## 3 PAH Formation--Mechanistic Features  
Given their chemical diversity, PAH formation and development occur through a complex set of reactions, mainly through condensation and cyclization from smaller organic molecules; under an array of specific conditions (carbon source, environment), complex approaches entailing PAH structural expansion have been investigated [3; 6; 47]. As such, the endeavor to provide an appropriate growth model has yielded several representations. The most important types of PAH formation mechanisms identified over the last few decades are based on acetylene addition reactions, vinylacetylene addition reactions, and radical reactions (Figure 3) [11; 47; 48; 49]. Hydrogen abstraction and acetylene or carbon addition (HACA). This mechanism was originally presented by [PERSON] et al. (1984) [50] and later introduced as HACA [51] in the framework of PAH growth in ethane-, ethylene-, and acetylene-fueled flames. The mechanism is depicted as a two-step process involving hydrogen abstraction (activation of the aromatic molecule), followed by the addition of acetylene (C\({}_{2}\)H\({}_{2}\)) in the radical position. The advantages of HACA stem from its continuous PAH formation/growth. This is, in turn, attributed to the relatively low reversibility of the reaction and the increased affinity of hydrogen atoms due to low energy barriers throughout the process [52]. Nonetheless, given its reaction rate, the HACA growth mechanism is argued to underperform in comparison to the relatively quick PAH formation process [47; 53]. Other pathways similar to HACA have been proposed in order to understand PAH chemistry. One of the alternatives is the Bittner-Howard process, involving the sequential addition of two C\({}_{2}\)H\({}_{2}\) molecules to afford a C\({}_{4}\)H\({}_{4}\) chain, which subsequently leads to additional ring formation [54]. However, these alternative pathways have been deemed unrealistic or limited under low-pressure or high-temperature flame conditions in terms of the residence time of unstable radicals [47,55]. The Diels-Alder mechanism, traditionally involving the reaction of acetylene with an olefin to afford a cyclic compound [56], was proposed by [PERSON] and [PERSON] (2000) [57]. It involves the formation of a Diels-Alder adduct by the cycloaddition of acetylene, followed by the loss of hydrogen and the resulting bay region ring closure [57]. However viable, it should be considered that the DA mechanism is accompanied by high energy barriers throughout the process, along with low reaction rates [47,58]. Hydrogen abstraction and vinylacetylene addition (HAVA). Similar to acetylene, vinylacetylene is widely available in combustion flames and involved in PAH formation through the hydrogen abstraction vinyl acetylene addition mechanism, hence the HAVA terminology. It can be achieved with vinylacetylene (VA), without the need for additional carbon species, through the addition of reactive VA (double and triple bonds) to a PAH radical, followed by cyclization [59,60]. Explorations of the HAVA mechanism deemed it considerably viable under different conditions (low energy barriers of reaction, wide temperature and pressure ranges) [59]. Methyl addition cyclization (MAC). The methyl radical is the most abundant alkyl radical in combustion flames among potential radical species. Due to the prevalence of the methyl radical, MAC plays an important role in PAH formation [61,62]. It is initiated with the formation of ethyl or propyl chains on the PAH structure (addition of 2-3 methyl radicals), followed by hydrogen elimination and subsequent ring closure [63]. As observed in a study simulating the formation of pyrene from phenanthrene, MAC appears not to be  
Figure 3: Schematic representation of the main PAH growth reaction mechanisms. competitive with HACA, given that C\({}_{2}\)H\({}_{2}\) surpasses the methyl radical in occupying the armchair position radical sites [64]. Ethynyl radical addition (HAERA).