Editing Contrail (section)

==Condensation trails as a result of engine exhaust==
[[File:Qantas Boeing 747-400 VH-OJU over Starbeyevo Kustov.jpg|thumb|Contrails of a [[Boeing 747-400|Boeing 747-438]] from [[Qantas]] at {{cvt|11000|m|ft}}]]

Engine exhaust is predominantly made up of water and carbon dioxide, the combustion products of hydrocarbon fuels. Many other chemical byproducts of incomplete hydrocarbon fuel combustion, including [[volatile organic compounds]], [[Inorganic compound|inorganic]] gases, [[polycyclic aromatic hydrocarbons]], [[Oxide|oxygenated]] organics, [[alcohols]], [[ozone]] and particles of soot have been observed at lower concentrations. The exact quality is a function of engine type and basic combustion engine function, with up to 30% of aircraft exhaust being unburned fuel.<ref>{{Cite journal |title = Biological and health effects of exposure to kerosene-based jet fuels and performance additives|year = 2003|doi = 10.1080/10937400306473|s2cid = 30595016|last1 = Ritchie|first1 = Glenn|last2 = Still|first2 = Kenneth|last3 = Rossi Iii|first3 = John|last4 = Bekkedal|first4 = Marni|last5 = Bobb|first5 = Andrew|last6 = Arfsten|first6 = Darryl|journal = Journal of Toxicology and Environmental Health, Part B|volume = 6|issue = 4|pages = 357–451|pmid = 12775519|access-date=}}</ref> (Micron-sized metallic particles resulting from engine wear have also been detected.{{cn|reason=this may be true but the quantities would be extremely small and vanishingly less than the water and co2: let's see the exact amounts detected|date=May 2024}}) At high altitudes as this water vapor emerges into a cold environment, the localized increase in water vapor can raise the [[relative humidity]] of the air past [[dew point|saturation point]]. The vapor then condenses into tiny water droplets which freeze if the temperature is low enough. These millions of tiny water droplets and/or ice crystals form the contrails. The time taken for the vapor to cool enough to condense accounts for the contrail forming some distance behind the aircraft. At high altitudes, supercooled water vapor requires a trigger to encourage deposition or condensation. The exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapor to condense rapidly. Exhaust contrails usually form at high altitudes; usually above {{convert|8000|m|ft|abbr=on}}, where the air temperature is below {{convert|-36.5|C|0|lk=on}}. They can also form closer to the ground when the air is cold and moist.<ref>{{cite web|url=http://science-edu.larc.nasa.gov/contrail-edu/faq.php|title=Contrail Education – FAQ|work=nasa.gov|url-status=dead|archive-url=https://web.archive.org/web/20160408184845/http://science-edu.larc.nasa.gov/contrail-edu/faq.php|archive-date=8 April 2016}}</ref>

A 2013–2014 study jointly supported by NASA, the German aerospace center DLR, and Canada's National Research Council NRC, determined that [[biofuel]]s could reduce contrail generation. This reduction was explained by demonstrating that biofuels produce fewer soot particles, which are the nuclei around which the ice crystals form. The tests were performed by flying a [[Douglas DC-8|DC-8]] at cruising altitude with a sample-gathering aircraft flying in trail. In these samples, the contrail-producing soot particle count was reduced by 50 to 70 percent, using a 50% blend of conventional Jet A1 fuel and HEFA (hydroprocessed esters and fatty acids) biofuel produced from [[camelina]].<ref>{{cite news |url= http://aviationweek.com/technology/week-technology-march-20-24-2017 |at= Paper published in ''Nature'', Rich Moore & Hans Schlager, authors |work= Aviation Week & Space Technology |title= The Week in Technology |date= 20–24 March 2017 |url-access= subscription}}</ref><ref>{{cite news |url= https://www.ainonline.com/aviation-news/business-aviation/2017-12-24/biofuels-could-reduce-contrail-formation-research-finds |title= Biofuels Could Reduce Contrail Formation, Research Finds |author= Sean Broderick |date= 24 December 2017 |access-date=13 October 2021}}</ref><ref>{{cite journal |title= Biofuel blending reduces particle emissions from aircraft engines at cruise conditions |author= Richard H. Moore|display-authors=et al |journal= Nature |volume= 543 |issue= 7645 |pages= 411–415 |date= 15 March 2017|doi= 10.1038/nature21420 |pmid= 28300096 |pmc= 8025803|bibcode= 2017Natur.543..411M |s2cid= 4447403|url= https://elib.dlr.de/112943/1/Moore_et_al_Nature_2017.pdf |archive-url=https://web.archive.org/web/20190427124937/https://elib.dlr.de/112943/1/Moore_et_al_Nature_2017.pdf |archive-date=2019-04-27 |url-status=live}}</ref>