Airplane Pollution: Understanding The Impact On Our Environment

Aviation is a significant contributor to global warming and climate change. Airplanes emit carbon dioxide (CO2) from burning fuel, and their emissions are around 1 billion tons of CO2 per year, more than the emissions of most countries. In addition to CO2, planes also emit other gases and pollutants such as ozone, methane, water vapour, soot, sulfur aerosols, water contrails, and nitrogen oxides. These emissions have serious environmental and health consequences, including an estimated 6,800 to 16,000 premature deaths per year. With the demand for air travel projected to increase, it is crucial to address the pollution caused by airplanes and transition to a more sustainable aviation industry.

Carbon dioxide emissions

Carbon dioxide (CO2) emissions from aircraft come from burning fuel. In 2019, global aviation emitted around 1 billion tons of CO2, accounting for about 2.5% of global CO2 emissions from fossil sources and land use. This share has varied between 2% and 2.5% since the mid-1990s but has noticeably increased since 2010.

CO2 emissions from commercial aviation have increased in recent years, with the industry's contribution to global emissions rising as commercial air traffic grows. In 2013, global CO2 emissions from commercial aviation were 707 million tons, rising to 920 million tons in 2019, a roughly 30% increase over six years. The United States, which has the world's largest commercial air traffic system, accounted for 200 million tons (23%) of the global CO2 emissions in 2017.

The number of people flying and the amount of cargo transported have quadrupled between 1990 and 2019, while flying has become more than twice as energy efficient. This efficiency has come from improved design and technology, larger planes that can carry more passengers, and a higher 'passenger load factor'. As a result, the carbon efficiency of traveling one kilometer has also more than doubled. In 1990, one passenger-kilometer emitted 357 grams of CO2, which dropped to 157 grams by 2019.

To reduce the climate and health impacts of aviation, Klöwer and colleagues have suggested two main strategies: a sustained annual decrease in air traffic by 2.5% or a transition to a 90% carbon-neutral fuel mix by 2050. Other proposed solutions include using flight paths that avoid contrail formation, building more efficient planes with post-emission controls, using a mix of sustainable fuels, and electrifying short-range flights with renewable energy sources.

Regulations and standards have been put in place to address CO2 emissions from aircraft. In 2016, the United Nations International Civil Aviation Organization (ICAO) established CO2 emission standards for new aircraft, with more restrictive efficiency standards for designs certified after 2020 for commercial jets and 2023 for business jets. The EPA also set CO2 emission standards for US aircraft under the Clean Air Act, matching or exceeding ICAO requirements. Aircraft manufacturers such as Boeing, Airbus, and others are already meeting and exceeding these standards, and airlines are expected to improve their fleet performance as they adopt newer aircraft.

Nitrogen oxides emissions

Nitrogen oxides (NOx) are one of the emissions produced by aircraft engines during jet kerosene combustion. NOx emissions from air travel affect the atmospheric concentrations of methane (CH4) and ozone (O3), which are greenhouse gases (GHGs). While O3 remains in the atmosphere for 2-8 weeks, CH4 can persist for at least 10 years, and over a 100-year timeframe, it is about 34 times as powerful as CO2. The formation of O3 by aircraft is similar to the formation of smog by road traffic. However, due to increased UV radiation at high altitudes, O3 is formed more effectively at cruising altitudes.

NOx emissions from aircraft engines lead to an initial increase in O3, resulting in a warming effect that can last for several months. This is followed by a long-term decrease in CH4 and O3, which has a cooling impact. Despite the subsequent decrease in CH4 and O3, the initial increase in O3 concentration is not offset. NOx emissions also contribute to the formation of smog-like conditions and air pollution near airports, with potential consequences for public health.

The AERONOX project investigated the emissions of NOx from aircraft engines and global air traffic at cruising altitudes. Simplified calculations indicate that aviation's contribution to radiative forcing (RF) will increase by factors of 3.0–4.0 by 2050 compared to the 2000 value, representing 4–4.7% of total RF (excluding induced cirrus). Aviation-induced RF contributes to climate change and the overall warming effect of aviation activities.

To address the issue of NOx emissions, one proposed solution is to employ post-emission controls, similar to catalytic converters in cars, to clean the exhaust as it leaves the airplane engine. However, retrofitting old planes with such technology is not cost-effective, so new aircraft would need to be built with these systems in place. Governments and regulatory bodies may need to provide incentives or mandate the adoption of more efficient aircraft with improved emission controls to address this issue effectively.

Soot emissions

Aircraft emit soot (black carbon) and sulfate aerosols. Dark soot particles absorb solar radiation and have a warming effect on the environment. This effect is particularly pronounced when soot is deposited on snow and ice, darkening the light surface and reducing its albedo effect (dark surfaces absorb more radiation than light surfaces).

The formation of soot is a result of the incomplete combustion of fossil fuels, including kerosene, which results in the formation of carbon-rich (>60%) by-products called char and condensates. Char and soot can be measured as elemental carbon (used in atmospheric sciences) or black carbon (used in soil and sediment sciences). The use of jet fuels with high aromatics and naphthalene concentrations increases soot formation, leading to persistent contrail cirrus.

To mitigate soot emissions, several solutions have been proposed:

• Reducing the aromatics content of fossil jet fuel down to 8-10% can be achieved without significant costs and could significantly lower non-CO2 effects.
• Using clean fuels, such as a 90% carbon-neutral fuel mix, to reduce the amount of pollutants released into the air.
• Employing post-emission controls, similar to catalytic converters in cars, to clean the exhaust as it leaves the airplane engine.
• Using flight paths that avoid contrail formation in cold, high-humidity atmospheric areas known as Ice Super Saturated Regions (ISSRs).

Water vapour emissions

Aircraft engines emit gases such as nitrous oxides (NOx), sulfur dioxide (SO2), and water (H2O), as well as particulate matter (soot). These emissions, when released at high altitudes, alter the chemical and physical properties of the atmosphere, leading to an increase in greenhouse gases and the potential formation of persistent contrail cirrus clouds.

The use of hydrogen fuel in aircraft has been proposed as a way to reduce NOx emissions. Hydrogen combustion produces significantly less NOx than kerosene fuel, which can contribute to upper tropospheric background ozone levels. However, hydrogen-fuelled aircraft are estimated to produce up to 2.6 times more water vapour than kerosene-fuelled aircraft. This increase in water vapour emissions could have potential implications for the climate, as water vapour is a greenhouse gas and can contribute to overall warming.

While the exact methods for calculating emissions from hydrogen-fuelled aircraft are still being refined, initial studies indicate that hydrogen-fuelled aircraft emit significantly more water vapour than their kerosene-fuelled counterparts. This difference in water vapour emissions is due to the increased mass and drag associated with hydrogen combustion, requiring more hydrogen to be burned and resulting in higher H2O emissions.

Overall, water vapour emissions from aircraft, particularly those powered by hydrogen fuel, have the potential to significantly impact the climate and contribute to global warming. While the use of hydrogen fuel can reduce NOx emissions, the increased water vapour emissions present a new challenge in the fight against climate change.

Aircraft noise

The primary source of aircraft noise is the engines, particularly during takeoff and climb. Jet engines, or gas turbine engines, produce a significant amount of noise, including the distinctive "buzzsaw" sound generated when the fan blades reach supersonic speeds. High-bypass-ratio turbofans also contribute to fan noise, and the high-velocity jet exiting the engine can cause turbulence, further adding to the overall noise level.

In addition to engine noise, there are other factors that contribute to aircraft noise pollution. These include aerodynamic noise, which is caused by the airflow around the aircraft's surfaces, especially during low-altitude, high-speed flight. The aircraft's systems, such as the cockpit, cabin pressurization, conditioning systems, and Auxiliary Power Units (APUs), can also generate significant noise. In propeller aircraft, the propellers themselves are a major source of noise, in addition to the aerodynamic noise.

Helicopters produce a unique form of aircraft noise. Their main and tail rotors create aerodynamically induced noise, while the main gearbox and transmission chains contribute to mechanically induced noise. Additionally, specialized electronic equipment in military aircraft can also generate internal noise.

While aircraft noise is a challenging issue, advancements in noise reduction technologies offer some hope. For example, the Pratt & Whitney PW1000G engine has significantly reduced noise levels in several aircraft models by optimizing the fan speed. Similarly, the PowerJet SaM146 engine in the Sukhoi Superjet 100 employs 3D aerodynamic fan blades and a specially designed nacelle to minimize noise.

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