## 4 Ambient Levels  
The total concentration of IVOCs varies in different environmental conditions. In Pasadena in the USA, the average concentration of IVOCs was measured as 6.3 \(\pm\) 1.9 \(\upmu\)g\(\cdot\)m\({}^{-3}\), comparable to that of OAs [11]. Notably, there was no significant difference in the IVOC concentrations between weekdays and weekends, indicating that other petroleum sources also contribute significantly to IVOC emissions. In Canada, indoor IVOC concentrations ranged from 14.4 to 69.0 \(\upmu\)g\(\cdot\)m\({}^{-3}\), while outdoor concentrations ranged from 4.2 to 8.1 \(\upmu\)g\(\cdot\)m\({}^{-3}\)[64]. The indoor IVOC concentrations were approximately six times higher than those outdoors, suggesting that indoor temperature and humidity conditions significantly affect the formation of IVOCs. Specifically, temperature was significantly positively correlated with the concentrations of branched alkanes and UCM, with higher temperatures leading to higher compound concentrations [65]. Relative humidity was significantly positively correlated with the concentrations of oxidized IVOCs (such as fatty acids), with high humidity potentially promoting the formation of these compounds [66]. In ambient air, several varieties of IVOCs can be detected. The atmospheric concentration of hexylcyclohexane is approximately 1.3 \(\upmu\)g\(\cdot\)m\({}^{-3}\), while the emissions of C\({}_{12}\)-C\({}_{14}\) n-alkyl cyclohexanes from diesel trucks range from 14.9 to 26.2 \(\upmu\)g\(\cdot\)km\({}^{-1}\). These figures indicate that C\({}_{12}\)-C\({}_{14}\) n-alkyl cyclohexanes have a certain concentration level in the atmosphere and account for a significant proportion of traffic emissions [18]. The average ambient concentration of IVOCs was (5.1 \(\pm\) 0.8) \(\upmu\)g\(\cdot\)m\({}^{-3}\) in Shanghai Yangshan Port in China [51], while it was (58.5 \(\pm\) 27.0) \(\upmu\)g\(\cdot\)m\({}^{-3}\) in winter and (6.8 \(\pm\) 3.7) \(\upmu\)g\(\cdot\)m\({}^{-3}\) in summer in urban Shanghai [10]. These data suggest that ship emissions contribute less to IVOCs than vehicle emissions and further confirm the positive correlation between temperature and IVOC concentrations. In low-temperature conditions, the volatility of many IVOCs decreases, causing them to exist more readily in the atmosphere in gaseous form. Figure 3 summarizes the atmospheric concentrations of IVOC species, in both gas and particle phases, observed at different sites. The spatial distribution of gaseous PAHs shows that the Haidian District in Beijing, China [67], has significantly higher concentration levels than other urban areas. In regard to observations of gaseous long-chain alkanes in several typical regions in China (Shanghai [68], the Pearl River Delta urban agglomeration [69], Guangzhou [36], Ezhou [70], Jinan [71], Harbin [72], as well as rural and suburban areas of the North China Plain [69]), the concentration distribution consistently decreases with an increasing carbon number. In contrast, monitoring data from urban areas in Europe and America (London, UK [73], Paris, France [32], Pasadena, USA [11]) do not show such a carbon number-dependent concentration gradient. This may be attributed to the higher use of clean energy in European and American cities, significantly reducing the contribution of fossil fuel combustion sources to IVOCs. Notably, the concentration levels of gaseous PAHs in open ocean environments (the North Atlantic, South Pacific, and Indian Ocean) [74] are significantly lower than terrestrial observations, reflecting the low pollution characteristics of typical marine environments. Observation data indicate that, except for the higher values observed in the Haidian District, Beijing, the concentrations of particulate-phase IVOCs at other sites generally do not exceed 2 parts per trillion by volume (pptV), significantly lower than those of gaseous components (Figure 3).