# Prediction and Comparison of In-Vehicle CO\({}_{2}\) Concentration Based on ARIMA and LSTM Models
## 3 Results  
### Analysis of In-Vehicle CO\({}_{2}\) Concentration  
The mean values of temperature, humidity, CO\({}_{2}\) concentration, and rate of CO\({}_{2}\) increase during the six field tests are presented in Table 6. Temperature and humidity remained relatively stable due to the operation of the vehicle's air conditioning system. The CO\({}_{2}\) concentration measurements for each field test are presented in Figure 6. During testing, the windows were closed, and internal circulation was activated, resulting in an increase in the CO\({}_{2}\) concentration over time. Due to real-time changes in vehicle speed causing an unstable air change rate, there were fluctuations observed during the growth process of CO\({}_{2}\) concentrations, similar to the field test results from [PERSON] et al., [PERSON] et al., and [PERSON] et al. [6; 13; 28]. The impact of different fan speeds on the increase in the in-vehicle CO2 concentration is illustrated in Figure 6a. After 20 min, the CO\({}_{2}\) concentration levels exceeded 4000 ppm in T1-S1 and T2-S4, with rates of CO\({}_{2}\) increase of 3.52 ppm/s and 3.02 ppm/s, respectively. The higher air speed at the air conditioning outlet in S4 results in an increased frequency of air exchange between the cabin and its surrounding environment, leading to a lower rate of CO\({}_{2}\) increase. The impact of different numbers of occupants on the increase in the in-vehicle CO\({}_{2}\) concentration is illustrated in Figure 6b. With three occupants, the CO\({}_{2}\) concentration reaches 4633 ppm after 20 min, exhibiting a rate of CO\({}_{2}\) increase of 3.52 ppm/s. With one occupant, the CO\({}_{2}\) concentration increases to 2152 ppm at a rate of 1.43 ppm/s after 20 min. The comparison indicates that with three occupants in the vehicle, the CO\({}_{2}\) concentration attained after 20 min is 2.53 times higher than that with only one occupant, and the rate of CO\({}_{2}\) increase is 3.03 times greater. Figure 6c illustrates the rate of CO\({}_{2}\) increase within a vehicle at varying driving speeds. Disregarding the impact of road conditions, when traveling at an average speed of 85.89 km/h on an expressway, the CO\({}_{2}\) concentration reaches 3176 ppm after 20 min, with a rate of CO\({}_{2}\) increase of 2.30 ppm/s. When traveling at an average speed of 55.85 km/h on a suburban road, the CO\({}_{2}\) concentration reaches 4228 ppm, with a rate of CO\({}_{2}\) increase of 3.17 ppm/s after 20 min of driving. The test data showed that a lower rate of CO\({}_{2}\) increase will be caused by increasing driving speed, and a lower concentration level will be reached over the same period. The phenomenon can be attributed to the reason that as the speed of driving increases, so does the external dynamic pressure. The static pressure decreases, and the leakage of air from the inside to the outside of the vehicle is accelerated, resulting in a reduction in the rate of CO2 increase. The impact of different interior vehicle volumes on the increase in the in-vehicle CO2 concentration is illustrated in Figure 6d. The T6-Honda Fit exhibits a CO2 concentration of 4870 ppm after 20 min, with a rate of CO2 increase of 3.72 ppm/s. Compared to the T1-Nissan Sylphy, which reaches 4633 ppm of CO2 concentration after 20 min, the concentration of the Nissan-Sylphy is 237 ppm smaller than that of the Honda-Fit. Since the interior space of the Nissan-Sylphy is 0.3 m\({}^{3}\) larger than that of the Fit, the calculation shows that each 1 m\({}^{3}\) increase in the volume of the vehicle reduces the CO2 concentration by 790 ppm. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test Number** & **Temperature (°C)** & **Relative Humidity (\%)** & **CO2 (ppm)** \\ \hline T1 & 22.78 & 32.07 & 2734 \\ T2 & 25.10 & 37.69 & 2489 \\ T3 & 17.14 & 38.84 & 1161 \\ T4 & 18.49 & 36.75 & 2427 \\ T5 & 25.21 & 28.69 & 1905 \\ T6 & 23.82 & 28.70 & 2866 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Average temperature and humidity and CO2 concentration of each test.