evidence that the plumes in both types of fireworks contain spheres that are hollow or core-shell, resembling bubble dynamics. Figure 8(a1-a18,b,c1-d2) illustrate the schematics of the two types of fireworks: HAS and LAS. Specifically, Figure 8(a1-a4) depict the firing process of professional fireworks, where mortar-style mechanisms are frequently used. Figure 8(a4) shows a series of images that such fireworks result from explosions. Figure 8(a5) is an illustration of the applied airborne collection in this study. The SEM analysis of the airborne samples indicates that the formation of the microspheres may occur via the Ostwald Ripening effect through contact recrystallization [35]. During the firework explosion, the intense heat can cause various metal compounds to vaporize or sublimate and then condense as they cool. Additionally, solid particulates undergoing a heating process can become hollow core-shell structures [36]. In materials science, it is postulated that large particles grow at the expense of smaller particles of the same solid species [37]. The SEM analysis of the airborne samples from professional fireworks reveals that most of the trapped particles are large and spherical in shape. Figure 8(a6) shows a sphere approximately 20 \(\upmu\)m in diameter with several smaller spherical particles deposited on its surface. Figure 8(a7) represents a higher-resolution SEM image of the boxed area in Figure 8(a6), revealing morphological details of the smaller spheres. It is evident that the microsphere has several smaller spheres attached to its surface (Figure 8(a7)). The variation in the diameter of the smaller particles may be because, during the growth process, the smaller particles continuously shrink as they are consumed by the larger particles. The smaller particles have different diameters, with the largest approximately two micrometers in size. Backscattering SEM imaging analysis reveals that both large and smaller particles have the same contrast, indicating a uniform material composition (Figure 8(a8)). This strongly suggests that the growth of the larger particles resulted from the coalescence of smaller particles into larger ones (Figure 8b). Another feature observed in the samples from HAS is the appearance of microspheres with core-shell morphology.