How does nitrogen gas fly the planes

What do aircraft emissions do?

Aircraft exhaust has two main components: carbon dioxide (CO2) and water vapor. In addition, there are nitrogen oxides, which either contribute to the formation or depletion of ozone. Both CO2 and water vapor and ozone are infrared-active gases - they are able to absorb heat radiation and also to emit it again. As a result, air traffic contributes to the warming of the atmosphere, says the director of the Institute for Atmospheric Physics at the German Aerospace Center (DLR). He estimates that the total contribution of air travel to the human contribution to climate change is around three percent.

Carbon dioxide is distributed evenly

Ulrich Schumann is on the trail of emissions

Kerosene consists of 86 percent carbon and 14 percent hydrogen. Because carbon combines with oxygen from the air during combustion, for every kilogram of kerosene that an aircraft burns, 3.15 kilograms of CO2 are released from the turbines. "Since the CO2 is long-lived in the atmosphere, it mixes evenly across the earth," explains Schumann.

Because it can migrate through all layers of the atmosphere, it does not matter whether the gas was emitted at an altitude of 10,000 meters or near the ground. The share of air traffic in climate change can thus be calculated very easily: Around 2.2 percent of man-made CO2 is due to air traffic. Road traffic accounts for around 14 percent and other modes of transport such as ship and rail together make up 3.8 percent.

In the case of water vapor, it is more difficult to clearly calculate the effects on the climate. When one kilogram of kerosene burns, 1.23 kilograms of water vapor are produced. When the hot and humid combustion gases mix with cold ambient air, the water vapor condenses and tiny water droplets are formed. In the cold air at around minus 40 degrees Celsius, the droplets freeze and form tiny ice crystals that are visible as contrails.

What happens then depends primarily on where the aircraft is currently: in the lower troposphere or in the higher stratosphere. "The troposphere is the well-mixed layer of the atmosphere in which the weather takes place," explains Schumann. "Above that is the stratosphere, which is very stable, and there is very little mixing."

DLR uses the Falcon research aircraft to measure aircraft emissions

Water vapor warms and cools

The stratosphere is very dry. The volume of water vapor in the air is less than 0.01 per thousand. There, the ice crystals from the contrails evaporate very quickly. But in the troposphere, where the air can be saturated with water vapor, how the contrails behave depends very much on the weather.

"If the air is humid, the ice particles absorb more moisture from the environment. Then the ice particles grow and the condensation streaks in the sky can expand into cirrus clouds. They act as a kind of condensation nucleus for moist air, can grow in their water mass and become thick clouds "says the atmospheric researcher. This is the case in around 10-20 percent of all flight routes. "In this respect, air traffic with its contrails actually increases the cloudiness of the earth," emphasizes Schumann.

However, the effects of these clouds on the climate are contradictory. On the one hand, the short-wave sunlight is partly reflected back into space by the contrails during the day. "To put it simply, one could say: The contrails cast a shadow on the earth. And in the shadow it is cooler." On the other hand, the ice particles in these clouds also absorb long-wave infrared radiation, some of which is reflected back to the earth. This effect is important day and night. Which of the two effects predominates is still considered to be the "crucial question in contrail research," but previous studies have shown "that the warming effect of contrails predominates," says Schumann.

Effect of soot still not understood

Aircraft exhaust also cause tiny soot particles. These are only between 5 and 100 nanometers in size. It is clear that the hot water vapor is deposited on these particles shortly after the aircraft turbine, among other things, when it condenses. But even without contrails, the particles remain in the atmosphere for many days. The researchers therefore suspect that these soot particles, if they are distributed in the atmosphere, can still act as condensation nuclei for cloud formation after days and weeks. The soot competes with other particles such as desert dust or acid droplets to form ice in the atmosphere, which complicates matters.

Contrails can turn into thick clouds in high humidity

In order to find out how soot affects cloud formation, DLR is researching not only aircraft emissions, but also dust clouds that are distributed in the atmosphere around the world after major forest fires. However, the results so far are contradicting itself. "It's not that easy, there has not yet been a clear answer," says the atmospheric researcher. It is not even known whether the soot will ultimately increase or decrease the cloud formation.

Good ozone - bad ozone

Aircraft engines get very hot. The temperatures in the combustion chamber can reach 2000 degrees Celsius locally. "At such temperatures, nitrogen from the air is also bound and converted into nitrogen oxide (NO and NO2)," explains the researcher. This nitrogen oxide can then develop two opposing effects: "At high altitudes, the nitrogen oxides lead to ozone depletion through photocatalytic processes. At low altitudes, the nitrogen oxides lead to ozone increase - so there are exactly the opposite effects."

However, it is now clear that an average decrease in ozone can only be expected in the higher stratosphere from 16,000 meters, where mainly supersonic aircraft are on the move. At the usual altitudes of commercial aircraft, i.e. below 12,000 meters, the nitrogen oxides, on the other hand, lead to an increase in ozone.

This ozone, like CO2 and water vapor, then acts as a greenhouse gas. In any case, the ozone hole over the Antarctic could not be plugged with air traffic emissions. This is because the planes fly far below the ozone layer, which protects us from the ultraviolet radiation. And the ozone usually does not get to the top.

Author: Fabian Schmidt
Editor: Brigitte Osterath