# Prediction and Comparison of In-Vehicle CO\({}_{2}\) Concentration Based on ARIMA and LSTM Models
## 2 Materials and Methods
### In-Vehicle CO\({}_{2}\) Concentration by Field Dynamic Testing
#### 2.1.2 Test Location and Process  
The field test site was situated in Guilin, Guangxi, and the test was conducted on 14-15 December 2022, under clear weather conditions. Given that the concentration of CO\({}_{2}\) inside a vehicle is influenced by various factors such as ventilation settings, the number of occupants, driving speed, and vehicle characteristics [13,14], this research monitored changes in CO\({}_{2}\) concentration across these different variables. We conducted a total of six field tests, and the specific operational conditions during each test are presented in Table 5. T1-T2 were designed to monitor and compare the impact of varying fan speeds on the rate of CO\({}_{2}\) increase. The fan speed of the Nissan-Sylphy is categorized into seven different levels. We selected level 1 (S1) and level 4 (S4) for the purpose of testing. The fan speeds at the vents of S1 and S4 were measured as 0.756 m/s and 1.648 m/s, respectively. The purpose of T3 and T1 is to monitor and compare the impact of varying numbers of passengers on the rate of CO\({}_{2}\) increase. T4-T5 were designed to monitor and compare the impact of varying vehicle speeds on the rate of CO\({}_{2}\) increase. T1-T5 were conducted in a Nissan-Sylphy, and T6 in a Honda-Fit to assess the effect of different interior spaces on the rate of increase in CO\({}_{2}\). The S1 fan speed at the air vent of the Honda-Fit was measured to be 0.723 m/s, which is approximately equivalent to that of the Nissan-Sylphy. The reference data for this research were obtained from the T1 test results, with all other tests compared against it. Our field test design refers to the test methodology described in reference [14].  
We selected three driving routes: 12 km of urban roads, with a speed range of 0-40 km/h; 15 km of suburban roads, with a speed range of 0-80 km/h; and 28 km of highways, with a speed range of 60-120 km/h. The duration for each test was controlled at 20 min, and the driving route is illustrated in Figure 4. During the test, the windows were closed, and the air conditioning system was set to internal circulation mode. Prior to each experiment, the instrument underwent calibration in an outdoor open area. Subsequently, fresh air was introduced into the vehicle by opening its doors and windows until the CO\({}_{2}\) concentration inside reached an equilibrium with that of its surroundings. Only then did all personnel enter the vehicle and commence testing. Each measurement lasted for a duration of 20 min, during which data were recorded at intervals of 5 s, resulting in a total collection of 1440 sets of data.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Test Variable** & **Test Number** & **Test Vehicle** & **Fan Speed** & **Driving Road** & **Number of Occupants** \\ \hline \multirow{2}{*}{Fan speed} & T1 & Nissan-Sylphy & S1 & Urban road & 3 \\  & T2 & Nissan-Sylphy & S4 & Urban road & 3 \\ \multirow{2}{*}{
\begin{tabular}{c} Number of \\ occupants \\ \end{tabular} } & T3 & Nissan-Sylphy & S1 & Urban road & 1 \\  & T4 & Nissan-Sylphy & S1 & Suburban road & 3 \\ \multirow{2}{*}{Driving speed} & T5 & Nissan-Sylphy & S1 & Expressway & 3 \\  & T6 & Honda-Fit & S1 & Urban road & 3 \\ \hline \end{tabular}
\end{table}
Table 5: Test conditions.