National Emissions of Greenhouse Gases and Air Pollutants from Commercial Aircraft in the Troposphere over South Korea

This study estimated greenhouse gas (GHG) and air pollutant emissions from aircraft in the troposphere (aircraft cruise altitudes, 1 12 km) over South Korea over a two-year period (2009 2010) using an activity-based (Landing and TakeOff (LTO) cycle) methodology. Both domestic and international LTOs covering 4 major airports and 11 smaller airports in South Korea were considered. The annual mean GHG emissions (CO2, N2O, CH4, and H2O) in the troposphere (1 12 km) over South Korea during the study period were approximately 3.5 × 103, 3.4 × 10-2, -6.6 × 10-2, and 1.4 × 103 kiloton (kt) yr-1, respectively. The tropospheric air pollutant emissions (CO, NOx, VOCs, and PM2.5) were approximately 3.0, 20, 1.0, and 0.2 kt yr-1, respectively. The monthly GHG and air pollutant emissions showed no significant variations. The GHG and air pollutant emissions during cruises over the South Korean airspace were significant contributors to (e.g., about 80% for NOx and about 75% for CO2) the total national aviation emissions including the emissions at airports, boundary layer and the free troposphere.


INTRODUCTION
The emissions of air pollutants [nitrogen oxides (NO x ), volatile organic compounds (VOCs) and particulate matter (PM)] and greenhouse gases (GHGs) from land transportation, such as roads, rail and inland shipping have been reported to have a significant impact on the atmosphere and climate change (Uherek et al. 2010).Although aviation emissions [2.7 Teragram (Tg) yr -1 of NO x in 2006; Wilkerson et al. 2010] comprise a small fraction of the global NO x emissions from man-made and natural sources, NO x emissions from subsonic aircraft might have a pronounced impact on the atmospheric chemical composition (Lee et al. 1997;Gauss et al. 2006).Aviation also plays an important role in long-range transportation and is expected to grow gradually in the future.The annual passenger traffic growth rate between 2000 and 2007 was 5.3% yr -1 , showing a 38% increase in passenger traffic (Lee et al. 2009).This growth trend is expected to continue over the next 20 years with world passenger traffic growing at 5% annually due to the large demand in the Asia Pacific, Middle East and Latin American air transportation markets (Boeing Commercial Airplanes 2012).In 2006 the global emissions of CO 2 , H 2 O, NO x , and CO from aircraft were estimated to be 162 (Tg-C), 233, 2.7, and 0.68 Tg yr -1 , respectively, with 93% of the world's aviation fuel consumption in the Northern Hemisphere (69% between 30 and 60°N) and 75% above 7 km for geographic coverage (Wilkerson et al. 2010).CO 2 emissions in East Asia account for 11% of global aviation CO 2 emissions (Wilkerson et al. 2010).
Accurate estimations of aircraft traffic air pollutant emissions, such as NO x , VOCs, and PM, are essential for examining their impact on the air quality in surface source regions, such as the vicinity of large cities even in the free troposphere where other emission sources are less significant.The ozone (O 3 ) concentrations in the boundary layer and free troposphere can be affected by NO x and VOCs emissions from aircraft, depending on the sensitivity of O 3 precursors (e.g., NO x and VOCs) to O 3 concentration (NO x -limited vs. VOC-limited) (Kleinman et al. 1997;Sillman 1999).The maximum impact of aircraft emissions on the O 3 surface concentration might occur near airports within the surface layer during the night due to the rapid O 3 titration by NO emitted from aircraft, which is a more important impact of NO x emissions than VOC emissions on O 3 destruction (Pison and Menut 2004).In the cruising altitudes in the upper troposphere and lower stratosphere, NO x emissions from aircraft are expected to increase the O 3 concentration (Hidalgo and Crutzen 1977;Johnson et al. 1992;Schumann 1997;Dameris et al. 1998;Kentarchos and Roelofs 2002;Grewe et al. 2002;Gauss et al. 2006).For example, Gauss et al. (2006) reported an aircraft-induced maximum increase in the zonal-mean O 3 concentrations ranging from 3.1 (in September) to 7.7 ppb (in June), accompanying that in the zonal-mean total reactive nitrogen (NO y ) from 156 (August) to 322 ppt (May).In addition, environmental concerns regarding the increase in fine PM emissions from the aircraft around large airports have recently increased (Webb et al. 2008;Lobo et al. 2012).
In contrast to other major anthropogenic emission sources, aircraft largely emit air pollutants and GHGs at flight altitudes in the free troposphere and lower stratosphere, where the lifetimes of the exhaust products as well as secondary photochemical products, e.g., O 3 , are much longer than those at the surface.Aviation emissions including contrails combined with this relatively longer lifetime can contribute to a change in the radiative forcing (RF) in the climate (Travis et al. 2002;Lee et al. 2010).This impact also occurs through direct (i.e., warming by CO 2 and H 2 O) and indirect (via atmospheric chemistry: O 3 formation and CH 4 reduction) effects.The total aviation RF has increased by 14% (excluding induced cirrus) from 2000 to 2005.The total aviation RF in 2005 was approximately 55 mW m -2 , which comprised 3.5% of the total anthropogenic forcing (Lee et al. 2009).
Compared to Europe and the USA (Wilkerson et al. 2010 and references therein), only a few studies have examined the regional aircraft emission data in Asia (Fan et al. 2012;Song and Shon 2012).This study used an activitybased methodology to estimate the national emissions of GHGs (CO 2 , N 2 O, H 2 O, and CH 4 ) and air pollutants (NO x , CO, VOCs, and PM) in the South Korean airspace over a two-year period (2009 -2010).To the best of the authors' knowledge this study provides the first landing and takeoff (LTO)-based aircraft emission estimates of both GHGs and air pollutants in the highly expanding air traffic regions in East Asia, covering both the surface and free troposphere (aircraft cruise altitudes, 1 -12 km).The spatiotemporal distribution (including geographical and monthly emissions) of these emissions was also characterized.

Study Location
The emissions of GHGs (e.g., CO 2 , N 2 O, CH 4 , and H 2 O) and air pollutants (NO x , CO, VOCs, and PM) from aircraft were calculated based on four major international airports in South Korea: Incheon (International Civil Aviation Organization (ICAO) code: RKSI), Gimpo (RKSS), Gimhae (RKPK), and Jeju International Airports (RKPC) (Fig. 1).Detailed information on these four airports can be found in the report by Song and Shon (2012).For domestic flights, thirty domestic air traffic routes involving 4 major airports were considered for the geographical distribution of aircraft emissions in Korean airspace (Fig. 1).For international flights, twelve international air traffic routes involving 4 major airports were considered (Fig. 1 and Table 1).

Estimation Methods of Aircraft Emissions
Aircraft emissions depend on the following factors: the number and types of aircraft operations; types and efficiency of aircraft engines; fuel used; length of flight; power setting; time in operation mode; and to a lesser degree, the aircraft exhaust gases emission altitude (IPCC 2006).To calculate the aircraft emissions, aircraft operations were divided into LTO cycles (taxi-in and out, start-up, approach, take-off and climb-out) and cruising.In general, the methods for estimating the emissions from aviation sources can be categorized into 3 types (Tier 1 -3).Briefly, the Tier 1, 2, and 3 methods are based on the aggregate fuel consumption quantity data, the number of LTOs, and actual flight movement data including the number of LTOs, respectively [IPCC 2006;European Environment Agency (EEA) 2009].A detailed description of these methods can be found in EEA (2009).In this study the LTO cycles in the boundary layer and actual flight pathways in the free troposphere (Tier 2) were used to estimate the national aircraft emissions in the boundary layer and at cruising altitudes (1 -12 km) in the South Korean airspace.This method is generally more accurate than the fuel consumption based method (Tier 1) because the activity (LTO) based method represents actual aviation situations (aircraft type, flight route, etc.).However, there are some limitations in applying Tier 2 methodology to obtain the correct data on fuel use and emission factors.The emission factors and fuel use factors are based on the fuel use of average aircraft.In fact, the average aircraft is different from the specific aircraft type and engine used in Korea.This can cause uncertainty in the aircraft emissions.The limitations in the fuel use and emission factors are discussed further below.
The aircraft emissions in cruise were calculated using Eq. ( 1): E ij = Emission (kg yr -1 ) of species (j) for the aircraft type (i).LTO i = Number of LTO for the aircraft type (i).F i = Fuel consumption for the aircraft type (i) and cruising route (1000kg).EF ij = Weighted-average emission factor (kg/1000kg-fuel) of species (j) for the aircraft type (i) associated with the total number of engine models.The yearly chemical species emissions at each airport (E ij ) were calculated from the monthly emission summation for all aircraft types based on monthly LTOs.The numbers of LTOs at the three airports, RKSS, RKPK and RKPC, and the RKSI airport were obtained from the Korea Airport Corporation (KAC, http://www.airport.co.kr/) and Incheon International Airport Corporation (IIAC, http://www.airport.kr/), respectively.The fuel consumption (use) for the specific aircraft type in the flight route was calculated using regression analysis using the standard flight distances (e.g., 125, 250, 500, 750, and 1000 nm), their corresponding standard fuel uses and the actual cruising distances (Table 2).The standard flight distances (in nm) and their corresponding fuel uses for the specific aircraft type are available from the European Monitoring and Evaluation Programme (EMEP)/ EEA Guidebook website (http://www.eea.europa.eu/emepeea-guidebook,EEA 2009).The flight distance for each domestic and international route in Korean airspace was estimated using the coordinates (latitude, longitude) of transit fixes in flight routes between two airports obtained from the Enroute Chart-ICAO (Office of Civil Aviation Ministry of Land, Transport and Maritime affairs, 2012, Aeronautical Information Services, http://ais.casa.go.kr/).Chemical species emission factors such as CO, NO x , and VOCs for each aircraft type were calculated based on fuel use.The emission factor for specific flight distance between two airports was estimated using regression analysis (Table 2) using standard flight distances and their corresponding emission factors (http://www.eea.europa.eu/emep-eea-guidebook,EEA 2009).Since the emission factors for certain types of aircraft such as A300, A319, and A321 were not available, those factors were replaced with A310, A320, and A320, respectively.This might cause a limitation in accurate aircraft emission estimation.The emission factors for CO 2 and PM 2.5 were adopted from Table 3-3 of EEA 2009.The emission factors for N 2 O and CH 4 were adopted from Santoni et al. (2011) and that for H 2 O was obtained from Vay et al. (1998).The emission factors used in this study are summarized in Table 3.An estimation of military aircraft emissions was excluded for national security reasons.Furthermore, the GHG and air pollutant emissions with altitude (surface to 12 km) were discussed using this study (1 -12 km) and our previous study (Song and Shon 2012).The airport ground-level (apron, taxi, and runway) emissions were adopted from Song and Shon (2012), while the emissions within the boundary layer (between ground level and 1km) were adopted using the emissions for approach and climb-out modes.

Emissions of GHGs in the Free Troposphere
Table 4 lists the number of monthly LTOs according to the aircraft cruising route in the free troposphere during the study period, 2009 -2010.The busy domestic routes were the routes between SS (RKSS) and PC (RKPC) and between SS (RKSS) and PK (RKPK) airports (Table 4a).For example, the number of LTOs for the route between SS and PC were 55543 and 58599 in 2009 and 2010, respectively, which was 38 -40% of the 30 domestic routes.A slight variation in monthly LTOs was observed during the study period, showing the highest values in August ( 2009) or October (2010) and the lowest in February.The busiest international route based at the 4 major international airports (RKSI, RKSS, RKPK and RKPC) was the route between SI (RKSI, Incheon) and China, followed by the route between SI and Japan (Table 4b).The number of LTOs for these China and Japan routes were 38 -39% and 28 -29% of the total international routes, respectively, due to the rapid increase in tourism and trade between the two countries.Detailed information on the LTOs and aircraft types at 4 international airports was reported by Song and Shon (2012).Briefly, the dominant aircraft type was B737 at RKSS, RKPK and RKPC (accounting for 56 -69% of the total LTOs).The dominant types at RKSI were A330 (17 -18%), B747 (18 -19%) and B777 (16%).Tables 5a and 6a    domestic and international routes during the study period.
The GHG emissions from international routes were a factor of 4 higher than those from domestic routes.As shown in Table 5a the highest domestic GHG emissions occurred in the route between SS and PC.No significant changes in the yearly GHG emissions were observed between the two years (≤ 4%).The negative CH 4 emissions suggest a decrease in atmospheric CH 4 concentration through chemical reactions in the atmosphere involving NO x emissions.The highest international route of GHG emissions were observed in the route between SI and China followed by the route between SI and Japan (Table 6a).The GHG emissions for the route between SI and China ranged from 940 to 1,000 for CO 2 , 0.0093 to 0.0098 for N 2 O, -0.018 to -0.019 for CH 4 , and 366 to 390 kilogram (kt) yr -1 for H 2 O.The yearly variations in emissions from the domestic and international routes were insignificant in the two year monitoring period (< 8%).The magnitude of total CO 2 emission (4.8 Tg yr -1 ) estimated from aircraft (in 2010) in the boundary layer and free troposphere over South Korea in this study was similar to that (3.8 Tg yr -1 ) estimated for domestic civil aviation in China in 2010 (Fan et al. 2012) and in the boundary layer of UK airports (2.4 Tg yr -1 ) in 2005 (Stettler et al. 2011).The present estimate of national CO 2 emission from civil aviation in 2009 was 0.8% of the global civil aviation (162 Tg-C yr -1 ) in 2006 (Wilkerson et al. 2010).Note that the Chinese emissions estimated using fuel consumption and domestic flights considered only the cruise phase and did not include international cruises.The UK emissions [estimated using the activity (LTO)-based methodology] considered the emissions from airports only.The total aircraft CO 2 emissions at all four major airports in 2009 derived from the current study accounted for approximately 0.84% of the national annual CO 2 emissions (540 Tg yr -1 in 2009) in South Korea, estimated using the 1996 IPCC Guidelines for National Greenhouse Gas Inventories (GIR 2011, http:// www.gir.go.kr/og/hm/gs/a/OGHMGSA010.do).Note that the national GHG inventory excludes international air traffic emissions (and ground support equipment (GSE) emission) and includes the emissions throughout the full domestic flight path (cruise phase).
Table 7a and Fig. 2 present the monthly GHG emissions in the free troposphere for both domestic and international routes during 2009 -2010.The monthly variations in 2009 were similar to those in 2010.In general, the monthly emission variations in the domestic routes (≤ 32%) were slightly higher than that in the international routes (≤ 10%).The monthly GHG emissions were highest in August (2009) or October (2010) for the domestic route, whereas the international routes were highest in January (2009) or August (2010) due to a temporal difference in the number of international passengers.The mean monthly CO 2 , N 2 O, CH 4 , and H 2 O emissions for the domestic routes in 2009 (and 2010) were 56 ± 7 (58 ± 4) kt month -1 , 566 ± 42 (571 ± 37) kg month -1 , -1097 ± 81 (-1105 ± 71) kg month -1 , and 22 ± 2 (23 ± 1) kt month -1 , respectively.The mean monthly CO 2 , N 2 O, CH 4 , and H 2 O emissions for the international routes in 2009 (and 2010) were 225 ± 9 (242 ± 10) kt month -1 , 2210 ± 90 (2379 ± 103) kg month -1 , -4278 ± 174 (-4605 ± 200) kg month -1 , and 88 ± 4 (94 ± 4) kt month -1 , respectively.A distinct seasonal difference in GHG emissions was observed between domestic and international routes due to the different demands for flights.For example, the GHG emissions for the domestic route were highest in summer, whereas those for the international route were highest in winter.
The emissions of GHGs (and air pollutants) differed according to the aircraft type (Fig. 3).Moreover, the aircraft type that emitted the dominant amount of GHG emissions differed according to the cruise type.For example, the aircraft type emitting the highest GHG emissions for the domestic routes was the B737 (e.g., 517 -536 kt yr -1 for CO 2 , 5.2 -5.3 ton yr -1 for N 2 O, -10 ton yr -1 for CH 4 , 205 -209 kt yr -1 for H 2 O) followed by A300 and A321.The GHG emissions for the B737 comprised 47% of the total emissions.For the international route, the aircraft type emitting the highest GHG emissions was the B747 (e.g., 1258 -1306 kt yr -1 for CO 2 , 12 -13 ton yr -1 for N 2 O, -24 to -25 ton yr -1 for CH 4 , 490 -509 kt yr -1 for H 2 O).The GHG emissions for the B747 were 26 -47% of the total amount of emissions.

Air Pollutant Emissions in the Free Troposphere
Tables 5b and 6b list the air pollutant emissions, such as CO, NO x , VOCs and PM 2.5 , in the free troposphere for domestic and international routes during the study period.
The air pollutant emissions from international routes were a factor of 4 -11 higher than those from domestic routes.Similar to GHGs, the highest domestic route of air pollutant emissions occurred in the route between SS and PC, where the domestic air pollutant emissions ranged from 235 to 238 for CO, 1590 to 1637 for NO x , 41 to 43 for VOCs, and 20 to 21 ton yr -1 for PM 2.5 .The yearly amount of air pollutant emissions remained relatively constant over the two years (≤ 1%) (Table 5b).In the case of international routes, the highest air pollutant emissions also occurred in the route between SI and China followed by the route between SI and Japan (Table 6b).Compared to the emissions from domestic routes, the yearly variations in emissions from the international routes were slightly larger between the two years (7 -8%).The NO x to VOCs emission ratios in the free troposphere ranged from 18 to 40.
Compared to the previous pollutant emissions from aviation, the air pollutant emissions, such as CO, VOCs, and NO x , estimated in the boundary layer and free troposphere over South Korea in 2010 were similar to those in China (Fan et al. 2012) and UK airports (Stettler et al. 2011).For example, the total CO, VOCs, and NO x emissions in China were 40, 4.6, and 154 kt yr -1 , respectively, whereas those in UK airports were 11.7, 1.8, and 10.2 kt yr -1 , respectively.Current estimates of national CO, VOCs, and NO x emissions from civil aviation in 2009 were 1.1, 1.9, and 0.93% that of global civil aviation (0.679, 0.098, and 2.656 Tg yr -1 ) in 2006, respectively (Wilkerson et al. 2010).
Table 7b and Fig. 2 present the monthly air pollutant emissions in the free troposphere for both domestic and international routes during 2009 -2010.The monthly emissions from the international routes were a factor of 4 -11 higher than those from the domestic routes.The monthly variation trend in 2009 was similar to that in 2010.In general, monthly emission variations in the domestic routes (≤ 18%) were slightly higher than those in the international routes (≤ 11%).Like GHGs, the monthly air pollutant emissions showed the highest values in August (2009) or October (2010) for domestic routes, whereas those for the international routes were observed in January (2009) or August (2010).The mean monthly CO, NO x , VOCs, and PM 2.5 emissions for domestic routes in 2009 (and 2010) were 43 ± 3 (43 ± 3), 292 ± 20 (296 ± 20), 7.4 ± 0.5 (7.4 ± 0.6), and 3.7 ± 0.3 (3.7 ± 0.2) ton month -1 , respectively.The mean monthly CO, NO x , VOCs, and PM 2.5 emissions for international routes in 2009 (and 2010) were 200 ± 8 (214 ± 9), 1325 ± 53 (1422 ± 62), 7.4 ± 0.5 (7.4 ± 0.6), and 14 ± 1 (15 ± 1) ton month -1 , respectively.A distinct seasonal difference in air pollutant emissions was observed between domestic and international routes.For example, the air pollutant emissions for the domestic routes were highest in summer, whereas those for the international routes were highest in winter.
The air pollutant emissions emitted from both domestic and international routes differed according to the aircraft type (Fig. 3).For example, the aircraft type emitting the highest CO emission for domestic routes was the B737 (e.g., 464 -471 ton yr -1 , 58% of total emissions) followed by the B747 and A300, and that emitting the highest NO x emission was also the B737 (e.g., 1885 -1919 ton yr -1 , 33%) followed in order by the A300 and A330.For VOCs, the aircraft type emitting the highest emission was the B747 (e.g., 32 -37 ton yr -1 , 22%) followed in order by the B737 and A330.The B737 showed the highest PM 2.5 emissions (33 -34 ton yr -1 , 47%) followed in order by the A300 and A321.For the international route, the aircraft type emitting the dominant emission was different from that for the domestic route.The aircraft type emitting the highest air pollutant emissions was the B747 followed by the A330.For example, the international cruise emissions for CO, NO x , VOCs, and PM 2.5 for B747 were 1381 -1431, 7546 -7825, 472 -488, and 80 -83 ton yr -1 , respectively.

Comparison of the Emissions Related to the Aircraft Flight Geographic Coverage
The air pollutant and GHG emissions from aircraft activities at four major international airports and 11 small-scale airports located in South Korea, including cruise mode in the free troposphere were compared during 2009 -2010 (Fig. 4 and Table 8).Detailed discussion of the air pollutant and GHG emissions at four major international airports and 11 small-scale airports in the boundary layer were reported by Song and Shon (2012) and Shon et al. (2013), respectively.The total air pollutant and GHG emissions at the 11 airports ranged from 4.8 to 12% at the four major airports.The GHG emissions, such as CO 2 , N 2 O, CH 4 , and H 2 O, in the cruise mode were predominant in the GHG emissions from aviation, accounting for more than 64% (52% for international route) of the national aircraft GHG emissions.The yearly CO 2 , N 2 O, CH 4 , and H 2 O emissions for the cruise mode were 3367 to 3590 kt yr -1 , 33 to 35 ton yr -1 , -64 to -68 ton yr -1 , and 1320 to 1400 kt yr -1 , respectively.The GHG emissions at the four international airports in the boundary layer (derived from approach, climb out, startup, takeoff taxi in, and taxi out modes) were significant (21 to 33% of national aircraft GHG emission), except for CH 4 (3%) (Song and Shon 2012).The total GHG emissions from the 11 small-scale airports in the boundary layer were insignificant (≤ 3%) (Shon et al. 2013).
The air pollutant emissions in cruise mode were dominant in the air pollutant emission from aviation, accounting for 39 to 86% (32 to 69% for international routes) of the national aircraft emissions.For example, the yearly emissions, such as CO, NO x , VOCs, and PM 2.5 for the cruise mode were 2919 to 3078, 19409 to 20577, 989 to 1052, and 215 to 228 ton yr -1 , respectively.The air pollutant emissions at the four international airports in the boundary layer were significant (20 to 54% of national aircraft emission), except for PM 2.5 (13%).The total air pollutant emissions from the 11 small-scale airports in the boundary layer were small (≤ 7%).
Figure 5 shows the total CO 2 , NO x , and VOC emissions with altitude from international and domestic aircraft over South Korea in 2010.Strongly enhanced CO 2 and NO x emissions occurred at flight altitudes of 10 -12 km (the upper troposphere) where contrails predominantly form, whereas strongly enhanced VOC emissions occurred at both the surface and altitudes of 10 -12 km.Unlike other emission gases, VOC emissions were significantly higher in the start-up operational mode so that the VOC emissions were also higher at the surface (Song and Shon 2012).The CO 2 and NO x emissions within the boundary layer (approximately ≤ 1 km) accounted for approximately 30 -32% and 19 -31% of their peak emissions at the altitudes (10 -12 km), respectively.
The current methodology for estimating aircraft emissions in the boundary layer presents two main sources of uncertainty (LTOs and LTO emission factors).Song and Shon (2012) reported a detailed discussion of the emission uncertainty in the boundary layer.Fuel consumption should be included in the aircraft emission uncertainty in cruise mode.As mentioned in section 2.2, estimating the cruise mode emissions involves calculating the aircraft type fuel consumption using regression analysis using the standard flight distance, corresponding standard fuel use and actual cruising distance.The fuel consumption error might be negligible due to the strong correlation coefficient (r 2 > 0.92) between the regression result and observations (actual fuel consumption).

SUMMARY AND CONCLUSIONS
The emissions of GHGs (e.g., CO 2 , N 2 O, CH 4 , and H 2 O) and air pollutants (NO x , CO, VOCs, and PM) from aircraft in the boundary layer and free troposphere over South Korea in 2009 -2010 were calculated using an activity-based (LTO) methodology.The busiest domestic and international routes showed the highest air pollutant and GHG emissions between Gimpo (Seoul) and Jeju (island) airports and the route between Incheon and China, respectively.The air pollutant and GHG emissions from the international routes were significantly higher than those from domestic routes (by a factor of 4 to 11).In general, there was no distinct emission difference between the two years (2009 -2010).The month of highest air pollutant and GHG emissions differed according to the route (international vs. domestic).For example, for the domestic routes, the highest monthly air pollutant and GHG emissions occurred in August (2009) or October (2010), whereas those for the international routes occurred in January (2009) or August (2010).In the free troposphere air pollutant and GHG emissions were dominant from aviation, accounting for 64 -97% and 39 -86% of the national aircraft emissions (including 4 major international airports and 11 small-scale airports), respectively.Of the air pollutants and GHGs, the CO emissions were a dominant contributor only in the boundary layer from the 4 major international airports (54%) to the national aircraft CO inventory, whereas the emissions from other pollutants and GHGs were less than 40%.
The NO x to VOC emission ratio from aircraft with different altitudes plays an important role in atmospheric chemistry (e.g., the production of loss of O 3 ) at different altitudes.The NO x to VOC emission ratio (range of 3 -12) in the boundary layer was somewhat lower than that (range of 18 -40) in the free troposphere.This suggests that the impact of the emission difference between the altitudes on increases or decreases in O 3 concentrations can be very significant.The different magnitudes of GHGs at different altitudes might influence the atmospheric environment and climate change.Therefore, future studies should assess the impact of aircraft emissions on the air quality (e.g., O 3 ) near airports and free troposphere as well as climate change.

Fig. 1 .
Fig. 1.Air routes for domestic and international flights and geographical distribution of CO 2 and NO x emissions (in 2010) for flight routes over South Korea.CHN-bound, JPN-bound, and SEA-bound routes indicate international routes to China, Japan, and Southeast of Asia, respectively.Flight information region (FIR) represents the airspace over South Korea.

Fig. 4 .
Fig. 4. Comparison of GHG and air pollutant emissions between the boundary layer and cruising altitude.

Table 1 .
Twelve international air traffic routes, including one or several sub routes, in the airspace over South Korea.

Table 2 .
Regression analysis for fuel consumption and NO x , VOCs, and CO emission factors using standard flight distances.

Table 3 .
present the GHG emissions, such as CO 2 , N 2 O, CH 4 , and H 2 O, in the free troposphere for GHG and air pollutant emission factors for aircraft types (in g kg -1 of fuel).
Note: Values represent emission factors calculated from the cruise distance of 450 km, which covers the highest number of LTOs during the study period.

Table 4 .
The number of monthly LTOs for the aircraft cruising route in the free troposphere during the study period of2009 -2010.

Table 7
Fig.2.Monthly distribution of GHG and air pollutant emissions calculated from both domestic and international cruise modes.