Carbon emissions of Provincial Administrative Regions from the airlines in China


 This paper seeks to calculate the overall emissions of China’s 413 main domestic routes in 2018, containing the Landing and Take-Off (LTO) emissions and Climb/Cruise/Descent (CCD) emissions. First, the standard calculation method of LTO emissions proposed by the International Civil Aviation Organization (ICAO) is applied to calculate the LTO emissions of 413 routes and 40 airlines. Next, the modified Fuel Percentage Method (MFPM) is used to calculate the CCD emissions of main aircraft types at various distances. Then the overall emissions are split into the Provincial Administrative Regions (PARs) on the routes to discuss the probable carbon compensations, which is the core to build an aviation carbon trading mechanism. China Southern Airlines, China Eastern Airlines, and Air China are the top three airlines in emissions. Guangdong has the most carbon emissions from airlines, but it may get the third most carbon compensation from the airlines.


Introduction
China's domestic civil aviation industry has recently ushered in rapid development 1,2 . In 2019, the total turnover of China's domestic routes was 82.95 billion ton-kilometers, an increase of 7.5% over the year 2018; Domestic routes completed 852.02 billion passenger-kilometers, a rise of 8.0% over the year 2018; Domestic air routes completed 7.86 billion ton-kilometers of cargo and mail, an increase of 4.1% over the year 2018 3 . The rapid expansion of carbon emissions accompanies the rapid development of the civil aviation industry. China's domestic airline carbon emissions increased by 95.3%, from 38.5 million tons in 2012 to 75.2 million tons in 2019 (This data is calculated based on total turnover data and unit emission data of CAAC 4 ).
In addition to rapid growth, aviation emissions have other impacts 5,6 . First, it isn't easy to be transformed by vegetation because of its high emission. The aircraft cruising stage is generally more than 10,000 meters, which is too far away from the ground vegetation and cannot be transformed easily. Second, the retention time is long, and the harm of mixing with particulate matter is more than twice the harm of carbon dioxide emitted. Third, although aircraft generally fly in the stratosphere, there is still much water vapor, suspended solid particles, impurities, etc. These are easy to mix with carbon dioxide and increase harm.
Many countries and organizations have proposed market-based mechanisms to solve the aviation emission problem, such as the EU Emission Trading Scheme (EU ETS) 7

and the Carbon
Offsetting and Reduction Scheme for International Aviation (CORSIA) proposed by ICAO 8 . The market-based mechanism is recognized as an essential measure to solve the problem of carbon emissions. However, the EU ETS focuses on the flights in the EU, and the CORSIA only covers international flights 9 . Therefore, China also should build its carbon trading mechanism to control the fast-growing aviation emissions. As we all know, the main body of aviation carbon emissions is airlines, and aviation carbon emissions have prominent characteristics of regional transfer. For example, a flight from Beijing may emit the most emissions in Hebei rather than Beijing. Therefore, it is crucial to calculate the historical emissions of each airline to each Provincial Administrative Region (PAR).
Furthermore, the current research on carbon emissions in China's civil aviation industry uses ICAO Carbon Emissions Calculator Methodology to calculate carbon emissions. Still, they mainly focus on specific airlines and routes or the country's overall aviation carbon emissions measurement. Because its carbon emission data system of various routes and airlines is not yet complete, it also has limitations, such as incomplete aircraft coverage and incomplete applicability to routes within the specific country. For example, the ICAO Carbon Emissions Calculator Methodology can calculate the carbon emission of A320 but has not shown the difference between A320-214 and A320-232. Furthermore, China has a vast territory, and its flights are not straight, so that the same aircraft may have different emission intensities in China and other countries.
This study calculates the CCD emissions and LTO emissions of 413 routes whose daily flights are equal to or larger than 3 (containing 319 transfer routes) and 40 Chinese airlines. Then we apply the ranging function of the Baidu map 10 to split the distance of these routes into 31 Chinese PARs in 2018 and calculate the carbon emissions of each airline in each PAR on each route. First, it should be noted that the emissions of shared flights are computed on the carrier airlines. Then, the carbon emissions of each airline in each airport in the Landing and Take-Off (LTO) stage are calculated based on the proposed method of ICAO 11 . Then this study adds the emissions in the LTO stage to the split emissions to get the final emissions of the airlines to each PAR. Finally, this study calculates the airlines' compensations to the PARs to simulate the emission trading mechanism.

Carbon emissions per kilometer of different aircraft types.
This study calculates the carbon emissions per kilometer of 42 types of aircraft at different flight distances. The exact method can be found in Section "Method." Comparing with the ICAO Carbon Emissions Calculator Methodology, our results are calculated from the actual operations of the domestic routes in China and cover more detailed aircraft types. The results (See Supplementary Table 1 for details) show that small passenger aircraft such as CRJ900 and E190 LR equipped with high-density economy class and business class which can accommodate 86-90 passengers are more environmentally friendly on short-and medium-haul routes from 0 to 2000 kilometers.
On the other hand, medium-sized passenger aircraft and upgraded new passenger aircraft such as Airbus A320 series (320-214, 320-232, 320-271N) and Boeing 737 series (737 MAX 8, 737-800) show better emission reduction performance on medium and long-distance routes of 2000-4000 kilometers, with the characteristics of larger passenger capacity (150-200) and higher fuel efficiency. While advanced wide-body airliners on long-haul routes, such as Boeing 777, Boeing 787, Airbus 380, Airbus 330, are leading in passenger capacity. Generally, they are equipped with economy class, business class, first-class, and the aircraft fuel consumption, carbon emissions, and operation and maintenance costs are relatively high.
Carbon emissions of the LTO stage of each route. This study applies the proposed method of ICAO to calculate the emissions of the LTO stage. The emissions of each aircraft in a single LTO cycle can be found in Supplementary Table 2. The results show that the total LTO emissions of the 413 routes are 9,551,644 tons. This study applies Fig.1a to deliver the top 10 routes and bottom ten routes in LTO emissions. Fig.1b displays the top 10 airlines and bottom ten airlines in LTO emissions. In Fig.1a and Fig.1b, the green ones are the top 10 ones and the red ones are the bottom ones. The most significant LTO emission route is Beijing-Shanghai, and the minor emission route is Haikou-Zhuhai. From the calculation method of LTO emissions, we can know that the emissions have direct relationships with the flight frequency and the aircraft type. In 2018, Beijing-Shanghai's flight frequency was 38854, which is the busiest route in China. Furthermore, the aircraft in this route contains many 747-400, 747-8I, 777-300ER, 330-343E, and 330-243E, and these five aircraft types are the top 5 in a single LTO. For example, the single LTO emissions of 747-400 are 11.193 tons, while the least one CORP MA-60 only emits 1.564 tons in a single LTO. Therefore, Beijing-Shanghai has the most significant LTO emissions. Correspondingly, the flight frequency of Haikou-Zhuhai is 2807, and its aircraft type is mainly 737-800, whose single LTO emissions are about 2.816 tons. Therefore, Haikou-Zhuhai has the most negligible LTO  The largest LTO emission airline is China Southern, and the least one is Joy Air. China Southern has 245 routes among the 413 routes, which is the most among the 40 airlines. In addition, its aircraft has many types whose single LTO emission is high, such as 777-300ER, 330-343E, and 330-243E. These two factors lead to the highest LTO emissions of China Southern. On the other hand, joy Air's route is mainly Hefei-Xi'an, and its aircraft type is CORP MA-60 (the most negligible emission in a single LTO), which has minor LTO emissions.
Overall  Comparing these airlines, we can see that due to the significant differences in fleet size and route resources among airlines, the emissions of domestic routes have an enormous difference. 0.00 1,000,000.00 2,000,000.00 3,000,000.00 4,000,000.00 5,000,000.00 6,000,000.00 7,000,000.00 8,000,000.00 9,000,000.00 10,000,000.00 This study applies the ranging function of the Baidu map to split the distance of 413 routes into the 31 PARs, and the overall emissions of the airlines to the PARs are shown in Fig.3. Unfortunately, the data of Taiwan, Hong Kong, and Macao is vacant.

Fig. 3 Emissions of the airlines to the PARs
It should be noted that except for these PARs, about 2,144,815 tons of emissions happen at sea. About 101 routes have emissions at sea, which are larger than about 21 PARs. As shown in Fig.3, Guangdong has the most carbon emissions from airlines, approximately 3,826,568 tons, 1,056,468 tons more than the second PAR Jiangxi. This is because Guangdong has the headquarters of China Southern Airlines and has two of the busiest airports in China: Guangzhou Baiyun Airport and Shenzhen Baoan Airport. Among the 413 routes, 119 routes take off, land, or pass through Guangdong. These factors cause Guangdong to bear the most emissions.
The other PARs with significant carbon emissions from airlines are Jiangxi, Hunan, Shandong, and Hubei, 2,770,100 tons, 2,567,880 tons, 2,557,272 tons, and 2,526,656 tons, respectively. Jiangxi, Hunan, and Hubei are in the middle of China, and many routes must pass through these PARs. The routes taking off, landing, or passing through these three PARs are 119, 109, and 126, respectively. Many hectic routes pass through it for Jiangxi, such as Guangzhou-Shanghai, Shanghai-Shenzhen, Guangzhou-Beijing, Beijing-Shenzhen, Chongqing-Shanghai, Chengdu-Shenzhen, and Guangzhou-Hangzhou. Many routes with large passengers pass through Hubei, such as Guangzhou-Beijing, Beijing-Shenzhen, Chengdu-Shanghai, and Chongqing-Shanghai. Hunan is also on many busy routes, such as Beijing-Shenzhen and Chongqing-Shanghai. Furthermore, the Wuhan Tianhe Airport in Hubei and Changsha Huanghua Airport in Hunan are also hectic airports in China. Shandong is also the only place for many busy routes, such as Beijing-Shanghai, Guangzhou-Beijing, and Beijing-Shenzhen. Shandong's Jinan Yaoqiang Airport and Qingdao Liuting Airport are also bustling airports. These factors lead these PARs to bear the carbon emissions of many routes.
It should be noted that although Hebei is next to Beijing and Tianjin, the only two municipalities in North China, it only ranks tenth in carbon emissions. Two reasons can explain this result. First, the 102 routes only have a short part in Hebei, so the total emissions are relatively small. Second, most of China's busiest routes are concentrated in the Yangtze River Delta and Pearl River. Among the top 20 routes with the most significant flight frequency, except Chengdu-Beijing, Xishuangbanna-Kunming, and Beijing-Xi'an, the other routes are in the Yangtze River Delta and Pearl River Delta.
The least five PARs are Tianjin, Qinghai, Tibet, Heilongjiang, and Ningxia, 429,195 tons, 290,770 tons, 265,470 tons, 225,305 tons, and 205,780 tons, respectively. The number of routes taking off, landing, or passing through these five PARs is 35, 17, 3, 15, and 27, respectively. However, these five PARs are not the least five ones in route amount. The route amount of Jilin and Xinjiang is less than Tianjin and Ningxia, but the emissions they are exposed to are more significant than the latter two. Two reasons can explain it. First, Tianjin and Ningxia are too small. The area of Tianjin is 11,966 square kilometers, and that of Ningxia is 66,400 square kilometers, which causes them to cover only a tiny part of the routes. Conversely, the areas of Jilin and Xinjiang are 187,400 square kilometers and 1,660,000 square kilometers. Second, for Xinjiang, although the number of routes taking off, landing, or passing through it is 18, many routes are only in Xinjiang. For example, in Xinjiang, the flight frequency of Urumqi-Kashgar is 12022, ranking 35th among these 413 routes; Hotan-Urumqi has 7833 flights in 2018, ranking 89th; Yining-Urumqi has 7119 flights, ranking 105th among these routes.
Main routes and airlines in emissions to each PAR. We summarize the top three routes that emit to each PAR, as shown in Tab.1.
As shown in Tab.1, some PARs have undertaken many carbon emissions from the routes, but these routes do not start or end on the PAR, such as Shandong, Jiangsu, Fujian, Jiangxi, and Henan. These PARs are on China's busiest routes and cover a large part of them. For example, Shandong is on the two busiest routes, Guangzhou-Beijing and Beijing-Shanghai, account for 32.3% and 32.9% of the total distances of these two routes. Some PARs' emissions are from the routes starting or ending, such as Beijing, Shanghai, Xinjiang, Yunnan, Hainan, and Heilongjiang. Beijing and Shanghai are the most developed cities in China, so they have many routes as the starting and ending points, with many emissions. Xinjiang, Yunnan, Hainan, and Heilongjiang are on the border of China, and few routes pass through them, so their main routes with the most emissions are the ones starting or ending in them.
Regarding emitting the most significant emissions, Beijing-Shanghai's route ranks first in four PARs (Beijing, Shanghai, Jiangsu, and Tianjin), covering the most PARs among the 413 routes. As the route emitting the second-largest emissions, Beijing-Shenzhen covers Beijing and Jiangxi, Chengdu-Shanghai covers Sichuan and Hubei, Guangzhou-Shanghai covers Shanghai and Guangdong, Harbin-Shanghai covers Heilongjiang and Liaoning, and Kunming-Shanghai covers Hunan and Guizhou. As the route emitting the third-largest emissions, Guangzhou-Beijing covers Hubei and Guangdong, Guangzhou-Chengdu covers Guangxi and Guizhou, and Guangzhou-Hangzhou covers Jiangxi and Zhejiang. For China, these routes cover more provinces and have significant emissions. Regarding emitting the second and third largest emissions, China Eastern covers 11 PARs and 8 PARs, the most among the 40 airlines. Moreover, with Shanghai Hongqiao Airport and Shanghai Pudong Airport as the centers, China Eastern Airlines has established an aviation network covering Eastern, Central, and Western China, becoming the second-largest source of domestic routes.
Airlines' carbon compensation to the PARs. After splitting the emissions of each airline into each PAR, this study will discuss the airlines' carbon compensation to the PARs. According to the principle of who is responsible for discharging, the airlines emitting emissions should offer some carbon compensation to the PARs. In 2018, China carried out carbon trading pilot projects in seven provinces and cities, and the carbon trading prices of these seven pilot projects are different. According to statistics 14,15 Fig. 4, the ranking of many PARs has changed a lot compared with Fig.3. The most noticeable change is in Beijing, whose emissions rank 18th but compensations rank 1st among these 31 PARs. This change is closely related to the carbon trading price. The price of Beijing in 2018 is 52.72 yuan/ton, much higher than the other PARs, so its compensations are the most. In addition, due to the relatively low price, Hubei changes from the 5th carbon emission to the 12th compensation, and Chongqing changes from the 19th carbon emission to the 29th compensation. Guangdong bears the most emissions, but its received payment only ranks third. This result indicates that the carbon trading price has a noticeable influence on the PARs.
The overall compensations that the airlines need to pay are shown in Fig.5.  Fig. 2 and Fig. 5, we can find that the order of several pairs of airlines has exchanged, such as Capital and Juneyao, Chengdu and West Air, Loong and Okay, Tibet and 9 Air, China Express and Chongqing, and Ruili and Fuzhou. In Fig. 2, Capital, Chengdu, Loong, Tibet, China Express, and Ruili rank 11 th , 16 th , 18 th , 21 st , 23 rd , and 27 th . However, they rank 10 th , 15 th , 17 th , 20 th , 22 nd , and 26 th in Fig. 5. This result indicates that the carbon trading price has a certain impact on various airlines. As the carbon trading price of different regions is different, the airlines whose routes pass through the regions with high prices may need to bear more compensation.

Discussion
This study focuses on calculating the overall emissions of the airlines to the Provincial Administrative Regions (PARs) and discussing the compensations that the airlines should pay to the PARs according to the principle of who is responsible for discharging. The overall emissions contain the CCD emissions and LTO emissions of 413 routes and 40 airlines in 2018, and the overall emissions are split into 30 PARs. Therefore, this research can provide a reference for China to build a domestic aviation carbon trading mechanism. The main conclusions and policy recommendations are as follows: First, the carbon emissions per kilometer of different aircraft types are additional under different flight distances. Small passenger aircraft such as CRJ900 and E190 LR are more environmentally friendly on short-and medium-haul routes from 0 to 2000 kilometers. Medium-sized passenger planes and upgraded new passenger planes such as the Airbus A320 series and Boeing 737 Series have shown better emission reduction performance on medium and long-haul routes of 2000-4000 kilometers. Advanced wide-body passenger aircraft such as Boeing 777, Boeing 787, Airbus 380, Airbus 330 that execute long-distance routes have relatively high fuel consumption, carbon emissions, and operation and maintenance costs.
Second, the LTO emissions have direct relationships with flight frequency and aircraft type. For example, some aircraft, such as 747-400, 747-8I, 777-300ER, 330-343E, and 330-243E, have more emissions in a single LTO. Although these aircraft have better performance in medium and long-haul routes due to their extensive range and large passenger capacity, their significant emissions in a single LTO are not friendly to the local environment. On the other hand, although Some aircraft are only suitable for short-distance routes, such as E195LR, E190 LR, CRJ900, and CORP MA-60, they have fewer single emissions and fewer impacts on the environment of the airport.
Third, China Southern Airlines, China Eastern Airlines, and Air China are the top three airlines in emissions, while LJ Air, Joy Air, and Changsha Airlines are the bottom three. Due to the significant differences in fleet size and route resources among airlines, emissions are enormous. Therefore, in the aspect of carbon compensations that need to be paid, the top airlines in emissions are also at the top, and the lower airlines in emissions are also ranked lower. However, the order of several pairs of airlines has been exchanged, which indicates that the carbon trading price has a particular impact on the airlines.
Fourth, Guangdong has the most carbon emissions from airlines, but it may get the third carbon compensation from the airlines. Two reasons lead to the significant emissions in Guangdong: the headquarters of China Southern Airlines and the large number of routes taking-off and landing it. Furthermore, the PARs with the most negligible emissions are not those with the minor routes. Due to the relatively low price, Hubei and Chongqing have a noticeable change in the ranking among these 31 PARs, which shows that the carbon trading price has an apparent influence on the PARs.
Finally, because some PARs are on China's busiest routes and cover a large part of them, although the routes do not start or end on them, they have undertaken many carbon emissions from these routes. For example, Beijing-Shanghai's route ranks first in emissions in four PARs, and China Southern has the most significant emissions in 16 PARs. As a result, different routes and airlines have distinctly different impacts on the PARs.
It should be noted that the overall emissions are calculated through the standard LTO stage.
The carbon emissions due to delays are not considered. In this paper, the traditional method is adopted to split various routes into various PARs, and the aircraft transfer caused by temporary weather is not considered. Furthermore, this study has not evaluated the emissions from air cargo. Therefore, further investigation can calculate the emissions caused by delay, aircraft transfer, and air cargo.

ICAO standard method to calculate LTO emissions
The take-Off and Landing stage (LTO) refer to the aircraft's whole process during takeoff and landing. This stage defined by ICAO includes four states, including approaching, taxiing, taking-off, and climbing, which defines climbing as the boundary layer from the end of aircraft takeoff to the aircraft's flight out of the atmosphere. Therefore, this paper uses the standard LTO cycle definition specified by ICAO to calculate the fuel consumption, including all activities at an altitude below 3000 feet (915m) near the airport. Therefore, this stage is not directly related to the route. In addition, the climbing process requires higher fuel consumption than the cruise phase at a constant altitude. Thus, the Climb, Cruise, and Descent cycle (CCD) are defined as all activities that occur at the height of 3000 feet (915 meters). Thus, fuel use accounts for most of the whole voyage and is directly related to the flight distance.
The calculation formula of carbon emissions in LTO stage is: is the emissions in the LTO stage; is the standard thrust of the engine of aircraft type a (unit: kg); is the number of engines of aircraft type a； is the thrust setting of stage m; is the working time of phase m， is the fuel consumption rate. 3.157 is the emission coefficient of fuel. The value range of m is 1, 2, 3, and 4, respectively corresponding to the four stages of takeoff and landing in the aircraft flight process: takeoff, climb, approach and taxiing. According to the standard LTO cycles defined by ICAO, when the aircraft is taking off, its engines are at 100% thrust and working time is 0.7 minutes; when the aircraft is climbing, its engines are at 85% thrust and working time is 2.2 minutes; when the aircraft is approaching, its engines are at 30% thrust and working time is 4 minutes; when the aircraft is taxiing, its engines are at 7% thrust and working time is 26 minutes. Therefore, in a standard LTO cycle, the total working time is 32.9 minutes.
The fuel consumption rate is calculated as: is the total number of airlines with aircraft type a; j is the type of engine of the aircraft; is the number of aircraft type a equipped with engine type j; is the fuel consumption rate of engine type j under the m setting. The data is from the ICAO Aircraft Engine Emissions Databank 11 . This formula is based on the weighted average of all possible engine types of the domestic routes in China. The LTO emission of each aircraft can be found in Supplementary Table 2.

Modified Fuel Percentage Method (MFPM)
The CCD emissions of the flight ( ) can be calculated by As we only consider the CCD section in this study, so we define the is the cruising distance, is the cruising speed, is the fuel consumption ratio when the aircraft is cruising, is the lift-drag ratio when the aircraft is cruising. The value of and has direct relationships with the aircraft type. We define = , and then for the cruising task section, the