Detection of CO2 and CH4 Concentrations on a Beijing Urban Road Using Vehicle-Mounted Tunable Diode Laser Absorption Spectroscopy

Abstract

The analysis of greenhouse gas emission characteristics on urban roads is of great significance for understanding the sources and sinks of urban greenhouse gases and their changing patterns. Based on tunable diode laser derivative absorption spectroscopy technology, which features high resolution, high sensitivity, and fast response, a vehicle-mounted system capable of simultaneously detecting CO2 and CH4 has been developed. The system has a response time of 0.38 s, with detection sensitivities of 5 ppb for CH4 and 0.2 ppm for CO2, power consumption of approximately 4.8 W, a weight of less than 3 kg, and dimensions of 255 mm × 275 mm × 85 mm. Using this system, monitoring campaigns were conducted on the same road in Beijing, running north–south, during different time periods in April and June 2023. The results show that there is little correlation between changes in CO2 and CH4 concentrations on the road, and these gas concentrations exhibit different influencing factors and spatiotemporal characteristics. The CO2 concentration on the road is primarily related to the degree of traffic congestion and does not exhibit significant seasonal variations. The average CO2 concentration measured on the road is much higher than the global average CO2 concentration during the same period. On the other hand, the CH4 concentration on the road is not strongly correlated with traffic congestion but is closely related to the leakage of methane from specific emission wells or covers. The CH4 concentration is higher in the morning, gradually decreases as the sun rises, and then increases again after sunset. The CH4 concentration measured at night in June is significantly lower than that in April, reflecting some seasonal variation. The CH4 concentration on the Beijing urban road is slightly higher than the global average CH4 concentration during the same period. The vehicle-mounted experiments verified the feasibility of using this self-developed system for vehicle-mounted detection of greenhouse gas concentrations on urban roads. The research results can provide data for analyzing the spatial pattern of regional carbon sources and sinks.