Numerous studies have attributed recorded warming to the combined effect of greenhouse gases and other anthropogenic influences. According to statistics from EPA, the largest share of greenhouse gas emissions is transportation, followed by electricity production, industry, commercial and residential, agriculture, and land use . But it has been difficult to separate these influences because they work interactively to increase the daily mean surface temperature.
During Beijing 2008 Olympic games, Beijing municipal government implemented a large-scale short-term traffic restriction policy to alleviate the traffic congestion problem. During the Olympic Games from July 20 to September 20, 2008, vehicles with even and odd license plate numbers were alternately allowed on road. Only the vehicles with even plate numbers can be allowed on the road on one day, and the vehicles with odd plate numbers on the following day . This traffic restriction provided an extraordinary opportunity to observe and examine the impact of traffic volume variation on the urban thermal environment.
We derived daytime and nighttime remotely sensed MODIS land surface temperature data during 2008 Olympics with Odd-and-Even Traffic restrictions, and construct the multi-variate statistical model with traffic volume variation as a dummy variable to isolate the impacts. Urban heat budget was estimated considering the net radiation, albedo, wind speed, etc over urban zones in Beijing.
Our analyses show that cutting off about half of the traffic volume has led to a marked decrease in the mean surface temperature by 1.5°C – 2.4°C and a shrinkage of the heat extent by 820 km2 in Beijing. Previous studies suggest that the whole anthropogenic heat from buildings and vehicles may contribute 2-3 °C temperature increase, thus negligible in commercial and residential areas. In comparison, the heat effect of the mobile anthropogenic source alone estimated from our study based on actual meteorological and thermal remote sensing data is significantly larger than the combined anthropogenic sources reported in the previous simulated studies .
This research represents the first effort in the quantitative assessment of the contribution of traffic flow volume in shaping and regulating the urban thermal environment. Results suggest that the impact of urban traffic on heat intensity is significantly larger than previously thought, and the management of urban traffic and vehicle fossil fuel use should be included in the future urban heat mitigation plan.
 US EPA, O. Inventory of U.S. Greenhouse Gas Emissions and Sinks (US EPA, 2018).
 Li, R. & Guo, M. Effects of odd–even traffic restriction on travel speed and traffic volume: evidence from Beijing Olympic games.
 Taha, H., Akbari, H., Sailor, D. & Ritschard, R. Causes and effects of heat islands: sensitivity to surface parameters and anthropogenic heating. Lawrence Berkeley Lab. Rep. 29864 (1992).
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