The Environmental Consequences of Engine Emissions in Air and Road Transport

Abstract

This study evaluated the environmental consequences of engine emissions from road and air transport on three commonly traveled routes between Berlin and the cities of Frankfurt, Paris, and Barcelona. The focus was on CO₂ emissions due to their significant impact on climate change. By collecting and analyzing comprehensive data on vehicle and aircraft emissions, this study aimed to compare the CO₂ output of each transport mode under different passenger load scenarios. The findings indicate that air transport is generally more efficient in terms of CO₂ emissions per passenger for longer distances. For instance, the CO₂ emissions per passenger ranged from 22.6 kg to 79.8 kg for air transport and from 64.7 kg to 220.8 kg for road transport, demonstrating reductions of approximately 65% to 72%. This study underscores the importance of considering both distance and passenger load when evaluating the environmental impact of different transport modes and highlights the need for a multifaceted approach to reducing transport emissions, including technological innovations, policy interventions, and behavioral changes.

1. Introduction

Engine emissions are a significant contributor to environmental pollution, affecting air quality, human health, and global climate [1]. Both air and road transport are essential for modern society, facilitating the movement of people and goods over various distances [2]. However, they also produce substantial amounts of pollutants, raising concerns about their environmental consequences [3,4].

Air transport, involving aircraft powered predominantly by jet engines, is a rapidly growing sector due to increasing globalization and demand for travel. Aircraft emissions occur at high altitudes and primarily consist of carbon dioxide (CO₂), nitrogen oxides (NOx), particulate matter (PM), sulfur oxides (SOx), and water vapor [5]. CO₂ is the most significant greenhouse gas emitted by aircraft, contributing to global warming [5,6,7,8]. NOx emissions at high altitudes can lead to the formation of ozone, a potent greenhouse gas, and contribute to the depletion of the stratospheric ozone layer [5,6,7,8]. The International Civil Aviation Organization (ICAO) and other regulatory organizations have been working on setting standards and promoting technologies to mitigate these emissions, but the sector’s rapid growth poses ongoing challenges [9].

Road transport is another major source of emissions, primarily from vehicles powered by internal combustion engines [10]. These vehicles emit a range of pollutants, including CO₂, NOx, carbon monoxide (CO), hydrocarbons (HC), and PM. The CO₂ emissions from road transport are a significant contributor to climate change, while NOx and PM have direct adverse effects on air quality and public health [11,12]. Urban areas experience elevated levels of these pollutants due to their high vehicle density and traffic congestion [11,12]. Various measures, such as the introduction of emission standards, the promotion of electric vehicles, and improvements in fuel efficiency, have been implemented to reduce the emissions from road transport [13].

The comparison between air and road transport emissions is complex due to differences in fuel types, operational patterns, and regulatory frameworks [14]. While air transport contributes more significantly to high-altitude emissions, having broader climatic effects, road transport has a more direct impact on urban air quality and public health [15,16]. Understanding the environmental consequences of these emissions is crucial for developing effective policies and strategies to mitigate their impact.

This study aimed to evaluate the environmental consequences of engine emissions owing to both air and road transport. By examining the existing data and conducting a comparative analysis, this study aimed to provide insights into the relative impacts of these transport modes and to suggest potential measures for reducing their environmental footprint. This research provides a novel comparative analysis of CO₂ emissions from air and road transport on commonly traveled routes, highlighting the critical role of passenger load and distance in determining environmental efficiency. The practical significance lies in its implications for policy making and technological innovation, offering clear recommendations for reducing the carbon footprint of transportation.

1.1. Development of Emissions from Road Transport

The evolution of the emissions from road transport has been as a result of technological progress, regulatory action, and growing environmental awareness. In the early 20th century, the widespread adoption of the internal combustion engine revolutionized transportation, making personal and commercial travel faster and more efficient [17]. Initially, little attention was paid to the environmental impact of these engines, but as the number of vehicles on the road grew, so did the levels of pollutants they emitted. This led to significant air quality issues, particularly in urban areas, and laid the groundwork for future regulatory efforts [18].

By the mid-20th century, the increasing density of vehicles in cities began to produce noticeable environmental problems. Smog became a common issue in many metropolitan areas, caused primarily by emissions HC and NOx from vehicle exhausts [19]. These pollutants, in the presence of sunlight, form ground-level ozone, a key component of smog, which poses serious health risks. The infamous smog episodes in Los Angeles during the 1940s and 1950s highlighted the need for regulatory intervention to control vehicle emissions [20]. The response to these growing concerns came in the form of the Clean Air Act of 1970 in the United States, which established the first national emissions standards for vehicles. These standards targeted reductions in CO, HC, and NOx [21]. Similar regulatory measures were adopted in other countries, driving the development of new technologies to reduce emissions. Innovations such as the catalytic converter, which reduces harmful emissions by converting exhaust gases into less harmful substances, and improvements in fuel injection systems were significant milestones during this period [22].

Despite these technological advancements, the rapid increase in the number of vehicles meant that overall emissions continued to rise. In the 1980s and 1990s, further tightening of emission standards and the introduction of onboard diagnostics systems in vehicles allowed for the better monitoring and control of emissions [23]. The issue of CO₂ emissions, which are directly linked to fuel consumption and thus to climate change began receive attention.

In response to the growing concerns about global warming, in the late 20th and early 21st century, the introduction of fuel efficiency standards aimed at reducing CO₂ emissions from road transport began [24]. These standards encouraged the development of more fuel-efficient engines and vehicles. Additionally, there was a push toward alternative fuels such as ethanol, biodiesel, and compressed natural gas, which produce fewer greenhouse gases than traditional gasoline and diesel [25]. The 21st century has also witnessed the rise in electric vehicles as a promising solution to reduce emissions from road transport. Electric vehicles, which produce no tailpipe emissions, have the potential to significantly lower CO₂ emissions, especially when powered by renewable energy sources [26]. Governments around the world have introduced incentives for electric vehicle adoption, including tax credits, rebates, and the development of charging infrastructure. This shift toward electrification represents a significant step forward in addressing the environmental impact of road transport [27].

However, the transition to cleaner transportation is not without challenges. The production of batteries for electric vehicles involves significant environmental and social issues, including the mining of rare earth metals and the energy-intensive manufacturing process [28]. Moreover, the overall environmental benefit of electric vehicles depends on the electricity mix used to charge them. In regions where electricity is still heavily reliant on fossil fuels, the reduction in emissions may be less significant [29]. Despite these challenges, the overall trend in road transport has been toward reducing emissions and mitigating their environmental impact. While significant progress has been made in reducing harmful emissions, ongoing efforts are necessary to address the remaining challenges and ensure that road transport becomes more sustainable in the future [30].

1.2. Development of Emissions from Air Transport

The development of emissions from air transport has mirrored the industry’s rapid growth and technological advancements over the past century. Initially, the environmental impact of aircraft emissions was not a major concern. However, as commercial aviation expanded, it became clear that the sector contributed significantly to atmospheric pollution [31]. The introduction of jet engines, while revolutionizing air travel with their speed and efficiency, also brought about new environmental challenges due to their emission profile [32].

In the early years, the primary focus was on addressing noise pollution caused by jet engines, which was a major concern for communities near airports. It was not until the 1970s that the broader environmental impacts of aircraft emissions began to attract attention [33]. Studies started to highlight the role of aviation in contributing to air pollution and its potential impacts on climate change. The key pollutants identified include carbon dioxide CO₂, NOx, PM, SOx, and water vapor [34]. One of the unique aspects of aircraft emissions is that they occur at high altitudes, which can amplify their impact on the atmosphere [35]. For example, the NOx emissions from aircraft can lead to the formation of ozone (O₃) in the upper atmosphere, which acts as a potent greenhouse gas. Additionally, the water vapor emitted by aircraft can form contrails and cirrus clouds, which also contribute to global warming. These high-altitude emissions have complex interactions with the atmosphere and are a significant area of study for climate scientists [36].

The ICAO has been instrumental in addressing the environmental impact of aviation. In the 1980s, the ICAO began setting standards for aircraft engine emissions, focusing initially on NOx and later expanding to include other pollutants [37]. These standards have driven technological advancements in engine design, resulting in more fuel-efficient and cleaner-burning engines. For instance, the development of high-bypass turbofan engines has significantly reduced the fuel consumption and emissions per passenger-kilometer [38]. Despite these technological improvements, the overall growth in air traffic has often outpaced the gains made in emission reductions. The aviation industry has seen exponential growth in passenger numbers and cargo volumes, driven by globalization and the increasing demand for air travel [39]. This growth has led to a continuous rise in the total emissions from the sector, prompting further regulatory measures and initiatives to mitigate its environmental impact [40].

In recent years, there has been a strong focus on developing sustainable aviation fuels to reduce the carbon footprint of air travel. Sustainable aviation fuels are produced from renewable sources such as agricultural residues, municipal waste, and algae, which can significantly lower CO₂ emissions compared to conventional jet fuel [41]. Several airlines and aviation companies have begun adopting sustainable aviation fuels, and regulatory organizations are working to create supportive frameworks. However, the production and scalability of sustainable aviation fuels remain challenges that need to be addressed.

The aviation industry is also exploring other innovative technologies to reduce emissions. Electric and hybrid-electric aircraft are being developed, with the potential to revolutionize short-haul flights by reducing or eliminating CO₂ emissions [42]. Additionally, improvements in air traffic management and operational efficiencies can further reduce fuel burn and emissions. The concept of more efficient flight paths and optimized cruising altitudes are examples of operational changes that can contribute to emission reductions [43].

Finally, international cooperation is important for addressing the global nature of aviation emissions. Initiatives such as the Carbon Offsetting and Reduction Scheme for International Aviation aim to stabilize CO₂ emissions from international flights at 2020 levels by requiring airlines to offset their emissions growth beyond this baseline [44]. Such initiatives represent a collaborative effort to balance the industry’s growth with its environmental responsibilities. The development of emissions from air transport reflects the industry’s technological advancements, regulatory efforts, and the increasing awareness of its environmental impact. While significant progress has been made in reducing emissions per flight, the overall growth in air traffic continues to pose environmental challenges [45]. Continued innovation, regulatory measures, and international cooperation will be essential in achieving a sustainable aviation future.

1.3. Importance of Evaluating Environmental Consequences

Understanding the environmental consequences of engine emissions in air and road transport is critical for several reasons. Firstly, these emissions have a direct impact on air quality, which in turn affects human health. Pollutants such as NOx and PM are known to cause respiratory and cardiovascular diseases [46]. Evaluating these impacts helps to inform public health policies and initiatives aimed at reducing the health burden associated with air pollution.

Secondly, engine emissions contribute significantly to global climate change. CO₂ is a major greenhouse gas emitted by both aircraft and road vehicles, trapping heat in the atmosphere and leading to global warming. This warming effect results in more frequent and severe weather events, the melting of ice caps, rising sea levels, and disruptions to society [47]. By assessing the extent of these emissions, it is possible to better understand their role in climate change and develop strategies to mitigate their impact.

In addition to CO₂, other gases such as CH₄ and O₃ have a role in climate change [48]. For instance, the NOx emitted from aircraft at high altitudes can lead to the formation of ozone [49]. Understanding these complex chemical interactions requires comprehensive evaluations of the emissions from different transport modes. Such evaluations help in forming accurate climate models and in predicting future climate scenarios.

Another critical reason for evaluating engine emissions is to inform and guide regulatory frameworks. Governments and international organizations have set various emission standards and regulations to control the release of harmful pollutants. By providing empirical data on the actual emissions from air and road transport, researchers can assess the effectiveness of these regulations and suggest improvements, where necessary [50]. This ensures that policies remain relevant and effective in reducing environmental pollution.

Economic implications also underscore the importance of evaluating environmental consequences. Pollution and climate change can have significant economic costs, including healthcare expenses, loss of productivity, and damage to infrastructure [51]. By understanding the sources and impacts of engine emissions, policymakers can implement cost-effective measures to mitigate these costs. For instance, investments in cleaner technologies and alternative fuels can reduce emissions and, consequently, their associated economic burdens.

Moreover, public awareness and education are important components of environmental protection efforts. Detailed evaluations of engine emissions provide valuable information that can be disseminated to the public, raising awareness about the environmental and health impacts of air and road transport [52]. This knowledge can drive behavioral changes, such as increased use of public transport and carpooling, and support policies promoting sustainable practices.

Technological innovation is another area where evaluating environmental consequences plays a pivotal role. Identifying the most harmful emissions and understanding their impacts can spur the development of cleaner technologies [53]. For example, advancements in electric and hybrid vehicles, as well as improvements in aircraft engine efficiency, are driven by the need to reduce emissions. Continuous evaluation ensures that these technologies evolve to meet environmental goals.

Evaluating the environmental consequences of engine emissions supports international cooperation and efforts to address global environmental challenges [54]. Issues such as climate change and air pollution do not respect national borders; thus, coordinated international action is necessary. Comprehensive data and analysis provide a basis for international agreements and collaborative efforts to reduce emissions and mitigate their impacts on a global scale.

In summary, evaluating the environmental consequences of engine emissions from air and road transport is vital for protecting public health, mitigating climate change, informing regulatory policies, reducing economic costs, raising public awareness, driving technological innovation, and fostering international cooperation. Through detailed assessments, it is possible to develop effective strategies to address the environmental challenges posed by these essential modes of transport.

2. Materials and Methods

This study aimed to perform a comparative analysis of the environmental consequences of the emissions from road and air transport on three common routes of varying lengths. The selected routes were major highway routes and the corresponding flight paths between Berlin, Germany, and three other major cities: Frankfurt, Paris, and Barcelona. These routes are well traveled, with several flights operating daily between the two airports.

To accurately compare emissions, comprehensive data were collected for both road and air transport. The comparison of CO₂ emissions between road and air transport was based on standardized emission models from the ICAO for air transport and the European Environment Agency (EEA) for road transport. These models included the following:

• Air transport model utilizes the ICAO’s Carbon Emissions Calculator, which estimates emissions based on flight distance, aircraft type, and load factors.
• Road transport model utilizes the EEA’s Computer Programme to Calculate Emissions from Road Transport (COPART) model. This model accounts for vehicle type, fuel type, and driving conditions to estimate emissions.

The CO₂ emissions per kilometer for different vehicle and aircraft types were used and multiplied by the distances traveled to ensure comparability and consistency. For road transport, the data included vehicle type, average daily traffic volumes obtained from transportation departments or the relevant authorities, standardized emission factors from databases, and fuel consumption rates. For air transport, the data encompassed the common aircraft models used on the selected routes, the number of flights per day and annual flight data, emission factors for different phases of flight (takeoff, cruising, landing) from sources such as the ICAO, and fuel burn rates for the selected aircraft types.

To accurately compare emissions, the data were collected for both road and air transport, focusing exclusively on CO₂ emissions due to their significant impact on climate change. CO₂ is the primary greenhouse gas emitted by the transport sector, and it is critical to understand and mitigate its effects.

The total emissions are calculated by multiplying the number of vehicles by the distance traveled and the emission factor, using the formula:

Emissions = ∑ (Number of vehicles × Distance × Emission Factor)

(1)

For air transport, emissions are calculated by applying emission factors of flight and multiplying them by the number of flights and distance:

Emissions = ∑ (Number of flights × Distance × Emission Factor)

(2)

The analysis involved comparing the total CO₂ emissions for each route. The emissions for each route were aggregated and compared between road and air transport. The environmental impact assessment focused on the contribution of CO₂ emissions to climate change, given that CO₂ is a major greenhouse gas with a well-understood impact on global warming.

We acknowledge limitations in data availability and accuracy for traffic volume, fuel consumption, and emission factors. Assumptions made in the calculations are also discussed. Furthermore, the scope of this study is highlighted, noting that it focused on specific routes.

By providing a detailed comparative analysis, this study aimed to offer valuable insights into the relative environmental impacts of these two transport modes, informing policy decisions and guiding future research directions. This comprehensive approach ensured a thorough evaluation of the environmental consequences, contributing to a better understanding of how to mitigate the impacts of transport emissions.

3. Results

The aim of this study was to compare and analyze the environmental consequences of emissions from road and air transport on similar routes, specifically focusing on the routes between Berlin Brandenburg Airport (BER) and Frankfurt Airport (FRA), Paris Charles de Gaulle Airport (CDG), and Barcelona El Prat Airport (BCN). By analyzing the CO₂ emissions for the different modes of transport on these routes, the aim was to identify the most sustainable options and provide recommendations for reducing the environmental impact of transportation.

Table 1. Overview of routes.

Transport	Route	Distance	Average Time
Road transport	BER–FRA	553 km	5 h 31 min
Road transport	BER–CDG	1036 km	11 h 10 min
Road transport	BER–BCN	1887 km	18 h
Air transport	BER–FRA	430 km	1 h 10 min
Air transport	BER–CDG	857 km	1 h 40 min
Air Transport	BER–BCN	1500 km	2 h 45 min

contains an overview of the routes, distances, and average travel times for the analyzed routes.

Table 2. Type of vehicle considered for road transport [55].

Vehicle Specification	Volkswagen Passat B8 2.0 TDI BMT 150 HP 4Motion Advance Specification
Range	1466 km	Engine displacement	1698 cm3
Fuel	Diesel	Fuel consumption	3.9 L/100 km
Fuel tank capacity	66 L	Horsepower	110 kW
CO2 emissions	117 g/km	Emission standard	EURO VI

outlines the specifications of the vehicles used for road transport. It includes details on the vehicle’s range, engine displacement, fuel type, fuel consumption, fuel tank capacity, horsepower, CO₂ emissions, and emission standard. This information is critical for calculating the emissions associated with road transport for the given routes.

The Volkswagen Passat B8 2.0 TDI BMT 150 HP 4Motion Advance was chosen for this study for several reasons: popularity, fuel efficiency, emission standard compliance technical specifications, and availability of data. The Volkswagen Passat is a widely used vehicle across Europe and globally, making it a representative choice for a mid-sized family car. Its popularity ensures that the findings of this study are relevant and relatable to a broad audience. The Passat B8 2.0 TDI is known for its fuel efficiency, with a consumption rate of 3.9 L per 100 km. This makes it an excellent example of modern, efficient diesel engines that comply with stringent emission standards. The vehicle meets the EURO VI emission standard, which is one of the strictest regulations in the world for exhaust emissions from vehicles. This ensures that the emissions data collected are relevant to current and future regulatory environments. The car’s technical specifications, including a range of 1466 km and CO₂ emissions of 117 g per kilometer, provided a solid basis for accurate and meaningful emission calculations over the distances studied. Detailed specifications for the Volkswagen Passat B8 2.0 TDI were readily available, making it easier to obtain accurate data for this study. These included information on fuel consumption, engine displacement, and emissions. With a horsepower of 110 kW, the Passat provides a good balance between performance and efficiency, making it a practical choice for long-distance travel, as considered in this study. The chosen vehicle was capable of comfortably covering the distances specified in this paper (e.g., BER–FRA, BER–CDG, BER–BCN) without requiring frequent refueling, which is important for comparing the overall environmental impact of road versus air transport. By choosing the Volkswagen Passat B8 2.0 TDI, we ensured that the findings are applicable to a commonly used vehicle type, providing practical insights into the environmental consequences of road transport emissions.

Table 3. Aircraft route—BER–FRA [56].

Aircraft Specification	Airbus A321-100 (Lufthansa)
Total capacity	200	Length	44.51 m
Range	3000 km	Wingspan	34.10 m
MTOW	83,000 kg	Height	11.76 m
CO2 emissions	7.9–9.48 kg/km	Max. cruising speed	840 km/h

outlines the specifications of the Airbus A321-100 aircraft used by Lufthansa for the route from Berlin to Frankfurt. It includes details on passenger capacity, physical dimensions (length, wingspan, height), operational range, maximum takeoff weight, CO₂ emissions per kilometer, and maximum cruising speed. This information was essential for calculating the environmental impact of air transport on this specific route.

The BER–FRA route is one of the busiest and most frequently operated domestic routes in Germany. It serves as a vital connection between the capital city, Berlin, and one of Europe’s largest financial hubs, Frankfurt. This route is crucial for business travelers and tourists alike. The Airbus A321-100 is a commonly used aircraft for this route due to its suitable range, capacity, and operational efficiency. Lufthansa frequently deploys this model on the BER–FRA route, making it representative of typical operations on this flight path. The A321-100 is known for its fuel efficiency and relatively lower CO₂ emissions compared to older aircraft models. Its emissions range between 7.9 to 9.48 kg per kilometer, making it a relevant choice for studying the environmental impact of air transport. With a total capacity of 200 passengers, the A321-100 can accommodate a significant number of travelers, which is required for busy domestic routes like BER–FRA. This ensured that the emissions data per passenger are realistic and applicable to typical flight operations. The aircraft’s range of 3000 km, maximum cruising speed of 840 km per hour, and structural dimensions (length, wingspan, height) make it well-suited for the 430 km journey between Berlin and Frankfurt.

Table 4. Aircraft route—BER–CDG [57].

Aircraft Specification	Airbus A220-300 (Air France)
Total capacity	148	Length	38.7 m
Range	6112 km	Wingspan	35.1 m
MTOW	67,585 kg	Height	11.5 m
CO2 emissions	6.32–7.9 kg/km	Max. cruising speed	980 km/h

outlines the specifications of the Airbus A220-300 aircraft used by Air France for the route from Berlin to Paris. It includes details on passenger capacity, physical dimensions (length, wingspan, height), operational range, maximum takeoff weight, CO₂ emissions per kilometer, and maximum cruising speed. This information was essential for calculating the environmental impact of air transport on this specific route.

The BER–CDG route is a significant international route connecting two major European capitals. It is frequently traveled by business and leisure passengers, making it an important route for both Air France and travelers. The Airbus A220-300 is commonly used by Air France for this route due to its efficient performance and suitable capacity. It is known for its modern design and fuel efficiency, making it a representative choice for this study. The A220-300 was designed for efficiency, with CO₂ emissions ranging from 6.32 to 7.9 kg per kilometer. This lower emission rate compared to older models makes it a relevant choice for assessing environmental impact. With a total capacity of 148 passengers, the A220-300 is well-suited for medium-haul routes like BER–CDG.

Table 5. Aircraft route—BER–BCN [58].

Aircraft Specification	Airbus A320-200 (Vueling)
Total capacity	180	Length	37.57 m
Range	6112 km	Wingspan	35.8 m
MTOW	73,500 kg	Height	11.76 m
CO2 emissions	7.9–9.48 kg/km	Max. cruising speed	955 km/h

outlines the specifications of the Airbus A320-200 aircraft used by Vueling for the route from Berlin to Barcelona. It includes details on passenger capacity, physical dimensions (length, wingspan, height), operational range, maximum takeoff weight, CO₂ emissions per kilometer, and maximum cruising speed.

The BER–BCN route is a significant international route connecting two major European cities, Berlin and Barcelona. This route is frequently used by both business and leisure travelers, making it an important connection for Vueling and passengers. The Airbus A320-200 is a commonly used aircraft for this route due to its versatility, efficiency, and suitability for longer-haul flights. It is a popular choice among airlines for routes of this nature. The A320-200 is designed for operational efficiency, with CO₂ emissions ranging from 7.9 to 9.48 kg per kilometer. With a total capacity of 180 passengers, the A320-200 is well-suited for the BER–BCN route. The aircraft’s range of 6112 km and a maximum cruising speed of 955 km per hour enable it to efficiently cover the 1500 km distance between Berlin and Barcelona.

For the comparative analysis of emissions between air and road transport, it was essential to include data on the CO₂ emissions per passenger for each type of aircraft used in this study. The specific emission values for each aircraft were as follows (with a load factor of 100%): Airbus A321-100 operated by Lufthansa emitted between 39.5 to 47.4 g of CO₂ per kilometer per passenger; Airbus A220-300 operated by Air France emitted between 39.5 to 60 g of CO₂ per kilometer per passenger; and Airbus A320-200 operated by Vueling emitted between 43.9 to 63.2 g of CO₂ per kilometer per passenger. Additionally, the average load factors for these airlines in 2023 were 83% for Lufthansa, 95% for Air France, and 91% for Vueling.

These emission values and load factors were crucial for the analysis to calculate the total CO₂ emissions for both one-passenger and four-passengers scenarios. By incorporating these factors, it was possible to provide a more accurate and realistic comparison of emissions for different modes of transport over the same route. Calculating the emissions for different passenger scenarios helped to illustrate the impact of vehicle occupancy on the overall environmental footprint, highlighting the efficiency of higher occupancy in reducing per capita emissions.

Comparative Analysis

For the road transport calculations, a CO₂ emission rate of 117 g per kilometer was used for the Volkswagen Passat B8 2.0 TDI. The detailed calculations for the specified routes were as follows:

• BER–FRA

Total CO₂ Emissions = Distance × CO₂ Emission Rate

Total CO₂ Emissions = 553 km × 117 g/km = 64.7 kg

2.

BER–CDG

Total CO₂ Emissions = Distance × CO₂ Emission Rate

Total CO₂ Emissions = 1036 km × 117 g/km = 121.2 kg

3.

BER–BCN

Total CO₂ Emissions = Distance × CO₂ Emission Rate

Total CO₂ Emissions = 1887 km × 117 g/km = 220.8 kg

The CO₂ emissions for air transport were calculated per passenger based on the aircraft’s load factor and total emissions per kilometer. Here are the detailed calculations for each route:

• BER–FRA

Total CO₂ Emissions = Distance × Average CO₂ Emission Rate

Total CO₂ Emissions = 430 km × 8.69 kg/km = 3736.7 kg

$${\mathrm{CO}}_2\mathrm{Emissions}\mathrm{per}\mathrm{Passenger}={\displaystyle \frac{{\mathrm{Total}\mathrm{CO}}_2\mathrm{Emissions}}{\mathrm{Total}\mathrm{Seats}\times \mathrm{Load}\mathrm{Factor}}}$$

$${\mathrm{CO}}_2\mathrm{Emissions}\mathrm{per}\mathrm{Passenger}={\displaystyle \frac{3736.7\mathrm{kg}}{200\mathrm{seats}\times 0.83}}=22.6\mathrm{kg}$$

2.

BER–CDG

Total CO₂ Emissions = Distance × Average CO₂ Emission Rate

Total CO₂ Emissions = 857 km × 7.11 kg/km = 6094.2 kg

$${\mathrm{CO}}_2\mathrm{Emissions}\mathrm{per}\mathrm{Passenger}={\displaystyle \frac{{\mathrm{Total}\mathrm{CO}}_2\mathrm{Emissions}}{\mathrm{Total}\mathrm{Seats}\times \mathrm{Load}\mathrm{Factor}}}$$

$${\mathrm{CO}}_2\mathrm{Emissions}\mathrm{per}\mathrm{Passenger}={\displaystyle \frac{6094.2\mathrm{kg}}{148\mathrm{seats}\times 0.95}}=43.5\mathrm{kg}$$

3.

BER–BCN

Total CO₂ Emissions = Distance × Average CO₂ Emission Rate

Total CO₂ Emissions = 1500 km × 8.69 kg/km = 13,035 kg

$${\mathrm{CO}}_2\mathrm{Emissions}\mathrm{per}\mathrm{Passenger}={\displaystyle \frac{{\mathrm{Total}\mathrm{CO}}_2\mathrm{Emissions}}{\mathrm{Total}\mathrm{Seats}\times \mathrm{Load}\mathrm{Factor}}}$$

$${\mathrm{CO}}_2\mathrm{Emissions}\mathrm{per}\mathrm{Passenger}={\displaystyle \frac{\mathrm{13,035}\mathrm{kg}}{180\mathrm{seats}\times 0.91}}=79.8\mathrm{kg}$$

Table 6. CO₂ emissions for one passenger.

Route	Transport	CO2 Emissions
BER–FRA	Road	64.7 kg
	Air	22.6 kg
BER–CDG	Road	121.2 kg
	Air	43.5 kg
BER–BCN	Road	220.8 kg
	Air	79.8 kg

presents the CO₂ emissions for one passenger traveling between Berlin Brandenburg Airport and three different destinations—Frankfurt Airport, Paris Charles de Gaulle Airport, and Barcelona El Prat Airport—using either road or air transport. The data in Table 6 clearly indicate that for all three routes, air transport was more efficient in terms of CO₂ emissions per passenger compared to road transport. This efficiency was most pronounced on shorter routes, such as BER–FRA, where air transport emissions were approximately one-third of the road transport emissions. Even on longer routes like BER–BCN, air transport remained the more environmentally friendly option, with less than half the CO₂ emissions of road transport.

Table 7. CO₂ emissions for four passengers.

Route	Transport	CO2 Emissions
BER–FRA	Road	64.7 kg/4 (16.2 kg/1)
	Air	90.4 kg/4 (22.6 kg/1)
BER–CDG	Road	121.2 kg/4 (30.3 kg/1)
	Air	174 kg/4 (43.5 kg/1)
BER–BCN	Road	220.8 kg/4 (55.2 kg/1)
	Air	319.2 kg/4 (79.8 kg/1)

presents the CO₂ emissions for four passengers traveling between Berlin Brandenburg Airport and three different destinations—Frankfurt Airport, Paris Charles de Gaulle Airport, and Barcelona El Prat Airport —using either road or air transport. The values are presented as total emissions for four passengers and emissions per individual passenger. When considering four passengers traveling together, road transport consistently emits less CO₂ per passenger across all three routes than air transport. This finding highlights the efficiency of road transport in reducing per capita emissions when traveling in groups.

For the BER–FRA route, air travel emitted significantly less CO₂ per passenger compared to road transport. Specifically, the emissions for air travel amounted to 22.6 kg of CO₂ per passenger, whereas road transport emitted 64.7 kg of CO₂ per passenger. When considering a scenario with four passengers, road transport became more competitive, emitting 16.2 kg of CO₂ per passenger. Despite this improvement, air travel still maintained lower emissions per passenger at 22.6 kg of CO₂.

On the BER–CDG route, air travel also proved to be more efficient in terms of CO₂ emissions for a single passenger. Air travel emitted 43.5 kg of CO₂ per passenger compared to road transport’s 121.2 kg of CO₂ per passenger. However, when the scenario involved four passengers, road transport’s emissions per passenger decreased to 30.3 kg of CO₂, which is lower than air travel’s 43.5 kg of CO₂ per passenger, indicating that road transport became more efficient with higher occupancy.

For the BER–BCN route, air travel emitted less CO₂ for a single passenger, with emissions at 79.8 kg of CO₂ per passenger compared to 220.8 kg of CO₂ per passenger for road transport. In a four-passenger scenario, road transport’s emissions per passenger dropped to 55.2 kg of CO₂, which is still a little lower than air travel’s 79.8 kg of CO₂ per passenger. Thus, air travel remained the more efficient mode in terms of CO₂ emissions for this route, even when considering multiple passengers.

Figure 1 illustrates the comparison of CO₂ emissions per passenger for road and air transport on the routes between Berlin and three major destinations: Frankfurt, Paris, and Barcelona. The data show that air transport was significantly more efficient in terms of CO₂ emissions per passenger, particularly for longer distances. Figure 1 also compares the traffic volumes on these routes with the corresponding CO₂ pollution levels, highlighting the relationship between traffic density and emissions.

In summary, for shorter routes like BER–FRA, air travel is more efficient in terms of CO₂ emissions for both individual and group travel. For longer routes, such as BER–CDG and BER–BCN, the advantage of air travel diminishes, and road transport can be more efficient, especially when considering multiple passengers. This analysis underscores the importance of considering both distance and passenger load when evaluating the environmental impact of different transport modes. As such, it is possible to better understand the trade-offs and make more informed decisions to reduce the environmental footprint of our transportation choices.

4. Discussion

The findings of this study highlight the importance of evaluating CO₂ emissions from different transport modes over similar routes. The comparative analysis focused on the environmental consequences of road and air transport between Berlin and three major destinations: Frankfurt, Paris, and Barcelona. The study areas, including Germany, France, and Spain, have varying traffic intensities. Berlin Brandenburg Airport (BER) and Frankfurt Airport (FRA) are among the busiest in Europe, with high air traffic intensities, contributing significantly to CO₂ emissions. Similarly, the major highways connecting these cities experience heavy road traffic, further contributing to CO₂ emissions. Understanding these traffic intensities is crucial for accurately assessing the environmental impact and implementing effective mitigation strategies. By analyzing CO₂ emissions, it is possible to understand the relative environmental impacts and identify the most sustainable transport options.

On the BER–FRA route, air transport was found to be significantly more efficient than road transport in terms of CO₂ emissions per passenger. For a single passenger, air transport emitted only 22.6 kg of CO₂ compared to 64.7 kg for road transport. This indicates that for shorter distances, such as the 430 km flight to Frankfurt, air transport can be a more environmentally friendly option. Even when considering four passengers, air transport remained more efficient, albeit the margin was narrower, highlighting the importance of load factors in emission calculations.

The BER–CDG route presented a more complex picture. For one passenger, air transport again showed lower emissions (43.5 kg) than road transport (121.2 kg). However, when four passengers were considered, road transport’s emissions per passenger (30.3 kg) were lower than those of air transport (43.5 kg). This suggests that for medium distances, the efficiency of road transport can surpass that of air transport, especially when vehicles are fully occupied. This underscores the potential environmental benefits of carpooling and maximizing vehicle occupancy.

For the long-distance BER–BCN route, air transport was more efficient for a single passenger, with emissions of 79.8 kg of CO₂ compared to 220.8 kg for road transport. However, for four passengers, road transport’s emissions per passenger were reduced to 55.2 kg, lower than air transport’s 79.8 kg. This demonstrates that while air transport is generally more efficient for long distances, road transport can become competitive with higher occupancy rates.

According to the ICAO, CO₂ equivalents is a standard metric used to compare the emissions from various greenhouse gases based on their global warming potential. This study focused on CO₂ but acknowledges that a complete evaluation should consider CO₂ equivalents for a more comprehensive assessment. Moreover, the findings indicate that air transport becomes less efficient compared to road transport when passenger loads are below a certain threshold. This threshold varies depending on the route and vehicle efficiency but generally aligns with findings by other researchers such as Dahlmann et al. [7], who suggested that optimizing passenger loads is critical for minimizing emissions.

The findings of this study are consistent with those in the literature, such as those of Roy et al. [1] and Farzaneh et al. [2], which also highlight the efficiency of air transport for long distances and the advantages of higher occupancy rates in road transport. These studies support the conclusion that a multifaceted approach, including technological innovations and policy interventions, is essential for reducing transport emissions.

One critical aspect to consider is the methodology used in this study, which focused exclusively on CO₂ emissions due to their significant impact on climate change. While CO₂ is the primary greenhouse gas emitted by both air and road transport, other pollutants such as NO_x and PM also play crucial roles in environmental and public health impacts. A comprehensive life cycle assessment would provide a more holistic understanding of the environmental consequences, including the production, operation, and disposal phases, of vehicles and aircraft.

The choice of vehicle and aircraft for this study, i.e., the Volkswagen Passat B8 2.0 TDI for road transport and various Airbus models for air transport, was based on their common usage and the availability of detailed emission data. These choices ensured that the findings are relevant and applicable to typical travel scenarios. However, advancements in technology, such as the development of electric vehicles and sustainable aviation fuels, are likely to influence future emission profiles and should be considered in ongoing research.

While this study focused on specific routes in Europe, the methodologies and findings are applicable to other regions with adjustments for local conditions. Future research could expand the scope to include different countries and continents, considering regional variations in vehicle types, fuel quality, and transport infrastructure.

Furthermore, this study highlights the importance of load factors in determining the environmental efficiency of transport modes. Higher occupancy rates significantly reduce per capita emissions, making road transport more competitive in scenarios involving multiple passengers. This finding emphasizes the need for policies and initiatives that promote higher vehicle occupancy, such as carpooling incentives and efficient public transport systems.

Despite the overall higher efficiency of air transport for longer distances, it is important to acknowledge the environmental challenges associated with aviation. High-altitude emissions, including water vapor and NO_x, have complex interactions with the atmosphere, potentially amplifying their climate impact. Therefore, technological innovations and regulatory measures aimed at reducing aviation emissions are important for sustainable air transport development. It is also important to note that aircraft engine pollution and road vehicle pollution differ not only in composition but also in the altitude at which they are emitted. Aircraft emissions primarily occur at high altitudes, which can affect the upper atmosphere and contribute to the formation of ozone and contrails, impacting global climate [41]. In contrast, road vehicle emissions occur at ground level, directly affecting urban air quality and human health [45]. The mixing of these pollutants depends on atmospheric conditions, but both contribute to overall environmental pollution and have cumulative health impacts, particularly in areas with high traffic intensities.

International cooperation and regulatory frameworks play a vital role in mitigating the environmental impacts of transport emissions. Initiatives like the ICAO’s Carbon Offsetting and Reduction Scheme for International Aviation and the European Union’s emission standards for road vehicles are essential in driving reductions in CO₂ emissions. Continued efforts to strengthen these regulations and promote cleaner technologies are necessary to achieve long-term sustainability goals.

The use of alternative fuels and hybrid engines significantly impacts transport emissions. Sustainable aviation fuels, such as those derived from biofuels, can reduce CO₂ emissions by up to 80% compared to traditional jet fuels [61]. Similarly, hybrid-electric vehicles and fully electric vehicles offer substantial reductions in emissions for road transport [59]. For instance, electric vehicles powered by renewable energy sources can reduce CO₂ emissions to nearly zero during operation, although the entire lifecycle emissions must be considered [62].

In conclusion, this study provides valuable insights into the comparative environmental impacts of road and air transport on similar routes. While air transport generally exhibits lower CO₂ emissions per passenger for long distances, road transport can be more efficient with higher occupancy rates. This analysis underscores the need for a multifaceted approach to reducing transport emissions, including technological innovation, policy interventions, and changes in travel behavior. By considering both distance and passenger load, it is possible to make more informed decisions to minimize the environmental footprint of the transportation choices and work toward a more sustainable future.

5. Conclusions

The comparative analysis of the CO₂ emissions from road and air transport on routes between Berlin and three major destinations—Frankfurt, Paris, and Barcelona—provided significant insights into the relative environmental impacts of these transportation modes. The comparative analysis of CO₂ emissions revealed that air transport emitted significantly less CO₂ per passenger for longer distances. For example, the BER–FRA route produced 22.6 kg of CO₂ per passenger for air transport versus 64.7 kg for road transport, a reduction of approximately 65%. For medium and long distances, air transport continued to show lower emissions per passenger, highlighting the importance of passenger load in determining the most environmentally friendly transport mode. This study focused on CO₂ emissions due to their critical role in climate change, providing a clear understanding of how different travel options contribute to global warming.

This study has limitations, including its focus on specific routes and the exclusive consideration of CO₂ emissions. Future research could expand the scope to include a wider range of routes, additional pollutants such as NOx and PM, and life cycle assessments of vehicles and aircraft.

This study underscores the importance of considering both distance and passenger load when evaluating the environmental impact of different transport modes. While air transport generally exhibits lower CO₂ emissions per passenger for longer distances, road transport can be more efficient with higher occupancy rates. These findings suggest the need for a multifaceted approach to reducing transport emissions, including technological innovation, policy interventions, and changes in travel behavior.

Technological advancements, such as the development of electric vehicles and sustainable aviation fuels, are likely to influence future emission profiles and should be considered in ongoing research. International cooperation and regulatory frameworks are crucial in mitigating the environmental impacts of transport emissions. Continued efforts to strengthen these regulations and promote cleaner technologies are necessary to achieve long-term sustainability goals.

In conclusion, air transport generally exhibits lower CO₂ emissions per passenger for longer distances than road transport. Practical recommendations include promoting higher occupancy rates in road transport through carpooling and public transport incentives and advancing the adoption of alternative fuels and hybrid engines in both air and road transport. Policy interventions and technological innovations are critical to achieving these goals and reducing the overall environmental footprint of transportation.