GHG accounting for pubilc transport in Xiamen city, China

Abstract

How to account for GHG emissions for public transport is now a key issue for low-carbon city development. This study provides a method to evaluate carbon footprinting for public transport systems in Xiamen city, China across the life cycle. This method, which was based on the life cycle assessment approach including three components – infrastructure, fuels and vehicles – was presented to account the GHG emissions of public transport. The GHG emissions of the two kinds of public transport systems (bus rapid transit [BRT] and normal bus transit [NRT]) in Xiamen City were compared. Results showed that the average carbon emissions of the BRT system was 638.44 gCO₂e per person, and that of the NBT system was 2,088.38 gCO₂e. If we only took the direct carbon emissions of fuel consumption in the vehicle operation into consideration, the average carbon emissions were, respectively, approximately 149.08 gCO₂e per person and 260.84 gCO₂e per person by BRT and NBT system. The results indicated that the effects of energy saving from the BRT system are better than NBT system, which is related to the features of the BRT system such as large volume, energy-saving and environment-friendly vehicle type and exclusive right-of way.

Figure 1.  The system boundary of the GHG accounting for urban public transport system.

Adapted with permission from [29].

Figure 2.  Carbon footprints flowchart of the normal bus transit system of Xiamen city, China in 2009 (tCO₂e per year).

Figure 3.  Carbon footprints of the normal bus transit system of Xiamen city, China in 2009 (tCO₂e per year).

ADR: Assembly, disposal and recycling.

Figure 4.  Carbon footprints for the infrastructure of the bus rapid transit system of Xiamen city, China in 2009 (tCO₂e per year).

Reproduced with permission from [29].

Figure 5.  GHG emissions of the vehicles and fuels of the bus rapid transit system of Xiamen city, China in 2009 (tCO₂e per year).

Reproduced with permission from [29].

Figure 6.  Carbon footprints of the bus rapid transit system of Xiamen city, China, in 2009 (tCO₂e per year).

ADR: Assembly, disposal and recycling.

Carbon emission reduction and ‘low carbon economy’ development have become the mainstream of the international community for addressing climate change. Transport has played an especially important role in responding to the challenge of averting dangerous climate change [1]. Transportation is one of the most important sources of energy consumption and GHG emissions and its proportion in carbon emission is rapidly increasing year by year. In 2050, as much as 30–50% of the total CO₂ emissions are projected to come from the transport sector, compared with today’s 20–25% [2]. A series of studies have accounted for GHG emissions on road building, vehicles production, fuel consumption and other aspects. Huang et al. built a model for pavement construction and maintenance based on life cycle assessment method, and calculated the energy consumption and GHG emissions of pavement rehabilitation the A34 road in the UK [3]. Other studies have estimated the historical trends of energy demand and the associated GHG emissions in the road transport sector and future trends under different policy scenarios [4–7]. These studies only considered the direct emissions (vehicle operation) and ignored the upstream emissions (road construction and vehicle manufacture) and the downstream emissions (decommissioning and recycling). However, the upstream and downstream emissions were proven to have significant impact on the whole GHG emissions [8]. It is therefore of critical importance to fully account for the GHG emissions of the transportation and to assess the possible and effective reduction measures. To date, new research interests are to use the life cycle assessment (LCA) to analyze and evaluate the energy consumption and environmental emissions of transportation. These studies assessed energy use, GHG emissions and criteria pollutant emissions associated with the full life cycle of various transportation activities. There are two leading transportation life cycle models that deserve to be mentioned: the life cycle emissions model (LEM) [9] and the GHGs, regulated emissions, and energy use in transportation (GREET) model [10].

‘Carbon footprint’ has become a widely used term and model in the public argument about responsibility and abatement action against the threat of global climate change. It had a great increase in public appearance over the last few months and years, and is now a buzzword widely used across the media, the government and in the business world [11]. The carbon footprint model has been applied in many scales, such as product, household, industry, transport, construction, water supply and medical treatment [12–21]. However, GHG accounting studies focusing on public transport mainly concentrated on certain aspect of transport activities, such as construction of infrastructure as roads, vehicle production, vehicle fuels and a comprehensive analysis of the traffic pressure on the environmen. Yet, there is still a lack of GHG accounting on the entire transport system, including the full life cycle carbon footprint of road construction, use, destruction and recycling disposal as well as vehicle production, operation, scrap and recycling process [22]. Bus rapid transit (BRT) systems have been identified as an inexpensive and efficient public transportation option [23–25]. Several studies have evaluated the efficiency and environmental effect of the BRT system through cost–benefit analyses or analysis of different scenarios [25–28]. In addition, the previous article has concretely analyzed and assessed the life cycle emissions of the BRT system in Xiamen City [29].

In this study, an evaluation model of GHG accounting for urban public transport was proposed based on the theory of carbon footprint assessment. Then, a comparative analysis of two kinds of public transport systems in Xiamen City (BRT and normal bus transit [NBT]) was presented. The results showed that BRT had more advantages in carbon emission reduction in comparison with NBT.

Background of Xiamen public transport system

Xiamen is a coastal city in South eastern China, which looks out to the Taiwan Strait and borders Quanzhou to the north and Zhangzhou to the south. It has direct jurisdiction over 6 districts as Siming, Huli, Jimei, Haicang, Tong’an and Xiang’an with an area of approximately 1,573 km². Its registered population is approximately 1.77 million, and the resident population is approximately 2.52 million. Being one of China’s earliest special economic zones in the 1980s, Xiamen has experienced a rapid development. From 1981 to 2009, urban built-up area has been enlarged into 212 km² from the inner cities of 12 km² and the GDP has increased to 161.9 billion from 700 million. With the population growth and urban expansion, the total passenger traffic volume of Xiamen City has increased gradually. Buses serve as the main form of public transportation in the city, with taxis and ferries used to a lesser extent. In 2008, there was a total of approximately 816 million passengers per year from Xiamen City public passenger transport, of which bus played a dominant role; the passenger capacity was 588 million people per year, accounting for 72.05% of the total passengers, and the passenger capacity of taxis was approximately 228.13 million, 27% of the total (Table 1).

By the end of 2008, a more comprehensive public transportation network was formed. There were 3,011 buses in use and a total of 218 bus lines including 3 BRT lines, 18 BRT connecting lines and 197 NBT lines (containing 30 CMB routes and 42 peasants-passengers lines). With a total of 10 taxi companies and 4,209 vehicles in Xiamen, the service capacity and quality of urban public transportation has improved significantly.

The BRT system (Project I) in Xiamen City went into operation at the beginning of September 2008, which is one of principal arterial routes in Xiamen. The BRT system has a total length of 54 km with three trunk routes (BRT Line 1, 2 and 3) and 20 tie lines to match. In addition, 120 trunk route buses (with a capacity of approximately 95 passengers each), which travel on separate bus lanes and 100 feeder line buses (capacity of 53 passengers) are used in the BRT system, to satisfy a demand of 240,000 passenger trips per day [101].

Methods

▪ System boundary

The system boundary determines the scope for carbon footprint, for example. which life cycle stages should be included in the GHG accounting [30]. Clearly the definition of system boundary plays a significant role on the calculating the results of carbon footprint. In this study, the carbon footprint transport system can be measured by the carbon footprint of industrial products across the full life cycle, just because the public transport system can be viewed as a special industrial product. Generally speaking, public transport systems are composed of three components, namely infrastructure (road and bus station), fuels and vehicles [31]. The life cycle phases of the public transport system are illustrated in Figure 1. The boundary for public transport infrastructure includes the following processes: raw material production, transportation and construction, operation and maintenance and decommissioning and recycling [32,33]. The boundary for vehicle includes the following phases: raw material recovery and extraction, transportation and material processing, material production and fabrication, vehicle component production, vehicle assembly, operation, disposal and recycling [10]. A fuel cycle is a complicated process, including upstream emissions associated with drilling, exploration and production, crude oil transport, refining, fuel transport, storage and product retail, as well as downstream disposal or recycling of oil products [9]. Owing to the limited data, this study did not calculate the carbon footprint of transport phase for the finished products of public vehicles and fuels, which needs to be supplemented and improved in future studies.

▪ Data sources

The data regarding the infrastructure construction of the BRT and NBT system in Xiamen City were directly from Xiamen Municipal Administrative Construction Headquarters and Xiamen Municipal Administrative Construction Exploitation Parent Company. For the infrastructure operation and maintenance, including the electricity consumption of the bus stops and the road lamps, the data were from Xiamen Municipal Works and Gardens Administration Bureau. In addition, the parameters of vehicles in Xiamen BRT and NBT system were provided by Xiamen King Long Motor Group Company. The data regarding the vehicle components were from Xiamen Metal Recycle Company. The data regarding the annual urban transport were predominantly from the Annual Report on the Development of Transportation, Posts and Telecommunications of Xiamen City (in Chinese) during 2006–2008 and so on.

▪ Calculating methodology

Calculating methodology of infrastructure GHG emission

The GHG emissions of infrastructure mainly come from the five parts: infrastructure material production, construction, operation&maintenance, decommissioning and disposal & recycling. We analyzed the infrastructure GHG emissions from the following five phases, respectively.

Infrastructure material production

Infrastructure material production contains four processes, namely burdens from raw materials extraction (e.g., drilling for oil and mining for iron ore), transportation and processing, refinement of raw materials into engineered materials, and manufacturing (e.g., extrusion of steel or aluminum). Carbon emissions mainly include three GHG: CO₂, methane (CH₄) and nitrous oxide (N₂O) expressed as CO₂ equivalent (CO₂e); Equation 1. Based on the IPCC 1996, the global-warming potential for the GHGs is that CO₂ is 1, CH₄ is 21 and N₂O is 310 [34–36].

Equation 1

where: Im (tCO₂e) is the GHG emissions in infrastructure material production; n is the number of building materials and elements; qi (t or m³)is the amount of material i; wi(%) is a proportion for waste of material i produced during the erection; EFCO₂,i (g/t) is the CO₂ emission factor in the material i production phase; EFCH4,i (g/t) is the CH₄ emission factor in the material i production phase; and EFN₂O,i (g/t) is the N₂O emission factor in the material i production phase.

Construction

The GHG emissions of construction phase includes two parts: carbon emissions from the construction process and transportation of construction materials, which mainly covers shipping of materials from manufacturing site to construction sites as well as the transportation to landfills/recyclers [33].

Equation 2

where, TE (gCO₂e/km·t) is the transport emission; FC (MJ/km) is the fuel consumption, the amount of fuel added to the engine, using lower heating value, converted from l/km; EO (g/MJ) is the emissions from engine operation, g emissions per MJ work energy output from the shaft of the engine, converted from g/kwh, while EP (g/MJ) is the emissions from fuel production for transportation of construction materials.

The energy consumption of construction mainly includes electricity used for power tools and lighting as well as diesel fuel used by heavy equipment at the construction site. Activities include site preparation, structural and envelope installation, mechanical, electrical equipment installation, and interior finishing. Energy and environmental flows associated with the construction process could not be developed directly, since there was no record of equipment use or operational hours. Estimates for construction energy consumption in the literature range from 1.2 to 10% of embodied energy [37]. The higher values in the Cole studies (6.5–10.0% of material embodied energy) included transportation burdens for construction workers. Therefore, in this study 5% of total embodied energy is used to account for both structure and interior according to the literature [38].

Operation & maintenance

Operation phase activities consist of lighting (road lamps) and equipment operation including escalators, card readers and air-conditions. The emissions can be estimated by the electricity consumption of these equipments. Based on the emissions factors (1TJ coal equivalent releases 92.64 tCO₂, 10.00t CH₄ and 1.40 t N₂O ) from the IPCC (1996), we can obtain the carbon emissions; see Equation 3.

Equation 3

where: E0 (TJ) is annual energy burdens of electrical equipments; n is the number of types of electrical equipment in site; E_j is the number of equipment j; E_j (kwh) is the annual electricity consumption of equipment j.

During the maintenance phase, we mainly consider the emissions from the materials production and the road repairing. The materials used in maintenance include cement, sand gravel and asphalt. The energy for road repairing is concluded to be 5% of total embodied energy of the materials used in maintenance as the construction phase.

Decommissioning

This GHG emission of this phase mainly derives from the demolition phase. The decommissioning energy for this study is calculated using 90% of the total energy in construction stage [39].

Disposal & recycling

The waste construction material can be divided into recoverable and unrecoverable materials. As for unrecoverable materials, only the transportation of the materials from the build site to disposal site can release carbon emissions. The disposal of recoverable materials includes the transportation as the unrecoverable materials and the reduced emissions from the recycling phase; see Equation 4.

Equation 4

Where: Id (t) is the carbon emissions from the disposal of waste construction materials; n is the number of types of waste construction materials; Wi (t) is the total weight of waste construction material i; Ri (%) is the recycling ratio of waste construction material i; Di (km) is the average distance from the build site to recycling site of waste construction material i; di (km) is the transportation distance from the build site to the end-disposal site of waste construction material i; and Tc (tCO₂e/t·km) is the carbon emissions per unit construction material by different type of shipping.

Calculating methodology of vehicle & fuel carbon footprints (GREET model)

The carbon footprint of public transport vehicles is derived from two aspects: fuel and vehicles. The energy consumption, pollutant emissions and capital investment existed in all stages of the life cycle of the vehicle, so the vehicle operation phase is not the sole concern when we measure the carbon footprint of vehicle and fuel. Only by implementing research on all the stages of the full life cycle can we really compare the energy saving and emission reduction effects of different vehicles and fuel. In this part the GHGs regulated emissions, and energy use in transportation (GREET) model developed by the Argonne National Laboratory is used as a research simulation tool, which has been widely used in North America since 1995. In the study on energy use aspects, the GREET model includes calculation methods of total energy (energy in non-renewable and renewable sources), fossil fuels (petroleum, fossil natural gas and coal, together), petroleum, coal and natural gas. The GREET model can calculate three main GHGs (CO₂, CH₄, N₂O) emissions and five standard emissions (volatile organic compounds [VOC], CO, NOx, PM10 and SOx). The GREET model takes the 1 million BTU (British thermal unit) output in every stage as a standard and calculates the emissions and energy use of the stage. The model integrates the emissions and energy use of all stages in order to get the final results from well-to-wheel.

Fuel carbon footprints: well-to-tank (WTT)

In the GREET model, CO₂ emission factor (g/106BTU fuel output) is calculated by carbon balance method, namely the carbon in the burning process fuel minus the carbon in the combustion emissions as VOCs、CO and CH₄, and the remaining carbon is changed into CO₂ can be calculated by Equation 5:

Equation 5

Where: CO₂ j k is the CO₂ emission factor of fuel j in the combustion process with the combustion technology k (g/106BTU); Densityj is the density of process fuel j; LHVj is the low calorific value of process fuel j; C_ratioj is the carbon content of process fuel j; VOCj k is the CO emission factor of fuel j in the combustion process with the combustion technology k (g/106BTU); CH4 j k is the CH₄ emission factor of fuel j in the combustion process with the combustion technology k (g/106BTU); 0.43 is the carbon content of CO; 0.85 is the average carbon content of VOC emissions; 0.75 is the carbon content of CH₄; 0. The former technical parameters of fuel in this model are contained in the fuel-specs of the GREET model.

Fuel carbon footprints: tank-to-wheel (TTW)

Direct emissions occur during the tank-to-wheel phase of the fuel. GHG emissions per kilometer [12] are calculated based on the consumption of each fuel type and the CO₂e emissions per liter of fuel. Equation 6 calculates emissions per km for different vehicle categories [40].

Equation 6

Where EFKM,i (gCO2e/km) is the transport emissions factor per distance of vehicle category i; ECx,I (litre/km) is the energy consumption of fuel type x in vehicle category i; EFCO₂,x (gCO₂/l) is the CO₂ emission factor for fuel type x; EFCH₄,x (gCO₂e/l) is the CH₄ emission factor for fuel type x; EFN₂O,x (gCO₂e/l) is the N₂O emission factor for fuel type x; and Ni is the total number of vehicles in category i, Nx,i is the number of vehicles in vehicle category i using fuel type x.

The GHG emission per passenger trip for different vehicle categories can be calculated by Equation 7:

Equation 7

Where EFP,i (g/passenger trip) is the transport emissions factor in vehicle category i; EFKM,i (gCO₂e/km)is the emissions from vehicle category i; DDi (km/year)is the total distance driven by vehicle category i; Pz (passenger trip) is the total passenger capacity of vehicle category i.

Vehicles carbon footprints

In this article, the carbon footprint calculation of vehicles has been divided into three parts: vehicle component production, vehicle operation and vehicle assembly and disposal and recycling (ADR), based on the fundamental parameters of vehicles life cycle of GREET model.

Vehicles carbon footprints: vehicle components production

The processes of vehicle components production phase are: the raw material recovery; raw materials transportation and processing; and material production, fabrication, and processing. The GHG emission in producing the materials and components can be calculated by Equation 8:

Equation 8

where: Vm (tCO₂e/t) is the GHG emissions from vehicle material production; n is the number of vehicle materials and elements; qi is the amount of material i; while the waste of material i produced during the manufacture is denoted by wi. EFCO₂,i (g/t) is the CO₂ emission factor in the material i production phase; EFCH₄,i (g/t) is the CH₄ emission factor in the material i production phase; EFN₂O,i (g/t) is the N₂O emission factor in the material i production phase.

Vehicles carbon footprints: assembly, disposal & recycling (ADR)

Owing to lack of related data in China, our study calculated that the emission related to a middle passenger car assembly, disposal and recycling is 1.33 t CO₂e according to the GREET (2.7) model [29].

Vehicles carbon footprints: vehicle operation

The vehicle operation phase is the same with the fuel TTW phase on the basis of different life cycle. Therefore, we only need to calculate the carbon footprint once in the fuel life cycle.

Results

On the basis of the carbon footprint calculation methods presented above, we individually calculate and analyze the carbon footprints of urban public transport system in Xiamen city (NBT and BRT).

▪ Carbon footprints of the NBT system

Carbon footprints of the infrastructure

Infrastructure carbon footprints are annualized based on the life span of the project. This is owing to the fact that emissions from material production, transport, construction, decommissioning and recycling occurs at the beginning or the end of the infrastructure life cycle, while other direct emissions such as operation and maintenance are annual. Not annualizing the upstream and downstream emissions would thus grossly overstate emissions in the first year and would not be compatible with the approach of monitoring annually emissions [40]. The total carbon footprints of the NBT infrastructure was 20,472,516.20 tCO₂e and the average carbon footprint was 1,023,625.81 tCO₂e per year based on a 20-year life span (Table 2). Materials production accounted for the majority of total GHG emission, approximately 56.23% of the carbon footprint. The second highest carbon footprint is from maintenance activities, accounting for 24.90%. Construction and decommissioning were responsible for 3.10 and 24.90% of the infrastructure carbon footprint, respectively.

Infrastructure material production

Until the end of 2008, the road lengths of the NBT system in Xiamen was approximately 3,115.23 km and the public transport hub area, 147,834 m². As the normal bus did not have the exclusive right-of-way, the road distribution rate was about 51.97% according to the passenger capacity statistics of Xiamen in between 2003–2008. The total GHG emission of infrastructure material production of NBT system was 13,273,314.71 tCO₂e in total, of which asphalt was the largest contributor, contributing approximately 68.74% to carbon emissions. The details of the data for the consumption and emissions of infrastructure materials are shown in Table 3.

Infrastructure transport & construction

Table 4 shows that the carbon emissions for the materials transportation phase was 66,599.23 tCO₂e, of which the emissions for sand were the largest. In this study, the carbon emissions for the equipment and labor power in the construction phase was 663,665.74 tCO₂e by the 5% of the embodied energy of the required construction materials [38]. According to the former results, the total GHG emissions for the construction phase were 730,264.96 tCO₂e.

Infrastructure maintenance

Table 5 shows that the annual commuted material GHG emissions of the NBT system infrastructure maintenance were 275,060.80 tCO₂e and the GHG emissions emitted by the materials transportation were 4,561.21 tCO₂e. As mentioned in methodology section, the energy for road repairing was concluded to be 5% of total embodied energy of the materials used in maintenance as the construction phase. The GHG emissions of equipment and labor power for road repairing were 13,753.04 tCO2e. The sum of the above three items were the total GHG emissions of infrastructure maintenance.

Infrastructure decommissioning

The carbon emissions of the decommissioning phase were calculated by the 90% of the total construction carbon emissions: 11,945.98 tCO₂e.

Carbon footprints of the vehicles

Usually, the vehicle type of the NBT system is the same as the connecting line of the BRT system; therefore, the carbon footprints of materials productions were substituted for the average of the connecting line vehicles (XMQ6127G and XMQ6891G) of BRT system, in that, the carbon footprint from material production of every vehicle was 28.30 tCO₂e [29]. There are 2551 buses of the NBT system in Xiamen city, China; in total the carbon footprint of materials production was 72,199.19 tCO₂e. Based on the calculation methods, the carbon footprint from the vehicle ADR phases was approximately 338.36 tCO₂e. To conclude, the carbon footprint of the NBT system was approximately 72537.55 tCO₂e.

Carbon footprints of the fuels

Gasoline and diesel fuels are used by the NBT system. Table 6 shows that the total carbon footprint of the operation phase from the NBT system in Xiamen city in 2009 was 147,177.04 tCO₂e, of which the direct carbon footprint per capital was 260.84 gCO₂e.

Carbon footprints of the NBT system in total

Figures 2 & 3 show the carbon footprints of the NBT system in Xiamen city, China. Using 2009 as an example, the result was 1,178,361.13 tCO₂e, among which, the materials production, maintenance and vehicles operation make up the top three contributors to the carbon footprint, responsible for 56.32, 24.90 and 12.49%, respectively.

▪ Carbon footprints of the BRT system

Carbon footprints of the infrastructure

The total carbon footprints of the BRT infrastructure (Project I) was 2,101,871.28 tCO₂e and the average carbon footprint was 42,037.43 tCO₂e per year based on a 50-year life span (Figure 4). Since BRT roads are mostly viaduct styles that the usage period is 50 years compared with the NBT 20 years. Operation and maintenance activities accounted for 55.49% of infrastructure total GHG emission. Materials production deducted the emission reduction from recycling accounted for approximately 39.78% of total life cycle emission. Transport activities including shipping of materials from manufacturing site to construction site and the transportation to landfills/recyclers accounted for only approximately 0.36% of the total life cycle GHG emissions. Construction and decommissioning were responsible for 2.30 and 2.07% of the infrastructure carbon footprint, respectively.

Different from the NBT, cement, steel and sand were the largest contributors to embodied energy and emission. The detailed data for the consumption and emissions of infrastructure materials have been listed in a previous study [29]. The total GHG emission of infrastructure material production was 1,010,008.62 tCO₂e and 20,200.1 tCO₂e per year.

It is worth noting that the BRT infrastructure operation phase includes the night-time view project (54%) on the exclusive roads apart from the nights, elevators, card readers and other electrical equipments (46%) at bus station. There are some waste construction materials (steel bars and aluminum profiles) of the BRT infrastructure, which can be protected for recycling. The energy consumption in the re-machining process of the recycled steel bars is 20–50% of the original production; here we chose 40% for calculation. The recovery factor of the steel bars is 0.50. The energy consumption of recycled aluminum profiles is 5–8% of original production of aluminum and here 4% is acceptable. The recovery factor for aluminum is 0.95 [39,41]. The distance from the building site to disposal site is 5 km for the unrecoverable materials and 10 km for the recoverable materials [39].

Carbon footprints of the vehicles and fuels

Figure 5 showed that fuel consumption accounted for the majority of total GHG emissions, approxiately 94.02% of the vehicle carbon footprint. Vehicle material production and ADR activities accounted for 5.77 and 0.21% of the total vehicle emission, respectively. According to the local statistics of buses by Xiamen Metal Recycling Company, a default retirement age of 10 years for vehicles was used in this article.

There are two main kinds of vehicles of BRT system in Xiamen City, the trunk route vehicle (XMQ6127G) and the feeder line vehicle (XMQ6891G). The configuration parameters of vehicles comes from the homepage of Xiamen Kinglong Auto Group [102] and the ration of vehicle materials are obtained by interview with workers in Xiamen Metal Recycling Plant.

Carbon footprints of the BRT system in total

When compared with NBT system, Figure 6 showed the total carbon footprint of the BRT in Xiamen City. The total carbon footprint was 55,927.07 tCO₂e per year. Infrastructure operation, infrastructure material production, fuel consumption and infrastructure maintenance activities accounted for the first four parts of the total carbon footprints. Whereas other parts together merely represent 5.04% of the total emissions.

▪ Comparative analysis on the carbon footprint of the BRT systen and the NBT system

Table 7 illustrated the carbon footprint analysis results of the BRT and NBT system in Xiamen city in 2009. The average carbon emissions of the BRT system were 4.18 g CO₂e per kilometer and 638.44 g CO₂e per person based on the life cycle, comparing those of 6.08 per kilometer and 2,088.38 g CO₂e per person emitted by the NBT system. If we took only the direct carbon emissions of fuel consumption in the vehicle operation into consideration, the average carbon emission of the BRT system was approximately 149.08 g CO₂e per person, and 260.84 g CO₂e from the NBT system, only accounting for 23.35 and 12.49% of the related total life cycle carbon emissions, respectively. Therefore, the per capita carbon footprint and per capita direct carbon emissions of BRT system were lower than NBT system and the effects of energy saving of BRT was more advantageous, which is related to the features of the BRT system such as large volume, energy-saving and environment-friendly vehicle type and exclusive right-of-way.

Discussions & future perspective

After 2005, there was a big challenge facing the development of public transport in Xiamen city. In terms of the percentage of bus travel, although it is a high level in Xiamen city, there is still a wide gap between Xiamen city and the international level of 40–80%. NBT is still the main public transport and the BRT is in its infancy. Owing to the traffic congestion, the running speed public vehicles dropped to 16 km/h at peak time. A lack of station facilities, inadequate transportation capacity access to the island and the single structure of transport services cannot satisfy needs to the travel long distances of the Bay City. Therefore, there is an urgent need to adjust the public transport network architecture and construct the multilevel, large capacity and rapid public transport system.

In the urban public transport system of Xiamen City, if we only consider the direct carbon emissions of fuel consumption in the vehicle operation, the average per capital carbon emission of the BRT system was approximately 149.08 gCO₂e, and 260.84 gCO₂e from the NBT system, respectively. It is interesting to find that the direct carbon emissions of fuel consumption in the vehicle operation only accounted for 23.35 and 12.49% of the total carbon footprint of the BRT and NBT system. The emissions would be underestimated if ignoring other parts of emissions in the life cycle of the public transport system. This can be confirmed by the report of GHG Emissions from the US Transportation Sector (1990–2003). According to the EPA [31], the total life cycle emissions for the US nation’s transportation sector are estimated to be 27–37% higher than direct fuel combustion emissions.

Both the GHG emissions per km and per capita from the BRT system were lower than the NBT system. The reasons came from the larger volume and lower energy-consumption of the BRT system. Based on this study, we can initially conclude that Xiamen should gradually establish the low-carbon public transportation system which is dominated by the BRT and based on NBT.

Carbon footprint assessment is a new measure to account for the GHG emissions and still at the starting stage on the related studies of China. This article built upon the carbon footprint assessment frame of urban public transport system, which is a useful supplement for the transport carbon footprint methodology. Moreover we have evaluated the carbon footprint of the two main public transport systems (NBT and BRT) by this model, expanding the research area and enriching the research cases, as the best experience for other cities in China. Some work remains to be further studied.

First, we chose the parameters on construction materials production, vehicles ADR and emission factors, which were referred to foreign literatures. In fact, some gap existed between China and abroad. Future research should focus on determining the parameters for China. In future studies, the parameters selection of carbon footprint model should be focused on ensuring a result of accuracy and objectivity. At the same time, owing to the data accessibility, input-output assessment methodology is difficult to apply to real carbon footprint calculation. Second, owing to limited time and data sources, this article only made comparative analysis on the carbon footprint of the BRT system and the NBT system. In the future, the research should be gradually expanded to taxis, private cars and other vehicles.

Table 1.  Passenger capacity of Xiamen city, China public traffic system in 2008.

Key indicators	Passenger capacity per year (million)	Proportion of the total (%)
Bus rapid transit	2,375.27	2.91
Normal bus transit	41,180.9	50.46
Community minibus	15,243.83	18.68
Public traffic aggregate	58,800	72.05
Taxi	22,813	27.95
Total	81,613	100.00

Data from [42].

Table 2.  Carbon footprints of the infrastructure of the normal bus transit system.

Category	GHG emissions (tCO2e per year)
Infrastructure material production	663,665.74
Infrastructure construction	36,513.25
Infrastructure maintenance	293,375.05
Infrastructure decommissioning	29,864.96
Infrastructure recycling	82.73
Aggregate	1,023,625.81

Table 3.  Consumption and carbon emissions for the infrastructure construction materials of the normal bus transit system.

Material	Consumption (t)	Actual GHG emission (tCO2e)	Commuted GHG emission (tCO2e)
Cement (in concrete)	1,149,200.07	3,132,222.94	1,627,816.26
Sand	6,777,580.78	2,995,582.26	1,556,804.10
Gravel	12,605,079.63	1,857,081.17	965,125.09
Asphalt	474,736.96	17,555,453.66	9,123,569.27
Aggregate		25,540,340.03	13,273,314.71

Table 4.  GHG emissions for the materials transportation of normal bus transit system.

Freight	Origin	Destination	Materials transport distance (km)^†	Actual GHG emissions (tCO2e)	Commuted GHG emission (tCO2e)
Cement (in concrete)	Longyan, Fujian	Xiamen, Fujian	142	12,966.79	6,738.84
Sand	Zhangzhou, Fujian	Xiamen, Fujian	50	26,927.33	13,994.13
Gravel	Zhangzhou, Fujian	Xiamen, Fujian	50	50,079.98	26,026.57
Asphalt	Maoming, Guangdong	Xiamen, Fujian	1,012	38,175.27	19,839.69
Aggregate				128,149.37	66,599.23

^†The distances were measured by Google map.

Table 5.  Annual GHG emissions for the infrastructure maintenance of the normal bus transit system.

Material	Material amount per unit area (kg)	Maintenance area (m^2)	Material amount (t)	Material GHG emissions (tCO2e)	Commuted material GHG emission (tCO2e)	Transportation GHG emissions (tCO2e)	Commuted transportation GHG emissions (tCO2e)
Cement (kg)	36.54	2,180,661	79673.50	78,876.77	40,992.26	898.98	467.20
Gravel (m^3)	0.27	2,180,661	862707.37	381,302.85	198,163.09	3,427.54	1,781.29
Sand (m^3)	0.13	2,180,661	462328.04	68,113.87	35,398.78	1,836.83	954.60
Asphalt (kg)	14.90	2,180,661	32497.95	974.94	506.68	2,613.27	1,358.12
Aggregate				529,268.42	275,060.80	8,776.62	4,561.21

Table 6.  The operation condition of the normal bus transit system in Xiamen city, China in 2009.

Category	Values
Total road length	3115.23 km
Vehicle numbers	2,551
Total passenger capacity per year	56,424.73 million
Total operating distance per year	193,654,000 km
Total carbon footprint per year	147,177.04 tCO2e
Direct carbon footprint per person	260.84 gCO2e/person

Table 7.  A comparison list of carbon footprint from the bus rapid transit and the normal bus transit systems in Xiamen city, China, in 2009 (tCO2e).

Category	BRT	NBT
Infrastructure
Materials production	20,200.17	663,665.74
Construction	1,107.57	36,513.25
Operation	17,273.74
Maintenance	6,055.29	293,375.05
Decommissioning	870.42	29,864.96
Recycling	-3,469.76	82.73
Vehicles and fuels
Vehicle material production	801.25	7,219.92
Assembling, disposal and recycling	29.18	338.36
Vehicle operation	13,059.21	147,177.04
Aggregate	55,927.07	1,178,361.13
GHG emission (gCO2e/km)	4.18	6.08
GHG emission (gCO2e/person)	638.44	2088.38

BRT: Bus rapid transit; NBT: Normal bus transit.

Life cycle assessment (LCA)

Analyze the environmental influences including energy use, resource consumption and pollutant emissions across the full life cycle of a product for production, use, disposal, recycling and other phases.

Carbon footprint

Amount of carbon emitted over the life stages of a product including goods and services or a measure of the exclusive total amount of carbon emission that is directly and indirectly caused by an activity including individuals, organizations, sectors and so on, expressed in CO₂ equivalents.

Carbon footprint assessment

Method of accounting GHG emission, which has three different approaches: life cycle assessment (LCA), input-output assessment (IOA) and hybrid life cycle assessment (hybrid LCA).

CO₂ equivalent (CO₂e)

A measure for describing how much global warming a given type and amount of GHG may cause, using the functionally equivalent amount or concentration of CO₂ as the reference.

Global-warming potential (GWP)

Relative measure of how much heat a GHG traps in the atmosphere, comparing the amount of heat trapped by a certain mass of the gas in question to the amount heat trapped by a similar mass of CO₂, which is expressed as a factor of CO₂ (whose GWP is standardized to 1), CH₄ is 21, and N₂O is 310 accroding to IPCC 1996.

Executive summary

Introduction

▪ This article built the carbon footprint assessment frame of urban public transport system and accounted for the GHG emissions of the two main public transport system (normal bus transit [NBT] and bus rapid transit [BRT]) by life cycle assessment methodology from three components as infrastructure, fuels and vehicles.

Direct fuel GHG emissions

▪ The average direct fuel GHG emissions of the BRT system was approximately 149.08 gCO2e per person and 260.84 gCO2e per person from the NBT system, only accounting for 23.35 and 12.49% of the related total life cycle carbon emissions, respectively.

Indirect GHG emissions

▪ The indirect emissions mainly come from energy consumption by the life cycle of infrastructure, vehicles and the upstream process of fuel, of which the largest emissions share of BRT system was concentrated in the upstream material production activities of both infrastructure and vehicles, which accounted for 31.33% of the total carbon footprint and the largest carbon emission contributor to NBT system is the material extraction activities, accounting for 56.60; road maintenance the second largest contributor 24.90%.

Comparative analysis

▪ The per capita carbon footprint and per capita direct carbon emissions of BRT system were lower for the NBT system and the effects of energy saving of BRT was more advantageous, which is related to the features of the BRT system such as large volume, energy-saving and environment-friendly vehicle type and exclusive right-of-way.

Police relevance

▪ Xiamen city should gradually establish the low-carbon public transportation system which was dominated by the BRT system and based on the NBT system.

Financial & competing interests disclosure

This study was supported by Chinese Academy of Sciences (KZCX2-YW-450), International Cooperation Program of State Commission of Science and Technology of China (2009DFB90120), National Natural Science Foundation of China (No. 71003090) and Public Welfare Project on Environment Protection (No. 201009055). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. Writing assistance was utilized from Marian Rhys of Simply better.