## 1 Introduction
### Automotive Natural Exposure Test  
The experiment was performed at the hygothermal natural environment test station in Qionghai City, Hainan Province, China. The test site is located at 19\({}^{\circ}\)14\({}^{\prime}\) N, 110\({}^{\circ}\)28\({}^{\prime}\) E and 10 m above sea level. The perfect ground drainage facilities eliminate the interference of local climate formed by rainwater accumulation on the measurement of test environmental parameters. The test object was an independent brand car, which was parked on a wide and flat lawn and maintained distances of more than three times the vehicle height from other test vehicles. To keep most of the interior trim parts under heavy heat load, the front windshield of the car was oriented towards the equator, and the engine, doors and windows were all closed. The natural environment parameters outside the car were monitored in real-time by a small weather station; the surface temperature of the parts and the microenvironment parameters inside the automobile were measured by the temperature and humidity sensors as well as the radiometers. Meanwhile, the test parameters were gathered by the PC-2 WC data collector at a frequency of 5 min. The main test equipment and measurement parameters are shown in Table 1. Twenty-five sensor measuring points were arranged on different types of auto-parts, as shown in Figure 1, among which there were 10 temperature measuring points on the exterior surface of the car, including (numbers refer to Figure 1): (1) front bumper; (2) radiator grille; (3) right headlights; (4) engine cover; (5) triangular plate of right rearview mirror support; (9) exterior door; (13) the center of exterior roof; (16) rear right lights; (17) rear bumper; and (25) right front wheel shield. There were 13 measuring points on the interior surface of the car: (6) dashboard; (7) steering wheel; (8) airbag cover; (10) interior door; (14) interior roof; (15) rear windowsill; (18) interior trim panel on B-pillar; (19) headrest; (20) seat; (21) carpet; (22) central console; (23) cigar lighter cover; and (24) lower dashboard decoration panel. The air temperature and humidity parameters in the micro-environment of the cabin were collected by measuring Points 11 and 12, respectively. Sensors on typical measuring points were installed as shown in Figure 2.  
The weathering external factors and their cumulative values throughout the year vary greatly among different parts, as shown in Figure 3. The daily maximum and annual solar radiation outside the car are 1155.0 W\(\cdot\)m\({}^{-2}\) and 5166.0 MJ\(\cdot\)m\({}^{-2}\), respectively, which are much higher than those of interior trim parts in the cabin. Due to the heat storage and greenhouse effect in the enclosed cabin, the air temperature in the cabin is obviously higher than that outside the car; therefore, the temperature and high temperature accumulation time of the dashboard are significantly higher than those of the exterior parts. In the cabin, the annual radiation, daily maximum temperature and high temperature accumulation time of the parts near the window are much higher than those of interior trim parts shielded by the car body. The test data of a typical sweltering summer day were selected from the whole year. As shown in Table 2 and Figure 4, the surface temperature of automobile presents a highly non-uniform distribution phenomenon. On the one hand, there are great differences in the surface temperature of different parts, among which the dashboard temperature is as high as 87.8 \({}^{\circ}\)C, while the maximum temperatures of the shielded cigar lighter cover and lower dashboard decoration are merely 65.6 and 59.3 \({}^{\circ}\)C, respectively. On the other hand, diurnal temperature differences of the parts are very remarkable, among which those of cigar lighter cover and lower dashboard decoration are just 36.6 and 30.2 \({}^{\circ}\)C, respectively, while that of dashboard is up to 59.3 \({}^{\circ}\)C.