3.2. Interior Concentration changes in Vehicles of Brand A
It had been commonly found that interior temperature was an especially important factor influencing the test results [
22,
29]. As shown in
Figure 2, the ratios (C
S/C
E) of concentrations of the confirmed compounds and TVOC measured in Brand A in sunshine condition (C
S) to that in the environment test chamber (C
E) were greater than 1, indicating that the VOCs and TVOC pollution concentrations in the three model vehicles increased sharply when the temperature rose from 28.7 °C to 61.5 °C. In addition, as illustrated in
Figure 3, when the in-vehicle temperature was 25 °C (in the environment test chamber), the concentrations of the eight in-vehicle confirmed compounds were all lower than their respective limited values in the national standard GB/T 27630-2011. However, when in-vehicle temperature increased, the concentrations of formaldehyde, acetaldehyde, and acrolein in Vehicle A1-1 (
Figure 3a) and that of styrene, formaldehyde, acetaldehyde, and acrolein in Vehicle A2-1 (
Figure 3b) were 1.89, 1.92, 1.44, 1.40, 7.84, 2.1, and 1.76 times more than their corresponding limited values in the national standard GB/T 27630. Therefore, in-vehicle high temperature was helpful for the evaporation and off-gassing of more VOCs from vehicle interior trims, for the main reason that the release amount of such VOCs as organic solvents, adhesives and additives contained in the interior trim materials could increase more when in-vehicle temperature rose [
34,
35]. In addition, the reports also showed that the concentrations of VOCs in vehicle interiors increased in concert with the temperature [
25,
27,
36]. Reducing in-vehicle temperature could slow down the VOC emissions from the vehicles’ interior materials.
3.3. Interior VOC Type changes in Vehicles of Brand A
Table 2 shows the types of the top 18 VOCs, excluding benzene, toluene, xylene, ethylbenzene, styrene, butyl acetate and undecane, identified in interior air samples of Vehicle A1-1, Vehicle A2-1, Vehicle A3-1, and changes of VOC types under different static conditions. In general, the most alkanes were in the three vehicles under each test condition, but there were more alkanes in the environment test chamber than in the sunshine condition. While in the sunshine condition, there were more other types of compounds, such as alcohols, ketones, benzenes, alkenes, aldehydes, esters, than that in environment test chamber (shown in
Table 2). This was because the exposure of the vehicles to direct sunlight in the sunshine condition could lead to interior temperatures up to 61.5 °C, therefore the surface temperatures of the interior materials would be higher, which could cause the volatilization of various chemical substances with different boiling points from the interior surfaces. Thus, the chemical composition and types of VOCs were changed under different static conditions. In conjunction with high temperature, the transmission of solar radiation through glass windows could induce photochemical reactions and the production of degradation of byproducts, which could also cause changes in the chemical composition and types of VOCs.
An environment with a high concentration of VOCs could pose a very large health hazard to drivers and passengers. However, considering that few drivers and passengers would stay in such high-temperature vehicles, it was necessary to further study the changes in the concentrations and types of VOCs in the vehicles under driving conditions when they were exposed to direct sunlight.
3.4. Interior Concentration changes in Vehicles of Brand B and C
To further evaluate the differences in VOC concentrations in the vehicles’ interior, air samples were also collected under driving conditions. According to the test results, the concentrations of eight confirmed compounds inside the vehicles of Brand B and C in the environment test chamber were commensurate, and the TVOC concentrations inside Brand C were obviously higher than that of Brand B. In addition, the concentrations of eight confirmed compounds and TVOC inside Brand B and C in the environment test chamber (CE) and under driving conditions (CD(0–30min), CD(60–90min)) were also compared and discussed.
As shown in
Figure 4, in general, the concentration changes of eight confirmed compounds inside six vehicles of Brand B were C
D(0–30min) > C
D(60–90min) ≥ C
E. But there were some exceptions. Inside the vehicle B1–1, the concentration change of benzene was C
D(0–30min) = C
D(60–90min) > C
E, that of xylene was C
D(60–90min) > C
D(0–30min) > C
E, that of ethylbenzene and styrene were C
D(60–90min) > C
D(0–30min) = C
E, and that of acrolein was C
E > C
D(0–30min) = C
D(60–90min). Inside the vehicle B1-2, the concentration change of formaldehyde was C
D(0–30min) > C
D(60–90min)>C
E, while that of other compounds were C
E > C
D(0–30min) ≥ C
D(60–90min). Inside the vehicle B2-1, the concentration change of styrene was C
E > C
D(0–30min) = C
D(60–90min), and that of formaldehyde was C
D(0–30min) > C
E > C
D(60–90min). Inside the vehicle B2-2, the concentration change of ethylbenzene was C
E = C
D(0–30min) > C
D(60–90min). Inside the vehicle B3-1 and B3-2, the concentration change of acetaldehyde was C
D(0–30min) > C
E > C
D(60–90min). Whereas, as shown in
Figure 5, the change trends of concentrations inside six vehicles of Brand C were clearly different with that inside Brand B, which generally were C
E > C
D(0–30min) > C
D(60–90min). But there were also some exceptions. Inside the vehicle C1-1, the concentration changes of toluene and formaldehyde were C
D(0–30min) > C
E > C
D(60–90min). The concentration changes of acetaldehyde inside vehicle C1-2, that of formaldehyde and acetaldehyde inside C1-3 and that of toluene inside C1-4 were all C
E > C
D(60–90min) > C
D(0–30min). Inside vehicle C1-4, the concentration change of xylene was C
E > C
D(0–30min) = C
D(60–90min), that of acrolein was C
D(60–90min)> C
E = C
D(0–30min). Similarly, as shown in
Figure 6, the TVOC concentration changes inside the six vehicles of Brand B were all C
D(0–30min) > C
D(60–90min) > C
E, while that inside Brand C were C
E > C
D(0–30min) > C
D(60–90min), which had the same trend as that of eight confirmed compounds.
As the air conditioning mode inside the tested vehicles was adjusted to internal circulation, and the infiltration air flow through joints and leaks in vehicle envelopes was the predominant airflow that could affect pollutant transportation inside vehicle cabins [
17,
37]. Under driving conditions, the vehicles ran at a certain speed, which could accelerate the air exchange inside and outside vehicles. Therefore, the concentration changes of VOCs and TVOC inside the vehicles should be C
E > C
D(0–30min) > C
D(60–90min) in theory, which was consistent with the results tested inside the vehicles of Brand C. But based on the test results of Brand B, except for some substances inside the vehicle B1-1 and B1-2, C
D(0–30min) were generally higher than C
E and C
D(60–90min). That was because the six vehicles of Brand B were all parked and enclosed under direct sunlight (
Table 1) before the experiments were carried out under driving conditions, which could accelerate volatilization of VOCs from vehicle interior trims due to the high temperature inside the vehicles. Therefore, it was reasonable that more pollutants were collected inside the vehicles of Brand B at the first 30 min under driving conditions than that in environment test chamber, even though the temperature was the same under these two conditions. However, although the vehicle C1-5 and C1-6 were also parked and enclosed under direct sunlight for 2 h before running on the vehicle proving ground, the C
D(0–30min) of VOCs and TVOC were actually lower than their C
E. That was because different vehicle brands have different air exchange rates. And the higher speed of the vehicles of Brand C than that of Brand B could lead to the increase of the air exchange rate inside and outside Brand C on one hand, while on the other hand, there might be (no air sampling) less pollutants emitted from the interior trims of Brand C than that of Brand B under direct sunlight, and then the concentration of pollutants inside Brand C would decrease rapidly at a higher air exchange rate.
As the vehicles continued to run at a certain speed, the concentration of VOCs and TVOC inside the vehicles would decrease with time due to the air exchange inside and outside the vehicles. Thus, C
D(60–90min) of VOCs and TVOC inside Brand B were less than their C
D(0–30min). But the air exchange rate value determined whether a longer driving process was required to reduce the concentration of airborne pollutants in the vehicles to be consistent with or lower than that in the environment test chamber. According to the test results shown in
Table 3, the C
D(60–90min) of eight compounds and TVOC were 1.00~3.27 times and 0.99~7.93 times more than their C
E, demonstrating that after a period of driving, the concentrations of air pollutants inside the vehicles of Brand B were decreased to the same level with or close to that in the environment test chamber (
Figure 4 and
Figure 6a). If the air conditioning mode was switched to external circulation (the concentration of ambient air pollutants should meet the requirements in
Section 2.2.1), it would take a shorter time to reduce the concentration of air pollutants in the vehicles. The above results further indicated that the concentration values obtained by the standard method (in environment test chamber) were close to the actual exposure level for drivers and passengers.