## 2 Literature Review  
Keywords were used to find the related literature since 2010 in Web of Science. The search string was AK = ((\"bike-share\" OR \"free-floating bike-sharing\" OR \"public bike\" OR \"public bicycle\" OR \"dock-less bike-sharing\") OR (\"life cycle assessment\" OR \"emission reduction\" OR \"GHG emission\" OR \"carbon emission\" OR \"modal shift\")). Bike sharing provides convenient and environmentally friendly trip modes for people because it does not generate air and noise pollution [34]. Although the emergence of bike sharing instead of other transport modes leads to emission reduction, the production, operation, and disposal of bikes will generate GHG emissions from the perspective of the life cycle. Existing studies have mainly focused on the use phase. The GHG emission reduction of bike sharing is mainly achieved by replacing other transport modes in the trip, such as motor vehicles and public transport. [PERSON] [31] assessed a bike-sharing program in Portland and found that the bike-sharing program could improve air quality and increase sports activity rates for bike-sharing users. [PERSON] et al. [35] assessed the impact of the Bike to Work Day on the bike mode shift by a questionnaire. The study found that 4% of participants started regular bike commuting through the event, with estimated 3.1 million regional VMT reductions and 1200 tons of GHG emission reductions per year. [PERSON] and [PERSON] [30] evaluated the environmental benefits of the FFBS in Shanghai from the perspective of space and time. When the trip distance of the FFBS was greater than 1 km, FFBS was considered to replace the car for the trip. Through a questionnaire survey, [PERSON] et al. [36] divided the modal shift threshold between different transport modes to determine the different transport modes replaced by public bikes at different distances in Hohhot, and found public bikes can reduce carbon emissions by 4381.28 tons. [PERSON] et al. [37] proposed a bike share emission reduction estimation model based on considering factors, such as the trip distance, trip purpose, trip start time, public transport accessibility, and historical distribution of trip mode selection, to estimate the alternative traffic mode of a shared bicycle trip. However, these studies only considered the GHG emission reduction of FFBS in the use phase, and did not perform a comprehensive analysis from the perspective of the life cycle. Recently, some scholars analyzed the environmental benefits of the bike sharing system (BSS) by using life cycle assessment. [PERSON] et al. [19] conducted life cycle assessment and comparison of the station-based BSS and free-floating BSS in the United States. Through scenario analysis, they quantified the impact of different modal shift structures caused by BSS on greenhouse gas emissions and the overall standardization environment. [PERSON] et al. [38] used life cycle assessment to compare private bicycle, smart dock BSS, and smart bike BSS, and evaluated the impact of the use of electronic equipment on the environmental benefits of BSS. [PERSON] et al. [39] used a questionnaire survey to understand the change of the trip mode after the introduction of shared bikes for directly use. On this basis, the change of the trip behavior and the life cycle environmental impact of shared bikes were modeled. Sensitivity analysis showed that aging, rising rent, and an increasing number of shared bikes had a negative impact on the environmental benefits of shared bikes. From the perspective of the life cycle, [PERSON] [29] quantitatively analyzed the carbon footprint of different transport modes, obtained the data of the modal shift by a questionnaire, and analyzed the environmental benefit of FFBS through scenario analysis. [PERSON] et al. [40] calculated the emission reduction threshold of FBBS by assuming different disposal methods. According to the total amount of FFBS in Beijing, it was found that it takes at least 686 days for an FBBS to reach the net emission equilibrium point.