## 6 Conclusions  
According to the application characteristics of the spraying method, the emulsified asphalt was selected to act as the carrier, and the materials for preparing nano-TiO\({}_{2}\) photocatalytic coating were determined by comprehensive comparison and selection. The test results indicated that the properties  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Spraying Amount (g/m\({}^{2}\))** & **0** & **111.1** & **222.2** & **333.3** & **444.4** & **611.1** & **Method [24]** \\ \hline Permeability coefficient C\({}_{\text{w}}\) (mL/15 s) & \textgreater{}400 & \textgreater{}400 & \textgreater{}400 & \textgreater{}400 & \textgreater{}400 & \textgreater{}400 & T0730 \\ \hline \end{tabular}
\end{table}
Table 13: Effect of spraying amount on permeability of asphalt mixture.  
Figure 10: Effect of spraying amount on the skid resistance of asphalt pavement.  
of prepared nano-TiO\({}_{2}\) photocatalytic coating met the Chinese specifications technical requirements. Meanwhile, the color of the coating (seen in Figure 5) was the same as that of the asphalt after setting, having offset the deficiency of different colors between traditional photocatalytic coating and the pavement. Therefore, the coating could be applied directly onto the surface of the pavement using an asphalt distributor, just like fog seal, to play a better role in the degradation of exhaust gas. The coating could be applied to pavement surfaces that are located in densely-populated areas, environmentally-sensitive areas, service areas and parking lots, and so on.  
An exhaust gas degradation test device was developed, and its test steps were described in detail. The evaluation indexes, cumulative degradation rate, and degradation efficiency were put forward.  
The test results of the performance of exhaust gas degradation of nano-TiO\({}_{2}\) photocatalytic coating showed that, when nano-TiO\({}_{2}\) content was changed in the range from 0-8% and the spraying amount was changed in range from 0-333.3 g/m\({}^{2}\), the performance of the photocatalytic coating on the degrading exhaust gas was significantly improved with the increase of nano-TiO\({}_{2}\) content and photocatalytic coating spraying amount. When over those dosages, the efficacy of photocatalytic coating was not significantly improved regarding exhaust gas degradation.  
In practical projects, the amount of spraying should be controlled strictly because the nano-TiO\({}_{2}\) photocatalytic coating can influence the skid resistance of the asphalt pavement to a certain extent. It is recommended that the spraying amount of nano-TiO\({}_{2}\) photocatalytic coating should not exceed 550 g/m\({}^{2}\). The nano-TiO\({}_{2}\) photocatalytic coating had little effect on the water permeability performance of asphalt pavement. As such, excessive dosage cannot effectively degrade exhaust gas but thickens the oil membrane, which leads to an inadequate skid resistance of the pavement, which is unsafe and uneconomical. Therefore, 8% nano-TiO\({}_{2}\) in coating and a 400 g/m\({}^{2}\) spraying amount (considering the loss of construction, etc.) were finally chosen to be the optimum parameters based on the photocatalytic properties as well as economic reasons.  
[PERSON] designed the experiment and wrote the manuscript; [PERSON] and [PERSON] conducted experiments, analyzed experimental data and wrote the manuscript; [PERSON] and [PERSON] helped conducting tests. This research was funded by \"the National Natural Science Foundation of China, grant number 51478052\", and \"Guangdong Transportation Department, grant number 2013-01-002\". The authors declare no conflict of interest.