## 4 Results and Discussion
### Carbon Cost of the Case Bridge
#### 4.3.2 Carbon Trading System  
Material production and transportation stage  
The annual average carbon emissions of enterprises are obtained by querying the annual carbon emissions of the building materials used by steel and concrete enterprises; then, using the Beijing Environment Exchange's quota verification method based on the total historical emissions, the enterprise's total annual carbon emissions are obtained. Since there are many companies involved in this stage, it is not realistic to perform calculations for each company. Therefore, all building material production companies are combined into one and calculated as a single typical building material factory. The average annual carbon emissions are 774,000 tons. According to Equations (8) and (9), the carbon allowance of typical building materials factories at this stage based on data from 2020 is calculated as 712,100 tons. According to Equation (6), based on the average carbon trading price of 89.1 CNY per ton announced by the Beijing Environment Exchange on 3 June 2020, the carbon cost at this stage is 60,103.16 CNY. Construction stage  
According to Equations (8) and (9) from the Beijing Environment Exchange carbon allowance calculation method, the carbon allowance of construction enterprises per year is 30,936.18 tons. According to Equation (7), based on the average carbon trading price of 89.1 CNY per ton announced by the Beijing Environment Exchange on 3 June 2020, the carbon cost is 1484.09 CNY. Figure 8 shows the carbon cost under the carbon trading system. Figure 7: The carbon cost of the bridge construction stage under the carbon taxing system. analyzed. (1) The carbon emission calculation results show that the carbon emissions of the material production stage are the largest during the construction period of the bridge, accounting for 92.1% of the entire construction stage, and the carbon emissions generated by the production of steel bars account for 62% of the entire material production stage. The carbon emissions of the steel bars and concrete of the bored piles are the highest in the material production stage and the transportation stage, so it is necessary for the material manufacturing industry to take carbon reduction measures to control carbon emissions. (2) The carbon cost calculation results show that the carbon cost under the carbon taxing system is significantly higher than that under the carbon trading system, and the carbon cost in the material production stage accounts for a very high proportion of the carbon cost in the construction period, reaching approximately 98%. Therefore, monetizing carbon emissions is conducive to clarifying the environmental impact of bridge construction. However, this paper also has various limitations. This paper only measures the carbon emissions and carbon costs during the bridge construction period, and in the future, the carbon emissions and carbon costs of the whole life cycle of the bridge can be studied. In addition, there are many types of building components and materials involved in a bridge, and it is difficult to quantify all materials and components in one study. Therefore, only the carbon emissions of key components and major materials are considered in this paper, and all components and materials can be quantitatively analyzed in the future. Conceptualization, [PERSON] ([PERSON]) and [PERSON]; methodology, [PERSON] ([PERSON]); software, [PERSON]; validation, C.W. ; formal analysis, [PERSON]; investigation, [PERSON] and K.G. ; resources, [PERSON] ([PERSON]); data curation, K.G. ; writing--original draft preparation, [PERSON] ([PERSON]), [PERSON] and C.W. ; writing--review and editing, [PERSON] and W.L. ; visualization, [PERSON] ([PERSON]); supervision, [PERSON]; project administration, [PERSON]; funding acquisition, [PERSON] ([PERSON]). All authors have read and agreed to the published version of the manuscript.