## 4 Results analysis
### Analysis of Vehicular Emissions of BTH Region
#### 4.1.2 Analysis of the Impact of Traffic Policies on Vehicle Pollutants  
Figure 4 shows the vehicular emissions for different scenarios and can be used to analyze the pollutant emission mitigation potential of different vehicle policies. As per the results, implementation of the stringent China VI emission standard is effective for mitigating vehicular emissions, especially CO and HC emissions. The standard is specifically projected to afford CO, HC, NO\({}_{x}\), and PM emission reductions of 49.08%, 49.00%, 4.88%, and 0.00%, respectively, in 2020. As also indicated in Figure 4, the adoption of an EVs policy could reduce CO, NO\({}_{x}\), HC, and PM emissions from vehicles by 25.01\(\times\)10\({}^{3}\), 1.07\(\times\)10\({}^{3}\), 4.11\(\times\)10\({}^{3}\), and 0.08\(\times\)10\({}^{3}\) ton, respectively, by 2020. However, approximately 2.6\(\times\)10\({}^{9}\), 3.2\(\times\)10\({}^{9}\), and 3.7\(\times\)10\({}^{9}\) kWh of electricity would have to be added by 2020, 2025, and 2030, respectively, to meet the power requirements of EVs. This implies the emission of additional air pollutants (SO\({}_{2}\), NO\({}_{x}\), PM, and CO\({}_{2}\)) through the increased electricity generation. Further analysis is thus required to explore the comprehensive impact of the adoption of EVs on pollutant emissions.  
The sources of electricity for EVs generally include coal-based power and renewable energy. In this study, it is assumed that electricity from coal-based power was locally generated and renewable power means solar, wind power and imported electricity. Based on different power sources of EVs, there would be different impacts on the environment. Figure 5 shows the additional CO\({}_{2}\) and air pollutant (PM, SO\({}_{2}\), and NO\({}_{x}\)) emissions that result from the additional electricity generation required by the adoption of EVs. As can be seen, an EVs policy promises to effectively mitigate CO and HC emissions in all the scenarios. However, it increases SO\({}_{2}\) and CO\({}_{2}\) emissions. Moreover, no reduction in NO\({}_{x}\) and PM emissions would be achieved by the EVs policy if the power required by the EVs were entirely generated by coal-based power. Actually, in such a case, the total NO\({}_{x}\) emission would increase by 0.72\(\times\)10\({}^{3}\) ton by 2020, 0.87\(\times\)10\({}^{3}\) ton by 2025, and 1.32\(\times\)10\({}^{3}\) ton by 2030. Conversely, if the power for EVs was generated 50% by coal-based and 50% from renewable sources, as in scenario S3, the EVs policy would reduce NO\({}_{x}\) emission by 0.17\(\times\)10\({}^{3}\) ton by 2020, 0.21\(\times\)10\({}^{3}\) ton by 2025, and 0.24\(\times\)10\({}^{3}\) ton by 2030. If the EVs power was entirely generated from renewable sources, as in scenario S4, the policy would remarkably reduce NO\({}_{x}\) emission by 1.07\(\times\)10\({}^{3}\) ton by 2020, 1.29\(\times\)10\({}^{3}\) ton by 2025, and 1.49\(\times\)10\({}^{3}\) by 2030. These results indicate that the vehicle-emissions could be reduced directly; but through the extra electricity generated from local region, leading to additional air-pollutants. Thus, an EVs policy should be enhanced by increasing the ratio of the needed power generated from renewable sources.  
Figure 4: _Cont._Figure 4: _Cont._