## 3 Materials and Methods
### Method
#### 3.3.1 The Environmental Impact Calculations  
The production phase of FFBS mainly includes four main processes: raw material production, processing, parts assembly, and transportation. FFBS is a traffic tool for single people. Hence, the number of passenger-kilometers traveled equals the number of vehicle-kilometers traveled [29,44]. The emission coefficient \(EF_{p}\) of FFBS at the production stage can be calculated as:  
\[EF_{p}=\frac{PEC}{D\cdot F\cdot T}=\frac{\sum_{k=1}^{K}\text{C}_{k}\cdot\theta_{ k}}{D\cdot F\cdot T} \tag{1}\]  
where:  
\(PEC\) is the GHG emission generated by FFBS;  
\(D\) is the average trip distance (km) of a FFBS trip;  
\(F\) is the average annual turnover number of FFBS;  
\(T\) is the service life of FFBS (years);  
\(\text{C}_{k}\) is the kth energy consumption required by FFBS; and  
\(\theta_{k}\) is the GHG emission factor of the kth energy. Taking Nanjing as an example, the main manufacturing plant of FFBS is located in Danyang City. The average distance of trucks that can load 100 FFBS from Danyang to Nanjing is 94 km, so the average delivery distance of each bike is 0.94 km. The GHG emissions in the delivery process for the production phase can be calculated as:  
\[GHG_{t}=\frac{D_{t}\text{C}_{t}\theta_{t}}{A} \tag{2}\]  
where:  
\(GHG_{t}\) is the GHG emission when the unit FFBS is transported to the destination;  
\(D_{t}\) is the distance between the production place and the destination;  
\(\text{C}_{t}\) is the energy consumption of the unit delivery truck used;  
\(\theta_{t}\) is the GHG emission factor of the energy; and  
\(A\) is the load FFBS number of per transportation truck available. The GHG emission reduction effect of FFBS is the difference of GHG emissions between using FFBS and other transport modes if FFBS was not available in the same trip. In the use phase, public transport and motor vehicles generate GHG emissions due to energy consumption. Therefore, for other transportation modes, such as bus, metro, taxi,  
Figure 1: System boundary of the free-floating bike sharing system. and private car, the emission factor (\(EF_{u_{i}}\)) of the mode i per km in the use phase is obtained by considering the GHG emissions related to energy consumption [29]. The GHG emission per km of transport mode \(i\) in the use phase \(EF_{u_{i}}\) is formulated as:  
\[EF_{u_{i}}=C_{i}\cdot\theta_{i} \tag{3}\]  
where \(C_{i}\) is the energy consumption per km of transport mode i. The GHG emissions of other transport modes in the use phase is \(GHG_{u_{i}}=EF_{u_{i}}\cdot Distance_{u_{i}}\). \(Distance_{u_{i}}\) is the trip distance of the alternative transport mode i for the associated FFBS trip. \(Distance_{u_{i}}=v_{i}\cdot t_{i}\). \(v_{i}\) is the average operating speed of the ith transport mode in Nanjing. The average operating speeds of taxi, private car, bus, and metro are 25.60, 26.00, 18.00, and 45.40 km/h, respectively [45; 46]. \(t_{i}\) is the time spent by using alternative transportation mode i to complete the trip, which is obtained by question Q5 or question Q7. According to the \"Didi Platform Green Transport White Paper 2020\" released by Didi Chuxing, which is the world's leading mobility technology platform, Figure 2 shows the GHG emission factors of different transport modes at the use phase [47]. The calculation is temporarily replaced by the electric bike data when the alternative transportation mode chosen by the respondents is other. The operation phase of FFBS mainly includes two main processes: rebalancing and maintenance [40]. The rebalancing of FFBS mainly uses three types of vehicles, namely, small trucks, three-wheel electric vehicles, and Iveco vehicles. According to the data collected from an FFBS enterprise in Nanjing, the condition of dispatching vehicles in Nanjing is shown in Table 1, and the rebalancing proportion from this enterprise is 10%. The enterprise occupies a large market share in Nanjing and is representative. Therefore, the number of FFBS rebalanced daily in Nanjing is 10% of the total amount of FFBS.