Spatial distribution of greenhouse gases (CO 2 and CH 4 ) on expressways in the megacity Shanghai, China.

Carbon dioxide (CO ₂ ) and methane (CH ₄ ) are the two major greenhouse gases (GHGs) in the atmosphere that contribute to global warming. Vehicle emissions on expressways cannot be neglected in the megacity Shanghai because oil accounts for 41% of the total primary energy consumption, and the expressway network carries 60% of the total traffic volume. The spatial distributions of CO ₂ and CH ₄ concentrations were monitored in situ on the expressways and in road tunnels using a mobile vehicle. The average CO ₂ and CH ₄ concentrations were 472.88 ± 34.48 ppm and 2033 ± 54 ppb on the expressways and 1308.92 ± 767.48 ppm and 2182 ± 112 ppb in the road tunnels in Shanghai, respectively. The highest CO ₂ and CH ₄ concentrations appeared on the Yan'an Elevated Road and the North-South Elevated Road, respectively, while their lowest values both occurred on the Huaxia Elevated Road passing through the suburban area. The hotspots of CO ₂ and CH ₄ were not consistent, suggesting that they have different sources. Tunnels had a "push-pull effect" on GHGs, and the traffic-congested Yan'an East Road Tunnel showed a dramatically increasing trend of GHG concentration from the entrance to the exit. This traffic-congested tunnel could accumulate a very high concentration of GHGs as well as other pollutants, which could introduce unhealthy conditions for both drivers and passengers. Significant correlations between CO ₂ and CH ₄ mostly appeared on the expressways and in the tunnels in Shanghai, suggesting the influences of vehicle exhaust. ΔCH ₄ /ΔCO ₂ (the slope of the linear regression between CH ₄ and CO ₂ ) and the CH ₄ /CO ₂ ratio could be used as indicators of vehicle exhaust sources because it increases from sources (e.g., road tunnels) to the observatories in the urban area.