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ARTICLE IN PRESS

SCIENCE OF THE TOTAL ENVIRONMENT XX (2008) XXXXXX

5

Fig. 5 Spatial distribution of vehicular NOx emissions for Beijing urban area in 2005.

and field investigation were applied in the grid-based method which reflects the actual vehicle activity on the roads. Table 2 illustrates that most of the activities of PCs and taxis occur in the downtown area and comprise over 90% of the total VKT in the urban area of Beijing. H o w e v e r , t h e m a c r o - s c a l e a p p r o a c h c o u l d n o t i d e n t i f y

included in the local statistical material. Since they are excluded from the vehicle emission inventories used in macro-scale methods, this could also lead to the underestimation of vehicular activity and emissions in the urban area of Beijing.

the spatial distributions of VKT accurately due to its intrinsic drawbacks. It underestimated by 8% and 63% the activity for PCs and taxies, respectively. Because the light duty vehicles (e.g. PCs and taxies) are the main sources of HC and CO emissions in the city, most of the vehicular HC and CO emissions are concentrated in the urban area of Beijing. Any differences between the macro-scale approach and vehicle actual activity may result in underestimating the vehicular HC and CO emissions in the urban area. (2) Because of Beijing's importance in China, there are many non-local vehicles and military vehicles driving in the urban area. However, these kinds of vehicles are not

In Beijing, the HDT use is restricted in the urban area so most of their activities occur in the suburbs. This contributes to high vehicular NOx emissions in suburban Beijing. How- ever, the macro-scale method ignores these different spatial distributions of vehicle activity for various fleets. Table 2 shows that, compared with the micro-scale approach, the macro-scale method will overestimate 51% and 211% of the vehicle activity for LDTs and HDTs, respectively. As trucks are the main source of NOx emission in the metropolitan cities in China (Wang et al., 2008), the macro-scale method appears to

overestimate the NOx emissions in the urban area of Beijing (Fig. 6).

3.3.

Impact of emission factors on emission estimations

When the VKT data is derived from the TDM model, it can be used for estimating emissions. Traditionally, the VKT data will be aggregated and multiplied by the average emission factor based on the average travel speed in the city to calculate the v e h i c l e e m i s s i o n s ( t r a v e l - b a s e d ) . I n t h i s s t u d y , t h e e m i s s i o n

factors and VKT data are not simply connected at the aggregate level; instead, they interact according to different

Macro-scale

11252

45

1285

1277

621

128

Micro-scale

12281

259

3472

411

413

252

Percent difference

8%

83%

63%

211%

51%

49%

Fig. 6 Macro- and micro-scale vehicle emission inventories for Beijing urban area.

Table 2 VKT for various vehicle types in macro- and micro-scale method, 104 km/day

Vehicle type

PC

SB

Taxi

HDT

LDT

Bus

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

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