## 3 Source Characteristics  
### Sources and Compositions  
IVOCs have extensive sources, mainly including primary emissions and secondary formations (Figure 2). Primary emissions are from anthropogenic and biogenic sources, involving fossil fuel combustion (e.g., motor vehicle exhausts, ship and aircraft emissions) [26; 27; 28], biomass combustion (e.g., wood and straw combustion, and garbage incineration) [29], industrial processes (e.g., chemical production and paint use) [30], catering [31], animal husbandy and plant emissions (e.g., terrestrial higher plants, aquatic plants, and phytoplankton) [32]. Secondary formations of IVOCs mainly occur via atmospheric chemical \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Definitions** & **Values** & **Units** \\ \hline CA & Contaminant concentration in air & & \(\text{\mu g\cdot m}^{-3}\) \\ ET & Exposure time & 3.7 & h/d \\ EF & Exposure frequency & 365 & d/y \\ ED & Exposure duration & 78.6 & y \\ AT & Averaging time & 78.6 \(\times\) 365 \(\times\) 24 & h \\ RFC & Reference concentration & & mg\(\cdot\)m\({}^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Meanings of relevant parameters used in human health risk assessment methods. reactions. In gas-phase oxidation reactions, VOCs react with hydroxyl radicals (OH-), the ozone (O3), or nitrate radicals (NO3-) to form intermediate products like peroxy radicals (RO2-) and epoxides. These intermediate products further react to generate low-volatility compounds, which are partially converted into IVOCs. Additionally, RO2- can undergo auto-oxidation to form low-volatility peroxides and hydroperoxides, and these products partially belong to IVOCs [7]. In regard to heterogeneous reactions, IVOCs react with sulfate ions on the surface of acidic particulate matter to form low-volatility compounds, such as sulfate esters, or undergo photochemical reactions on particle surfaces or in droplets under light to generate low-volatility compounds [33; 34], promoting the secondary formation of IVOCs. The currently identified species of IVOCs cover long-chain alkanes (C12-C22), branched cyclohexanes, sesquiterpenens, PAHs, monocyclic aromatic hydrocarbons, phenolic compounds, ketones, esters, organic acids, and heterocyclic compounds. The primary components of IVOCs emitted during fossil fuel combustion are aromatic hydrocarbons and long-chain alkanes [35; 36; 37]. Compared to diesel vehicle exhausts, gasoline vehicle exhausts contain an obviously higher concentration of aromatic hydrocarbons and a relatively lower concentration of long-chain alkanes. It has been reported that important components of aircraft exhausts are low molecular weight PAHs and long-chain alkanes above C12. During biomass burning, a large amount of oxygenated IVOCs (O-IVOCs), PAHs, and long-chain alkanes are released, and the IVOCs emitted from different biomass sources vary [38; 39]. For example, the pyrolysis of lignin in wood generates a large amount of methoxyphenols, and the combustion of deciduous plants commonly produces 2,6-dimethoxyphenol and guaiacol, while the burning of coniferous plants also emits guaiacol. The use of volatile chemical products (VCPs), such as paints, printing inks, adhesives, polishes, air fresheners, perfumes, and emulsions during industrial processes releases O-IVOCs, such as alcohols, esters, and ketones [40]. Biogenic emissions release sesquiterpenens, phenolic compounds, oxalic acid, long-chain alkanes, indole, and other compounds, with marine biogenic emissions releasing a higher amount of oxalic acid [41; 42; 43; 44]. The secondary formation of oxalic acid may occur through the oxidation of long-chain dicarboxylic acids  
Figure 2: Conceptual diagram of the sources and emission characteristics of IVOCs. and glyoxylic acid emitted by marine biota via multiphase reactions.