Protocol to calculate aircraft emissions for international air routes in South America

Summary Recently, aviation pollution has drawn important social attention. The protocol proposed in this paper can simultaneously calculate the overall emissions of six aviation pollutants (CO2, CO, HC, NOx, SO2, and PM2.5), including the landing and take-off emissions and climb/cruise/descent emissions. The international routes in South America during 2019–2021 are an example to illustrate the use of this protocol. This protocol can provide a methodological basis for calculating aviation pollutant emissions in different countries and regions. For complete details on the use and execution of this protocol, please refer to Cui et al. (2022b).1

to calculate the overall aircraft emissions. The CCD emissions are calculated through the Modified BFFM2-FOA-FPM method, and the LTO emissions are calculated based on the ICAO Aircraft Engine Emissions Databank. 6 Figure 1 summarizes the methods proposed in this paper.

Comparison with other methods
Aiming at the accounting method of gaseous pollutant emission of aero-engine during standard landing and take-off cycle, ICAO has successfully developed simple, advanced, and complex methods according to different calculation methods and data requirements since the 1970s. For example, based on the ICAO calculation system, the U.S. Environmental Protection Agency (EPA) puts forward the EPA (Environmental Protection Agency) method combined with the actual situation. Meanwhile, the European Environment Agency (EEA) has established the EMEP (European monitoring and evaluation program) cooperative action framework. Since then, ICAO has further improved the calculation method and proposed the ICAO carbon emission calculator, which can estimate the aviation emission per unit passenger based on the data of various aircraft types.

ICAO emission inventory method.
ICAO emission inventory calculation methods are divided into simple, advanced, and complex methods, and simple methods are divided into A and B. 7 Simple method A takes the number of LTO cycles and the emission index of various emissions as an essential reference, which can quickly evaluate the emissions during the aircraft LTO cycle. Still, the model does not consider the aircraft type's impact and each stage's operation time. Simple method B further considers the variation characteristics of fuel flow rate and emission index in different operation stages and uses the operation time set by ICAO under other rated thrusts in different settings. The emission index was found from the EEDB (Engine Emissions Data Bank) database to calculate each aircraft's emission of various pollutants. Researchers have carried out many studies on the establishment of aviation emission inventory and pollution simulation based on simple methods: Kesgin 8 and Altuntas 9 approximate estimated the aircraft emissions from domestic routes of multiple airports in Turkey and determined the emission characteristics of air pollutants in different operation stages of aircraft. Cokorilo 10 estimated aircraft engines' emission and carbon emission costs at Belgrade airport, Serbia, and proposed corresponding emission reduction plans.
In the advanced method, considering that the fuel flow rate and emission index will change with external thrust setting, flight level, atmospheric environment, and other factors in actual operation, the BFFM2 (Boeing Fuel Flow Method II) method is proposed for correction. Turgut et al. 11 used ICAO advanced method to quantify the emission of gaseous pollutants from thirty airports in Turkey.
The complex method is to apply the measured accurate data, performance information, operation time, fuel consumption, and thrust of an aero-engine under different loads and external conditions to the complex computer model to obtain the final pollutant emission. For example, Xu et al. 12 estimated the engine pollutant emissions of Shanghai Hongqiao International Airport and Shanghai Pudong International Airport in the taxiing phase based on the Aircraft Communication Addressing and Reporting System (ACARS) data.
The direct use of model reference value in ICAO's simple methods will uncertainty the accounting results. On the other hand, the advanced and complex methods further improved have highly accurate results. However, they have the limitations of high data requirements, complex implementation, high research cost, and are unsuitable for mass calculation. Therefore, the development of relevant research is relatively slow.

American EPA method.
American EPA method mainly aims to calculate emission inventory during the LTO cycle. 13 Its overall calculation idea is the same as ICAO's simple method B. Still, the difference is that the EPA method considers the correction of the maximum mixing layer height on the operation time of the aircraft during climbing and approach under different meteorological conditions. The air pollutant emission above the maximum mixing layer height will not affect the air quality near the ground. The ultimate mixing layer height is taken as the effective emission height of the aircraft, and the relationship between meteorological conditions and aero-engine emission is established. Zhou et al. 14 estimated the pollutant emission during the take-off and landing cycle of domestic civil aviation aircraft in 2015 based on the improved EPA method. They compared the calculation results with the calculation results of the guide method, providing a technical reference for revising the calculation method of aircraft emission in the guide. And Baxter et al. 15 used this method to calculate the air pollutant emissions of aircraft engines at Kansai International Airport in Japan.
Although the EPA method considers meteorological conditions and establishes the relationship between meteorological conditions and aero-engine emissions, it is helpful to understand the relationship between meteorological conditions and emissions. However, the calculation of emission inventory during the aircraft LTO cycle by the EPA method is the same as that of the ICAO simple B method. Therefore, there may be no significant difference between the two calculation results for a single aircraft. As a result, and because many organizations and projects apply ICAO recommended method, ICAO recommended method is the most effective method for LTO cycle pollutant emission evaluation. 16 3. European EMEP method. EMEP calculation method is divided into the level I, level II, and level III methods. Level III is divided into method A and method B. 17 For fuel consumption calculation in the LTO cycle and cruise phase, the level I method considers the factors, including emission index and fuel consumption in-flight phase and range type. The level II method further considers the influence of aircraft type based on the level I method. Level III method A is to model the emission list, average value of fuel consumption, and flight distance during a cruise in the LTO stage and analyze the fuel consumption level of each segment, which can be used in the transportation planning of airlines. Level III method B calculates the pollutant emissions in each stage of the whole trajectory based on the aircraft's flight ll OPEN ACCESS STAR Protocols 4, 101952, March 17, 2023 performance and the engine's rated dynamic parameters. Based on this, Pereira et al. 18 assessed the environmental impact of replacing existing aviation fuels with hydrogen or natural gas.
This method is an accounting method of pollutant emission of aircraft during the whole flight based on fuel statistical data. Still, it focuses on analyzing the emission characteristics of aero-engine from the fuel perspective and ignores the differences between engine types.

The ICAO Carbon Emission Calculator.
The ICAO Carbon Emission Calculator 6 is a universal method for calculating aviation carbon emissions. The ICAO methodology employs a distance-based approach to estimate an individual's aviation emissions using available data on aircraft types. The ICAO Carbon Emission Calculator requires users to enter the origin and destination airport of the flight. It is then compared with the published scheduled flights to obtain the type of aircraft used to serve the two related airports and the number of departures per aircraft. Then map each plane to one of the equivalent aircraft types to calculate the trip's fuel consumption based on the great circle distance between the airports involved in the journey. The load factor and passenger-to-freight ratio obtained from the traffic and operating data collected by ICAO are then applied to obtain the total fuel usage attributable to the passengers. Then, the system weights the average fuel consumption of the journey based on the take-off frequency of each equivalent aircraft type. The fuel consumption is then divided by the total number of economy class equivalent passengers, giving an average fuel burn per economy class passenger. The result is then multiplied by 3.157 to obtain the amount of CO 2 , which is the carbon footprint of each passenger. Some studies have focused on estimating aviation carbon emissions. For example, Wasiuk et al. 19 estimated the global emissions during 2005-2011, but they have not involved particular aircraft, routes, and airlines. Liu et al. 20 applied the ICAO method to estimate the emissions of the routes among 208 Chinese airports, but no discussion has been done on specific aircraft and distance segments.
However, there are some drawbacks to the approach provided by ICAO. First, the distance difference is not enough. For example, according to the VariFlight, 21 A320-214 flew between 360 km and 3,649 km on domestic routes in China in 2018, exceeding the methodology provided by the ICAO. Second, there is no distinction between specific aircraft. ICAO's calculation method only considers large sequences and does not consider differences between subsequences. For example, the A320 family has many families, such as the A320-100 and A320-200, with different engine types, which may lead to a significant difference in the carbon emissions of the two aircraft. 22,23 Third, various pollutants cannot be calculated at the same time.
To sum up, the accounting methods of aviation pollutants mainly used in the existing research are primarily formulated by European and American countries. In emission accounting, most pollutant emission indexes use ICAO reference values, and the calculation model is rarely improved and modified according to the actual situation. And it is not possible to calculate different pollutants at the same time.
Generally, the primary calculation methods for non-CO 2 emissions are the ICAO method (Boeing Fuel Flow Method 2 (BFFM2) method and First Order Approximation (FOA) method. 19,24 Therefore, this protocol proposes a Modified BFFM2-FOA-FPM method based on the Fuel Percentage Method (FPM). This method combines the BFFM2-FOA method and the Fuel Percentage Method (FPM), 25 which can calculate the CO 2 and non-CO 2 emissions simultaneously. Some papers have focused on calculating CO 2 and non-CO 2 emissions, but no unified approach exists to connect the two accounting methods. Furthermore, the calculation method of CO 2 emissions has not considered the difference of sub-series, and that of non-CO 2 emissions has not focused on the emissions in the CCD stage. Therefore, we built a new Modified BFFM2-FOA-FPM method to calculate CO 2 and non-CO 2 emissions from the CCD stage. The LTO emissions are calculated using the ll OPEN ACCESS International Civil Aviation Organization (ICAO). And our approach has considered the aircraft types' emission intensity. We divide the route distance into several groups and get the six pollutions of aircraft types in these different distances to ensure that the calculation results are accurate. Compared with existing research, our results cover more detailed aircraft types, and we corrected the results with the actual flight time to make it more real. In addition, our method can calculate the PM2.5 of the CCD stage. Existing studies focus on calculating PM2.5 in the LTO stage, while there are few corresponding calculation methods for PM2.5 in the CCD stage. The Modified BFFM2-FOA-FPM method can estimate the PM2.5 value of the CCD stage.
To more clearly compare the differences and connections between the methods in this paper and other methods, the main advantages and disadvantages of each method are summarized as shown in Table 1.

Proposed timing
The actual time necessary to realize each calculation step will depend strongly on the timing of collecting the flight information. The more route data to be collected, the longer it takes. It should be noted that flight information can also be obtained through purchase on the Variglight, but the price is very expensive, and it is difficult to ensure that clear and detailed flight information can be obtained, which is not conducive to data processing and use. Therefore, it is not recommended to purchase directly from the official Variglight. This paper uses South America's international routes as an example to provide the timing interval of each step. i. Get the route information between the two countries.
ii. Query all South American countries to get all international routes in South America, containing the origin airport and the destination airport.
Note: According to our statistics, there is little change in the weekly arrangement of international routes in South America, so we collect the weekly route information. For example, when the departure country was Colombia, and the arrival country was Venezuela in 2019, we can get six routes:

Get the LTO emissions.
We can get the six pollutions for each aircraft in an LTO cycle based on the engine type and engine number. Then multiplying by the flight frequency and accumulating, we can get the LTO emissions. 6. Calculate the emission intensity of the CCD stage. a. Divide the distance segments. The emission intensity of the same model at different distance sections is very different, so this paper first segments the distance. All routes are divided into distance segments according to distance.
b. Calculate the ratio cr in Modified BFFM2-FOA-FPM method for each aircraft in different distance segments. In the Modified BFFM2-FOA-FPM method, the primary unknown variable is ratio cr . We comprehensively calculated it by integrating the fuel volume data per unit flying time of each aircraft type and the ICAO Aircraft Engine Emissions Databank. Then, the value is corrected using the aircraft flight time collected earlier. c. Get the CO 2 emission intensity of each aircraft in each distance segment. If the ratio cr is determined, the CCD CO 2 emissions in each route can be obtained. Based on the distance of each flight, we can get the CO 2 emission intensity of each aircraft in each distance segment.
CRITICAL: [Carbon emission intensity has two applications: 1. It is used to calculate the emissions of transit routes. 2. Used to calculate the carbon emission intensity of the other five pollutants.] d. Calculate the emission intensity of other five pollutants. Based on the CO 2 emission intensity, we can get the fuel consumption intensity of each aircraft in each distance segment and then calculate the emission intensities of the other five pollutants. For SO 2 , the emission intensity is 3.870 g/kg multiplied by the fuel consumption intensity; For CO and HC, I j = I j0 Ã q 3:3 d 1:02 . q is the ratio of outside temperature to 288 K; d is the ratio of external pressure to sea level pressure. I j0 is the standard emission coefficient of an LTO stage of CO or HC (g/kg). We assume that the aircraft's cruising altitude is 10,000 meters, then we can obtain the relationship between the altitude of 0-10,000 meters and the temperature. We use interpolation integration to get q; According to the relationship between altitude and air pressure, the interpolation integration is used to obtain d. According to the ICAO aircraft engine emissions databank and fuel intensity value, we obtain the emission intensity of CO and HC.
For NOx, I NOx = I j0 Ã d 0:51 q 1:65 Ã exp À 19:0 Ã À 0:0063 À 0:622Ã4ÃPv ÁÁ . I j0 is the standard emission coefficient of a LTO stage of NOx (g/kg). q is the ratio of outside temperature to 288 K; d is ratio of external pressure to sea level pressure. 4 is atmospheric relative humidity; P is external pressure; Pv is atmospheric saturation pressure. The value of q, d, 4, P, and Pv is also obtained through interpolation integration. PM2.5 can be divided into Nonvolatile Component Fine Particles (NCFP) and Volatile Component Fine Particles (VCFP), which can be calculated by Modified BFFM2-FOA-FPM.
CRITICAL: [In this step, we can get a data sheet that contains the emission intensity of these six pollutants of different aircraft at different distances. This data sheet is based on the fuel emission intensity. Because the average flight distance and aircraft type are other in different regions, different regions correspond to different fuel emission intensities, which need to be recalculated for different areas and cannot be applied to all parts. It cannot be used in all regions. If we research a new region, the steps must be repeated.] 7. Calculate the emissions in the CCD stage.
The emission intensity of each aircraft type is multiplied by the flight distance, flight frequency, and other data to obtain the emission at the CCD stage.
CRITICAL: [If one flight is a transit flight and the aircraft take off and lands twice, the CCD emissions are the sum of two flight distances before and after; If the flight distance is three segments, the situation is the same.] It should be noted that the calculation of CCD emission intensity needs to coordinate the situation of all routes, so it is impossible to give an example for only one route. The detailed results can be found in Tables S4, S5, and S6.  Figure 3B. Comparing Figures 3A and 3B, we can also find that carbon dioxide accounts for the most significant proportion of various pollutants. For example, in 2019, the overall carbon dioxide emissions were 5,867,289.72 tons, far exceeding the second gas, Nitrogen Oxide (44,985.09 tons). In addition to CO 2 , CO and NOx are also relatively large. However, the proportion of non-carbon dioxide pollution varies in different years. For example, in 2019 and 2021, the content of NOx is slightly higher than that of CO, but in 2020, the content of CO will exceed that of the former.

Routes and airlines with the largest emissions of six pollutants during 2019-2021
To further calculate the detailed emissions of six gases from international routes in South America in 2019-2021, we screened the routes and airlines with the most significant emissions of six gases in In conclusion, LATAM Airlines has the highest frequency among the airlines with the most significant emissions, which is consistent with its status as one of the largest airlines in South America.
Emission intensity in the CCD stage of six pollutants during 2019-2021 As mentioned earlier, we divide the distance into 11 segments based on the distance range: 0-500 km, 501 km-1,000 km, 1,001 km-1,500 km, 1,501 km-2,000 km, 2,001 km-2,500 km, 2,501 km-3,000 km, 3,001 km-3,500 km, 3,501 km-4,000 km, 4,001 km-4,500 km, 4,501 km-5,000 km, and 5,001 km-5,500 km. Furthermore, we considered the difference between sub-series, such as 320-214 and 320-232. Then, we get the aircraft's emission intensity of the six pollutions from 2019 to 2021 based on the Modified BFFM2-FOA-FPM Method. Among them, the aircraft with the highest emission intensity in each distance segment and their intensities in 2019 are listed in Table 2. The emission intensity of other years can also be obtained through the supplemental tables. The proposed methods can analyze the emission intensity differences of different aircraft in the same series in detail.
To show the data more clearly, we selected the aircraft with the highest emission intensity in each distance segment in 2019. As shown in Table 2

Accuracy analysis
In this section, we discuss the accuracy of the results. We have not found the direct data of international routes in South America, and we can only deduce it through indirect data. According to flight

LIMITATIONS
It should be noted that the overall emissions are calculated through the standard LTO stage. Therefore, the emissions due to delays are not considered, and the aircraft transfer caused by temporary weather is not considered. Therefore, further investigation can calculate the emissions caused by delay, aircraft transfer, and air cargo.

TROUBLESHOOTING Problem 1
How to determine the specific flight time?
Flight time is crucial to the calculation of emission intensity. Still, in the app of Variglight, on the same route, even for the same aircraft type, the specific flight times of different airlines may be quite other, and how determining flight time is the key to the accuracy of the calculation.

Potential solution
Collect data for multiple weeks and take the median flight time as the specific flight time.

Problem 2
Differences in weekly flight arrangements. The airline's arrangements may not be the same every week. Sometimes there are flights this week, but there are no flights next week, or the aircraft types and frequencies of flights are changed.

Potential solution
Collect more data for a few weeks to see the overall situation. It is ignored if there are only one or two flights in three consecutive months. In terms of model and frequency, the model and frequency with the highest occurrence probability in three successive months are also taken as the calculation basis.

Problem 3
Incomplete information on Variglight.
At present, the Variglight is a convenient channel to obtain the detailed information of aircraft routes. When collecting data on the Variglight, we would encounter a lack of aircraft, flight time, flight distance, etc. This rarely happens, but sometimes the relevant data is critical.

Potential solution
If there is a lack of aircraft information, collect it for a few more weeks. If there is, follow the other weeks. If it is still lacking, replace it with the corresponding aircraft with the lowest carbon emission intensity according to the flight distance.
If there is a lack of flight time, collect more for a few weeks. If not, estimate according to the speed and flight distance of the aircraft.
If the flight distance is missing, enter the coordinates of the origin and destination airports on the map to calculate the 3-D distance.

Problem 4
The Variglight may restrict access.
When collecting data on the Variglight, because of the significant frequency of access, IP access is often restricted, affecting the progress of data collection.

Potential solution
There is no other way to deal with it. You can only pause the collection until the IP access restriction is lifted.

Problem 5
Some flights may temporarily stop at the standby airport or temporarily add a transit airport.
When collecting the flight data, some flights may temporarily choose alternate airports or add transit airports due to weather or air traffic control reasons.

Potential solution
Take it as a special case and ignore it, and use the information of normal flights as the information of this route.

RESOURCE AVAILABILITY
Lead contact Further information and requests should be directed to the lead author, Qiang Cui (cuiqiang@seu. edu.cn).

Materials availability
This study did not generate new unique materials.

Data and code availability
The CCD and LTO emissions of each route and airline for the six pollutions are shown in Tables S1, S2, and S3. For example, the first data in Table S1 represents the LTO emissions and CCD emissions of six pollutants on the Cali-Caracas route of Avior Airlines. The overall emissions of the airline on the route are the sum of the LTO emissions and CCD emissions.
The emission intensities of the six pollutions of each aircraft type can be found in Tables S4, S5, and S6. For example, the first data in Table S4 shows the CO2 emission intensity of each aircraft type in the 0-500 segment.
The data can also be obtained in the Aviation Emissions Accounting Databases: http://www.aeads. com.cn.
This paper does not use code.
Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.