Contrails, Chemistry, and Jet Fuel Emissions: A Second Look

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Contrails may be seen if temperature and weather is ideal. Image by NASA/ORAU Richard Moore.

The chemical dynamics of jet contrails are incomplete without a full, physical meaning.

The purely chemical descriptions add color and an intuitive understanding.

The rigorous descriptions of ‘microscopic surface phenomena’ in contrail formation (in the last century) have evolved as sophisticated techniques progressed from laboratory to computational simulation.

Although the first pioneers in the field of cloud science were a part of the quantum mechanics’ generation of pioneers, their methodology for studying cloud formation reflected both a (quantum) probabilistic attitude and a pragmatic philosophy to understand weather dynamics

Surface Phenomena Hold the Key to Explaining Clouds and Contrails

When the field of modern physical chemistry took shape in the early 20th century, stellar performances from researchers in the physical sciences framed a foundation for the following century of discovery.

Perhaps the first stellar performance came from Nobel laureate, Irving Langmuir.

Utilizing Langmuir’s Methods to Understand Cirrus Cloud Formation, Simplifying Assumptions from Langmuir’s Work on Light Bulbs Serve as a Model. Copyright by John Jaksich. All Rights Reserved.

Langmuir studied ‘surface phenomena,’ or physical-chemical dynamics of incandescent light filaments.

Although the study of incandescent bulbs seems far removed from cirrus clouds, this is how Langmuir came to understand the tungsten filament that interests atmospheric scientists.

Original incandescent light bulbs were not as efficient as modern incandescent bulbs – they contained a myriad of gases that aged the filament. Thus, Langmuir’s experiments showed that the hot interiors of incandescent bulbs heated the gases that reacted with the tungsten—decreasing the bulb’s expected life.

Symbolically the scenario looks like the image below: (The complete set of reactions are omitted for clarity. The chemical symbols are: Tungsten: W, and Oxygen: O.)

Scientists Use the Langmuir Ansatz to Explain Contrail Formation

In a first approximation to cirrus (or contrail) cloud formation, we need to understand that clouds are simply water crystals. Using a thermodynamic (heat and energy) argument we can compute how the pressures, volumes, and temperatures influence contrail/cirrus cloud formation, P = T (constant/Volume).

Langmuir’s Ansatz illustrates how water vapor becomes ice (through an intermediate stage). Copyright by John Jaksich. All Rights Reserved.

The equation governs how a designated temperature and pressure produce a ‘given amount of ice.’ The (constant/Volume) term (when graphed) can be regarded as the rate at which ice will form under atmospheric conditions. When discerning how water (both vapor and liquid) becomes crystalline from purely thermodynamic considerations, the type of ice (pure water and not heterogeneous mixtures of atmospheric constituents) is an important first approximation.

Real World Exhaust Mixtures form Contrails with Heterogeneous Species from Jet Exhaust

Understanding the heterogeneous nature of atmospheric ice and contrail dynamics gives way to comprehending ‘real world’ chemistry. Again, starting from an approximation, we are aware that contrails form in the presence of jet exhaust and surrounding atmosphere. From a sulfurous/water vapor model, results point towards (utilizing Langmuir’s Ansatz):

Reaction Scheme Showing Forms of Contrail Ice and Initial Intermediate Species, Water and Sulfuric Acid. Copyright by John Jaksich. All Rights Reserved.

Jet Emissions and Chemistry

The realities of flight dictate that there are a large variety of  mis-characterized emissions in jet contrails.

As with any combustible fuel, jet engines experience daily wear from routine to stressful flight conditions; the resultant emission is rarely benign (the formation of a contrail from only sulfurous species can be ruled out).

The differing species in jet exhaust include: unspent fuel, aluminum, sulfur, manganese, chromium, and barium.

In Australian research that cites studies performed by government agencies from the United States, Australia, and France, a virtual cornucopia of metals and other inorganic species have been found in and around major airports. These inorganic elements are considered toxic in many instances, and we normally avoid them – but they’re a normal part of jet operations.

The metallic species (i.e., manganese or chromium) result from the corrosion of sulfuric acid upon the jet engines, and the sulfuric acid  results from sulfur-containing jet fuel.

When the simplified conditions of jet exhaust meet with rumors about jet contrails being vehicles for poison gas, social experiments, or germ warfare, one is apt to come away bewildered and frightened, as well.

NASA scientists work to modify contrail ‘exhaust’ with special blends of fuels. Instruments mounted on the experimental jet are characterizing the soot and gases streaming from the DC-8. Image by NASA/Eddie Winstead.

Conspiracy theories take advantage of the fact that the Laws of Thermodynamics allow for a jet engine’s increasing inefficiency as time advances. However, common sense dictates that daily wear to a combustion engine results in inefficient operation – and jet emissions that may look scary but are, in reality, just exhaust.

Kinder and Gentler Jet Fuels

Research into ecologically-conscious jet fuel is underway. Research groups from around the world are attempting to remove the soot (metal-containing species in exhaust) and sulfuric acid species found in the average jet liner exhaust.

The thrust of some groups is to manufacture synthetic fuel; plant oils or gasification of coal are under investigation.

Despite current efforts to find a benign carbon substitute, carbon fuels continue to deposit GHGs (green house gases) into the atmosphere. We need a paradigm shift from easily-available carbon to an also easily-available alternative that is presently unknown.

Until then, we – and the environment – will await the paradigm-shift in energy and engine design.

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