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Travel-based (104 tons)
Grid-based (104 tons)
The method of integrating the traffic model and vehicle emission model to estimate traffic emissions presented in this study can be extended in various ways. For example, it can be used to evaluate the impact of urban land plan on traffic emissions. It can also be used to evaluate the effect of traffic management measures on traffic emission reduction. Future research will be necessary to determine whether these types of evaluations will improve traffic emissions modeling.
Table 3 – Travel- and grid-based total running emissions for Beijing urban area
speed values in each grid cell. In other words, grid VKT is aggregated and grid-based emission factors are assigned based on the grid average speeds estimated in travel demand models. The travel-based and grid-based vehicle emissions are presented in Table 3.
It shows that the grid-based method generally results in higher emissions than the travel-based method, and the percent difference of the emissions between these two methods are respectively 15%, 13% and 5% for HC, CO and NOx. Because the same total VKT were used in these two methods, the differences are mainly attributed to the different approaches of emission factor development. Therefore, we may underesti- mate the vehicle emissions in the Beijing urban area if the travel-based method is applied. Smit et al. (2008) also drew the similar conclusions in their study in Amsterdam, Netherlands.
Table 3 also shows that the NOx difference between these two methods is not as high as HC or CO. This is due to HC and CO emissions being more sensitive to the speed and driving mode than NOx, which can be seen in Fig. 2.
A bottom-up methodology using the TDM traffic model and MOBILE5B-China are applied to assess the traffic-induced emissions in the Beijing urban area in this study. The grid- based vehicular emissions of HC, CO and NOx are calculated based on grid-based vehicle activities and emission factors, and they show clear spatial distributions. The total vehicle emissions of HC, CO and NOx are respectively 13.33×104, 100.02×104 and 7.55×104 tons in Beijing urban area in 2005.
The result of the macro-scale approach to determine vehicular activities and emissions has been compared with the micro-scale methodology. When compared to the relatively realistic grid-based approach, the macro-scale approach under- estimated the VKT for PC, SB, taxi and bus. It also overestimated the activities of HDTs and LDTs in the Beijing urban area. The macro-scale approach provided acceptable deviations (−18.7 to −6.1%) for both HC and CO emissions. However, it may overestimate NOx emissions since it does not consider the traffic restriction of HDTs in the Beijing urban area.
The comparison of travel- and grid-based emission inven- tory indicates that the emissions calculated by these two methods differ from each other even though they have the equal VKT. Because the current emission models generally have a relationship between emission factors and speeds (Fig. 2), when VKT and emission factors are combined, the grid-based approach is likely to be more sensitive in measur- ing emission effects due to specific changes in each grid,
particularly those grids with low speeds (e.g., heavily con- gested area) and high speeds (e.g., the area with freeways).
This work was supported by the China National Nature Science Foundation (Project No. 50678092) and Beijing Environmental Protection Bureau. We also thank Mr. Chuck Freed formally of US. EPA for his aid in editing this paper. The contents of this paper are solely the responsibility of the authors and do not necessarily represent official views of the sponsors.
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Please cite this article as: Wang H, et al, A bottom-up methodology to estimate vehicle emissions for the Beijing urban area, Sci Total Environ (2008), doi:10.1016/j.scitotenv.2008.11.008