**4. Results and Discussion**

Chemical mass balance results demonstrated that carpet and seats are the most important VOCs source inside a new vehicle [58], so the bucket, bench seats and floor were chosen as the conventional VOCs sources in the cabin. Besides, the contaminant emission behavior of the dashboard and rear board were additionally investigated due to their prominent representation of high solar exposure.

Solar flux and average temperature variations and distributions with respect to soaking process are shown in Figures 8 and 9, respectively. The transmitted solar flux shows a decrease tendency on window No.3 and No.4, whereas with a sustained growth for the other envelope throughout the soaking period. Most direct solar irradiation (over 350 W) enters the cabin through the windshield and rear glass, falling on the dashboard and rear board. Only a small portion of the sun's rays (below 40 W) passes through the side windows. At noon, the sun moves directly above the vehicle with about 180◦ solar incidence angle, causing a similar solar load on both side body. In Figure 8b, the surface temperature rises steadily with the increase of solar intensity. The highest average temperature with more than 60 ◦C occurs at the dashboard and rear board, while the floor has the lowest temperature with about 44 ◦C. The average temperature of bucket seat No.1 is slightly higher than that of No.2. This is due to the reason that the former receives more solar load from both the windshield and left glass (Figure 9a). The temperature on the two bucket seats tends to be the same at noon because of the comparable sun exposure (Figure 9b). The heat is difficult to transfer around the interior surface by conduction due to low thermal conductivity, exacerbating the thermal imbalance and local overheating [59].

**Figure 8.** The solar flux (**a**) and temperature (**b**) variations from 10:00 am to noon.

**Figure 9.** The solar heat distributions at 10:00 am (**a**) and noon (**b**); The temperature distributions at 10:00 am (**c**) and noon (**d**).

Figure 10 displays the temperature distributions of airflows in the driver plane. The air temperature near the hot interior surface is substantially higher than that which is far away from it; this is because of where the thermal boundary layers exist. In addition, the upper air owns a higher temperature because of the hot air rising and more heat sources. The hotter air gradually develops towards the floor, forming obvious temperature stratification phenomenon. The temperature in the driver's head position reaches 59 ◦C. At 10:00 am, the airflow crosses the bucket to the rear compartment, while it turns into a flow recirculation in the located temperature layer at 11:00 am and noon. Our analysis shows that the temperature distribution is primarily affected by solar radiation and airflow itself. Due to the heat exchange and direct solar heating, the temperature rises as much as 30 ◦C for direct exposed cabin surfaces and 10 ◦C for shaded ones, respectively, which supports the claim that solar radiation is a necessity in the thermal environment simulation.

**Figure 10.** Temperature distributions of airflows with respect to the three time cases at driver plane.

Figure 11 shows the VOCs distributions in the whole cabin at 10:00 am and noon, respectively. And the driver plane was selected in the compartment to analyze the VOCs distributions under solar radiation in Figure 12. It is clearly noticed that the VOCs concentration at the hotter contaminant source is remarkably larger due to the strong dependence between the VOCs emission and the surface temperature. The dashboard and rear board are exposed to the strongest sunlight uniformly, therefore releasing more TVOC at the per unit area. The VOCs emission from the higher temperature area at the carpet increases approximately five-fold compared to the unexposed region. From the view of time, the concentration is about 3–4 times higher at noon than 10:00 am. Besides, the paths of concentration distribution and dispersion are significantly different in the four cases. A remarkable pollutant plume above the hotter surfaces is observed in Figure 12, which confirms that the near-wall thermal buoyancy flows are captured by the adopted turbulence and radiation models. The higher VOCs concentration distributes below the driver's knees, which is difficult to diffuse above the dashboard and rear board by natural convection (Figure 11a,b). Most TVOC is concentrated on the driver's head and above. As the concentration increases, a small amount of TVOC moves towards the floor under the driving of concentration difference. On the roof, the pollutants released from the dashboard and rear board tend to form a bridge of high concentration contaminants (Figure 11c,d). All pollutants gather near the surface sources and diffuse throughout the surroundings without ventilation. It demonstrates that thermal buoyancy and natural convection play an important role in dissipating pollutants from the contaminant source to the adjacent air in the enclosed environment.

**Figure 11.** VOCs concentration emitted from carpet at 10:00 am (**a**) and noon (**b**); TVOC released from dashboard, rear board and seats at 10:00 am (**c**) and noon (**d**).

Table 6 lists the amount of pollutants released by materials from 10:00 am to noon, which was calculated based on the Equation (10). There is a total of 35.08 μg VOCs emitted from carpet and 19 mg TVOC released from other interiors, which far exceeds the national standards of many countries. The seats become the largest source of pollutants due to the larger surface area. Moreover, as illustrated in Figure 10, above the seats are the places where pollutants are most likely to accumulate. It will pose a great threat to health if the level of pollutant exposure cannot be mitigated effectively by natural ventilation or HVAC systems when drivers and passengers re-enter the car.

**Figure 12.** The pollutant distributions on the driver plane from 10:00 am to noon.


**Table 6.** The total pollutants mass (VOCs and TVOC) released from interior surfaces.
