The trail of contrails
Humans have undeniably affected the atmosphere and air quality of our planet. However, because most gasses are invisible to the human eye, it can be hard to physically see some of the changes we’ve brought about. Smog, exhaust from cars and trucks and smoke from chimneys and fires are a few of the visible signs, but one of the most intriguing visual manifestation of how we’ve changed the atmosphere are the jet trails left behind by airplanes. These lines of clouds are called contrails, and anyone who lives close to an airport or commonly used airspace sees them every day. This paper will examine what contrails are, how they are made and what effects they might have on our atmosphere. These effects are particularly important because of the rapid growth of the airline industry, which has been increasing two and a half times the average rate of economic growth since 1960. (6)
Most modern commercial airplanes run on jet engines that burn fossil fuels, usually kerosene, which is a hydrocarbon liquid obtained from crude oil (5). The process used by the engine to convert fuel to energy is internal combustion, the same process used by a normal car engine, where the fuel is combined with oxygen to produce heat, water, and carbon dioxide. Carbon dioxide is of course one of the most common greenhouse gasses that contributes to global warming, and according to NASA, aircraft are responsible for four percent of the annual carbon dioxide emissions from fossil fuels across the globe. It is estimated that the world’s 16,000 commercial air jet aircraft produce 600 million tons of carbon dioxide per year, nearly equal to the amount produced per year by all human activities in Africa (6). (1)
Higher carbon dioxide emissions are the result of incomplete combustion, which occurs at the lower power levels the jet engines operate at during taxiing, and landing. The higher the temperature inside the engine, the more efficiently the fuel is burned. Unfortunately, higher engine temperatures come with their own pollution problems. One of these is the increased amount of nitrogen oxide that is emitted. Nitrogen oxide leads to the formation of ozone in the atmosphere, and although ozone is necessary in the upper atmosphere (about fifteen miles high), it can be a harmful pollutant at ground level. Since most commercial jet aircraft only operate about a mile high, this is a significant concern. NASA reported that “In 1993, a study of toxic emissions at Chicago’s Midway Airport revealed that arriving and departing planes released more pollutants than the industrial pollution sources in the surrounding 16-square-mile area. A more recent study at London’s Heathrow airport showed that aircraft contributed between 16 and 35 percent of ground level NOx concentrations.” (1)
Are these gasses, carbon dioxide and nitrogen oxide, what make up the visible contrails left by airplanes? In fact, they are not. These emissions are the same gasses produced by automobiles and are largely invisible. However, understanding the production of these gasses helps us to understand what contrails really are. When a jet engine operates, it combines fuel and air to produce heat and energy. Carbon dioxide is one byproduct of this reaction. Steam is another. There is water vapor in the air, and under the intense heat of the combustion reaction, this vapor is heated up into steam. This, combined with the fact that water itself is a byproduct of an internal combustion reaction, means that about a gallon of water being produced per gallon of fuel burned. As a result, when the engine operates at a higher power and more fuel is burned, more steam is produced, and increase at the same time that nitrogen oxide increases. (3)
Because the air temperature at the altitude airplanes operate at, is far below freezing, the steam quickly freezes into ice particles, which along with any dissolved particles, or aerosols in the air, are what make up the visible contrail. If the air is very dry, there will be little water vapor present and no contrail will be formed. The more moisture present in the air, the larger the contrail will be and the longer it will last. Sailors have used this as sign to help them judge the weather conditions for many years. (2)
It seems then, that in the face of more significant pollutants like carbon dioxide and nitrogen oxide, contrails are relatively harmless. However, they can have serious affects on the lower atmosphere. In 1998 NASA ran an experiment where they flew a jet in circles and found that the contrails they created ultimately coalesced into a cirrus cloud covering 1400 square miles (3). However, the effects of contrails on the atmosphere are still currently being debated. The Intergovernmental Panel on Climate Change, affirms that contrails can from into cirrus clouds, and asserts that both cirrus clouds and contrails themselves contribute to global warming themselves (4).
Other groups believe that contrails actually contribute to global dimming, or the blocking of light and heat from reaching the earth’s surface. They point to the average increase of one degree Celsius in the temperature over the United States during the three day suspension of commercial air travel after September 11 2001 as evidence that contrails, possibly because of the aerosols within them, deflect solar radiation. (3)
Either way, it is certain that contrails, the vapor trails left behind by airplanes that we see every day, play a define role in the climate of our planet and the health of the atmosphere.