# Prediction and Comparison of In-Vehicle CO\({}_{2}\) Concentration Based on ARIMA and LSTM Models
## 1 Introduction  
Individuals spend a significant amount of time in indoor environments, including automobiles. A survey conducted among Americans revealed that 88% of their day is spent indoors, and 7% is spent in a vehicle [1, 2]. Therefore, indoor air quality is closely linked to the health, comfort, well-being, and productivity of occupants [3]. Furthermore, the air pollution levels within a vehicle's interior environment exceed those of indoor living environments [4]. Therefore, we should pay attention to the air quality issue within the vehicle's interior environment, and the number of studies in this field is increasing [5]. To ensure thermal comfort while driving, people will close windows and use air conditioning, particularly during winter and summer. In a tropical country like Thailand, air conditioning is utilized throughout almost the entire year due to the hot climate, and people commonly utilize recirculation (RC) ventilation mode because it can reduce fuel consumption [6]. When occupants operate air conditioning based on their personal preferences in a vehicle, they may not be aware of the issues caused by RC ventilation mode and closedwindows: the elevated concentration of carbon dioxide (CO\({}_{2}\)) in a vehicle due to human respiration within a small, enclosed space and a low air ventilation rate. In order to investigate the level of CO\({}_{2}\) concentration in vehicles during driving, numerous scholars have conducted field measurements of CO\({}_{2}\) concentration in small vehicles. For instance, [PERSON] et al. [7] observed that after 15 min of driving, the CO\({}_{2}\) concentration in a vehicle reached 5000 ppm. [PERSON] et al. [6] observed that the concentration of CO\({}_{2}\) increased to 10,000 ppm after 33 min of driving. In a survey of taxi air quality, it was discovered that CO\({}_{2}\) levels reached 3000 ppm within 45 min of operation and remained above this threshold for over four hours [8]; [PERSON] et al. [9] conducted a random selection of 51 private vehicles and monitored their CO\({}_{2}\) levels during daily driving, revealing that 96% of the vehicles exceeded 1000 ppm, and 16% surpassed the threshold of 5000 ppm. In addition to small vehicles, CO\({}_{2}\) concentrations in large vehicles have also attracted the attention of numerous scholars. [PERSON] [10] discovered that the concentration of CO\({}_{2}\) reached 4000 ppm when a bus was fully loaded, which is ten times higher than the ambient concentration; [PERSON] et al. [11] conducted an investigation on the concentration of CO\({}_{2}\) in a campus tour bus. Throughout the test, the maximum and average CO\({}_{2}\) concentrations in the driver zone were 2511 ppm and 1831 ppm, respectively. Meanwhile, the passenger zone reached 2647 ppm and 2047 ppm, respectively. [PERSON] et al. [12] monitored the concentration of CO\({}_{2}\) in buses during peak commuting hours and found a mean value of 1039 ppm. However, few of the above-mentioned studies have focused on variations in CO\({}_{2}\) concentrations across diverse driving conditions. Furthermore, CO\({}_{2}\) and particulates are key indicators for assessing in-vehicle air quality [13]. Current research has shown that there are fewer studies on in-vehicle CO\({}_{2}\) concentrations compared to in-vehicle particulate studies [14]. This indicates a relative scarcity of in-vehicle CO\({}_{2}\) research. Currently, there are a limited number of standards pertaining to CO\({}_{2}\) concentration in passenger vehicles. Consequently, reference is generally made to indoor air quality standards. Table 1 presents the recommended or specified CO\({}_{2}\) values proposed by various institutions. Among them, the carbon dioxide concentrations specified by ASHRAE, OSHA, the Chinese National Center for Disease Control and Prevention, French rules, and the Taiwan Environmental Protection Administration in China pertain to indoor air quality. The Hong Kong Environmental Protection Administration in China has developed specific standards for CO\({}_{2}\) levels in buses.