Quantifying Carbon Emissions Generated by Monorail Transits: A Life Cycle Assessment Approach

The use of rail transits results in the generation of a large amount of carbon emissions. Throughout the life cycle of a rail transit system, huge amounts of carbon are emitted, which contributes to the threat posed by carbon emission on the city ecosystem. Despite the many methods previously proposed to quantify carbon emissions from rail transit systems, a method that can be applied to measure carbon emissions of monorail systems is yet to be developed. We have used the life cycle assessment (LCA) method to propose a method that can be used to quantify carbon emissions from monorail transits. The life cycle of a monorail transit system was divided into four stages (production, construction, use, and end-of-life). A monorail transit line segment in Chongqing, China, was selected for a case study. The results show that the “use” stage of the monorail transit line system significantly increases (93.2%) carbon emissions, while the “end-of-life” stage does not contribute significantly to the total carbon emitted. The processes of generation of steal, concrete, and cement are the three leading processes that contribute to the emission of carbon dioxide. The percentages of carbon emitted during these processes are 32%, 29.6%, and 13.3%, respectively. Prestressed concrete activity accounts for the largest proportion (91.1%) of the total carbon emissions. The results presented herein can potentially help in realizing sustainable development and developing green transportation.


Introduction
Global climate changes have aroused great concern worldwide as they have seriously affected social sustainability [1]. One important way to address the problem of climate change is to reduce carbon emissions [2]. e transportation sector facilitates human travel and helps in economic development. However, a large amount of energy is consumed during transportation, resulting in substantial carbon emissions. It has been reported that the transportation sector accounts for 14% of total carbon emissions worldwide [3]. Moreover, carbon emissions from the transportation sector have increased by 29% from 1990 to 2009 [4].
e monorail system uses a single rail instead of two rails as used in traditional rail systems during operation [5]. e monorail systems are low-noise systems that are safe to use. ey are characterized by good climbing ability and small turning radii [6,7]. At present, monorails are in operation in many countries worldwide, such as Japan, Germany, and the United States of America. Egypt, ailand, Brazil, China, and a few other countries are actively building their monorail systems. is provides a glimpse into the great potential for further advances of monorail transits in the future. Huge quantities of raw materials (such as cement, concrete, and steel) are required for the efficient development of infrastructure required to build the monorail systems. e production of these raw materials is often accompanied by an enormous amount of carbon emissions. As large-scale construction of monorail systems will be realized in the near future, it is necessary to quantify carbon emissions to realize sustainable development.
Life cycle assessment (LCA) is a method of assessing the impacts exerted by products on the environment during their lifetime [8]. In general, process-based LCA consists of four steps: (1) goal and scope definition, (2) inventory analysis, (3) environmental impact assessment, and (4) interpretation [8]. e LCA method has been widely used to evaluate the environmental performance of rail transits in recent years (Table 1). Table 1 presents the data and research contexts of 25 selected works in which LCA was applied to research into transport systems. Among these 25 selected studies, 12 articles focused on HSR systems, 9 on metro or subways, and 6 on railways. Results obtained by studying light rail transits (LRTs) were reported in two papers. To the best of our knowledge, there are no reports on the carbon emissions from monorail transit systems. It has also been observed that the life cycle has been described and categorized differently by different research groups. Researchers chose the length of the life cycle based on the characteristics of the system. e results obtained in different cases were different. Researchers analyzed databases from their own country or nearby countries to match their local situation. For example, CLCD is generally used as the database for Chinese research. e object of research considered in this paper is a monorail transit in Chongqing, China. Hence, CLCD was selected as the database. Table 1 demonstrates that LCA can provide a systematic framework to evaluate the life cycle environmental impacts of various rail transit systems. However, there are some research gaps: (1) To date, researchers have widely focused on metro and HSR. To the best of our knowledge, carbon emission has not been quantified from the perspective of the whole life cycle for the case of monorail transit. (2) Researchers have widely studied the four stages of rail transit systems (i.e., production, construction, use, and end-of-life), but a detailed study on the carbon emissions caused by materials production and construction works in the maintenance stage of the rail transit systems has not been conducted.
e aim of the present work is to quantify carbon emissions attributable to monorail transit systems using an LCA approach, with a monorail transit line in Chongqing, China, as a case study. e research work here is organized in four sections. Section 1 provides a brief introduction to the Chongqing monorail line 2 in China. Section 2 briefly introduces the methodologies of the LCA calculation. In Section 3, the case study for calculating the carbon emissions during the four stages of a monorail transit line in Chongqing is presented; the calculation results are interpreted; the observations are discussed; the uncertainty analysis and sensitivity analysis are conducted; recommendations are given. In section 4, the conclusions are derived and a summary of the key findings are presented. e results presented are expected to serve as a source of information and data that can be used to conduct LCAs in the future.

Materials and Methods
is study is based on a process-based LCA method. e different steps involved in the execution of the LCA method are defined in ISO 14040 [8]: goal and scope definition, life cycle inventory analysis, environment impact analysis, and interpretation. e results obtained following the LCA method reveal the potential environmental impacts of the monorail system (from cradle to grave). e methodology is shown in Figure 1.

Goal and Scope: Definition.
e goal of this study is to assess the carbon emissions of a monorail transit line segment during the whole life cycle of its operation. e system boundary of the monorail transit was defined based on the guidelines presented in the EN 15978 standard [34]. Specifically, the process is divided into the production (associated with the extraction and upstream production, transport to a factory, and manufacturing processes), construction (associated with the processes of transportation to the working site and installation), use (associated with the use, maintenance, repair, and replacement of the system and the usage of operational energy), and end-of-life (associated with the processes of demolition, material transport, waste processing, and disposal) stages. e basic assumptions made have been presented: (1) e life cycle length of the system is 50 years. e lifespan is the same as the lifespan assumed in previous works on a Chinese subway [22] and a light rail case [33]. (2) Vehicles were absent from the system, and this could be attributed to the lack of data. Studies have been previously conducted under similar situations, and the results have been reported [17,[19][20][21][22][26][27][28][29][30][31]. (3) All the wastes are landfilled, and carbon emissions caused by waste transportation during the process of waste processing and waste disposal are primarily considered. (4) Operational energy consumption remains the same every year (this assumption helps simplify the calculations). (5) All the materials are locally available, and the transportation distance is 50 km.
e detailed system boundary is illustrated in Figure 2.

Inventory
Analysis. e second step in LCA involves the process of inventory analysis. In this step, information on carbon emission factors associated with different materials and resource (material and energy) consumption at all stages is collected.

Carbon Emission Factors.
We chose data presented in the Chinese Core Life Cycle Database (the most authoritative database for LCA in China) [35] to conduct our research to effectively reflect the Chinese conditions. e baseline emission factors of China's regional power grid (from 2011 to 2017) in Chongqing were obtained based on the data from the Ministry of Ecology and Environment of the People's Republic of China [36].  [37] is studied to determine the machine-teams involved in the different working activities occurring in the construction Step 3 Environmental impact analysis (Carbon emission)

Material consumption
Step 2 Inventory analysis  [38] is considered to determine the machine-teams involved in the maintenance works associated with the use stage. e data presented in the Expense Standard of Machine-Team of Highway Project (JTGT_3833-2018) [39] are analyzed to obtain the unit energy consumption of different machineteams. e China Urban Rail Transit Almanac 2019 [40] is referred to, to determine the amount of electricity consumed by the monorail transit lines and stations. e carbon emissions of the monorail transit line are obtained by multiplying the electricity carbon emission factors with the total quantity of electricity.

Environmental Impact Assessment.
e third step involves environmental impact assessment or life cycle impact assessment [41]. Results obtained using the various analytical methods reveal that carbon emission (for monorail transits) is realized in four stages. Total carbon emission is calculated using the following equation: where C tot , C PD , C CS , C US , and C EL correspond to the amounts of carbon emitted during the life cycle, the production stage, the construction stage, the use stage, and the end-of-life stage, respectively. e amount of carbon emissions produced in the production stage is equal to the carbon emission factors of various materials multiplied by the corresponding material quantities.
is can be represented by the following equation: where CF MF,i represents the carbon emission factors of the i − th material and Q MF,i represents the consumed quantities of the i − th material. Carbon emissions produced during the construction stage are primarily generated during the transportation of the construction materials and consumption of energy (by different construction machines). e amount of carbon emissions generated in the construction stage can be represented by the following equation: where CF mt,i represents the carbon emission factor for the i − th construction material transportation mode, Q mt,i represents the quantity of the i − th transported construction materials, and D i represents the distance between the production factory and the construction site. Here, it is assumed that road freight is the only transportation mode adopted. CF cf,i represents the carbon emission factor of the i − th consumed fuel mode in the construction stage, Q cf,i represents the quantity of the i − th fuel consumed in the construction stage, CF e represents the regional carbon emission factor for electricity, and Q ce represents the quantity of electricity utilized by the construction machines during the construction stage. e carbon emissions during the use stage are primarily generated during maintenance works and the consumption of electricity by the vehicles and stations. e amounts of the carbon emissions produced during the use stage can be determined using the following equation: where C US represents the amount of carbon emissions generated from the monorail line in the use stage, C op represents the carbon emissions produced during the operational phase, and C mtp represents the carbon emissions produced during the maintenance phase.
In (5) and (6), C sy represents the annual carbon emissions generated from the monorail line in the course of its operation, Y is the service life in the operational stage, and Computational Intelligence and Neuroscience Q oe represents the amount of electricity consumed during the operational stage.
In the maintenance phase, the origin of carbon emissions can be attributed to the processes of material production and fuel consumption (consumed by construction machines). e amount of carbon emissions produced in the maintenance stage can be calculated using the following equation: where CF mt,i represents the carbon emission factors of the i − th material, Q mt,i represents the consumed quantities of the i − th material, CF mfc,i represents the carbon emission factor for the i − th consumed fuel (consumed by machines during the maintenance process), Q mfc,i indicates the quantity of the i − th fuel consumed (realized during the maintenance process), and Q me represents the quantity of electricity utilized by the construction machines during the maintenance process. e carbon emissions generated during the end-of-life stage are produced during the consumption of energy by the construction machines during the processes of demolition and waste transportation. e amount of carbon emission produced during the end-of-life stage can be calculated using the following equation: where CF f d,i represents the carbon emission factor corresponding to the consumed fuel mode (disposal process), Q fc,i represents the quantity of the i − th fuel consumed during the disposal process, Q de refers to the quantity of electricity utilized by the construction machines during the disposal process, CF dm t,i represents the carbon emission factor for the i − th disposal transportation mode, Q dm t,i represents the quantity of the i − th transported materials, and D i indicates the distance between the construction site and the disposal area.

Interpretation.
In this step, the conclusions are arrived at based on the results obtained during inventory analysis and impact assessment. e key stages, working activities, and materials (used throughout the life cycle) that affect the extent of carbon emission produced during the functioning of a monorail transit were identified. e uncertainty analysis and sensitivity analysis methods were used, and the data were analyzed using Oracle Crystal Ball software to determine the possible range of life cycle carbon emissions of a monorail transit. e parameters that affect the results are also identified.  Figure 3.

Carbon Emissions in the Production Stage.
According to equation (2), the carbon emissions produced during the production of the materials primarily used for production are shown in Table 2.
e total amount of carbon emissions produced during the production of main materials was 3,365.43 t.

Carbon Emissions Produced during the Construction
Stage.
e carbon emissions produced during the construction stage are primarily generated during the processes of material transportation and construction. Equation (3) and the assumptions made were taken into consideration to calculate the amount of carbon emissions produced during the process of transportation of the materials (Table 3).
e total amount of carbon emissions produced during the transportation of materials was 113.49t. e amount of carbon emissions produced during the process of on-site construction was calculated (equation (3), Table 4). e carbon emission factors corresponding to electricity and diesel are 1.294 t CO 2 /MWh and 3.664 kg/kg, respectively. e amount of carbon emissions produced in the area of the chosen line segment was calculated to be 5,653.58 t. When the amount of carbon emissions produced during the transportation of the main material is taken into account, the total amount of carbon emissions generated during the construction stage is calculated to be 5,767.18 t.

Carbon Emissions Produced during the Maintenance Phase of the Use Stage.
e assumptions were taken into account, and the carbon emissions produced during the maintenance phase were calculated (equation (7), Table 5). e guidelines presented in the Budget Norm of Maintenance and Strengthening of Highway Bridges (YNG/T B02-2011) [38] were followed, and it was assumed that each time, one concrete treatment, 100 m-long expansion joint, 10 dm 3 -volume bearings, 1000 m-long cracked concrete, and 400 m 3 -area concrete beam section were maintained. According to the Budget Norm of Maintenance and Strengthening of Highway Bridges (YNG/T B02-2011) [38], the maintenance works are carried out once every ten years.
e total amount of carbon emissions produced during material, electricity, and fuel consumption during the construction of the machine was calculated to be 16.87 t/ 10 years. If the life cycle is considered to be 50 years long, the total carbon emission is calculated to be 60.51 t.     e annual power consumption amounts and the annual passenger turnover were determined by analyzing the statistical data presented in the China Urban Rail Transit Yearbook 2019 [40]. e amount of carbon emission produced was calculated accordingly. e amount of carbon emissions produced by monorail line 2 (during its operation in 2018) was calculated to be 34,029.22 t. e total amount of carbon emissions produced during the 50-year-long life cycle was calculated to be 1,701,461 t. e amount of carbon emissions produced per unit length was 54255.77 t/km. e amount of carbon emissions produced by the chosen part of line 2 (length: 2.4 km) was 130,213.85 t. Equation (4) was used to calculate the total amount of carbon emissions generated during the whole use stage. e amount was calculated to be 130,274.36 t.

Carbon Emissions Produced during the End-of-Life
Stage. Equation (8) was used, and the assumptions made were considered to calculate the amount of the carbon emissions produced during the process of structure demolition and waste disposal during the end-of-life stage. e engine-driven air compressor within 9 m 3 /min was primarily used during the demolition process. e total amount of energy consumed during the demolition process was calculated to be 11,916.83 kWh. e total amount of carbon emissions generated during the demolition process was calculated to be 356.35 t.
e total amount of carbon emissions generated during the process of waste transportation was calculated to be 113.49 t, and the total amount of carbon emissions produced during this stage was 469.84 t. Computational Intelligence and Neuroscience

Amount of Carbon Emissions Produced during Different
Stages. A comparison of the amounts of carbon emissions produced during different stages of the life cycle of the material is presented in Figure 4(a) and Figure 4(b). Analysis of the figures indicates that the maximum amount of the carbon emissions generated during the life cycle of the monorail transit is produced during the use stage (93.2%). e amount produced in this stage is significantly higher than the amount produced during the other stages. is can be primarily attributed to the fact that the amount of carbon emissions produced during the process of energy consumption during the entire 50-year-long operation period is considered during the study of the operational phase. A huge amount of carbon emissions is produced during this phase. Various maintenance works are carried out during the life cycle of the material. Each stage requires a certain amount of manpower and materials for completion. Different types of machinery are also required for the effective execution of the process. Maintenance work is carried out once every 10 years. us, a total of five maintenance cycles are carried out during the whole life cycle. is results in the generation of a large amount of carbon emissions in the use stage. A considerable amount of carbon emissions is also produced during the construction stage (4.1%). is can be attributed to the fact that a huge quantity of energy is consumed by different working machines during this process. e amount of carbon emissions generated during the production stage (2.4%) is slightly lower than the amount generated during the construction stage. It has also been observed that the amount of carbon emissions generated from the end-of-life stage is only 0.3% of the total amount generated, an insignificant amount as compared to emissions produced in other stages.

Carbon Emissions from Different Materials.
Figures 5(a) and 5(b) provide a comparison of the amounts of carbon emissions produced during the use of different materials. As the figures show, there are significant differences in the amount of carbon emissions generated by different types of materials. e top 11 construction materials that contribute the most toward the production of carbon emissions are listed in Figure 5(a). e cumulative amount of the carbon emissions generated by these 11 materials is 3365.15 t, accounting for almost all of the total carbon emissions generated during the production stage. Specifically, the use of steel (32%), concrete (29.6%), cement (13.3%), and mortar (13%) results in the production of the maximum amount of carbon emissions during the production stage.
is can be attributed to the fact that a monorail line is a reinforced concrete structure that requires the use of a large amount of steel (1,376.36 t), concrete (3,549 e total amount of carbon emissions produced is large as the consumption and carbon emission factors of these materials are significantly high. erefore, the key to improving the environmental performance of monorail transit lies in decreasing the amount of carbon emissions generated by these materials. It has been previously reported [42] that an efficient water reducing agent can be used to replace cement during the process of concrete production. is can help in reducing the amount of carbon emissions produced during the material production stage. e concrete strength can be improved simultaneously to reduce the amount of cement and concrete used. Furthermore, designers could consider using more amounts of renewable construction materials (such as renewable concrete) in the design stage [43]. Mortar, bricks, building blocks, gravel, sand, and other materials also contribute to the generation of carbon emissions in the production stage. e use of these materials results in the generation of insignificant amounts of carbon emissions.

Carbon Emissions: Analysis of Working Activities Occurring in the Construction Stage.
A comparison of the amounts of carbon emissions generated at different stages of the construction stage is presented in Figure 6. Fifteen working activities are considered.
e results reveal that prestressed concrete activity accounts for the largest proportion (91.1%) of the total carbon emissions produced in this stage. e amount of carbon emission produced during prestressed concrete activity is significantly higher than the amount produced during other construction activities. is can be primarily attributed to the fact that a huge quantity of electricity is consumed during the operation of machines single-cylinder slow-motion winch within 30 KN (2,681,021.27 kWh), single-cylinder slow-motion winch within 50 KN (934,667.96 kWh), Φ100 mm electric multistage water pump (≤120 m; 204,479.87 kWh), and concrete delivery pump (60 m 3 /h; 98,556.12 kWh). In addition, a considerable amount of carbon emissions is generated during the construction of the beam concrete (3.6%). e amount of carbon emissions generated during the processes of beam reinforcement (1.5%) and rebar processing (1.5%) is slightly lower than the amount produced during the process of beam concrete construction. e amount of carbon emissions attributable to other construction activities accounts for less than 1% of the total carbon emissions originating from all construction activities. is indicates that these activities do not significantly affect the environment.

Uncertainty Analysis: Construction Stage and Production
Stage.
e uncertainty of carbon emissions corresponding to material and fuel consumption can be attributed to construction errors. Uncertainty analysis was conducted using the Oracle Crystal Ball (Figure 7) to evaluate the possible range of carbon emissions for a monorail segment during its life cycle. Standard normal distributions were assumed for the input parameters, and the number of testing times was set to 100,000. e probability distributions corresponding to the target variables were normal distributions. e variation range calculated for carbon emissions was 100,000-180,000 t.

Life Cycle: Sensitivity Analysis.
e Oracle Crystal Ball was used to determine the sensitivity of carbon emissions during the life cycle of the monorail transit line. e results are shown in Table 6.
Analysis of the data presented in Table 6     consumed annually during the operation of the system significantly influence the amount of carbon emission produced by the monorail system during its lifetime. In addition, the quantities of single-cylinder slow-motion winch within 30 KN, single-cylinder slow-motion winch within 50 KN, C40 concrete, cement mortar (1 : 3), and brick 200 × 95 × 53 also exert a certain influence on the results. It was observed that the degrees of influence were almost the same.

Discussion of the Approach.
is calculation framework can be used to calculate the amount of carbon emission produced by a monorail line during its lifetime, but it can also be used to calculate the amount of carbon emissions produced by ordinary railway, light rail, subway, and other rail transportation systems. e calculation results can help design the monorail transit system, and the results can help reduce the amount of carbon emission produced at the initial stages of the process.
As there are few monorail lines in operation in China, it is difficult to obtain field data. It is challenging to verify the universality of the results calculated from the only data available. More data can be obtained in the future as the construction of monorail traffic has been planned. Our model will be further optimized to make it universal.

Recommendations for Future
Research. More attention should be paid to the following research directions: (1) Verification and application of the approach used by us should be realized by analyzing fundamental data. erefore, attention should be paid to obtaining more amounts of data to improve the developed approach.
(2) Some scenarios can be designed to simulate the carbon emission reduction effects based on the calculation results presented. is can potentially help in proposing helpful suggestions that can be exploited by designers and politicians for the sustainable development of the city.

Conclusions
We calculated the amount of carbon emissions produced during the operation of a monorail transit line (during its lifetime) using the LCA method. A segment of the Chongqing monorail line 2 was selected for the case study for validating the developed model. e following conclusions were drawn: (1) e life cycle of the monorail transit line can be theoretically divided into four stages: the production, construction, use, and end-of-life stages. e use stage contributes >90% of the carbon emissions produced during the lifetime of the monorail transit line. A significant amount of carbon emissions is produced during the construction and production stages. e amount of emissions produced during the end-of-life stage is less than the amount produced in the other three stages.
(2) Steel, concrete, and cement are the most important sources of carbon emissions. (3) Prestressed concrete activity accounts for the maximum proportion (91.1%) of the total carbon emissions. Significant amounts of carbon emissions are generated during the process of beam concrete construction (3.6%). e amounts of carbon emissions generated during the process of beam reinforcement (1.5%) and rebar processing (1.5%) are slightly lower than the amounts generated during the process of beam concrete construction.
In summary, the results presented herein can help gain a better understanding of the effects of the monorail transit industry on the ecosystem. e results can potentially help develop ideas that can be used by designers working in the rail transit industry to meet low carbon emission goals.

Abbreviations
ARMA: Autoregressive moving average CE: Carbon emissions CEF: Carbon emission factor GHG: Greenhouse gas HSR: High-speed rail IO: Input-output LCA: Life cycle assessment LRT: Light rail transit PKT: Passenger kilometer traveled VKT: Vehicle kilometer traveled PMT: Passenger mile traveled RTS: Rapid transit system SKM: Per seat per kilometer traveled VMT: Vehicle mile traveled.

Data Availability
e data used to support the findings of this study are available from the corresponding authors upon request.

Conflicts of Interest
e authors declare no conflicts of interest.